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Dynamics of cratons in an evolving mantle
Abstract
The tectonic quiescence of cratons on a tectonically active planet has been attributed to their physical properties such as buoyancy, viscosity, and yield strength. Previous modelling has shown the conditions under which cratons may be stable for the present, but cast doubt on how they survived in a more energetic mantle of the past. Here we incorporate an endothermic phase change at 670 km, and a depth-dependent viscosity structure consistent with post-glacial rebound and geoid modelling, to simulate the dynamics of cratons in an “Earth-like” convecting system. We find that cratons are unconditionally stable in such systems for plausible ranges of viscosity ratios between the root and asthenosphere (50–150) and the root/oceanic lithosphere yield strength ratio (5–30). Realistic mantle viscosity structures have limited effect on the average background cratonic stress state, but do buffer cratons from extreme stress excursions. An endothermic phase change at 670 km introduces an additional time-dependence into the system, with slab breakthrough into the lower mantle associated with 2–3 fold stress increases at the surface. Under Precambrian mantle conditions, however, the dominant effect is not more violent mantle avalanches, or faster mantle/plate velocities, but rather the drastic viscosity drop which results from hotter mantle conditions in the past. This results in a large decrease in the cratonic stress field, and promotes craton survival under the evolving mantle conditions of the early Earth.
... Cratons are the oldest continental nuclei on Earth, which have survived for billions of years (Pearson et al., 2021). The long-term resistance of the thick cratonic lithosphere (>180 km) to asthenospheric erosion is attributed to its distinctive chemical and physical properties, which are characterized by a lower density and higher strength compared to the surrounding mantle (Boyd, 1989;Jordan, 1975;Karato, 2010;Lenardic et al., 2003;Lenardic & Moresi, 1999;O'Neill et al., 2008;Peslier et al., 2010). Because the chemical buoyancy alone cannot account for the longevity and stability of cratons (Jordan, 1975;Lenardic et al., 2003), a high strength associated with sufficient viscosity contrasts (>3 to >3000 times) between the cratonic root (depleted and cold) and the underlying asthenosphere is thought to be the key factor characterizing typical stable cratons, such as the Slave, Kaapvaal, and Siberian cratons (Doucet et al., 2014;Karato & Wu, 1993;Kilgore et al., 2020;Kohlstedt et al., 1995;Lenardic & Moresi, 1999;O'Neill et al., 2008;Peslier et al., 2010Peslier et al., , 2017Sleep, 2003;Taylor et al., 2016;H. ...
... The long-term resistance of the thick cratonic lithosphere (>180 km) to asthenospheric erosion is attributed to its distinctive chemical and physical properties, which are characterized by a lower density and higher strength compared to the surrounding mantle (Boyd, 1989;Jordan, 1975;Karato, 2010;Lenardic et al., 2003;Lenardic & Moresi, 1999;O'Neill et al., 2008;Peslier et al., 2010). Because the chemical buoyancy alone cannot account for the longevity and stability of cratons (Jordan, 1975;Lenardic et al., 2003), a high strength associated with sufficient viscosity contrasts (>3 to >3000 times) between the cratonic root (depleted and cold) and the underlying asthenosphere is thought to be the key factor characterizing typical stable cratons, such as the Slave, Kaapvaal, and Siberian cratons (Doucet et al., 2014;Karato & Wu, 1993;Kilgore et al., 2020;Kohlstedt et al., 1995;Lenardic & Moresi, 1999;O'Neill et al., 2008;Peslier et al., 2010Peslier et al., , 2017Sleep, 2003;Taylor et al., 2016;H. Wang et al., 2014). ...
... Reducing the viscosity contrast between the cratonic root and the asthenosphere is necessary to facilitate craton destruction (Hirth & Kohlstedt, 1996;Z. X. A. Li et al., 2008;Liao et al., 2017;O'Neill et al., 2008;Peslier et al., 2010;C. Sun & Dasgupta, 2020;H. ...
Abstract The disruption of the mantle roots of cratons is common after cratonization. Craton destruction, which is characterized by severe lithospheric thinning, extensive thrust and extensional deformation, basin filling, and intense thermal activities, is relatively rare and is generally attributed to intensely reduced viscosity contrasts between the lithospheric mantle root and the underlying asthenosphere. However, the extent of the required viscosity contrast remains unclear. The North China craton (NCC) is a typical example of a partially destroyed craton, with its eastern part experiencing destruction in the Early Cretaceous. In this study, we measured the water content of clinopyroxene phenocrysts in Middle Jurassic lithospheric mantle‐derived, weakly alkaline volcanic rocks from the Shanhaiguan area. Our data and those of previous studies show that the lithospheric mantle in the eastern NCC contained substantial amounts of water (650–2,900 ppm) before craton destruction. In addition, the continuous supply of water by the subducted Paleo‐Pacific slab resulted in a more hydrous lithospheric mantle (up to ca. 9,000 ppm) during the craton destruction. The viscosity contrasts between the lithospheric mantle root and the asthenosphere (with an average viscosity of 3.7 × 1018 Pa s) were 2–8 and 0.3–2 before and during the craton destruction, respectively. Our study indicates that a drastic drop in the lithospheric mantle viscosity, which is controlled by the synergic effects of a high water content and elevated temperature, is required to induce craton destruction.
... There have been numerous lines of evidence suggesting the preservation of the Precambrian crustal basement and similarly old deeper mantle roots beneath cratons (e.g., Pearson, 1999;Richardson et al., 1984;Spetsius et al., 2002;Walker et al., 1989). Compared to the short-lived oceanic lithosphere that is recycled back into the mantle on a timescale of ∼100 Ma, the continental lithosphere is capable of resisting deformation and avoiding recycling as a whole over a timescale of about one order of magnitude longer (Lenardic et al., 2003;O'Neill et al., 2008). The longevity and stability of continents, in particular cratons, have been primarily attributed to the structure and intrinsic (physical and chemical) properties of the continental lithosphere, relative to the asthenosphere and oceanic lithosphere ( Figure 14 and Table 1). ...
... A significant reduction of the crustal contribution by scraping off the most buoyant upper crust would dramatically increase the density of the continental lithosphere and make it readily subductable (e.g., Capitanio et al., 2010). However, recent numeric modeling studies indicate that, although intrinsic buoyancy improves the stability of continental lithosphere, it is insufficient by itself to account for the longevity of cratons (Doin et al., 1997;François et al., 2013;Lenardic & Moresi, 1999;O'Neill et al., 2008). ...
... Geodynamical models argue that a plausible range of rheological contrast between a cratonic root and asthenosphere (2-3 orders of magnitude for constant stress) could prevent the cratonic root from convecting mantle erosion for over billions of years, even without considering the effects of the chemical buoyancy of the root (e.g., Lenardic & Moresi, 1999;O'Neill et al., 2008;Wang et al., 2014). If the intrinsic buoyancy is further involved in modeling, the value of required viscosity ratios could be further reduced to below two orders of magnitude (∼50 for constant stress, Wang et al., 2014). ...
There are various explanations for how the Earth’s continents form, develop, and change but challenges remain in fully understanding the driving forces behind plate tectonics on our planet.
... There have been numerous lines of evidence suggesting the preservation of the Precambrian crustal basement and similarly old deeper mantle roots beneath cratons (e.g., Pearson, 1999;Richardson et al., 1984;Spetsius et al., 2002;Walker et al., 1989). Compared to the short-lived oceanic lithosphere that is recycled back into the mantle on a timescale of ∼100 Ma, the continental lithosphere is capable of resisting deformation and avoiding recycling as a whole over a timescale of about one order of magnitude longer (Lenardic et al., 2003;O'Neill et al., 2008). The longevity and stability of continents, in particular cratons, have been primarily attributed to the structure and intrinsic (physical and chemical) properties of the continental lithosphere, relative to the asthenosphere and oceanic lithosphere ( Figure 14 and Table 1). ...
... A significant reduction of the crustal contribution by scraping off the most buoyant upper crust would dramatically increase the density of the continental lithosphere and make it readily subductable (e.g., Capitanio et al., 2010). However, recent numeric modeling studies indicate that, although intrinsic buoyancy improves the stability of continental lithosphere, it is insufficient by itself to account for the longevity of cratons (Doin et al., 1997;François et al., 2013;Lenardic & Moresi, 1999;O'Neill et al., 2008). ...
... Geodynamical models argue that a plausible range of rheological contrast between a cratonic root and asthenosphere (2-3 orders of magnitude for constant stress) could prevent the cratonic root from convecting mantle erosion for over billions of years, even without considering the effects of the chemical buoyancy of the root (e.g., Lenardic & Moresi, 1999;O'Neill et al., 2008;Wang et al., 2014). If the intrinsic buoyancy is further involved in modeling, the value of required viscosity ratios could be further reduced to below two orders of magnitude (∼50 for constant stress, Wang et al., 2014). ...
The continental crust is unique to the Earth in the solar system, and controversies remain regarding its origin, accretion and reworking of continents. The plate tectonics theory has been significantly challenged in explaining the origin of Archean (especially pre‐3.0 Ga) continents as they rarely preserve hallmarks of plate tectonics. In contrast, growing evidence emerges to support oceanic plateau models that better explain characteristics of Archean continents, including the bimodal volcanics and nearly coeval emplacement of tonalite‐trondjhemite‐granodiorite (TTG) rocks, presence of ∼1600°C komatiites and dominant dome structures, and lack of ultra‐high‐pressure rocks, paired metamorphic belts and ophiolites. On the other hand, the theory of plate tectonics has been successfully applied to interpret the accretion of continents along subduction zones since the late Archean (3.0–2.5 Ga). During subduction processes, the new mafic crust is generated at the base of continents through partial melting of mantle wedge with the addition of H2O‐dominant fluids from subducted oceanic slabs and partial melting of the juvenile mafic crust results in the generation of new felsic crusts. This eventually leads to the outgrowth of continents. Subduction processes also cause softening, thinning, and recycling of continental lithosphere due to the vigorous infiltration of volatile‐rich fluids and melts, especially along weak belts/layers, leading to widespread continental reworking and even craton destruction. Reworking of continents also occurs in continental interiors due to either plate boundary processes or plume‐lithosphere interactions. The effects of plumes have proven to be less significant and cause lower degrees of lithospheric modification than subduction‐induced craton destruction.
... There have been numerous lines of evidence suggesting the preservation of the Precambrian crustal basement and similarly old deeper mantle roots beneath cratons (e.g., Pearson, 1999;Richardson et al., 1984;Spetsius et al., 2002;Walker et al., 1989). Compared to the short-lived oceanic lithosphere that is recycled back into the mantle on a timescale of ∼100 Ma, the continental lithosphere is capable of resisting deformation and avoiding recycling as a whole over a timescale of about one order of magnitude longer (Lenardic et al., 2003;O'Neill et al., 2008). The longevity and stability of continents, in particular cratons, have been primarily attributed to the structure and intrinsic (physical and chemical) properties of the continental lithosphere, relative to the asthenosphere and oceanic lithosphere ( Figure 14 and Table 1). ...
... A significant reduction of the crustal contribution by scraping off the most buoyant upper crust would dramatically increase the density of the continental lithosphere and make it readily subductable (e.g., Capitanio et al., 2010). However, recent numeric modeling studies indicate that, although intrinsic buoyancy improves the stability of continental lithosphere, it is insufficient by itself to account for the longevity of cratons (Doin et al., 1997;François et al., 2013;Lenardic & Moresi, 1999;O'Neill et al., 2008). ...
... Geodynamical models argue that a plausible range of rheological contrast between a cratonic root and asthenosphere (2-3 orders of magnitude for constant stress) could prevent the cratonic root from convecting mantle erosion for over billions of years, even without considering the effects of the chemical buoyancy of the root (e.g., Lenardic & Moresi, 1999;O'Neill et al., 2008;Wang et al., 2014). If the intrinsic buoyancy is further involved in modeling, the value of required viscosity ratios could be further reduced to below two orders of magnitude (∼50 for constant stress, Wang et al., 2014). ...
... In addition to their significant age, cratons generally have a thick and cold lithosphere (Jordan 1975(Jordan , 1978Polet & Anderson 1995;Rudnick et al. 1998;Lenardic & Moresi 1999;Artemieva & Mooney 2002;Gung et al. 2003;Lenardic et al. 2003;King 2005;Cooper et al. 2006). Their endurance provokes a fundamental question about the special conditions that have protected them from the destructive forces of mantle dynamics, providing a longer survival time than any other type of lithosphere, for example, oceanic or noncratonic continental lithosphere (Lenardic & Moresi 1999;Shapiro et al. 1999;Lenardic et al. 2000Lenardic et al. , 2003Sleep 2003;King 2005;Cooper et al. 2006;O'Neill et al. 2008;Yoshida 2010Yoshida , 2012Wang et al. 2014). The craton stabilization ages determined by rhenium depletion peak around 3 Ga (Pearson et al. 1995a,b;Pearson & Wittig 2014). ...
... Thus, it is likely that the viscosity of cratons plays a more significant role in cratonic survival than does craton density. The long-term survival of high viscosity cratons has been studied numerically before (Lenardic & Moresi 1999;Lenardic et al. 2000Lenardic et al. , 2003O'Neill et al. 2008;Yoshida 2010Yoshida , 2012Wang et al. 2014); however, estimates of the appropriate viscosity for cratons remain controversial. ...
... Several studies have shown that there is a significant viscosity contrast between cratonic and noncratonic lithosphere. The estimated viscosity contrast calculated by Lenardic et al. (2003) was 1000 times whereas O'Neill et al. (2008) estimated a viscosity contrast of 50-150 times between cratons and their surroundings. In another study, using 2-D box model, Wang et al. (2014) had shown that a very small viscosity contrast (of the order of 10) can protect the cratons if non-Newtonian flow laws are considered. ...
Cratons are the oldest parts of the lithosphere, some of them surviving since Archean. Their long-term survival has sometimes been attributed to high viscosity and low density. In our study, we use a numerical model to examine how shear tractions exerted by mantle convection work to deform cratons by convective shearing. We find that although tractions at the base of the lithosphere increase with increasing lithosphere thickness, the associated strain-rates decrease. This inverse relationship between stress and strain-rate results from lateral viscosity variations along with the model's free-slip condition imposed at the Earth's surface, which enables strain to accumulate along weak zones at plate boundaries. Additionally, we show that resistance to lithosphere deformation by means of convective shearing, which we express as an apparent viscosity, scales with the square of lithosphere thickness. This suggests that the enhanced thickness of the cratons protects them from convective shear and allows them to survive as the least deformed areas of the lithosphere. Indeed, we show that the combination of a smaller asthenospheric viscosity drop and a larger cratonic viscosity, together with the excess thickness of cratons compared to the surrounding lithosphere, can explain their survival since Archean time. © The Author(s) 2019. Published by Oxford University Press on behalf of The Royal Astronomical Society.
... Seismic studies show that the crustal thickness of the eastern NCC is~30 km, while it is~45 km in the west [Bao et al., 2013;Cheng et al., These observations of the eastern NCC reactivation and the contrast between the eastern and western NCC provide a unique opportunity to study the stability and evolution of cratonic lithosphere. The stability of cratonic lithosphere is mainly controlled by lithospheric viscosity [e.g., Lenardic et al., 2003;O'Neill et al., 2008]. It has been proposed that the reactivation of the eastern NCC is largely caused by water-induced lithospheric viscosity reduction, while such effects on the western NCC may be small [e.g., Zhu et al., 2012a]. ...
... As an alternative model to the subduction model for reactivation of cratonic lithosphere [Lenardic et al., 2003;O'Neill et al., 2008O'Neill et al., , 2010, Wang et al. [2015] suggested that a gravitational instability model [e.g., Jaupart et al., 2007] may explain some of the main observations associated with the eastern NCC reactivation, including the episodic and long-lasting magmatic activities and the formation and foundering of the lower eclogitic crust. An important feature of the instability model by Wang et al. [2015] is that some of the destabilized cratonic lithospheric mantle, due to its compositional buoyancy, comes back to be part of the newly formed lithosphere, as first observed in laboratory studies [Jaupart et al., 2007;Fourel et al., 2013]. ...
... where η c and η h reflect the composition-and depth-dependence, respectively, and E * = E/(RΔT) is the dimensionless activation energy with R as the gas constant and E as the activation energy. Although it has been suggested that the viscosity contrast between the cratonic lithosphere and the asthenosphere may not exceed a factor of 100 [e.g., O'Neill et al., 2008], with the consideration of stress-dependent viscosity, we set η c to be 1000 for cratonic lithosphere and 1 for the normal mantle, respectively, following Wang et al. [2015]. The depth-dependence parameter, η h , is set to be 1 for upper mantle (i.e., above a depth of 410 km), 5 for the transition zone (i.e., between depths of 410 km and 660 km), and 30 for lower mantle (i.e., below a depth of 660 km) ( Figure 2c). ...
The eastern North China Craton (NCC) has undergone extensive reactivation during the Mesozoic and Cenozoic, while the western NCC has remained stable throughout its geological history. Geophysical and geochemical observations, including heat flux, surface topography, crustal and lithospheric thicknesses, and volcanism, show significant contrast between the eastern and western NCC. These observations provide constraints on thermochemical structure and reactivation process of the eastern NCC, thus helping understand the dynamic evolution of cratonic lithosphere. In this study, we determined the residual topography for the NCC region by removing crustal contribution to the topography. We found that the residual topography of the eastern NCC region is generally 0.3-0.9km higher than the western NCC. We computed a large number of two-dimension thermochemical convection models for gravitational instability of cratonic lithosphere and quantified surface heat flux and topography contrasts between stable and destabilized parts of cratonic lithosphere. These models consider different chemical buoyancy (i.e., buoyancy number B) and viscosity for the cratonic lithosphere. Our models suggest that to explain the difference in heat flux (25-30mW/m2) and residual topography (0.3-0.9km) between the eastern and western NCC regions, the buoyancy number B is required to be ~0.3-0.4. This range of B implies that as much as 50% of the original cratonic lithospheric material remains in the present-day eastern NCC lithosphere and its underlying shallow mantle and that the new lithosphere in the eastern NCC may be a mixture of the relics of old craton materials and the normal mantle.
... Cratonic lithosphere remains stable over billion-year timescales, eluding destruction by tectonic processes and asthenospheric convection [e.g., Jordan, 1978;Walker et al., 1989;O'Neill et al., 2008]. Geologic processes such as continental rifting, however, may signal destabilization of the cratonic mantle lithosphere. ...
... Geochemistry, Geophysics, Geosystems 10.1002/2015GC005779 between asthenosphere and lithosphere of at least 50 [O'Neill et al., 2008]. The viscosity range of the asthenosphere is 5310 17 25310 18 PaÁs [Pollitz et al., 1998;Sj€ oberg et al., 2000;Larsen et al., 2005]. ...
Water and other trace element concentrations in olivine (1–39 ppm H2O), orthopyroxene (10–150 ppm H2O), and clinopyroxene (16–340 ppm H2O) of mantle xenoliths from the Labait volcano, located on the edge of the Tanzanian craton along the eastern branch of the East African Rift, record melting and subsequent refertilization by plume magmas in a stratified lithosphere. These water contents are at the lower end of the range observed in other cratonic mantle lithospheres. Despite correlations between water content and indices of melting in orthopyroxene from the shallow peridotites, and in both olivine and orthopyroxene from the deep peridotites, water concentrations are too high for the peridotites to be simple residues. Instead, the Labait water contents are best explained as reflecting interaction between residual peridotite with a melt having relatively low water content (<1 wt.% H2O). Plume-derived melts are the likely source of water and other trace element enrichments in the Labait peridotites. Only garnet may have undergone addition of water from the host magma as evidenced by water content increasing toward the kelyphite rim in one otherwise homogeneous garnet. Based on modeling of the diffusion profile, magma ascent occurred at 4–28 m/s. In summary, plume-craton interaction appears to result in only moderate water enrichment of the lithosphere.
... This interpretation is generally based on the exposure of exclusively ancient, undeformed shields at low elevations with low relief (16). These shields overlie a thick, depleted lithospheric mantle (i.e., up to ∼250 km in thickness) that is inherently buoyant, viscous, and difficult to convectively remove (4,16,17). However, several of these cratons are capped by Phanerozoic large igneous provinces (LIPs)-rare, but giant outpourings of lava (>10 6 km 3 ) that were extruded within geologically short time frames (<5 Ma)-which are commonly linked to the arrival of plume heads beneath the plate (18). ...
Large igneous provinces (LIPs) are formed by enormous (i.e., frequently >106 km3) but short-lived magmatic events that have profound effects upon global geodynamic, tectonic, and environmental processes. Lithospheric structure is known to modulate mantle melting, yet its evolution during and after such dramatic periods of magmatism is poorly constrained. Using geochemical and seismological observations, we find that magmatism is associated with thin (i.e., ≲80 km) lithosphere and we reveal a striking positive correlation between the thickness of modern-day lithosphere beneath LIPs and time since eruption. Oceanic lithosphere rethickens to 125 km, while continental regions reach >190 km. Our results point to systematic destruction and subsequent regrowth of lithospheric mantle during and after LIP emplacement and recratonization of the continents following eruption. These insights have implications for the stability, age, and composition of ancient, thick, and chemically distinct lithospheric roots, the distribution of economic resources, and emissions of chemical species that force catastrophic environmental change.
... Additionally, having higher brittle yield stress in the cratonic lithosphere or the presence of mobile belts (weak zones) around cratons also helps with cratonic root stability. O'Neill et al. (2008) further showed cratonic stability in their models having a compositional viscosity ratio of 50-150 between the root and the asthenosphere along with a root/oceanic lithospheric yield stress ratio of 5-30. The authors also argued that hotter Archean mantle temperatures would result in lower viscosities and reduced stresses around cratons, thereby promoting cratonic survival. ...
Geophysical, geochemical, and geological investigations have attributed the stable behaviour of Earth's continents to the presence of their Archean cratonic roots. These roots are likely composed of melt-depleted, low density residual peridotite with high Magnesium number (Mg#), while devolatilisation from the upper mantle during magmatic events might have made these roots more viscous and intrinsically stronger than the convecting mantle.Several conceptual dynamic and petrological models of craton formation have been proposed. Dynamic models invoke far-field shortening or mantle melting events, e.g., by mantle plumes, to create melt-depleted and thick cratons. Compositional buoyancy and rheological modifications have also been invoked to create long-lived stable cratonic lithosphere. However, these conceptual models have not been tested in a dynamically self-consistent model. In this study, we present global thermochemical models of craton formation with coupled core-mantle-crust evolution driven entirely by gravitational forces.Our results with melting and crustal production (both oceanic and continental) show that formation of cratonic roots can occur through naturally occurring lateral compression and thickening of the lithosphere in a self-consistent manner, without the need to invoke far-field tectonic forces. Plume impingements, and gravitational sliding creates thrusting of lithosphere to form thick, stable, and strong lithosphere that has a strong resemblance to the Archean cratons that we can still observe today at the Earth's surface. These models also suggest the recycling of denser eclogitic crust by delamination and dripping processes. Within our computed parameter space, a variety of tectonic regimes are observed which also transition with time. Based on these results, we propose that a ridge-only regime or a sluggish-stagnant-lid regime might have been active on Earth during the Archean Eon as they offer favourable dynamics and conditions for craton formation.
... Although lithospheric thinning of the NCC has been recognized for several decades, the dynamics and mechanisms of this process remain controversial (Xu 2001;Zhang et al. 2007aZhang et al. , 2014Gao et al. 2008;Yang and Li 2008;Xia et al. 2013;Liu et al. 2019). The presence of depleted, refractory, buoyant and dry lithosphere is essential for the stability and longevity of cratons (Carlson et al. 2005;O'Neill et al. 2008;Peslier et al. 2010;Lee et al. 2011). However, the geochemical compositions of Cretaceous primitive basalts from the eastern NCC show that large amounts of water and crustal inputs were introduced into the NCC mantle prior to 125 Ma when the primitive basalts formed. ...
The modification of the Archaean lithospheric mantle root beneath the eastern North China Craton (NCC) has been noticed. However, the degree of modification and the characteristics of metasomatic agents for the NCC lithospheric mantle are still unclear. Here, we compile the geochemistry and ⁸⁷Sr/⁸⁶Sr and ¹⁴³Nd/¹⁴⁴Nd isotope ratios of Cretaceous primitive basalts (Yixian, Sihetun, Fangcheng and Feixian) from the eastern NCC and estimate the inputs of subduction-related elements into the Archaean lithospheric mantle beneath the NCC. These basalts show primitive (high MgO, Cr and Ni contents) and arc-like geochemical features (enrichments in LILEs and depletions in HFSEs) that indicate that the mantle sources were modified by fluids related to subducted crustal materials. Substantial proportions of subduction-mobile elements (e.g. ~95% Ba and ~90% Th) transferred to the basalt sources via hydrous melts. Thus, a large volume of fluids transferred into the lithospheric mantle. The data support the model that the NCC Archaean lithospheric mantle was weakened by hydrous melts, which resulted in a fusible weakened lithospheric mantle. Preliminary lithospheric thinning was induced by the extension of the NCC resulting from trench retreat of the Paleo-Pacific plate at ~140-120 Ma. Then, decompression melting of the lithospheric mantle caused pervasive melts in the weakened lithospheric mantle, resulting in the lithospheric mantle having a low viscosity comparable to the asthenospheric mantle, which principally reduced the lithospheric mantle.
... Electrical conductivity models also show enhanced H content in olivine beneath Lac de Gras with 150 ppm H 2 O at 200 km depths while the surrounding lithospheric mantle, i.e. most of the Slave cratonic root, is ''dry" with olivine containing <10 ppm H 2 O (Jones et al., 2014). A viscosity contrast of 3-12 Pa between a mantle lithosphere containing 10 ppm H 2 O in olivine and the asthenosphere (Fig. 13) at the LAB $220 km depth beneath the Slave craton (Chen et al., 2007;Schaeffer and Lebedev, 2014) is sufficient to prevent cratonic root delamination (Shapiro et al., 1999;Sleep, 2003;O'Neill et al., 2008). Thus, the xenoliths analyzed in this study cannot be representative of the central Slave cratonic lithosphere as a whole, and instead preferentially represent the 'wettest' and most metasomatized regions. ...
Whether hydrogen incorporated in nominally anhydrous mantle minerals plays a role in the strength and longevity of the thick cratonic lithosphere is a matter of debate. In particular, the percolation of hydrogen-bearing melts and fluids could potentially add hydrogen to the mantle lithosphere, weaken its olivines (the dominant mineral in mantle peridotite), and cause delamination of the lithosphere's base. The influence of metasomatism on hydrogen contents of cratonic mantle minerals can be tested in mantle xenoliths from the Slave Craton (Canada) because they show extensive evidence for metasomatism of a layered cratonic mantle. Minerals from mantle xenoliths from the Diavik mine in the Lac de Gras kimberlite area located at the center of the Archean Slave craton were analyzed by FTIR for hydrogen contents. The 18 peridotites, two pyroxenites, one websterite and one wehrlite span an equilibration pressure range from 3.1 to 6.6 GPa and include samples from the shallow (≤ 145 km), oxidized ultra-depleted layer; the deeper (∼145-180 km), reduced less depleted layer; and an ultra-deep (≥ 180 km) layer near the base of the lithosphere. Olivine, orthopyroxene, clinopyroxene and garnet from peridotites contain 30 - 145, 110 – 225, 105 – 285, 2 – 105 ppm H2O, respectively. Within each deep and ultra-deep layer, correlations of hydrogen contents in minerals and tracers of metasomatism (for example light over heavy rare-earth-element ratio (LREE/HREE), high-field-strength-element (HFSE) content with equilibration pressure) can be explained by a chromatographic process occurring during the percolation of kimberlite-like melts through garnet peridotite. The hydrogen content of peridotite minerals is controlled by the compositions of the evolving melt and of the minerals and by mineral/melt partition coefficients. At the beginning of the process, clinopyroxene scavenges most of the hydrogen and garnet most of the HFSE. As the melt evolves and becomes enriched in hydrogen and LREE, olivine and garnet start to incorporate hydrogen and pyroxenes become enriched in LREE. The hydrogen content of peridotite increases with decreasing depth, overall (e.g., from 75 to 138 ppm H2O in the deep peridotites). Effective viscosity calculated using olivine hydrogen content for the deepest xenoliths near the lithosphere-asthenosphere boundary overlaps with estimates of asthenospheric viscosities. These xenoliths cannot be representative of the overall cratonic root because the lack of viscosity contrast would have caused basal erosion of lithosphere. Instead, metasomatism must be confined in narrow zones channeling kimberlite melts through the lithosphere and from where xenoliths are preferentially sampled. Such localized metasomatism by hydrogen-bearing melts therefore does not necessarily result in delamination of the cratonic root.
... Such cratonic peridotites are less dense (Herzberg and Rudnick 2012) and more viscous (Dixon et al. 2004;Hirth et al. 2000) than the surrounding asthenosphere, made up of fertile peridotites richer in iron and containing more water. The cratonic keel is thus physically isolated from the convective mantle and preserved from delamination, providing its longevity (Jordan 1978;O'Neill et al. 2008;Sleep 2003). ...
Water and iron are believed to be key constituents controlling the strength and density of the lithosphere and, therefore, play a crucial role in the long-term stability of cratons. On the other hand, metasomatism can modify the water and iron abundances in the mantle and possibly triggers thermo-mechanical erosion of cratonic keels. Whether local or large scale processes control water distribution in cratonic mantle remains unclear, calling for further investigation. Spinel peridotite xenoliths in alkali basalts of the Cenozoic Tok volcanic field sampled the lithospheric mantle beneath the southeastern margin of the Siberian Craton. The absence of garnet-bearing peridotite amongst the xenoliths, together with voluminous eruptions of basaltic magma, suggests that the craton margin, in contrast to the central part, lost its deep keel. The Tok peridotites experienced extensive and complex metasomatic reworking by evolved, CaFe rich liquids that transformed refractory harzburgite to lherzolite and wehrlite. We used polarized Fourier transform infrared spectroscopy (FTIR) to obtain water content in olivine, orthopyroxene (opx), and clinopyroxene (cpx) of 14 Tok xenoliths. Olivine, with a water content of 0–3 ppm H2O, was severely degassed, probably during emplacement and cooling of the host lava flow. Orthopyroxene (49–106 ppm H2O) and clinopyroxene (97–300 ppm H2O) are in equilibrium. The cores of the pyroxene grains, unlike olivine, experienced no water loss due to dehydration or addition attributable to interaction with the host magma. The water contents of Opx and Cpx are similar to those from the Kaapvaal, Tanzania, and North China cratons, but the Tok Opx has less water than previously studied Opx from the central Siberian craton (Udachnaya, 28–301 ppm; average 138 ppm). Melting models suggest that the water contents of Tok peridotites are higher than in melting residues, and argue for a post-melting (metasomatic) origin.
Moreover, the water contents in Opx and Cpx of Tok peridotites are decoupled from iron enrichments or other indicators of melt metasomatism (e.g., CaO and P2O5). Such decoupling is not seen in the Udachnaya and Kaapvaal peridotites but is similar to observations on Tanzanian peridotites. Our data suggest that iron enrichments in the southeastern Siberian craton mantle preceded water enrichment. Pervasive and large-scale, iron enrichment in the lithospheric mantle may strongly increase its density and initiate a thermo-magmatic erosion. By contrast, the distribution of water in xenoliths is relatively “recent” and was controlled by local metasomatic processes that operate shortly before the volcanic eruption. Hence, water abundances in minerals of Tok mantle xenoliths appear to represent a snapshot of water in the vicinity of the xenolith source regions.
... One puzzling aspect about the Hf isotope variation of the Kaapvaal CLM at the time of B-LIP magmatism pertains to the common understanding that the buoyancy and strength imparted to cratons by melt depletion and dehydration of their underlying lithosphere are what allow continents to persist in a dynamic mantle (O'Neill et al., 2008). However, in lithosphere in which garnet is present as a residual phase of partial melting, Hf isotopes should evolve to highly radiogenic compositions over time due to garnet's high Lu/Hf ratio. ...
The Bushveld Large Igneous Province (B-LIP) comprises a diverse array of >30 magma bodies that intruded the Kaapvaal Craton at ∼2.06 Ga. In this paper we use zircon and bulk-rock Lu-Hf isotope data to determine whether the B-LIP formed in response to the arrival of a plume(s) from the deep mantle or by melting of the depleted upper mantle during foundering of an eclogitized residue at the base of the lithosphere. New zircon Hf isotope compositions for four B-LIP bodies yield intrusion-specific average εHf(2.06Ga) values that range from −20.7 ± 2.8 to −2.7 ± 2.8, largely consistent with literature zircon data for other B-LIP intrusions. Bulk-rock solution εHf(2.06Ga) values for a variety of B-LIP intrusions range from −2.1 ± 0.2 to −10.6 ± 0.2. Because the most radiogenic Hf isotope compositions across the entire B-LIP are nearly primordial, having an εHf(2.06Ga) close to 0, it is likely that the heat source of the B-LIP was a plume(s) from deep mantle. The Hf isotope data further suggests that individual intrusions in the B-LIP were produced by melting of three distinct source reservoirs (in addition to melts derived from the plume itself): 1) Subduction and plume modified continental lithospheric mantle; 2) Older (∼2.7 Ga) mafic-ultramafic plume-related material trapped in the Kaapvaal lithosphere; and 3) Mid- to upper Kaapvaal crust. The presence of ∼2.7 Ga mafic-ultramafic material in the Kaapvaal lithosphere may have acted to strengthen the lithosphere so that it was able to resist being dispersed by the arrival of the B-LIP plume at ∼2.06 Ga.
... * Melesse Alemayehu melesse555@yahoo.com and Wittig 2014). While melt depletion increases buoyancy and viscosity by the removal of FeO and H 2 O, respectively, which are essential to the longevity of the SCLM, metasomatism, and refertilization generally can have adverse effects by increasing density, lowering viscosity, and generating rheological weak zones (e.g., Jordan 1988;O'Neill et al. 2008;Wang et al. 2014). Thus, the SCLM can become unstable during various tectonomagmatic processes, such as plume impingement (e.g., Wang et al. 2015). ...
The behavior of sub-continental lithospheric mantle (SCLM) in extensional settings, up to successful rifting, plays an important role in geodynamics and in the global carbon cycle, yet the underlying processes and rates of lithosphere destruction remain poorly constrained. We determined platinum-group element (PGE: Os, Ir, Ru, Pt, and Pd) abundances and Re–Os-isotope systematics for well-characterized mantle xenoliths hosted in Cenozoic basalts from the northwestern plateau (Gundeweyn area) and southern rift zone (Dillo and Megado areas) of Ethiopia to provide new insights on the nature and timing of processes leading to the formation and transformation of the off-cratonic lithospheric mantle beneath the East Africa rift system (EARS). The whole-rock PGE concentrations are highly variable, with total PGE abundances ranging from 6.6 to 12.6 ppb for Gundeweyn, 11.5 to 23.3 ppb for Dillo, and 9.9 to 19.4 ppb for Megado mantle xenoliths. The 187Os/188Os ratios of the whole-rock mantle xenoliths vary from 0.1180 to 0.1287 for Gundeweyn, 0.1238 to 0.1410 for Dillo and 0.1165 to 0.1277 for Megado, compared to 0.130 for the Afar plume and ≥ 0.14 for the Kenya plume, with Re depletion ages up to 1.45 Ga for Gundeweyn, 0.64 Ga for Dillo, and 1.65 Ga for Megado mantle xenoliths. The regional differences between refertilizing agents recorded in mantle xenoliths from the plateau area and the rift systems reflect distinct tectonomagmatic settings: (1) low PGE abundances, with some retention of low 187Os/188Os in Gundeweyn peridotites, are ascribed to scavenging by early small-volume oxidizing melts, generated in the convecting mantle ahead of the arrival of the Afar plume. (2) Percolation of late-stage silicate/basaltic melts, associated with the arrival of hot mantle plume and lithosphere thinning in the rift setting, locally led to refertilization and sulfide precipitation and partial replenishment of the PGE (Dillo), with convecting mantle-like 187Os/188Os. Local enclaves of older, cryptically metasomatised mantle with unradiogenic Os (Megado) attest to the heterogeneous nature of melt–peridotite interaction at this stage (pervasive vs. focused melt flow). Highly depleted abundances of the compatible PGE are characteristic of SCLM affected by incipient rifting and percolation of oxidizing melts, here associated with the Afar and Kenya plume beneath the East Africa rift, and may be precursors to advanced degrees of lithosphere destruction/transformation.
... The viscosities at the bottom of the lithosphere (> 200 km depth), however, are the ones that warrant closer inspection to understand why cratonic roots do not get delaminated by the surrounding asthenosphere. A high viscosity contrast between the lithosphere and the asthenosphere (> 3 to > 50 times depending on estimated conditions) is necessary for the survival of a cratonic root for several billion years (Shapiro et al. 1999;Sleep 2003;O'Neill et al. 2008;Karato 2010;Wang et al. 2014). Viscosity estimates for the astheno-sphere can be inferred from post-glacial rebound and earthquake data (Pollitz et al. 1998;Sjöberg et al. 2000;Larsen et al. 2005;Fleming et al. 2007;Masuti et al. 2016). ...
... Current seismic readings provide little evidence that there is still a strong, deep and intact lithospheric root. This indicates that a large portion of the root has recently been removed [57]. The Archean Wyoming Province also has no seismically discernable root at present. ...
... However, buoyancy by itself cannot explain craton longevity (Doin et al., 1997;Lenardic and Moresi, 1999). In addition, viscosities at least a fac-tor 3-10 higher than surrounding mantle, often attributed to low contents of volatiles, are required to stabilise cratons against erosion by convective stresses and to localize deformation around rather than within them (Doin et al., 1997;Lenardic et al., 2000;O'Neill et al., 2008;Pollack, 1986;H. Wang et al., 2014). ...
The long-term stability of cratons has been attributed to low temperatures and depletion in iron and water, which decrease density and increase viscosity. However, steady-state thermal models based on heat flow and xenolith constraints systematically overpredict the seismic velocity-depth gradients in cratonic lithospheric mantle. Here we invert for the 1-D thermal structure and a depth distribution of metasomatic minerals that fit average Rayleigh-wave dispersion curves for the Archean Kaapvaal, Yilgarn and Slave cratons and the Proterozoic Baltic Shield below Finland. To match the seismic profiles, we need a significant amount of hydrous and/or carbonate minerals in the shallow lithospheric mantle, starting between the Moho and 70 km depth and extending down to at least 100–150 km. The metasomatic component can consist of 0.5–1 wt% water bound in amphibole, antigorite and chlorite, ∼0.2 wt% water plus potassium to form phlogopite, or ∼5 wt% CO2 plus Ca for carbonate, or a combination of these. Lithospheric temperatures that fit the seismic data are consistent with heat flow constraints, but most are lower than those inferred from xenolith geothermobarometry. The dispersion data require differences in Moho heat flux between individual cratons, and sublithospheric mantle temperatures that are 100–200 °C less beneath Yilgarn, Slave and Finland than beneath Kaapvaal. Significant upward-increasing metasomatism by water and CO2-rich fluids is not only a plausible mechanism to explain the average seismic structure of cratonic lithosphere but such metasomatism may also lead to the formation of mid-lithospheric discontinuities and would contribute to the positive chemical buoyancy of cratonic roots.
... The viscosities at the bottom of the lithosphere (> 200 km depth), however, are the ones that warrant closer inspection to understand why cratonic roots do not get delaminated by the surrounding asthenosphere. A high viscosity contrast between the lithosphere and the asthenosphere (> 3 to > 50 times depending on estimated conditions) is necessary for the survival of a cratonic root for several billion years (Shapiro et al. 1999;Sleep 2003;O'Neill et al. 2008;Karato 2010;Wang et al. 2014). Viscosity estimates for the astheno-sphere can be inferred from post-glacial rebound and earthquake data (Pollitz et al. 1998;Sjöberg et al. 2000;Larsen et al. 2005;Fleming et al. 2007;Masuti et al. 2016). ...
The concentration and distribution of water in the Earth has influenced its evolution throughout its history. Even at the trace levels contained in the planet’s deep interior (mantle and core), water affects Earth’s thermal, deformational, melting, electrical and seismic properties, that control differentiation, plate tectonics and volcanism. These in turn influenced the development of Earth’s atmosphere, oceans, and life. In addition to the ubiquitous presence of water in the hydrosphere, most of Earth’s “water” actually occurs as trace amounts of hydrogen incorporated in the rock-forming silicate minerals that constitute the planet’s crust and mantle, and may also be stored in the metallic core. The heterogeneous distribution of water in the Earth is the result of early planetary differentiation into crust, mantle and core, followed by remixing of lithosphere into the mantle after plate-tectonics started. The Earth’s total water content is estimated at times the equivalent mass of the oceans (or a concentration of 3900_{-3300}^{+32700}~\mbox{ppm} weight H2O). Uncertainties in this estimate arise primarily from the less-well-known concentrations for the lower mantle and core, since samples for water analyses are only available from the crust, the upper mantle and very rarely from the mantle transition zone (410–670 km depth). For the lower mantle (670–2900 km) and core (2900–4500 km), the estimates rely on laboratory experiments and indirect geophysical techniques (electrical conductivity and seismology).
The Earth's accretion likely started relatively dry because it mainly acquired material from the inner part of the proto-planetary disk, where temperatures were too high for the formation and accretion of water ice. Combined evidence from several radionuclide systems (Pd-Ag, Mn-Cr, Rb-Sr, U-Pb) suggests that water was not incorporated in the Earth in significant quantities until the planet had grown to~60-90% of its current size, while core formation was still on-going. Dynamic models of planet formation provide additional evidence for water delivery to the Earth during the same period by water-rich planetesimals originating from the asteroid belt and possibly beyond. This early delivered water may have been partly lost during giant impacts, including the Moon forming event: magma oceans can form in their aftermath, degas and be followed by atmospheric loss. More water may have been delivered and/or lost after core formation during late accretion of extraterrestrial material ("late-veneer"). Stable isotopes of hydrogen, carbon, nitrogen and some noble gases in Earth's materials show similar compositions to those in carbonaceous chondrites, implying a common origin for their water, and only allowing for minor water inputs from comets.
... Cratonic breakup and subsequent rifting has been studied both numerically [e.g., Sokoutis et al., 2007;Huismans and Beaumont, 2011;O'Neill et al., 2008;King, 2005;Lenardic et al., 2000Lenardic et al., , 2003Brune et al., 2014;Regenauer-Lieb et al., 2008;Rosenbaum et al., 2008;Weinberg et al., 2007;Regenauer-Lieb et al., 2006;Wang et al., 2015] and also via analog modeling techniques [Corti , 2012, and references therein]. However, the numerical studies have been for a homogeneous lithosphere without large scale weak zones resembling cratonic metasomatism and chemical heterogeneity. ...
Cratons form the stable core roots of the continental crust. Despite long term stability, cratons have failed in the past. Cratonic destruction (e.g., North Atlantic craton) due to chemical rejuvenation at the base of the lithosphere remains poorly constrained numerically. We use 2D petrological–thermomechanical models to assess cratonic rifting characteristics and mantle CO2 degassing in the presence of a carbonated subcontinental lithospheric mantle (SCLM). We test two tectonothermal SCLM compositions: Archon (depleted) and Tecton (fertilized) using 2 CO2 wt.% in the bulk composition to represent a metasomatized SCLM. We parameterize cratonic breakup via extensional duration (7-12 Ma; full breakup), tectonothermal age, TMoho(300-600∘C), and crustal rheology. The two compositions with metasomatized SCLMs share similar rifting features and decarbonation trends during initial extension. However, we show long-term (>67 Ma) stability differences due to lithospheric density contrasts between SCLM compositions. The Tecton model shows convective removal and thinning of the metasomatized SCLM during failed rifting. The Archon composition remained stable, highlighting the primary role for SCLM density even when metasomatized at its base. In the short-term, three failed rifting characteristics emerge: failed rifting without decarbonation, failed rifting with decarbonation, and semiâĂŞfailed rifting with dry asthenospheric melting and decarbonation. Decarbonation trends were greatest in the failed rifts, reaching peak fluxes of 94x104 kg m−3. Increased TMoho did not alter the effects of rifting or decarbonation. Lastly, we show mantle regions where decarbonation, mantle melting in the presence of carbonate, and preservation of carbonated mantle occur during rifting.
... Therefore, we vary the densities of these two layers respectively. A high viscosity root is widely suggested to be of key importance on craton longevity in mantle convection models, while buoyancy is of secondary or even minor importance O'Neill et al., 2008). However, the dynamics of craton evolution with the presence of subduction (Lenardic et al., 2003) could be very different than those of mantle convection models. ...
Cratons have remained stable for billions of years, despite of ongoing mantle convection and plate tectonics. The North China Craton (NCC), however, is abnormal, as it has experienced a destruction event during the Mesozoic and Cenozoic which was accompanied by extensive magmatism. Several lines of evidence suggest that the (Paleo-)Pacific plate played an important role in this event. Yet, the geodynamic link between subduction and craton destruction remains poorly understood, and it is unclear why there is no systematic spatial and temporal variation of magmatism related to subduction. Here, we perform 2-D petrological-thermomechanical simulations to investigate the influence of subduction dynamics and (de)hydration processes, on craton destruction. Results show that: (1) cold slabs may transport considerable amounts of water into the mantle transition zone; (2) the subducted slab triggers wet upwellings from the transition zone that result in partial melting. Subsequently formed buoyant melt regions percolate the base of the craton, which results in a mixed magma source, deriving from the continental mantle lithosphere (CML), the asthenosphere and the oceanic crust. This is consistent with the geochemical signatures observed in 90–40 Ma rocks in the NCC; (3) cratons are more prone to be destructed by mantle convection if they are more buoyant and the subducting plate has higher water content. Our results suggest that refertilization of the cratonic mantle lithosphere by slab-triggered wet upwellings is physically a plausible mechanism of decratonization. Our model might also be applicable to the destruction of Wyoming craton in North America.
... As a consequence of their longevity and strength to survive recycling events (O'Reilly et al. 2001;King 2005;Neill et al. 2008), cratons stand out as complex tectonic domains that have witnessed the evolutionary history of the continental lithosphere. Thanks to that, intracratonic basins, as poly-historic depocenters, generally host important records of the trajectory of the differentiated cratonic lithosphere through geological time (e.g.: Korsch and Lindsay 1989;Kaminski and Jaupart 2000;Spalletti and Limarino 2006;Kadima et al. 2011). ...
The intracratonic São Francisco basin covers almost the whole NS-trending lobe of the São Francisco craton, encompassing multiple and superimposed basin-cycles younger than 1.8 Ga. Underlain by a relatively thick and cold lithosphere, the basin contains three major Precambrian first-order sequences. The Mesoproterozoic to Early Neoproterozoic Paranoá-Upper Espinhaço sequence consists of a sand-dominated rift-sag succession that grades laterally into the sediments of a rift-passive margin basin developed along the western São Francisco plate between 1.3 and 0.9 Ga. The main occurrence of this sequence is associated with the NW-trending Pirapora aulacogen, a prominent graben nucleated in the early stages of São Francisco basin evolution (Paleoproterozoic?). The Neoproterozoic Macaúbas sequence and its correlatives record extensional events that affected the São Francisco-Congo in same time period of the dispersal of Rodinia. The Ediacaran Bambuí sequence covers large areas of the basin and marks the onset of a foreland basin-cycle triggered by the successive Brasiliano orogenies that involved the cratonic margins during the West Gondwana assembly. Diamictite-bearing successions of both Macaúbas and Bambuí sequences record important glacial ages that might have covered low latitudes during the Neoproterozoic. The Precambrian fill units of the basin are involved in foreland fold-thrust belts of opposite vergences, the Brasília, on west, Rio Preto on the north, and the Araçuaí, on the east. The Proterozoic assemblages are unconformably overlain by the Paleozoic Santa Fé Group, as well as the Cretaceous Areado, Mata da Corda and Urucuia groups. The glaciogenic Santa Fé Group is exposed in a few portions of the central and northern São Francisco basin and records the passage of the Gondwana through polar latitudes in the Late Carboniferous to Early Permian. The Lower Cretaceous Areado Group contains a package of sand-dominated strata deposited under arid to semi-arid conditions. They are overlain by Upper Cretaceous volcanic and epiclastic successions, generated during a magmatic event that affected large areas of the central and southeastern Brazil. This event is probably coeval with the deposition of the widespread Urucuia desertic successions and marks an important uplift phase of the Alto Paranaíba arch, a 350 km long and 80 km wide structure that separates the Paraná and São Francisco hydrographic and sedimentary basins. The Cretaceous cover assemblages are contemporaneous to the South Atlantic opening and the consequent separation of the São Francisco and Congo cratons.
... Numerical experiments (Doin et al., 1997) indicate that the high viscosity of the cratonic lithosphere relative to the surrounding mantle is an important ingredient for the stability of thick continental lithosphere for billions of years. Based on mantle convection models to simulate the present-day mantle convection state, O'Neill et al. (2008) proposed that craton stability is achieved when the cratonic mantle is 50-150 times more viscous than the asthenosphere. In the present work, I assumed that the viscosity ratio between the cratonic lithosphere and the asthenosphere is 100. ...
After decades of geological and geophysical data acquisition along with quantitative modeling of the long-term evolution of the landscape at divergent continental margins, the search for an explanation for the formation and evolution of steep escarpments bordering the coast is still a challenging task. One difficult aspect to explain about the evolution of these escarpments is the expressive variability of denudation rate through the post-rift phase observed in many margins. Here I propose that the interaction of small-scale convection in the asthenosphere with the base of the continental lithosphere can create intermittent vertical displacements of the surface with magnitude of a few hundreds of meters at the continental margin. These topographic perturbations are sufficient to produce an expressive variability in the rate of erosion of the landscape through the post-rift phase similar to the exhumation history observed along old divergent margins. I show that the vertical motion of the surface is amplified when a mobile belt is present at the continental margin, with lithospheric mantle less viscous than the cratonic lithosphere and, consequently, more prone to be partially eroded by the convective asthenosphere. I conclude that the influence of small-scale convection is not the primary explanation for the formation of high topographic features at divergent continental margins, but can be an important component contributing to sustain a preexistent escarpment. The present results are based on numerical simulations that combine thermochemical convection in the mantle, flexure of the lithosphere and surface processes of erosion and sedimentation.
... These add dimensions of complication and expand the parameter space, but will be an important avenue for further work. The asthenosphere in particular has been demonstrated to be fundamentally important in plate generation and lithospheric stresses (Richards et al., 2001;O'Neill et al., 2008;Hoink and Lenardic, 2009;King, 2015), but apparently does not exist on Venus . As a result we have not included an asthenosphere explicitly to allow a generalisation of our results to Venus-type planets, but the effect of this on planetary evolution should be further explored. ...
The tectonic regime of a planet depends critically on the contributions of basal and internal heating to the planetary mantle, and how these evolve through time. We use viscoplastic mantle convection simulations, with evolving core-mantle boundary temperatures, and radiogenic heat decay, to explore how these factors affect tectonic regime over the lifetime of a planet. The simulations demonstrate i) hot, mantle conditions, coming out of a magma ocean phase of evolution, can produce a “hot” stagnant-lid regime, whilst a cooler post magma ocean mantle may begin in a plate tectonic regime; ii) Planets may evolve from an initial hot stagnant-lid condition, through an episodic regime lasting 1-3Gyr, into a plate-tectonic regime, and finally into a cold, senescent stagnant lid regime after ∼10Gyr of evolution, as heat production and basal temperatures wane; and iii) the thermal state of the post magma ocean mantle, which effectively sets the initial conditions for the sub-solidus mantle convection phase of planetary evolution, is one of the most sensitive parameters affecting planetary evolution - systems with exactly the same physical parameters may exhibit completely different tectonics depending on the initial state employed. Estimates of the early Earth’s temperatures suggest Earth may have begun in a hot stagnant lid mode, evolving into an episodic regime throughout most of the Archaean, before finally passing into a plate tectonic regime. The implication of these results is that, for many cases, plate tectonics may be a phase in planetary evolution between hot and cold stagnant states, rather than an end-member.
... Another example of a geodynamical scenario in which particle distribution can be significantly affected is the long-term interaction between the base of the lithosphere and the convecting mantle. Here we test the CVI scheme on a 3-D model of a very viscous cratonic root in a much weaker thermochemically convecting mantle that has often been studied in 2-D situations [Lenardic et al., 2003;O'Neill et al., 2008;Wang et al., 2014Wang et al., , 2015. The computational domain is 660 km deep with a unit aspect ratio. ...
The particle-in-cell method is generally considered a flexible and robust method to model the geodynamic problems with chemical heterogeneity. However, velocity interpolation from grid points to particle locations is often performed without considering the divergence of the velocity field, which can lead to significant particle dispersion or clustering if those particles move through regions of strong velocity gradients. This may ultimately result in cells void of particles, which, if left untreated, may, in turn, lead to numerical inaccuracies. Here we apply a two-dimensional conservative velocity interpolation (CVI) scheme to steady state and time-dependent flow fields with strong velocity gradients (e.g., due to large local viscosity variation) and derive and apply the three-dimensional equivalent. We show that the introduction of CVI significantly reduces the dispersion and clustering of particles in both steady state and time-dependent flow problems and maintains a locally steady number of particles, without the need for ad hoc remedies such as very high initial particle densities or reseeding during the calculation. We illustrate that this method provides a significant improvement to particle distributions in common geodynamic modeling problems such as subduction zones or lithosphere-asthenosphere boundary dynamics.
... The chemical distinction of highly melt-depleted cratonic roots is considered to be the most important reason for the survival of Archean lithosphere [Boyd, 1989;Carlson et al., 2005]. Geodynamical research supports this hypothesis through numerical modeling by using reasonable density structure and mantle rheology Lenardic and Moresi, 1999;O'Neill et al., 2008;Wang et al., 2014]. ...
Geologically rapid (10s of Myr) partial removal of thick continental lithosphere is evident beneath Precambrian terranes such as North China craton, southern Africa and the North Atlantic Craton, and has been linked with thermo-mechanical erosion by mantle plumes. We performed numerical experiments with realistic viscosities to test this hypothesis and constrain the most important parameters that influence cratonic lithosphere erosion. Our models indicate that the thermo-mechanical erosion by a plume impact on typical Archean lithospheric mantle is unlikely to be more effective than long-term erosion from normal plate-mantle interaction. Therefore, unmodified cratonic roots that have been stable for billions of years will not be significantly disrupted by the erosion of a plume event. However, the buoyancy and strength of highly depleted continental roots can be modified by fluid-melt metasomatism, and our models show this is essential for the thinning of originally stable continental roots. The long-term but punctuated history of metasomatic enrichment beneath ancient continents makes this mode of weakening very likely. The effect of the plume impact is to speed up the erosion significantly and help the removal of the lithospheric root to occur within 10s of Myrs if affected by metasomatic weakening. This article is protected by copyright. All rights reserved.
... Cratons are lithospheric blocks that have remained stable for billions of years, and their longevity has often been attributed to their buoyancy and rigidity. Recent research suggests that these factors may not be sufficient to prevent cratons being destroyed by mantle convection (O'Neill et al., 2008). It has been suggested that it is the low water content of cratonic lithosphere which causes the high strength and allows cratons to survive for billions of years (Peslier et al., 2010). ...
The North China Craton (NCC) and Central Asian Orogen Belt (CAOB) in Northeastern China experienced a range of tectonic events during the Phanerozoic, dominated by lithospheric thinning of the eastern NCC in the late Mesozoic and Cenozoic. In order to better understand the tectonic evolution of the NCC and the CAOB, new broadband and long period magnetotelluric data were collected along a north-west to south-east trending profile that extended from the CAOB across the Yanshan Belt, the Tanlu Fault Zone to the Liaodong Peninsula. A two-dimensional (2-D) resistivity model was derived from inversion of the transverse electric mode, transverse magnetic mode and vertical magnetic field data.
A key question in the planetary sciences centers on the divergence between the sibling planets, Venus and Earth. Venus currently does not operate with plate tectonics, and its thick atmosphere has led to extreme greenhouse conditions. It is unknown if this state was set primordially or if Venus was once more Earth-like. Here, we explore Venus as an example of a planet that recently transitioned between tectonic regimes. Our results show that transitions naturally lead to substantial resurfacing and melt-generated outgassing from lithosphere-breaking events and overturns, with 3 to 10 bars of atmosphere generated per overturn over ~60–million year timescales and ~10 to 100 bars outgassed over billion-year time frames. We find that the observation of Venus with a thick greenhouse atmosphere and the inferences of currently low volcanic rates and previous prodigious volcanic rates are consistent with a planet that has undergone a transition in tectonics, suggesting that Venus once hosted clement surface conditions and was more Earth-like.
Long-lived (>2.5 Ga) cratons usually preserve ancient cold and refractory mantle roots, but how the deep roots survive from recycling back to the convective mantle remains open to debate. Here, the mechanism for preservation of Archean mantle roots is explored using the major-, trace-element and Sr-Nd isotopic systematics of kimberlites, the asthenosphere-derived magmas under cratons. A case study on ~480 Ma kimberlites of the North China Craton suggests that their segregation domains have pressures (~5 GPa) shallower than the lower boundaries of typical craton roots and potential temperatures (Tp) between those of the ambient asthenosphere (Tp= ~1400 oC) and the cold lithospheric roots of cratons (~1200 oC). The dataset of primary kimberlites worldwide records similar temperature variation in their segregation domains, which likely represent the lowermost (asthenospheric) part of a thick thermal boundary layer between conductive lithosphere and convective asthenosphere. Our calculation on mantle viscosity suggests that the asthenospheric part of the thermal boundary layer would show marked viscosity increase due to thermal offset from normal mantle adiabat. The resultant resistant uppermost asthenosphere can serve as a protective sheath that can protect the cratonic roots from being eroded and removed. Our proposed model emphasizes the longevity of cratons provided simply by the thermal contrast between the cold craton roots and the asthenosphere
Elevated contents of structurally-bound hydrogen in olivine, introduced to cratonic mantle during metasomatism, are widely believed to decrease the effective viscosity so that it is similar to the underlying asthenosphere (10¹⁸–10¹⁹ Pa⋅s) and susceptible to basal erosion. Nevertheless, large variations exist in H contents of mantle olivines from the Kaapvaal, Siberia, Slave, Tanzania and Wyoming cratons (0 to 320 ppmw) and the role of water as the main driver of rheological weakening is contentious. While recent experimental studies have shown that olivine has the capacity to host large quantities of fluorine, as well as hydrogen, the magnitude of this has not yet been established for cratonic mantle. Our new dataset for Kaapvaal craton peridotites shows that the style of metasomatism influences the addition of H2O and F to cratonic mantle. Silicic fluids derived from subducted slabs, and responsible for the pervasive orthopyroxene enrichment observed in the Kaapvaal (and many of the world's other cratons), deliver significant quantities of H2O but lesser amounts of F, whereas proto-kimberlite melts transport high quantities of both H2O and F. Kaapvaal mantle olivines are major hosts of both H2O (up to 105 ppmw) and F (up to 180 ppm), and we propose that a major increases in the bulk H2O (∼30%) and F (∼65%) of the Kaapvaal craton occurred over a short (20 Ma) time interval between the two main pulses of Cretaceous kimberlite emplacement in southern Africa. By combining the thermal structure of individual cratons with corresponding H2O data for olivine, we show that the effective viscosity contrast at the base of the mechanical boundary layer and asthenosphere below the Kaapvaal, Slave and Siberia cratons varies from 2 to 14. Some of the lowest effective viscosities (3.5×10¹⁷ Pa⋅s) occur in H2O-rich olivine in Siberia sheared peridotites from the base of the craton and are consistent with highly-localised metasomatism. More viscous peridotites (up to 1.5×10²¹ Pa⋅s) were entrained from the thermal boundary layer beneath the Kaapvaal and Tanzania cratons but residence times in this region are short due to convective overturn (<100 Ma) and this dehydrated mantle would offer only temporary protection to cratonic keels. Our viscosity estimates are based on the H content of olivine and, while the additional effect of structurally bound F in olivine is currently uncertain, we anticipate that it will have a similar effect on mantle rheology to H, so that the base of the mechanical boundary layer beneath cratons at the time and location of kimberlite generation is close to the tipping point for instability. The rapid build-up of volatiles associated with pervasive kimberlite activity may have been the catalyst for lithospheric thinning on the southern margin of the Kaapvaal craton, but more localised pulses of kimberlite activity occurred over longer time intervals in the Siberia and Slave cratons and so had a less profound effect on their stability. A prolonged and widespread subduction-flux of H2O to the Wyoming and N. China cratons, and subsequent sub-solidus partitioning in olivine, may have been the driver for the rheological weakening that ultimately led to delamination.
Despite being exposed to convective stresses for much of the Earth’s history, cratonic roots appear capable of resisting mantle shearing. This tectonic stability can be attributed to the neutral density and higher strength of cratons. However, the excess thickness of cratons and their higher viscosity amplify coupling to underlying mantle flow, which could be destabilizing. To investigate the stresses that a convecting mantle exerts on cratons that are both strong and thick, we developed instantaneous global spherical numerical models that incorporate present-day geoemetry of cratons within active mantle flow. Our results show that mantle flow is diverted downward beneath thick and viscous cratonic roots, giving rise to a ring of elevated and inwardly-convergent tractions along a craton’s periphery. These tractions induce regional compressive stress regimes within cratonic interiors. Such compression could serve to stabilize older continental lithosphere against mantle shearing, thus adding an additional factor that promotes cratonic longevity.
Despite being exposed to convective stresses for much of the Earth's history, cratonic roots appear capable of resisting mantle shearing. This tectonic stability can be attributed to the neutral density and higher strength of cratons. However, the excess thickness of cratons and their higher viscosity amplify coupling to underlying mantle flow, which could be destabilizing. To investigate the stresses that a convecting mantle exerts on cratons that are both strong and thick, we developed instantaneous global spherical numerical models that incorporate present‐day geoemetry of cratons within active mantle flow. Our results show that mantle flow is diverted downward beneath thick and viscous cratonic roots, giving rise to a ring of elevated and inwardly‐convergent tractions along a craton's periphery. These tractions induce regional compressive stress regimes within cratonic interiors. Such compression could serve to stabilize older continental lithosphere against mantle shearing, thus adding an additional factor that promotes cratonic longevity.
Geophysical, geochemical, and geological investigations have attributed the stable behaviour of Earth’s continents to the presence of their Archean cratonic roots. These roots are likely composed of melt-depleted, low density residual peridotite with high magnesium number (Mg#), while devolatilisation from the upper mantle during magmatic events might have made these roots more viscous and intrinsically stronger than the convecting mantle. Several conceptual dynamic and petrological models of craton formation have been proposed. Dynamic models invoke far-field shortening or mantle melting events, e.g., by mantle plumes, to create melt-depleted and thick cratons. Compositional buoyancy and rheological modifications have also been invoked to create long-lived stable cratonic lithosphere. However, these conceptual models have not been tested in a dynamically self-consistent model. In this study, we present global thermochemical models of craton formation with coupled core-mantle-crust evolution driven entirely by gravitational forces. Our results with melting and crustal production (both oceanic and continental) show that formation of cratonic roots can occur through naturally occurring lateral compression and thickening of the lithosphere in a self-consistent manner, without the need to invoke far-field tectonic forces. Plume impingements, and gravitational sliding creates thrusting of lithosphere to form thick, stable, and strong lithosphere that has a strong resemblance to the Archean cratons that we can still observe today at the Earth’s surface. These models also suggest the recycling of denser eclogitic crust by delamination and dripping processes. Within our computed parameter space, a variety of tectonic regimes are observed which also transition with time. Based on these results, we propose that a ridge-only regime or a sluggish-lid regime might have been active on Earth during the Archean Eon as they offer favourable dynamics and conditions for craton formation.
The composition of diamond-hosted inclusions provides insight into the character of the sub-cratonic lithosphere of the Guiana Shield. Guyana’s Paleoproterozoic diamonds preserve an inclusion suite comprised of forsterite (Fo ~89.3-91.8), enstatite, chromite, and Cr-pyrope. Raman thermobarometry of entrapped olivine and pyrope inclusions indicate entrapment pressures of ~5.3 – 7.0 GPa. Unpolarized Fourier transform infrared spectroscopic measurements of forsterite and enstatite inclusions produce low absorbances from OH. Using established calibrations, those absorbances indicate forsterite and enstatite contain median values of 26±14 and 14±18 ppm H2O, respectively, and suggest a high effective viscosity of 1023.7±2.1 Pa∙s for the lithospheric mantle. When combined with inclusion thermobarometry, diamond residence temperatures suggest paleo-geotherms ranging from 35 to 40 mW m-2. Low-to-moderate Fe2O3 content (1.9±0.8 wt.%) and low oxygen fugacity (log ƒO2 (ΔFMQ) -1.6±1.1) determined from chromite inclusions indicate crystallization in reducing conditions. Forsterite and chromite inclusions retain evidence for metasomatic alteration, including Mn-enrichment in forsterite and chromite rich in Zn. These characteristics indicate that the sub-cratonic lithosphere of the Guiana Shield experienced episodes of partial melting and fluid-driven metasomatism of dry, strongly viscous, and moderately-depleted garnet-spinel harzburgite. The Guiana Shield has been relatively stable since the Paleoproterozoic, meaning diamond inclusions may also provide the best means for understanding current conditions in the region’s lithospheric mantle.
The polygenic suite of on- and off-craton mantle xenoliths from the ensemble of cratons reveals the admixed and/or interstratified nature of the depleted and fertile mantle beneath the Indian shield. Most cratons are reactivated and exhibit decoupling of the crust and mantle post-Proterozoic. The SCLM beneath exhibits accretion and reworking to varying degrees. The cratons to the south of the CITZ are older and have a more evolved crustal structure than the ones to the north. The Conrad is less prominent beneath the Bundelkhand Craton, and the lower crust has a larger component of magmatic cumulates.
The viscosity of cratons is key to understanding their long term survival. In this study, we present a time-dependent, full spherical, three dimensional mantle convection model to investigate the evolution of cratons of different viscosities. The models are initiated from 409 Ma and run forward in time till the present-day. We impose a surface velocity boundary condition, derived from plate tectonic reconstruction, to drive mantle convection in our models. Cratons of different viscosities evolve accordingly with the changing velocity field from their original locations. Along with the viscosity of cratons, the viscosity of the asthenosphere also plays an important role in cratons' long term survival. Our results predict that for the long-term survival of cratons they need to be at least 100 times more viscous than their surroundings and the asthenosphere needs to have a viscosity of the order of 10²⁰ Pa-s or more.
Cratons are the most ancient parts of continents that are underlain by thick, cold, old and refractory lithospheric roots. However, how cratonic roots remain stable for billions of years and become remobilized later is still not well understood. The eastern North China Craton (NCC) is the best region to illuminate this issue because of its well-known lithospheric thinning and decratonization during the Mesozoic–Cenozoic. The thinning mechanism is debated because of limited constraints on the thermal-chemical conditions (lithology and P–T–H 2 O–fo 2 ) of the Archean lithospheric mantle before and during its removal. Here, we provide constraints on these thermal-chemical conditions for the Archean lithospheric mantle beneath the eastern NCC during its extensive thinning in the form of whole-rock chemical and Sr–Nd–Pb isotopic compositions and mineral (especially olivine) chemistry of the Early Cretaceous primitive basalts (MgO > 10 wt.%) from Yixian and Sihetun in the western Liaoning Province. Our data support a model in which the Yixian and Sihetun basalts were derived from metasomatized Archean lithospheric mantle under shallow (∼50–60 km), hot (∼1,290–1,350 °C) conditions. This indicates the existence of a relict (∼25 km) of the Archean lithospheric mantle during the Early Cretaceous, supporting gradual or episodic erosion of the eastern NCC lithospheric mantle. Furthermore, the NCC lithospheric mantle was not only widely rehydrated (>1,000 ppm H 2 O) but also highly oxidized (Δlogfo 2FMQ =+1.5∼+1.9 at 1.7–2.0 GPa) during its extensive thinning. Such rehydration and oxidization are demonstrated to be closely related to wet upwelling from the Mantle Transition Zone (MTZ) triggered by the deep subduction of the Paleo-Pacific oceanic slab in the period ∼200–125 Ma. We emphasize that the water released from the upwelling MTZ component and associated hydrous melt influx played a key role in the lithospheric thinning of the eastern NCC by oxidizing the lithospheric mantle and lowering its melting point, which led to redox melting, promoting the erosion of cratonic lithosphere. Our study provides key evidence for the role of deep volatile cycling from the MTZ in modifying thermal-chemical conditions and in the lithospheric thinning of cratons.
To constrain the density structure of Archean lithosphere, I assume that gravitational potential energy (GPE) per unit area at Archean spreading centers equals that for continents whose surfaces lay near sea level. The present-day balance of GPE limits average density deficits in Archean mantle lithosphere due to depletion of iron during its formation to ∼30–45 kg/m³. For an Archean volume of seawater equal to or greater than that today and heat loss approximately three times that today (e.g., at ∼3.5 Ga), plausible Archean structures, constrained by GPE balance, favor submerged spreading centers and continents. Emergent Archean continents would be allowed by a combination of the following (a) heat loss only twice that today, (b) less than approximately half of the present-day volume of thick crust, either continental or oceanic plateaus, and (c) deep Archean spreading centers (≳2 km below continents), which would require a thickness of Archean oceanic crust ≲25 km. If the volume of Archean seawater were 50% greater than that today, emergent spreading centers could have existed only as isolated unusual features, and emergent continents would require both limited extent of thick crust and spreading centers deeper that ∼2 km. The observed widespread development of continental margins and carbonate platforms at ∼2.7–2.5 Ga is consistent with heat loss roughly twice that today, a volume of seawater comparable with that today, and either (a) a volume of continental crust comparable to that at present and oceanic crust ≲30 km thick or (b) thicker oceanic crust, possibly 40–50 km, but with a volume of continental crust less than approximately half that today. Calculated temperatures at the Archean Moho are ∼700–900 °C. These calculations do not rule out a hot early Archean ocean (∼50–80 °C) and do not favor an early Archean emergence of life on land above sea level.
The view that cratons are tectonically and geomorphologically inert continental fragments is at odds with a growing body of evidence partly based on low-temperature thermochronology (LTT) studies. These suggest that large areas of cratons may have undergone discrete episodes of regional-scale Neoproterozoic and/or Phanerozoic heating, and cooling from modestly elevated paleotemperatures. Cooling is often attributed to the km-scale erosion of overlying low-conductivity sediments, rather than to removal of large sections of crystalline basement. Independent evidence for sedimentary burial includes: preservation of outliers, the sedimentary record in intracratonic basins, and sedimentary xenoliths entrained within kimberlites periodically emplaced into cratons. Further, stratigraphic and isotopic data from basinal sediments proximal to some cratons carry a record of the detritus removed, which can be linked temporally to cooling episodes in their inferred cratonic source areas. Differences in denudation rates reported from cratonic basement reconstructed from LTT data (long-term) and cosmogenic isotope and chemical weathering studies (short-term) reflect the strong contrast in erodibility potential between cover sediments since removed and the preserved crystalline rocks. Underlying processes involved in cratonic heating and cooling may include one of, or a complex interplay between: proximity to sediment sources from elevated orogens forming extensive foreland basins, structural deformation transmitted by far-field horizontal stresses from active plate boundaries, and the development of dynamic topography driven by vertical mantle stresses. Dynamic topography may also explain elevation changes observed in some cratons, where no clear deformation is apparent. LTT studies from classic cratons in Fennoscandia, Western Australia, Southern Africa, and Canada are reviewed, with emphasis on different aspects of their more recent evolution.
Growing evidence shows that lithospheric mantle beneath cratons may contain a certain amount of water that originated from dehydration of subducted slabs or mantle metasomatism. As water can significantly reduce the viscosity of nominally anhydrous minerals such as olivine, hydration-induced rheological weakening is a possible mechanism for the lithospheric thinning of cratons. Using 2D thermomechanical numerical models we investigated the influence of water on dislocation and diffusion creep of olivine during the evolution of cratonic lithosphere. Modeling results indicate that dislocation creep of wet olivine alone is insufficient to trigger dramatic lithospheric thinning within a timescale of tens of millions of years, even with an extremely high water content. However, if diffusion creep is incorporated, significant convective instability will occur at the base of the lithosphere and drive lithospheric mantle dripping, which results in intense lithospheric thinning. We performed semi-analytical models to better understand the influence of various parameters on the onset of convective instability. The convective instability promoted by hydration weakening drives lithospheric mantle dripping beneath cratons and thus provides a possible mechanism for cratonic thinning.
Cratons are strong and their preservation demonstrates that they resist deformation and fragmentation. Yet several cratons are rifting now, or have rifted in the past. We suggest that cratons need to be weakened before they can rift. Specifically, metasomatism of the depleted dehydrated craton mantle lithosphere is a potential weakening mechanism. We use 2D numerical models to test the efficiency of simulated melt metasomatism and coeval rehydration to weaken craton mantle lithosphere roots. These processes effectively increase root density through a parameterized melt-peridotite reaction, and reduce root viscosity by increasing the temperature and rehydrating the cratonic mantle lithosphere. The models are designed to investigate when a craton is sufficiently weakened to undergo rifting and is no longer protected by adjacent standard Phanerozoic lithosphere. We find that cratons only become vulnerable to rifting following large-volume melt metasomatism (~ 30% by volume) and thinning of the gravitationally unstable cratonic lithosphere from > 250 km to ~ 100 km; at which point its residual crustal strength is important. Furthermore, our results indicate that rifting of cratons depends on the timing of extension with respect to metasomatism. An important effect in the large-volume melt models is the melt-induced increase in temperature which must have time to reach peak values in the uppermost mantle lithosphere before rifting. Release of water stored in the transition zone at the base of a big mantle wedge may provide a suitable natural setting for both rehydration and refertilization of an overlying craton and is consistent with evidence from the eastern North China Craton. An additional effect is that cratons subside isostatically to balance the increasing density of craton mantle lithosphere where it is moderately metasomatized. We suggest that this forms intracratonic basins and that their subsidence and subsequent uplift, and cratonic rifting constitute evidence of progressive metasomatism of cratonic mantle lithosphere.
Oceanic plateaus develop by decompression melting of mantle plumes and have contributed to the growth of the continental crust throughout Earth's evolution. Occasional large-scale partial melting events of parts of the asthenosphere during the Archean produced large domains of precursor crustal material. The fractionation of arc-related crust during the Proterozoic and Phanerozoic contributed to the growth of continental crust. However, it remains unclear whether the continents or their precursors formed during episodic events or whether the gaps in zircon age records are a function of varying preservation potential. This study demonstrates that the formation of the continental crust was intrinsically tied to the thermoconvective evolution of the Earth's mantle. Our numerical solutions for the full set of physical balance equations of convection in a spherical shell mantle, combined with simplified equations of chemical continent–mantle differentiation, demonstrate that the actual rate of continental growth is not uniform through time. The kinetic energy of solid-state mantle creep (Ekin) slowly decreases with superposed episodic but not periodic maxima. In addition, laterally averaged surface heat flow (qob) behaves similarly but shows peaks that lag by 15–30 Ma compared with the Ekin peaks. Peak values of continental growth are delayed by 75–100 Ma relative to the qob maxima. The calculated present-day qob and total continental mass values agree well with observed values. Each episode of continental growth is separated from the next by an interval of quiescence that is not the result of variations in mantle creep velocity but instead reflects the fact that the peridotite solidus is not only a function of pressure but also of local water abundance. A period of differentiation results in a reduction in regional water concentrations, thereby increasing the temperature of the peridotite solidus and the regional viscosity of the mantle. By plausibly varying the parameters in our model, we were able to reproduce the intervals of the observed frequency peaks of zircon age determinations without essentially changing any of the other results. The results yield a calculated integrated continental growth curve that resembles the curves of GLAM, Begg et al. (2009), Belousova et al. (2010), and Dhuime et al. (2012), although our curve is less smooth and contains distinct variations that are not evident in these other curves.
Trace amount of water associated with the lattice defects of nominally anhydrous minerals (NAMs) can be measured using Fourier transform infrared spectroscopy (FTIR) and secondary ion mass spectrometry (SIMS). Lots of data on water in NAMs from different lithologies, especially mantle peridotite xenoliths, have been published. The water distribution in olivine from peridotite xenoliths often displays a diffusion profile with high water concentration in the core and low at the rim, which indicates water loss via diffusion during the ascent of host magma. On the other hand, water is homogeneously distributed in pyroxene and its concentration is typically interpreted to represent a mantle value. The water concentration of magma in equilibrium with NAM can be estimated using specific partition coefficient, from which the water content of parental magma and the mantle source can be inferred. The accuracy of this method, however, depends on the selection of appropriate partition coefficient for the system. Using hydrogen isotope compositions and H2O/Ce ratios of mantle NAMs, water source regions can be traced and water heterogeneity can be mapped in the upper mantle. Water plays an important role in the stability of cratonic mantle. The water contents and vertical distribution patterns can be significantly different among different cratonic mantles, which may result from different geologic activities. However, the mantle-plume interaction may not necessarily result in significant change of water content in cratonic mantle. The estimation of the water content in the upper mantle is still largely based on geochemical models due to the limitations of data on water in mantle NAMs.
Convective removal on lithospheric root, especially on cratonic lithosphere root is studied via numerical simulations in 2D models. The control parameters in the models include the width, x, stretching factor, gamma, viscosity ratio, eta(c), and chemical buoyancy number, B, or effective density contrast, Delta rho(tc). Numerical results show that mantle convection thins the thickened lithosphere, and (1) when B = 0 and eta(c) = 1, i. e., for general lithosphere, the root removal duration is scaled as 0. 0073 gamma(0.70)x(0.26), which means, for a 300 km thickness lithosphere with an initially equilibrium thickness of 120 km, and a root width of 300 km or 1500 km, it takes 225 Ma or 342 Ma to thin by sublithospheric convection to its equilibrium; (2) for small B and eta(c), the process to thin cratonic lithosphere is similar to that to thin general lithosphere, but the root removal duration is increased significantly, and the root removal duration is scaled as 0.0057 eta(0.52)(c) Delta rho(-0.21)(tc)gamma(0.789 eta c-0.36)x(0.04) With this scaling law, for a 300 km thickness lithosphere with an initially equilibrium thickness of 120 km, and a root width of 1500 km, the root removal duration is 1.18 Ga if eta(c)=10; and (3) for large B and eta(c) (B >= 0.2 and eta(c)>10), the process to thin cratonic lithosphere is very different. Because of the influences of chemical buoyancy and viscosity, the cratonic lithosphere is spread to adjacent lithosphere instead of mixing with underneath mantle. In these cases, the root removal duration is very long (>3 Ga).
The Hadean mantle was efficiently heated from high internal heat production, high rates of impact bombardment, and primordial heat from accretion. As a result a strong case is made for extremely high internal temperatures, low internal viscosities, and extremely vigorous mantle convection. Previous studies of mixing in such high-Rayleigh number convective environments indicate that chemically heterogeneous mantle anomalies should have efficiently remixed into the mantle on timescales of less than 100 Myr. However, 142Nd and 182W isotope studies indicate that heterogeneous mantle domains survived, without mixing, for over 2 Gyr-at odds with expected mixing rates. Similarly, concentrations of platinum group elements in Archean komatiites, purportedly due to the later veneer of meteoritic addition on the Earth, only achieve current levels at 2.7 Ga-indicating a time lag of almost 1 to 2 Gyr in mixing this material thoroughly into the mantle. Previous studies have sought to explain slow Archean mantle mixing via mantle layering due to endothermic phase changes, or anomalously viscous blobs of material, with limited efficacy. Here we pursue another explanation for inefficient mantle mixing in the Hadean: tectonic regime. A number of lines of evidence suggest that resurfacing in the Archean was episodic, and extending these models to Hadean times implies the Hadean was characterized by long periods of tectonic quiescence. We explore mixing times in 3D spherical-cap models of mantle convection, which incorporate vertically stratified and temperature-dependent viscosities. We show that mixing in stagnant-lid regimes is, at the extreme, over an order of magnitude less efficient than mobile-lid mixing, and for plausible Rayleigh numbers and internal heat production, the lag in Hadean convective recycling can be explained. The attractiveness of this model is that it not only explains the long-lived 142Nd and 182W anomalies, but also posits an explanation for the delay between accretion of the late veneer-between 4.5 to 3.8 Ga on a stagnant surface- and its fully mixed signature apparent in elevated PGEs in 2.7 Ga komatiites. It also provides an explanation for the 400 Myrs of immobility of the mafic protolith from which the Jack Hill zircons were sourced, and retards early heat loss from the mantle, providing a solution to the "Archean thermal catastrophe" of parameterized Earth evolution models.
The distribution of water concentrations in the upper mantle has drastic influence on its melting, rheology, and electrical and thermal conductivities and yet is primarily known indirectly from analyses of OIB and MORB. Here, actual mantle samples, eight peridotite xenoliths from Salt Lake Crater (SLC) and one from Pali in Oahu in Hawaii were analyzed by FTIR. Water contents of orthopyroxene, clinopyroxene and the highest measured in olivine are 116-222, 246-442, and 10-26 ppm weight H2O respectively. Although pyroxene water contents correlate with indices of partial melting, they are too high to be explained by simple melting modeling. Mantle-melt interaction modeling reproduces best the SLC data. These peridotites represent depleted oceanic mantle older than the Pacific lithosphere that has been refertilized by nephelinite melts containing <5 weight % H2O. Metasomatism in the Hawaiian peridotites resulted in an apparent decoupling of water and LREE that can be reconciled via assimilation and fractional crystallization. Calculated bulk-rock water contents for SLC (50 to 96 ppm H2O) are on the low side of that of the MORB source (50-200 ppm H2O). Preceding metasomatism, the SLC peridotites must have been even drier, with a water content similar to that of the Pali peridotite (45 ppm H2O), a relatively unmetasomatized fragment of the Pacific lithosphere. Moreover, our data show that the oceanic mantle lithosphere above plumes is not necessarily enriched in water. Calculated viscosities using olivine water contents allow to estimate the depth of the lithosphere-asthenosphere boundary beneath Hawaii at ∼90 km. This article is protected by copyright. All rights reserved.
The Protopangaea-Palaeopangaea model for the Precambrian continental crust predicts quasi-integrity reflecting a dominant Lid Tectonics defined by a palaeomagnetic record showing prolonged near-static polar behaviour during very long time intervals (~2.7–2.2, 1.5–1.2, and 0.75–0.6 Ga). Intervening times show polar loops radiating from the geometric centre of the crust explaining the anomalous Precambrian magnetic inclination bias. The crustal lid was a symmetrical crescent-shaped body confined to a single hemisphere on the globe comparable in form to the Phanerozoic supercontinent (Neo) Pangaea. There were two major transitions in the tectonic regime when prolonged near-static motion was succeeded by widespread tectonic-magmatic activity, and each coincided with the major isotopic/geochemical signatures in the Precambrian record. The first comprised a ~90° reconfiguration of crust and mantle at ~2.2 Ga terminating the long 2.7– 2.2 Ga static interval; the second was the largest continental break-up event in geological history and is constrained to the Ediacaran Period at ~0.6 Ga by multiple isotopic and geochemical signatures and the subsidence history of marine passive margins. Break-up of the lid at ~0.6 Ga defines a transition from dominant Lid Tectonics to dominant Plate Tectonics and is the primary cause of contrasts between the Precambrian and Phanerozoic aeons of geological times. The long interval of minimal continental motion in the mid-Proterozoic correlates with extensive emplacement of anorogenic anorthosite-rapakivi plutons unique to these times, and high-level emplacement was probably caused by blanketing of the mantle and comprehensive thermal weakening of the crust. Continental velocities were low during the two Proterozoic intervals characterized by profound glaciation (~2.4–2.2 and ~0.75–0.6 Ga) when partial or complete magmatic shutdown is likely to have reduced volcanic greenhouse gas production. Specific implications of Protopangaea-Palaeopangaea include: (i) support for recent evidence that 60–70% of the present continental crust had accreted by ~2.5 Ga; (ii) recognition from spatially constrained mineral provinces that sub-crustal lithosphere was already chemically differentiated by mid-Archaean times; (iii) strong axial alignment of younger greenstone belts, major Palaeoproterozoic shear zones, and later Meso– Neoproterozoic mobile/orogenic belts; (iv) concentration of anorogenic magmatism and progressive contraction of activity towards the orogenic margin subsequently to become the focus of Grenville (~1.1 Ga) orogenesis; and (v) late Neoproterozoic arc magmatism/tectonics at the instep margin of the continental crescent persisting until the Ediacaran continental break-up.
In contrast to the coastal regions of the South China Block (SCB), little is known about the subcontinental lithospheric mantle beneath the interior of the SCB. Mantle xenoliths entrained in Cenozoic basalts in eastern and central Guangxi Province, the interior of the SCB, include spinel harzburgites, clinopyroxene-poor lherzolites, lherzolites and olivine websterites. The mineral chemistry of the harzburgites and clinopyroxene-poor lherzolites is moderately refractory [Mg# value of olivine (Mg#Ol) = 90.2–91.3], whereas other lherzolite is more fertile (Mg#Ol = 89.3). Zoned olivines (Mg#Ol = 83.7–88.8) in the harzburgites and zoned olivine xenocrysts (Mg#Ol = 75.2–82) in the basalts reflects disequilibrium between olivines and the basaltic host melts during magma ascent. An olivine websterite (Mg#Ol = 87.5) is similar to the lherzolite in mineral chemistry. The REE patterns of clinopyroxenes in these xenoliths vary from LREE-depleted, to flat, to LREE-enriched patterns, and commonly exhibit positive Sr anomalies and negative Nb, Zr and Ti anomalies. The peridotitic xenoliths mostly experienced moderate to high degree of melt extraction (F = 10–20 %) and were modified by silicate metasomatism. We thus suggest that the harzburgites and clinopyroxene-poor lherzolites with high Mg#Ol values represent ancient (Proterozoic) lithospheric mantle, preserved beneath the Guangxi Province. In contrast, the minor, fertile (low-Mg#Ol) lherzolites represent lithospheric mantle accreted during the Phanerozoic, and a small amount of pyroxenite was produced via interaction between peridotite and silicate-rich melts. The mantle-accretion process that occurred beneath the SCB during Mesozoic to Cenozoic time extended into Guangxi Province. The lithospheric mantle beneath the interior of the SCB is heterogeneous, featuring various types of peridotite and co-existing pyroxenite. This heterogeneity also indicates that the lithospheric mantle beneath the regions affected by translithospheric faults could be wholly or partially replaced by the juvenile accreted mantle. In contrast, the most stable regions in the interior of the SCB probably are dominated by moderately to strongly refractory, ancient (Proterozoic) lithospheric mantle.
The physical conditions required to provide for the tectonic stability of cratonic crust and for the relative longevity of deep cratonic lithosphere within a dynamic, convecting mantle are explored through a suite of numerical simulations. The simulations allow chemically distinct continents to reside within the upper thermal boundary layer of a thermally convecting mantle layer. A rheologic formulation, which models both brittle and ductile behavior, is incorporated to allow for plate-like behavior and the associated subduction of oceanic lithosphere. Several mechanisms that may stabilize cratons are considered. The two most often invoked mechanisms, chemical buoyancy and/or high viscosity of cratonic root material, are found to be relatively ineffective if cratons come into contact with subduction zones. High root viscosity can provide for stability and longevity but only within a thick root limit in which the thickness of chemically distinct, high-viscosity cratonic lithosphere exceeds the thickness of old oceanic lithosphere by at least a factor of 2. This end-member implies a very thick mechanical lithosphere for cratons. A high brittle yield stress for cratonic lithosphere as a whole, relative to oceanic lithosphere, is found to be an effective and robust means for providing stability and lithospheric longevity. This mode does not require exceedingly deep strength within cratons. A high yield stress for only the crustal or mantle component of the cratonic lithosphere is found to be less effective as detachment zones can then form at the crust-mantle interface which decreases the longevity potential of cratonic roots. The degree of yield stress variations between cratonic and oceanic lithosphere required for stability and longevity can be decreased if cratons are bordered by continental lithosphere that has a relatively low yield stress, i.e., mobile belts. Simulations that combine all the mechanisms can lead to crustal stability and deep root longevity for model cratons over several mantle overturn times, but the dominant stabilizing factor remains a relatively high brittle yield stress for cratonic lithosphere.
Orogenic peridotites occur enclosed in Proterozoic gneisses at several localities in the Western Gneiss Region (WGR) of western Norway; garnet peridotites typically occur as discrete zones within larger bodies of garnet-free, chromite-bearing dunite and are commonly closely associated with pyroxenites and eclogites. The dunites of the large Almklovdalen peridotite body have extremely depleted compositions (Mg-number 92-93.6); the garnet peridotites have lower Mg-number (90.6-91.7) and higher whole-rock Ca and Al contents. Post-depletion metasomatism of both rock types is indicated by variable enrichment in the light rare earth elements, Th, Ba and Sr. The dunites can be modelled as residues after very high degrees (>60%) of melt extraction at high pressure (5-7 GPa), inconsistent with the preservation of lower degrees of melting in the garnet peridotites. The garnet peridotites are, therefore, interpreted as zones of melt percolation, which resulted in refertilization of the dunites by a silicate melt rich in Fe, Ca, Al and Na, but not Ti. Previous Re-Os dating gives Archaean model ages for the dunites, but mixed Archaean and Proterozoic ages for the garnet peridotites, suggesting that refertilization occurred in Proterozoic time. At least some Proterozoic lithosphere may represent reworked and transformed Archaean lithospheric mantle.
To understand the origin, modification, and preservation of continents on Earth, a multidisciplinary study is examining the crust and upper mantle of southern Africa. Xenoliths of the mantle brought to the surface by kimberlites show that the mantle beneath the Archean Kaapvaal craton is mostly melt-depleted peridotite with melt extraction accompanying crust formation in the Archean. Eclogitic xenoliths from the craton record subduction of altered oceanic crust beneath the craton at ca. 3 Ga. Proterozoic age peridotite found beneath the surrounding Proterozoic accretionary belts provides evidence for crust-mantle coupling and long-term stability of the upper 150 km of the lithosphere. Petrologic examination of Archean ultramafic magmas (komatiites) from South Africa indicates that some komatiitic magmas contain substantial quantities of water (>4 wt%). This finding strengthens the possibility that the cratonic lithosphere formed initially in a subduction zone setting, the demise of which led to accretion of the arc crust and thickening of the lithospheric mantle to create a stable, thick, continental lithosphere. Geochronologic studies of lower crustal xenoliths from the craton show a prolonged thermal evolution of the lower crust extending to 1 Ga. This thermal evolution is also reflected in ca. 1 Ga ages of some eclogitic diamond inclusions from the lithospheric mantle.
Heavy-mineral concentrates (garnets, chromites) and xenoliths from 21 Cretaceous–Tertiary kimberlite intrusions have been
used to map the lithospheric mantle beneath the Lac de Gras area in the central part of the Slave Province. Analyses of Nickel
Temperature (TNi) and Zinc Temperature (TZn) have been used to place garnet and chromite xenocrysts, respectively, in depth context. Paleogeotherms derived from both
xenoliths and concentrates lie near a 35 mW/m2 conductive model at T ≤ 900°C, and near a 38 mW/m2 model at higher T, implying a marked change in conductivity and/or a thermal transient. Plots of garnet composition vs TNi also show a sharp discontinuity in mantle composition at 900°C. Garnets from <145 km depth are ultradepleted in Y, Zr, Ti
and Ga, whereas those from greater depths (to ≥ 200 km) are similar to garnets from Archean mantle world-wide. Relative abundances
of garnet types indicate that the shallow layer consists of ∼60% (clinopyroxene-free) harzburgite and 40% lherzolite, whereas
the deeper layer contains 15–20% harzburgite and 80–85% lherzolite. T estimates on eclogite xenoliths show that all were derived from the deeper layer. Xenolith data and garnet compositions indicate
that the shallow layer is more magnesian (Fo92–94) than the deeper layer (Fo91–92), and both layers are more olivine rich than South African or Siberian Archean peridotite xenoliths. The composition and
sharply defined structure of the Lac de Gras lithosphere are unique within our current knowledge of Archean mantle sections.
The shallow layer of this lithosphere section is similar to peridotites from some highly depleted ophiolites from convergent-margin
settings, and may have formed in a similar situation during the accretion of the Hackett and Contwoyto terranes (magmatic
arc and accretionary prism, respectively) to the ancient continental Anton terrane at 2.6–2.7 Ga. The deeper layer is interpreted
as a plume head, which rose from the lower mantle and underplated the existing lithosphere at 2.6 Ga; evidence includes a
high proportion of the superdeep inclusion assemblage (ferropericlase–perovskite) in the diamond population. This event could
have provided heat for generation of the widespread 2.6 Ga post-tectonic granites. Proterozoic subduction from east and west
may have modified the cratonic root, mainly by introduction of eclogites near its base.
Starting with the Laramide orogeny and continuing through the Cenozoic, the U.S. Cordilleran orogen is unusual for its width, nature of uplift, and style of tectonic and magmatic activity. We present teleseismic tomography evidence for a thickness of modified North America lithosphere <200 km beneath Colorado and >100 km beneath New Mexico. Existing explanations for uplift or magmatism cannot accommodate lithosphere this thick. Imaged mantle structure is low in seismic velocity roughly beneath the Rocky Mountains of Colorado and New Mexico, and high in velocity to the east and west, beneath the tectonically intact Great Plains and Colorado Plateau. Structure internal to the low-velocity volume has a NE grain suggestive of influence by inherited Precambrian sutures. We conclude that the high-velocity upper mantle is Precambrian lithosphere, and the lowvelocity volume is partially molten Precambrian North America mantle.
We address the question of how convective processes control the thicknesses of oceanic and continental lithospheres. The numerical convection model involves a Newtonian rheology which depends on temperature and pressure. A repeated plate tectonic cycle is modeled by imposing a time-dependent surface velocity. One part of the surface, representing a continent, never subducts. The asymptotic equilibrium thickness of the lithosphere varies with the viscosity at the base of the lithosphere, but is not directly sensitive to the pressure dependence of the viscosity law and to the plate velocity. For small activation volumes, and average upper mantle viscosities deduced from postglacial rebound, the equilibrium plate thickness is more than 400 km (regimes 1 and 2). The equilibrium thickness of the oceanic lithosphere (around 100 km) implies that the viscosity in the asthenosphere is less than 7×1018 Pa s. Only models with strongly pressure-dependent viscosity laws (activation volumes greater than 9×10−6 m3/mol) are able to reconcile this value with the average upper mantle viscosity (5×1020 Pa s). For these models, there are two lithospheric thicknesses such that the heat supplied by convection at the base of the lithosphere equals the surface conductive heat flow (regime 3). They could be that of an aged oceanic lithosphere and that of a shield lithospheric root. They indeed appear as points of prefered thickness in our numerical models. However, convection triggered by the lateral density jumps at the boundaries between the root and the thinner lithosphere slowly destabilizes the thick lithosphere. A plausible degree of chemical buoyancy in a depleted lithospheric root does not prevent convective erosion. In our simulations, long-term stability of a cratonic lithospheric root is best achieved when its material is both buoyant and more viscous than the surrounding mantle. Extensive devolatilization of the refractory rocks forming the root is invoked to explain this viscosity increase.
Numerical simulations show that depth-dependent viscosity can increase
the wavelength of mantle convection. The physical mechanism behind this
phenomenon and its robustness with respect to model parameters remain to
be fully elucidated. Toward this end, we develop theoretical heat flow
scalings for a convecting fluid layer with depth-dependent viscosity.
Bottom and internally heated end-members are considered. For the former,
the viscosity structure consists of a high-viscosity central region
bounded from above and below by horizontal low-viscosity channels. For
internally heated cases, only a surface low-viscosity channel is
present. Theoretical scalings derived from boundary layer theory show
that depth-dependent rheology lowers the lateral dissipation associated
with steady state convective rolls, allowing longer aspect ratio cells
to form as the viscosity contrast between the channels and the central
region is increased. The maximum cell aspect ratio is estimated from the
condition that the pressure gradients that drive lateral flow in the
channels do not become so large as to inhibit vertical flow into the
channels. Scaling predictions compare favorably to results of numerical
simulations for steady state cells. As the Rayleigh number driving
convection is increased, small-scale boundary layer instabilities begin
to form. This increases lateral dissipation within the channels and the
preferred cell aspect ratio decreases as a result. Internally heated
simulations show that a near-surface high-viscosity layer, an analog to
tectonic plates, can suppress these small-scale instabilities. This
allows a low-viscosity channel to maintain large aspect ratio cells for
Rayleigh numbers approaching that of the present-day Earth.
The Paleozoic formations exposed to the West of the Main Uralian fault
belonged to a passive margin of Balica. The margin formed in the Early
Paleozoic as a result of break-up of a bigger continent. The rugged outline of
the margin, with promontories and recesses, predetermined many features
of the future Uralide foldbelt. Three stages of development of the margin
are established: a rift stage; a passive stage; and a collisional stage. Late
Cambrian to Early Ordovician rifting was followed by seafloor spreading
and subsidence of the newly formed margin. The margin developed a typical
topographic profile with a shelf, continental slope, and continental rise,
whose facial zones can be recognised in the rocks of the Uralides. The passive
margin stage was accompanied by weak tectonic activity, with rare, localised
episodes of rifting. The accumulation of sediments was governed by subsidence
and a strong dependence on sea-level changes. The change to the
collisional stage can be recognised in the accumulation of greywacke flysch,
which filled a deep-water basin and was deposited over the continental shelf.
These collision-related processes were strongly diachronous, starting in the
Late Devonian in the Southern Urals, and propagating northward, were they
ended in Pay-Khoy as late as the Jurassic.
1. INTRODUCTION
Before the advent of the theory of plate tectonics, the
Paleozoic passive margin of Baltica involved in the
Uralides was traditionally treated as a miogeosyncline
[e.g., Kheraskov, 1967]. Many other terms from the preplate
tectonics era could also be applied to what are now
considered the deep water bathyal formations of the
Mountain Building
Seismic tomography can provide unique information on the structure of the subcontinental lithospheric mantle (SCLM), but seismic velocity reflects both temperature and composition. We present a methodology for evaluating and isolating the relative contributions of these effects, which produces maps of regional geotherm and broad compositional constraints on the SCLM from the inversion of shear wave (Vs) seismic tomography. This approach uses model geotherms quantized in steps of 2.5 mW/m2 and three mantle compositions corresponding to typical Archean, Proterozoic, and Phanerozoic SCLM. Starting from an assumed composition for a volume of SCLM, lithospheric density at surface pressure and temperature is calculated for each geotherm at each point; the optimum geotherm is taken as the one yielding a density closest to the mean value derived from mantle xenoliths (3.31 g/cm3), since density varies with composition. Results requiring densities or geotherms outside the known natural range of these parameters worldwide require the choice of a different mantle composition. This technique, applied iteratively to a 275 km × 275 km Vs model developed by S. Grand (University of Texas, Austin), results in maps of the geotherm and regional density, which allow interpretation of SCLM composition within broad limits. These results can then be compared with local (paleo)geotherms and data for mantle composition, derived from xenolith suites. Application of this technique to the SCLM beneath Africa, Siberia, and North America shows good correlation with regional geological features, xenolith data, and other geophysical data.
During 1991–1994,the four year period covered by this report,there has been a shift in emphasis away from one‐dimensional (radial) mantle viscosity models and toward an understanding of the three‐dimensional (radial and lateral) structure of mantle viscosity. Our understanding of mantle viscosity comes primarily from two sources: studying the creep properties of mantle minerals under appropriate pressures and temperatures and using theoretical flow models to predict surface observables, such as: plate velocities, gravity, geoid, and heat flow. Laboratory measurements of deformation indicate that the rheology of upper mantle minerals such as olivine ((Mg,Fe) 2 SiO 4 ) is a strong function of temperatue, grain size, and stress [cf., Karato and Wu , 1993]. The deformation of minerals under mantle conditions generally follows a flow law of the form urn:x-wiley:87551209:media:rog1798:rog1798-math-0001 where ϵ is the deformation rate,σ is the deviatoric stress, μ is the shear modulus,d is the grain size of the rock, Q is the activation energy for the deformation mechanism,T is the temperature in Kelvin,R is the gas constant and A is a constant. Viscosity is defined as urn:x-wiley:87551209:media:rog1798:rog1798-math-0002 therefore, deformation is directly related to viscosity. Substituting equation (1) into equation (2), an effective viscosity, urn:x-wiley:87551209:media:rog1798:rog1798-math-0003 can be defined.For temperature changes of 100 degrees K, the viscosity changes by an order of magnitude at constant stress [cf., Karato and Wu ,1993].Changes of deviatoric stress by a factor of 2 change the viscosity by an order of magnitude [cf., Karato and Wu ,1993]. Other factors, such as partial pressure of oxygen and water content may also have important effects.
Cratons are underlain by thick, cold, and highly melt-depleted mantle roots, the latter imposing a chemical buoyancy that roughly offsets the craton's negative thermal buoyancy associated with its cooler thermal state. Petrologic/geochemical predictions of three endmember scenarios for the origin of cratonic mantle are discussed: (1) high-degree melting in a very hot plume head with a potential temperature >1650°C, (2) accretion of oceanic lithosphere, and (3) accretion of arc lithosphere. The hot plume scenario predicts that cratonic peridotites were formed by high degrees of melting at very high pressures (≥7 GPa), whereas the two accretion scenarios predict an origin by melting on average at lower pressures (
Archaean cratons are the stable remnants of Earth's early continental lithosphere, and their structure, composition and survival over geological time make them unique features of the Earth's surface. The Kaapvaal Project of southern Africa was organized around a broadly diverse scientific collaboration to investigate fundamental questions of craton formation and mantle differentiation in the early Earth. The principal aim of the project was to characterize the physical and chemical nature of the crust and mantle of the cratons of southern Africa in geological detail, and to use the 3D seismic and geochemical images of crustal and mantle heterogeneity to reconstruct the assembly history of the cratons. Seismic results confirm that the structure of crust and tectospheric mantle of the cratons differs significantly from that of post-Archaean terranes. Three-dimensional body-wave tomographic images reveal that high-velocity mantle roots extend to depths of at least 200 km, and locally to depths of 250-300 km beneath cratonic terranes. No low-velocity channel has been identified beneath the cratonic root. The Kaapvaal Craton was modified approximately 2.05 Ga by the Bushveld magmatic event, and the mantle beneath the Bushveld Province is characterized by relatively low seismic velocities. The crust beneath undisturbed Archaean craton is relatively thin (c. 35-40 km), unlayered and characterized by a strong velocity contrast across a sharp Moho, whereas post-Archaean terranes and Archaean regions disrupted by large-scale Proterozoic magmatic or tectonic events are characterized by thicker crust, complex Moho structure and higher seismic velocities in the lower crust. A review of Re-Os depletion model age determinations confirms that the mantle root beneath the cratons is Archaean in age. The data show also that there is no apparent age progression with depth in the mantle keel, indicating that its thickness has not increased over geological time. Both laboratory experiments and geochemical results from eclogite xenoliths suggest that subduction processes played a central role in the formation of Archaean crust, the melt depletion of Archaean mantle and the assembly of early continental lithosphere. Co-ordinated geochronological studies of crustal and mantle xenoliths have revealed that both crust and mantle have experienced a multi-stage history. The lower crust in particular retains a comprehensive record of the tectonothermal evolution of the lithosphere. Analysis of lower-crustal xenoliths has shown that much of the deep craton experienced a dynamic and protracted history of tectonothermal activity that is temporally associated with events seen in the surface record. Cratonization thus occurred not as a discrete event, but in stages, with final stabilization postdating crustal formation.
Cratons are areas of continental lithosphere that exhibit long-term
stability against deformation. Seismic evidence suggests that cratonic
lithosphere may have formed via thrust stacking of proto-cratonic
lithosphere. We conducted numerical simulations and scaling analysis to
test this hypothesis, as well as to elucidate mechanisms for
stabilization. We found that formation of cratonic lithosphere via
thrust stacking is most viable for buoyant and viscous lithosphere that
is thin and/or possesses low effective friction coefficients. These
conditions lead to low integrated yield strength within proto-cratonic
lithosphere that allows it to fail in response to convection-generated
stresses. Specifically, formation via thrust stacking is viable for
lithosphere with chemical to thermal buoyancy ratios of B = 0.75-1.5,
viscosity contrasts between the lithosphere and convective mantle of
Δη > 102 , and friction coefficients of μ =
0.05-0.1. Preservation depends on the balance between the chemical
lithosphere's integrated yield and convection-generated stresses. The
physical process of thrust stacking generates a thickened cratonic root.
This provides a higher integrated yield stress within cratons, which is
more conducive to stability subsequent to formation. Increased friction
coefficient values, due to dehydration, can also provide higher
integrated yield stresses within cratons. To provide long-term
stability, integrated yield stresses must be great enough to offset
future mantle convection-generated stresses, which can increase with
time as the mantle viscosity increases due to cooling. Thin or
rehydrated cratonic lithosphere may not provide stability against the
increasing convective stresses, thus providing an explanation as to why
some cratons are not long-lived.
Continental rifting along the largely non-volcanic southern margin of Australia occurred during two discrete phases, Syn-rift 1 (165--140 Ma) and Syn-rift 2 (90--83 Ma), but existing data are too sparse to map their spatial extent. The southern margin of Australia was studied using a new Bouguer gravity grid of filtered satellite data and corrected shipboard gravity data. Estimates of crustal thickness, depth to basement and interpretations of major basement-involved structures were derived from analyses of terrain-corrected Bouguer gravity data (radial power spectra, horizontal and vertical derivatives, analytic signal, and Euler 3-D deconvolution). Our gravity interpretations are constrained with seismic reflection, refraction and magnetic data, and are consistent with current palaeo-plate-tectonic models of the Southern Ocean. The crustal thickness and structure onshore varies between tectonic domains, thinning from 35 km onshore to approximately 10 km beneath the magnetic quiet zone offshore. Euler deconvolution solutions calibrated with seismic reflection data reveal two discrete sub-parallel rift systems beneath the shelf break. The tip of the western rift system overlaps that of the east, but they are separated by approximately 200 km. Each system terminates at or near to the boundary of the Gawler craton, suggesting that the craton served as an obstacle to rift propagation. Seismic data link these structures to Syn-rift 1 (165--140 Ma). The Ceduna fan, which contains up to 15 km of primarily post-rift strata, lies between the two rift zones. Euler deconvolution solutions with depth extent >5 km mark a NW-trending line between the two rifts beneath the fan, but no deep information is available to verify this structure. During Syn-rift II, a new rift zone developed oceanward of the Syn-Rift I along the western rift zone. Thus rifting migrated oceanward with time, probably in discrete jumps. Using our observations of the location and distribution of basement related structures, we have developed a new breakup model for Australia and Antarctica.
The composition of the subcontinental lithospheric mantle (SCLM) varies in a systematic way with the age of the last major tectonothermal event in the overlying crust. This secular evolution in SCLM composition implies quasi-contemporaneous formation (or modification) of the crust and its underlying mantle root, and indicates that crust and mantle in many cases have remained linked through their subsequent history. Archean SCLM is distinctively different from younger mantle; it is highly depleted, commonly is strongly stratified, and contains rock types (especially subcalcic harzburgites) that are essentially absent in younger SCLM. Some, but not all, Archean SCLM also has higher Si/Mg than younger SCLM. Attempts to explain the formation of Archean SCLM by reference to Uniformitarian processes, such as the subduction of oceanic mantle (“lithospheric stacking”), founder on the marked differences in geochemical trends between Archean xenolith suites and Phanerozoic examples of highly depleted mantle, such as abyssal peridotites, island-arc xenolith suites and ophiolites. In Archean xenolith suites, positive correlations between Fe, Cr and Al imply that no Cr–Al phase (i.e. spinel or garnet) was present on the liquidus during the melting. This situation is in direct contrast to the geochemical patterns observed in highly depleted peridotites from modern environments, which are controlled by the presence of spinel during melting. It is more likely that Archean SCLM represents residues and/or cumulates from high-degree melting at significant depths, related to specifically Archean processes involving major mantle overturns or megaplumes. The preservation of island-arc like SCLM at shallow levels in some sections (e.g. Slave Craton, E. Greenland) suggests that this specifically Archean tectonic regime may have coexisted with a shallow regime more similar to modern plate tectonics. Preliminary data from in situ Re–Os dating of sulfide minerals in mantle-derived peridotites suggest that much Archean SCLM may have formed in a small number of such major events >3.0Ga ago. The survival of Archean crust may have been critically determined by the availability of large plugs of very buoyant SCLM (a “life-raft model” of craton formation). Many Archean SCLM sections have been strongly affected by Proterozoic and Phanerozoic metasomatism, and much of the observed secular evolution in SCLM composition, at least through Proterozoic time, may reflect the progressive modification of relict, buoyant Archean lithosphere.
Continental cratons have not experienced major tectonic disruptions over a timescale of 109 years. The thickness of cratonic lithosphere also appears to have changed little over this timescale. These observations are often attributed to the presence of chemically buoyant and/or highly viscous subcratonic roots. Simple physical scaling relationships are developed to explore the buoyancy and/or viscosity conditions required to stabilize such roots against large-scale deformation and rapid remixing into the mantle. The scalings are tested using idealized numerical simulations with good general agreement. Applied to Earth, the scalings suggest that (1) buoyancy alone is unlikely to stabilize cratonic roots and (2) if root viscosity is to provide stability into the Archean, then roots must be 103 times as viscous as the mantle. Based on available experimental data, root dehydration cannot account for the required viscosity increase. Temperature-dependent viscosity can stabilize roots, but it does so at the expense of stagnating the entire mantle lithosphere, i.e., at the expense of sacrificing plate tectonics. This suggests that the plastic yielding properties of rocks at low temperatures will need to be more directly accounted for in future experiments exploring root stability.
Sub-calcic garnets encapsulated by diamonds from relatively young (90
Myr) kimberlites in southern Africa, yield ancient Sm-Nd and Rb-Sr model
ages (3,200-3,300 Myr). The chemistry and distribution of these and
associated sub-calcic garnets from kimberlite concentrate, indicate
diamonds formed following enrichment of residual sub-cratonic mantle
such as that remaining after widespread extraction of 3,500-Myr
komatiitic lavas.
Many hitherto puzzling features of the isotope and trace-element geochemistry of the Earth's mantle and crust can be explained if Earth history is punctuated by episodes of enhanced exchange between the lower and upper mantle. Such episodes would replenish the upper mantle with trace elements, and also cause rapid growth of continental crust. This picture is consistent with recent geophysical models in which two-layer convection alternates with episodes of penetrative or whole-mantle convection.
RECEIVED AUGUST 25, 2005; ACCEPTED MARCH 24, 2006; ADVANCE ACCESS PUBLICATION APRIL 28, 2006 Orogenic peridotites occur enclosed in Proterozoic gneisses at several localities in the Western Gneiss Region (WGR) of western Norway; garnet peridotites typically occur as discrete zones within larger bodies of garnet-free, chromite-bearing dunite and are commonly closely associated with pyroxenites and eclogites. The dunites of the large Almklovdalen peridotite body have extremely depleted com-positions (Mg-number 92–93Á6); the garnet peridotites have lower Mg-number (90Á6–91Á7) and higher whole-rock Ca and Al con-tents. Post-depletion metasomatism of both rock types is indicated by variable enrichment in the light rare earth elements, Th, Ba and Sr. The dunites can be modelled as residues after very high degrees (>60%) of melt extraction at high pressure (5–7 GPa), inconsist-ent with the preservation of lower degrees of melting in the garnet peridotites. The garnet peridotites are, therefore, interpreted as zones of melt percolation, which resulted in refertilization of the dunites by a silicate melt rich in Fe, Ca, Al and Na, but not Ti. Previous Re–Os dating gives Archaean model ages for the dunites, but mixed Archaean and Proterozoic ages for the garnet peridotites, suggesting that refertilization occurred in Proterozoic time. At least some Proterozoic lithosphere may represent reworked and transformed Archaean lithospheric mantle.
This study uses information on composition, thermal state and petrological thickness to calculate the densities of different types of subcontinental lithospheric mantle (SCLM). Data from mantle-derived peridotite xenoliths and garnet–xenocryst suites document a secular evolution in the composition of SCLM: the mean composition of newly formed SCLM has become progressively less depleted, in terms of Al, Ca, mg# and Fe/Al, from Archean, through Proterozoic to Phanerozoic time. Thermobarometric analyses of xenolith and xenocryst suites worldwide show that the mean lithospheric palaeogeotherms rise from low values (corresponding to surface heat flows of 35–40 mW/m2) beneath Archean terranes, to higher values (>50 mW/m2) beneath regions with Phanerozoic crust. The typical thickness of the lithosphere (defined as a chemical boundary layer), ranges from about 250 to 180 km, 180–150 km and 140–60 km for Archean, Proterozoic and Phanerozoic terranes respectively. The depth of this lithosphere–asthenosphere boundary corresponds to a temperature of 1250–1300°C. Using the estimated compositions, average mineral compositions and experimental data on the densities of mineral end-members (tables 1 and 2), we calculate mean densities at 20°C for Primitive Mantle (3.39 Mg m−3) and for SCLM of Archean (3.31±.016 Mg m−3), Proterozoic (3.35±0.02 Mg m−3) and Phanerozoic (3.36±0.02 Mg m−3) age. Curves of density and cumulative density versus depth, which take into account variations in geotherm with tectonothermal age, have been constructed for each age type of lithospheric section to assess the buoyancy of these columns relative to the asthenosphere, modelled as a Primitive Mantle composition. The density curves show that Archean SCLM is significantly buoyant relative to the asthenosphere at depths greater than about 60 km. Proterozoic sections deeper than about 100 km thick also are significantly buoyant. The buoyancy of Archean and Proterozoic SCLM sections, combined with their refractory composition, leads to high viscosities and explains the longevity and stability of old SCLM. Replacement of Archean lithosphere, as beneath the present-day eastern Sino–Korean craton, probably involves mechanical dispersal by rifting, accompanied by the rise of hot, fertile asthenospheric material. Fertile Phanerozoic lithosphere is buoyant when the geotherm is sufficiently high, as in many Cenozoic volcanic provinces. However, as the geothermal gradient relaxes toward a stable conductive profile, Phanerozoic SCLM sections thinner than about 100 km become denser than the asthenosphere, and hence gravitationally unstable. This could help to induce delamination of the SCLM and upwelling of asthenospheric material, beginning a new cycle. The tectonic consequences of such lithosphere replacement would include uplift and magmatism, and basin formation during subsequent thermal relaxation.
'Continental interior' was originally defined as a geographic concept on the example of Central Asia. Later James Dwight Dana used it in a geological context, when he argued that North America was an ideal continent with a low, old, stable interior and higher, younger, more active periphery. This picture was thought satisfactory from the viewpoint of fixist tectonics for more than a century, although it was clear that it did not account for the structure of Eurasia. Neither in Asia nor in Europe cratons and/or areas of gentle deformation outside orogenic belts coincide with the continental interior. Mobilist tectonics, and especially its plate tectonics version, made clear that in a world where continents are constantly united and re-dispersed, such non-coincidence with continental interior, however it may be defined, is exactly what one would expect. If, however, after isolation by rifting and/or transform-fault-displacement, no plate boundary cuts across a continent, the lithosphere beneath it would cool and thicken. If this lithosphere is also made up of high Mg/Mg + Fe residual material left after making basalt, it would be lighter than pristine mantle lithosphere. Such lithosphere would be resistant to subduction and to deformation. It would thus protect the portion of the continent overlying it, giving rise to a craton. Cratonic keels of 300 km or deeper can be generated by shortening a depleted mantle cushion that normally reaches down to some -150 km by similar to 50%. In the Archaean, high geothermal gradients would eclogitise the mafic bottom of a continent more mafic than those that are younger and prevent its upper surface from rising during such a shortening. This would keep the tops of Archaean cratons unmetamorphic or at low grades. If a continent with such a deep keel is left alone for a time period on the order of 1 Ga, continuous cooling will render it very strong. Only very large strike-slip systems can remain active as plate boundaries for a long time within a continental interior. Therefore, if a continental interior can be kept away from such a boundary for about 1 Ga, it inevitably will turn into a craton with low relief, probably a high percentage (areawise) of internal drainage, and high climatic continentality. Late Palaeozoic Gondwana-Land was one such region. Pre-Miocene Africa was perhaps another with many features inherited from Gondwana-Land. If India stops pushing, it is likely that Central Asia will turn into yet another one. Plume-controlled active rifting is the only way to destroy such a consolidated continental interior. It is thus the composition and the thermal state plus the state of stress of the lithosphere underlying a continent that determines its tectonic behaviour and not its crustal structure, nor its geography. Craton formation is largely a lithospheric process, not a crustal one. By the very nature of the process of craton formation, cratons commonly, but not always, originate within continental interiors, but continental interior alone can be assigned no tectonic connotation whatever. (C) 1999 Elsevier Science B.V. All rights reserved.
Continental cratons overlie thick, high-viscosity, thermal and chemical boundary layers, where the chemical boundary layers are less dense than they would be due to thermal effects alone, perhaps because they are depleted in basaltic constituents. If the continental tectosphere is the same age as the overlying Archaean crust, then the continental tectosphere must be able to survive for several billion years without undergoing a convective instability, despite being both cold and thick. Since platforms and shields correlate only weakly with Earth's gravity and geoid anomalies, acceptable models of the continental tectosphere must also satisfy this gravity constraint. We investigate the long-term stability of the continental tectosphere by carrying out a number of numerical convection experiments within a two-dimensional Cartesian domain. We initiate our experiments with a tectosphere (thermal and chemical boundary layers) immersed in a region of uniform composition, temperature, and viscosity, and consider the effects on the stability of the tectosphere of (1) activation energy (used to define the temperature dependence of viscosity), (2) compositional buoyancy, and (3) linear or non-linear rheology. The large lateral thermal gradients required to match oceanic and tectosphere structures initiate the dominant instability, a "drip" which develops at the side of the tectosphere and moves to beneath its center. High activation energies and high background viscosities restrict the amount and rate of entrainment. Compositional buoyancy does not significantly change the flow pattern. Rather, compositional buoyancy slows the destruction process somewhat and reduces the stress within the tectosphere. With a non-Newtonian rheology, this reduction in sb-ess helps to stiffen the tectosphere. In these experiments, dynamical systems that adequately model the present ocean-continent structures have activation energy E* greater than or equal to 180 kJ mole(-1) - a value about one third the estimate of activation energy for olivine, E* approximate to 520 kJ mole(-1). Although for E* approximate to 520 kJ mole(-1), compositional buoyancy is not required for the tectosphere to survive, the joint application of longevity and gravity constraints allows us to reject all models not containing compositional buoyancy, and to predict that the ratio of compositional to thermal buoyancy within the continental tectosphere is approximately unity.
Mantle Convection in the Earth and Planets is a comprehensive synthesis
of all aspects of mantle convection within the Earth, the terrestrial
planets, the Moon, and the Galilean satellites of Jupiter. The authors
include up-to-date discussions of the latest research developments that
have revolutionized our understanding of the Earth and the planets. The
book features a comprehensive index, an extensive reference list,
numerous illustrations (many in color) and major questions that focus
the discussion and suggest avenues of future research. It is suitable
as a text for graduate courses in geophysics and planetary physics, and
as a supplementary reference for use at the undergraduate level. It is
also an invaluable review for researchers in the broad fields of the
Earth and planetary sciences.
We have mapped the deep structure of the Slave craton by combining analysis the effective elastic thickness (Te) with data on mantle samples from numerous kimberlites. Three-dimensional mapping of the subcontinental lithospheric mantle (SCLM), using mantle-derived xenoliths and xenocrysts in kimberlites, has shown that much of the craton is underlain by a strongly layered SCLM; a highly depleted upper layer (low in basaltic components Ca, Al, Fe) is separated from a relatively fertile lower layer by a sharp boundary. This boundary lies at 140-150 km depth in the Lac de Gras area and shallows to
Almost all earthquakes on the continents are confined within a crustal
layer that varies in thickness (T s) from
about 10 to 40 km, and are not in the mantle. Variations in
T s correlate with variations in the
effective elastic thickness (T e), both
of them having similar values, although
T e is usually the smaller of the two.
These observations suggest that the lower crust, at least in some
places, is stronger than the mantle beneath the Moho, contrary to most
models of continental rheology. Thus the strength of the continental
lithosphere is likely to be contained within the seismogenic layer,
variations in the thickness of this strong layer determining the heights
of the mountain ranges it can support. The aseismic nature of the
continental mantle and the lower crustal seismicity beneath some shields
are probably related to their water contents.
We have inverted fundamental and higher-mode Rayleigh waveforms from 685 vertical component broadband seismograms with wave paths over North America to obtain an image of the upper mantle S velocity structure down to 660 km. Among the well-resolved features of the new model are (1) a high-velocity root beneath the North American craton which extends no deeper than 250 km except near the Archean core of the craton where depths of 350 km are reached, (2) a weak band of low-velocity along the eastern margin of the North American craton, which reaches into the transition zone, (3) a low velocity slab window beneath the western United States down to a depth of 300 km, (4) areas of low uppermost mantle velocity beneath the Cascade volcanoes, the Yellowstone hotspot track, the Colorado plateau, the Sierra Madre Occidental, and the grabens bordering the Jalisco block, and (5) a pronounced band of high velocities in the transition zone, coinciding with the expected location of the subducted trailing fragments of the Farallon plate. We introduce several improvements to the method of partitioned waveform inversion, which was used to compute the new model: rather than to correct for an estimated depth to the Mohorovicic discontinuity, we leave the crustal thickness as a free though yet poorly resolved parameter in the inversion; we also improve the windowing and filtering operator used to select uncontaminated waveforms.
Convection beneath continents may be associated with the generation of partially molten zones at depth. Experimentally determined non-Newtonian, temperature- and pressure-dependent creep laws for olivine were used to model upper mantle convection in the presence and absence of a melt phase. In our models, convection is chaotic and exhibits a strong time dependence. Using model ``dry'' and ``water-undersaturated'' solidus relations, partially molten regions develop at asthenospheric depths forming low-viscosity regions. The existence of these regions is linked to convection and consequently varies in space and time with an episodicity of 5 to 10 m.y., in good agreement with observations based on seismic tomography and the episodicity and volumetric abundance of surface volcanics. For an average continental heat flux of about 53.3 mW/m2, maximum melt fractions range between 0 and 2% (dry solidus) and 2 and 4% (water-undersaturated solidus). On the basis of experimental deformation studies, we employed an empirical relationship between stress and grain size to assess the grain size variations possible in the upper mantle. The grain sizes obtained (millimeters to centimeters) are in good agreement with grain sizes observed in natural examples. The smallest grain sizes occur along narrow zones of high stress within the deepest part of the lithosphere. The largest grain sizes are found in the overlying lithosphere and in partially molten asthenospheric regions. As our models scale well to natural observations, these results appear to preclude diffusion creep (n=1) as a viable upper mantle steady state deformation mechanism.
It is pointed out that the nature of the 670-km discontinuity in the
earth's mantle is of particular importance in geodynamics because of its
possible role of breaking mantle convection into two layers. In the
present investigation, the density change is considered as a change
which arises solely from a phase transition. The investigation is based
on a set of so-called 'extended Boussinesq equations' which include the
energy effects of adiabatic compression, latent heat, and frictional
heating. Attention is given to the constitutive equations and numerical
methods, the Boussinesq Limit Di=0, the finite dissipation number,
effective thermal expansivity, superplasticity, and layers of unequal
thickness.
We address the question of the depth extent of mantle high-velocity zones under ancient cratons by using seismic velocity maps from recent mantle tomographic studies. We divided old continents into two age provinces: 800 1700 Ma (Middle Proterozoic) and older than 1700 Ma (Archean and Early Proterozoic). The areas included in these age provinces are cross-correlated with the tomography at different depths to quantify the global occurrence of high-velocity anomaly (HVA) extensions beneath cratons. Statistically significant HVAs underlie the oldest cratons to a depth of 250 km. There is no significant consistent correlation between tomography and younger cratons. The older cratons have correlated HVAs extending to depths varying from
Mantle convection is the accepted paradigm for explaining the internal geological activity of the Earth. Its theoretical understanding is largely based on the simple concept of Rayleigh-Benard convection. Obviously the Earth's mantle is not so simple, and the consequences of various complications must be considered. This review concentrates on the principal mechanisms by which phase transitions influence mantle convection and on the various controlling influences from a perspective of geodynamical theory and numerical convection modeling. -from Author
The Antrim Plateau Volcanics, Australia’s largest Phanerozoic flood-basalt province, originally covered an area of at least 300 000 km2 across northern Australia. Stratigraphic constraints indicate that the Antrim Plateau Volcanics are of Early Cambrian age (ca 545–509 Ma), although previous attempts to date the Antrim basalts by radiometric methods have been inconclusive. We present an ion microprobe U–Pb zircon age of 513 ± 12 Ma for the ∼250 km-long Milliwindi dolerite dyke in the west Kimberley. The dolerite is geochemically identical to basalts of the Antrim Plateau Volcanics, and was probably a feeder dyke for basalts that have since been eroded. It is suggested that the Antrim Plateau Volcanics extended hundreds of kilometres further to the west than recognised previously and may have once covered part of the Kimberley block.
Continental cratons overlie thick, high viscosity, thermal and chemical boundary layers, where the chemical boundary layers are less dense than they would be due to thermal effects alone, perhaps because they are depleted in basaltic constituents. If the continental tectosphere is the same age as the overlying Archaean curst, then the continental tectosphere must be able to survive for several billion years without undergoing a convective instability, despite being both cold and thick. Sine platforms and shields correlate only weakly with Earth's gravity and geoid anomalies, acceptable models of the continental tectosphere must also satisfy this gravity constraint. We investigate the long-term stability of the continental tectosphere by carrying out a number of numerical convection experiments within a two-dimensional Cartesian domain. We initiate our experiments with a tectosphere (thermal and chemical boundary layers) immersed in a region of uniform composition, temperature, and viscosity, and consider the effects on the stability of the tectosphere of (1) activation energy (used to define the temperature dependence of viscosity), (2) compositional buoyancy, and (3) linear or non-linear rheology. The large lateral thermal gradients required to match oceanic and tectosphere structures initiate the dominant instability, a “drip” which develps at the side of the tectosphere and moves to beneath its center. High activation energies and high background viscosities restrict the amount and rate of entrainment. Compositional buoyancy does not significantly change the flow pattern. Rather, compositional buoyancy slows the destruction process somewhat and reduces the stress within the tectosphere. With a non-Newtonian rheology, this reduction in stress helps to stiffen the tectosphere. In these experiments, dynamical systems that adequately model the present ocean-contient structures have activation energy E*≥180 kJ mole−1 —a value about one third the estimate of activation energy for olivine, E*≈520 kJ mole−1. Although for E*≈520 kJ mole−1, compositional buoyancy is not required for the tectosphere to survive, the joint application of longevity and gravity constraints allows us to reject all models not containing compositional buoyancy, and to predict that the ratio of compositional to thermal buoyancy within the continental tectosphere is approximately unity.
Earth's near-surface layer, its lithosphere, is broken into quasi-rigid plates that form the upper thermal boundary layer for mantle convection. Since the discovery of plate tectonics, it has been widely conjectured but only recently demonstrated that this peculiar style of convection may be facilitated by an upper mantle low viscosity zone (LVZ) over which the plates glide easily. The LVZ, or ''asthenosphere,'' concept dates from 19th century investigations of isostatic support of mountain belts and is supported by modern evidence for a seismic low velocity zone and by studies of postglacial rebound and dynamic compensation of the Earth's gravity field. Here we show in both two-dimensional (2-D) Cartesian and 3-D spherical Earth models that combining a pronounced LVZ and a plastic yield stress to allow localized weakening of the cold thermal boundary layer results in a distinctly plate tectonic style of convection, with $30% toroidal surface motion for the 3-D case. Recycling of water into the upper mantle at subduction zones is a plausible cause of Earth's LVZ, whereas Venus is dry and lacks both an LVZ and plate tectonics.
The distribution of the heat producing elements within the lithosphere provides an important control on continental thermal regimes and the mechanical strength of the lithosphere. Moreover, the strong temperature dependence of lithospheric rheology suggests the possibility of an important feedback between deformation and the distribution of heat producing elements. Simple models for lithospheric rheology are used to illustrate how such feedback might serve as an important control on both the characteristic abundance of, and spatial variation in, the heat production elements in the crust. These models also imply that the organisation of heat producing elements is essential for the long-term tectonic stabilisation of the continental crust. This is particularly relevant to the evolution of cratons in early Earth history, wherein lies the most dramatic evidence for the role played by tectonic processes in achieving a stable ordering of the heat producing elements. ß 2002 Elsevier Science B.V. All rights reserved.
Do Archean ages obtained for diamonds from many of the world’s cratons constitute a strong constraint on the thermal state of the Archean continental lithosphere? The apparent longevity of diamonds obtained from cratonic kimberlites [Boyd et al., Nature 315 (1985) 387–389; Richardson et al., Nature 310 (1984) 198–202] has been used to infer the physical and chemical isolation of cratonic roots from the convecting mantle since 3 Ga. This would also provide an extremely strong constraint on the thermal history of the lithospheric mantle – requiring low temperatures at depth for its entire history. Recent evidence suggests, however, that the published ‘diamond’ ages may not represent the ages of the diamonds themselves, but significantly pre-date them [Shimizu and Sobolev, Nature 375 (1995) 308–311; Spetsius et al., Earth Planet. Sci. Lett. 199 (2002) 111–126]. We use a particle-in-cell finite element code to model the thermal stability of the continental lithosphere in a convecting mantle. The continental crust modulates the thermal conditions of the underlying mantle lithosphere, increasing the depth of the thermal boundary layer beneath the continent and providing a mechanism for stabilizing the sub-continental thermal field. If diamonds have survived in cratonic roots since the Archean, the conditions necessary for diamond stability must have existed in the Archean continental lithosphere, and those conditions must have remained relatively unperturbed for ∼3 Gyr [Boyd et al., Nature 315 (1985) 387–389]. Here, the longevity of the diamond stability field is explored for systems with chemically distinct continental crust and a strongly temperature-dependent mantle viscosity. Such models frequently produce the temperature conditions needed to form diamonds within the Archean lithosphere, but the temperature fluctuations experienced within the modeled mantle lithosphere are generally able to destroy these diamonds within 1 Gyr. Increasing the distance to active margins has only a marginal effect on the longevity of the diamond stability field. Convectively stable cratonic roots extend the lifetime of the diamond stability field in those regions. However, while the residence time of diamonds approaches the order of magnitude required (284–852 Myr), extremely fortuitous mantle conditions are required to explain Archean diamonds.
Sino-Korean Craton (SKC) in eastern China is an important natural laboratory for studying temporal change to the lithosphere because there is the jutaxposition of Ordovician diamondiferous kimberlites, Mesozoic lamprophyre-basalt and Cenozoic tholeiite-alkali basalts in this craton. While diamond inclusions, xenoliths and mineral concentrates in kimberlites indicate a thick (180 km), cold and refractory lithospheric keel beneath the SKC prior to the Palaeozoic, basalt-borne xenoliths reveal the presence of thin (<80 km), hot and fertile lithosphere in the Cenozoic. This indicates the dramatic change in lithospheric architecture during the Phanerozoic. Geochemical characterization of late Jurassic to recent basalts further suggests that the lithospheric destruction started since the Jurassic, probably due to the loss of physical integrity of the craton as a result of the Triassic collision between North China and Yangtze blocks. The replacement of old lithospheric keel by “oceanic” mantle has been accomplished during the late Cretaceous. Coupled thermo-mechanical and chemical erosion within the lithosphere-asthenosphere interface is considered as an important mechanism to thin the lithosphere. The lithospheric thinning may proceed with gradual upward migration of the lithosphere-asthenosphere boundary. Alternatively, the lithospheric thinning could proceed in the way that the old lithospheric mantle was penetrated and then desegregated by hot mantle materials which rise along vertical lithospheric shear zones and spread like mushroom clouds.
Zircon megacrysts from the Mir kimberlite, Yakutia, contain inclusions of chromite, chrome diopside, magnesian olivine, Ni-rich monosulfide solid solution and phlogopite. The mineral chemistry of the inclusion suite suggests that the zircons grew in a metasomatized peridotite matrix. Twenty-three zircons were chosen for U–Pb dating, Hf isotope and trace element determinations. The trace element data are typical of kimberlitic zircons worldwide. LAM-ICPMS U–Pb dating yields a weighted mean 206Pb/236U age of 353.6±2.5 Ma. Hf isotope measurements by LAM-MC-ICPMS yield ϵHf values of 3.0–9.2, and model ages (TDM) of 600–800 Ma. These data constrain the crystallization of the zircons to between 350 and 600 Ma. However, LAM-MC-ICPMS microanalysis of Os isotopes in sulfides included in three zircons yields TRD model ages of 2.37–2.92 Ga (TMA=2.39–3.19 Ga). To explain the discordance between the ages of the zircons and their sulfide inclusions, we suggest that these zircons grew in a metasomatized peridotite, which contained sulfides that were residual from ancient melting events. These sulfides, together with other peridotite phases, were trapped in the metasomatic zircon with little modification of their elemental or isotopic composition. This model has important implications for the interpretation of Re–Os model ages of sulfide inclusions in diamonds. Diamonds also could capture and preserve older sulfides during their growth or regrowth in mantle rocks, and the inclusions therefore do not necessarily date the formation of the diamond.
The influence of water on the dynamics of the oceanic upper mantle is re-evaluated based on recent experimental constraints on the solubility of water in mantle minerals and earlier experimental studies of olivine rheology. Experimental results indicate that the viscosity of olivine aggregates is reduced by a factor of ∼ 140 in the presence of water at a confining pressure of 300 MPa and that the influence of water on viscosity depends on the concentration of water in olivine. The water content of olivine in the MORB source is estimated to be810±490H10/6 Si, a value greater than the solubility of water in olivine at a confining pressure of 300 MPa (∼ 250H10/6 Si). We therefore conclude that the viscosity of the mantle in the MORB source region is 500±300 times less than that of dry olivine aggregates. The dependence of the solubility of water in olivine on pressure and water fugacity is used in conjunction with other petrological constraints to estimate the depth at which melting initiates beneath mid-ocean ridges. These calculations indicate that melting begins at a depth of ∼ 115 km, consistent with other geochemical observations. Owing to the relatively small amount of water present in the MORB source, only ∼ 1–2% melt is produced in the depth interval between the water-influenced solidus and the dry solidus. A discontinuity in mantle viscosity can develop at a depth of ∼ 60–70 km as a result of the extraction of water from olivine during the MORB melting process. In the mid-ocean ridge environment, the mantle viscosity at depths above this discontinuity may be large enough to produce lateral pressure gradients capable of focusing melt migration to the ridge axis. These observations indicate that the base of an oceanic plate is defined by a compositional rather than thermal boundary layer, or at least that the location of the thermal boundary layer is strongly influenced by a compositional boundary, and that the evolution of the oceanic upper mantle is strongly influenced by a viscosity structure that is controlled by the extraction of water from olivine at mid-ocean ridges.
Reliable evaluations of whether a phase transformation boundary with a negative Clapeyron slope will cause mantle convection to be stratified will require models to include good simulations of plates and plumes. Here numerical models of mantle convection are presented that incorporate reasonable simulations of subducting lithospheric plates and good simulations of rising low-viscosity mantle plumes, as well as being carefully scaled to other first-order parameters of the mantle. The models show that subducting plates and plume heads penetrate a phase transformation barrier more readily than does flow in constant viscosity convection models in which plates and plumes are poorly simulated. Plates can penetrate a larger magnitude of the Clapeyron slope than plume heads, with plume tails the least able to penetrate. These results, together with recent laboratory results on Clapeyron slopes and observational evidence that even weak plume tails reach the surface, suggest that subducting plates usually are not strongly resisted by this mechanism, and that putative episodes of mantle overturn would be much less dramatic than in some recent constant-viscosity models.
Major- and trace-element analyses of garnets from heavy-mineral concentrates have been used to derive the compositional and thermal structure of the subcontinental lithospheric mantle (SCLM) beneath 16 areas within the core of the ancient Laurentian continent and 11 areas in the craton margin and fringing mobile belts. Results are presented as stratigraphic sections showing variations in the relative proportions of different rock types and metasomatic styles, and the mean Fo content of olivine, with depth. Detailed comparisons with data from mantle xenoliths demonstrate the reliability of the sections.
In situ Re–Os analysis of sulfide phases in peridotite xenoliths from kimberlites in the Kaapvaal Craton has been used to analyse relationships between crustal events and the modification of the subcontinental lithospheric mantle (SCLM). Fifteen samples from the Western Terrane (Finsch, Kimberley and Jagersfontein) and 10 from the Southeastern Terrane (Northern Lesotho pipes) were serially sectioned to reveal the sulfide phases. The peridotites contain multiple generations of sulfides with widely varying Os contents, Re/Os and ¹⁸⁷Os/¹⁸⁸Os. Where ≥4 sulfides have been analysed in a sample, the TRD model age of whole-rock sample is younger than the maximum sulfide TRD, usually by 0.3–1.5 Ga. The Re–Os model ages of the whole-rock samples therefore represent mixtures, and are unlikely to date any specific geological event.
S–P travel-time residuals and receiver-function images are used to infer the Vp/Vs (compressional to shear wave velocity) ratio of the lithospheric mantle beneath southern African and the topography of the underlying 410-km discontinuity. Low Vp/Vs ratios provide evidence independent of geochemical observations for a highly depleted root (Mg#∼92–94) beneath the Kaapvaal craton. The receiver-function images, on the other hand, consistently show a flat 410-km discontinuity beneath the entire array. This observation, after combined with the results of geodynamical modeling, allows us to place limits on the thickness of this chemical boundary layer, which is between ∼160 and ∼370 km.
The stabilization of continental lithosphere to form cratons is accomplished by volatile loss from the upper mantle during magmatic events associated with the formation of continental crust. Volatile depletion elevates the solidus and increases the stiffness of the mantle residuum, thereby imparting a resistance to subsequent melting and deformation. Freeboard is maintained in part by the buoyancy associated with an increased Mg/(Mg + Fe) ratio in the mantle residuum following extraction of crustal material. Augmented subcratonic seismic velocities derive from the same shift in this ratio. The higher effective viscosity of the stabilized subcratonic upper mantle inhibits its entrainment in mantle convection, and locally thickens the conductive boundary layer. Heat approaching from greater depths is diverted away from the stiff craton to other areas that continue to transfer heat by convection, thus yielding a low surface heat flow within cratons.Cratonization by devolatilization and petrologic depletion was most effective in the Archean and has diminished in effectiveness over geologic time as the mantle temperature has fallen because of the declining store of internal heat. From the Archean to the present that ascending mantle material which has undergone partial melting has encountered the solidus at progressively shallower depth, has remained supersolidus over a smaller depth range, has temperatures which have exceeded the solidus by lesser amounts, has undergone diminishing degrees of partial melting, and has experienced less thorough devolatilization. At a given time the rate of production of continental crust is likely to be proportional to the depth extent and fraction of partial melting. Integration of the partial melt zone over time yields a growth curve that is similar to some continental crustal growth curves inferred from isotopic evolution.
Rock deformation experiments indicate that serpentinization can strongly influence the strength and tectonics of the oceanic lithosphere. Strength versus depth profiles, calculated for conditions appropriate for slow-spreading ridges, indicate that the presence of serpentinite can reduce the integrated strength of the lithosphere by up to 30%. Results from flexural fault models indicate that if serpentinization is isolated to fault zones, strain localization should be enhanced, providing an explanation for the variations in the style of normal faulting along slow-spreading ridge segments. At segment centers, where serpentinites are scarce, deformation is accommodated on closely spaced faults with small throws. At the segment ends, where serpentinites are most abundant, faults are widely spaced and have large throws.
We present a new upper mantle seismic model for southern Africa based on the fitting of a large (3622 waveforms) multi-mode surface wave data set with propagation paths significantly shorter (≤ 6000 km) than those in globally-derived surface wave models. The seismic lithosphere beneath the cratonic region of southern Africa in this model is about 175 ± 25 km thick, consistent with other recent surface wave models, but significantly thinner than indicated by teleseismic body-wave tomography. We determine the in situ geotherm from kimberlite nodules from beneath the same region and find the thermal lithosphere model that best fits the nodule data has a mechanical boundary layer thickness of 186 km and a thermal lithosphere thickness of 204 km, in very good agreement with the seismic measurement. The shear wave velocity determined from analyzes of the kimberlite nodule compositions agree with the seismic shear wave velocity to a depth of ∼150 km. However, the shear wave velocity decrease at the base of the lid seen in the seismic model does not correspond to a change in mineralogy. Recent experimental studies of the shear wave velocity in olivine as a function of temperature and period of oscillation demonstrate that this wave speed decrease can result from grain boundary relaxation at high temperatures at the period of seismic waves. This decrease in velocity occurs where the mantle temperature is close to the melting temperature (within ∼100 °C).