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Granite ascent and emplacement during contractional deformation in convergent orogens
Abstract
Based on a case study in the Central Maine Belt of west-central Maine, U.S.A., it is proposed that crustal-scale shear zone systems provide an effective focussing mechanism for transfer of granite melt through the crust in convergent orogens. During contractional deformation, flow of melt in crustal materials at depths below the brittle–plastic transition is coupled with plastic deformation of these materials. The flow is driven by pressure gradients generated by buoyancy forces and tectonic stresses. Within the oblique-reverse Central Maine Belt shear zone system, stromatic migmatite and concordant to weakly discordant irregular granite sheets occur in zones of higher strain, which suggests percolative flow of melt to form the migmatite leucosomes and viscous flow of melt channelized in sheet-like bodies, possibly along fractures. Cyclic fluctuations of melt pressure may cause instantaneous changes in the effective permeability of the flow network if self-propagating melt-filled tensile and/or dilatant shear fractures are produced due to melt-enhanced embrittlement. Inhomogeneous migmatite and schlieric granite occur in zones of lower strain, which suggests migration of partially-molten material through these zones en masse by granular flow, and channelized flow of melt carrying entrained residue. Founded on the Central Maine Belt case study, we develop a model of melt extraction and ascent using the driving forces, stress conditions and crustal rheologies in convergent, especially transpressive orogens. Ascent of melt becomes inhibited with decreasing depth as the solidus is approached. For intermediate a(H2O) muscovite-dehydration melting, the water-saturated solidus occurs between 400 and 200 MPa, near the brittle–plastic transition during high-T–low-P metamorphism, where the balance of forces favors (sub-) horizontal fracture propagation. Emplacement of melt may be accommodated by ductile flow and/or stoping of wall rock, and inflation may be accommodated by lifting of the roof at shallower crustal levels and/or sinking of the pluton floor. The resultant plutons have (sub-) horizontal tabular geometries with floors that slope down to the ascent conduits. Although these plutons may have locally discordant relations with country rock structures, when viewed at the crustal-scale, granite ascent and emplacement in convergent orogens are syn-tectonic processes.
... Likewise, some kilometre-scale regions of stromatic migmatite hosted in diatexite (a migmatite with high melt fraction; Brown, 1973) have been interpreted as crustal-scale high-strain magma transfer zones involving migration and/or draining of melt (e.g., Scott and Stevenson, 1986;Sleep, 1974) from adjacent less deformed migmatite, i.e., supra-solidus wall rocks (Brown and Solar, 1998a;Hasalová et al., 2011;Marchildon and Brown, 2003;Schulmann et al., 2008;Weinberg and Mark, 2008). Field studies link melt ascent and eventual emplacement of plutons based on the close association between regional deformation, migmatisation, dyking, and zones of strain localisation (e. g., Brown, 2013;Brown and Solar, 1998b;Castro, 1986;de Saint Blanquat et al., 1998;Hutton, 1988;Pitcher, 1979;Rosenberg, 2004;Vernon et al., 2012;Vigneresse, 1995;Zibra et al., 2014). Furthermore, as summarised by Cruden and Weinberg (2018), faults and shear zones from all geodynamic systems have been implicated as high-strain melt-migration pathways through both supra-and subsolidus rocks (normal (e.g., Gardner et al., 2020;Grocott et al., 1994Grocott et al., , 2009Grocott and Taylor, 2002;Hutton et al., 1990;Richards and Collins, 2004), thrust/reverse (e.g., Collins and Sawyer, 1996;Ingram and Hutton, 1994;Piazolo et al., 2020;Silva et al., 2022;Stuart et al., 2018aStuart et al., , 2018b, strike-slip (e.g., Guineberteau et al., 1987;Hutton, 1988;Tikoff and Teyssier, 1992), transpressional systems (e.g., Benn et al., 1999;Brown and Solar, 1998b;Denèle et al., 2008;McCaffrey, 1992;Vernon et al., 2012). ...
... Field studies link melt ascent and eventual emplacement of plutons based on the close association between regional deformation, migmatisation, dyking, and zones of strain localisation (e. g., Brown, 2013;Brown and Solar, 1998b;Castro, 1986;de Saint Blanquat et al., 1998;Hutton, 1988;Pitcher, 1979;Rosenberg, 2004;Vernon et al., 2012;Vigneresse, 1995;Zibra et al., 2014). Furthermore, as summarised by Cruden and Weinberg (2018), faults and shear zones from all geodynamic systems have been implicated as high-strain melt-migration pathways through both supra-and subsolidus rocks (normal (e.g., Gardner et al., 2020;Grocott et al., 1994Grocott et al., , 2009Grocott and Taylor, 2002;Hutton et al., 1990;Richards and Collins, 2004), thrust/reverse (e.g., Collins and Sawyer, 1996;Ingram and Hutton, 1994;Piazolo et al., 2020;Silva et al., 2022;Stuart et al., 2018aStuart et al., , 2018b, strike-slip (e.g., Guineberteau et al., 1987;Hutton, 1988;Tikoff and Teyssier, 1992), transpressional systems (e.g., Benn et al., 1999;Brown and Solar, 1998b;Denèle et al., 2008;McCaffrey, 1992;Vernon et al., 2012). ...
... While melt segregation and extraction in the anatectic zone are relatively well studied, less is known about melt transfer to the upper crust, especially through sub-solidus rocks. The current paradigm invokes two main mechanisms for melt transfer through the crust: flow in (1) dykes/hydrofractures or (2) shear zones (Benn et al., 1999;Brown, 2013;Brown and Rushmer, 1997;Brown and Solar, 1998b;Carvalho et al., 2016;Clemens and Mawer, 1992;Collins and Sawyer, 1996;Daczko et al., 2016;Denèle et al., 2008;Diener et al., 2014;Etheridge et al., 2021;Gardner et al., 2020;Ghatak et al., 2022;Grocott et al., 1994Grocott et al., , 2009Grocott and Taylor, 2002;Guineberteau et al., 1987;Hall and Kisters, 2016;Hasalová et al., 2008Hasalová et al., , 2011Hutton, 1988;Hutton et al., 1990;Ingram and Hutton, 1994;Kisters et al., 2009;Lee et al., 2018;McCaffrey, 1992;Meek et al., 2019;Mogk, 1992;Piazolo et al., 2020;Reichardt and Weinberg, 2012;Richards and Collins, 2004;Rosenberg, 2004;Sawyer, 2010;Silva et al., 2022;Stuart et al., 2018aStuart et al., , 2018bTikoff and Teyssier, 1992;Vernon et al., 2012;Weinberg and Searle, 1998;Yakymchuk et al., 2013). Granite (sensu lato) may be observed in shear bands and high-strain zones (Ashworth, 1976;Barr, 1985;Weinberg and Mark, 2008 and references therein;Hasalová et al., 2011;Carvalho et al., 2016). ...
Melt transfer and migration occurs through both supra- and sub-solidus rocks. Mechanisms of melt transfer include dyking, mobile hydrofracturing and diffuse porous melt flow where melt flow may or may not be channelized via instabilities or into high-strain zones of active deformation. Here, we highlight the microstructural- and outcrop-scale signatures of syn-deformational melt-migration pathways through high-strain zones that cut sub-solidus rocks. High-strain zones with high proportions (>10%) of macroscopic, internally undeformed, felsic or leucocratic material are readily interpreted as important melt-migration pathways and are most common in supra-solidus host rocks. However, it is challenging to recognise high-strain melt-migration pathways through sub-solidus rocks; these pathways may lack noticeable felsic or leucocratic components at the outcrop scale and share many macroscopic features in common with ‘classic' sub-solidus mylonite, such that the two are generally conflated. We contrast field and microstructural characteristics of ‘classic' mylonite originating from solid-state deformation with those of high-strain zones that also cut sub-solidus rocks yet have microstructural indicators of the former presence of melt. We compile several features allowing one to distinguish solid-state from melt-present deformation in high-strain zones that cut sub-solidus rocks. Our aim is to encourage geologists to assess such high-strain zones on a case-by-case basis, in view of sub-solidus (i.e., mylonitic) versus melt-present deformation. Such assessment is crucial as (1) rocks deformed in the presence of melt, even small percentages of melt, are orders of magnitude weaker than their solid-state equivalents, (2) melt-rock interaction in such zones may result in metasomatism, and (3) such zones may sustain long-lived melt migration and ascent enabling chemical differentiation at a crustal scale. With this contribution we aim to increase the ease of recognising this important subset of melt-migration pathways by assisting in clarity of description and interpretation of high-strain rocks.
... Anatexis is the process in which magma is formed by partial melting of crustal rocks (Ashworth, 1985). It plays an important role of active defor-mation and emplacement of crustal granites (Brown & Solar, 1998), especially when it comes from sediments (Harris et al., 2000). Different geochemical signature can evidence the supracrustal origin from granites (Frost et al., 2001;Whalen et al., 1987;Chappell & White, 2001;Laurent et al., 2014) and can relate to different tectonic setting (Batchelor & Bowden, 1985;Pearce, 1996). ...
... It plays an important role of active defor-mation and emplacement of crustal granites (Brown & Solar, 1998), especially when it comes from sediments (Harris et al., 2000). Different geochemical signature can evidence the supracrustal origin from granites (Frost et al., 2001;Whalen et al., 1987;Chappell & White, 2001;Laurent et al., 2014) and can relate to different tectonic setting (Batchelor & Bowden, 1985;Pearce, 1996). ...
... Furthermore, samples 18A, 18B, 21A and 21B showed negative anomalies of Sr, while the opposite is observed in samples 20A and 20B ( Figure 4B). All samples plotted between syn-collisional to post-orogenic fields in Batchelor & Bowden (1985) diagram ( Figure 4C). In Pearce (1996) tectonic diagram ( Figure 4D), samples 18A, 18B, 19A and 19B plotted on the edge between syncollisional to volcanic arc granites, while samples 21A and B plotted on the post-collision granites field. ...
During the transition of Rhyacian to Orosirian periods a collision between the Archean Divinópolis and Campo Belo metamorphic complexes occurred in southern São Francisco Craton (SFC) followed by the collapse of the orogen. In the same region, the Statherian period is marked by a huge intraplate magmatic event that is represented in the area by the Para de Minas dyke swarm. Peraluminous granitoid occurrences are reported in the region of Formiga and Itapecerica (Minas Gerais – Brazil), surrounding high metamorphic grade khondalitic paragneisses and banded iron formations. To contribute to the understanding of the tectonic context of the southern SFC during the Paleoproterozoic era, these granitic rocks are studied based on petrographic, geochemical, and monazite U-Th-Pb geochronological analyzes. They were characterized as S-type (metasedimentary origin) peraluminous metamonzogranites, with crustal geochemical signature, and genesis related to anatexis in syn- to post-collisional environment. The geochronological results yielded two groups of Orosirian (~1,90 Ga) and Statherian (~1,78 Ga) ages. These results are related, respectively, to the collapse of the Rhyacian-Orosirian orogen and to the regional warming, associated with the Avanavero-Xiong’er LIP that is represented in the area by the Pará de Minas dyke swarm.
... Early models involved the vertical ascent of magmas as diapirs (e.g., Bott et al., 1958;Willis-Richards and Jackson, 1989;Weinberg and Podladkicov, 1994;Burov et al., 2003). Later models involved processes such as compaction-driven melting to form a network of veins and channels (Brown and Solar, 1998;Searle, 1998, 2013;Weinberg and Mark, 2008;Searle et al., 2009;Sawyer et al., 2011), diking (Clemens and Mawer, 1992;Petford et al., 1993), emplacement by stoping and assimilation (Glazner and Bartley, 2006), emplacement into extensional or trans-tensional shear zones (Hutton and Reavy, 1992), emplacement into transpressional shear zones (Searle et al., 2016), and lateral emplacement as laccoliths or horizontal tabular intrusions (Cruden, 1998;Taylor, 2007). Mountain belts usually show limited depth exposures depending on their age and erosion levels. ...
... Leucosomes eventually link to form melt pathways along shear zones or dike-sill complexes, as seen along parts of the Himalaya (Searle et al., 2009) and the Karakoram Searle, 1998, 1999). Potential granite emplacement mechanisms include forceful diapirism and ballooning plutons (Weinberg and Podladkicov, 1994), diking (Petford et al., 1993;Reichardt and Weinberg, 2012), transtensional emplacement into fault zones (Hutton and Reavy, 1992), transpressional emplacement (D'Lemos et al., 1992;Brown and Solar, 1998), wall rock assimilation, laccoliths (Scaillet et al., 1995;Cruden, 1998), and wedgeshaped plutons (Cruden and Weinberg, 2018). Magma, once generated in the source region, can be transported to higher structural levels via fractures or shear zones (see Brown, 2007Brown, , 2013. ...
The Permian Cornubian granite batholith (295−275 Ma) in SW England includes seven major plutons and numerous smaller stocks extending for ∼250 km from the Isles of Scilly in the WSW to Dartmoor in the ENE. The granites are peraluminous and classified as crustal melt S-type, predominantly two-mica granites, and biotite or tourmaline monzo- and syenogranites, with subordinate minor topaz granite and lithium mica granite. The granites and their host rocks are pervasively mineralized with tin (cassiterite), tungsten (wolframite, ferberite), copper (chalcopyrite, chalcocite, bornite), arsenic (arsenopyrite), and zinc (sphalerite) mineralized lodes. Quartz-muscovite selvedges (greisen-bordered) also contain enrichment of lithophile elements such as boron (tourmaline), fluorine (fluorite), and lithium (lithium-micas such as lepidolite and zinnwaldite). They are derived from both muscovite and biotite dehydration melting of pelitic-psammitic rocks and intruded from a common source along the length of the batholith. Pressure estimates from andalusite and cordierite-bearing hornfels in the contact metamorphic aureole (150 ± 100 MPa) show that the granites intruded to 3 km depth. Cupolas around the Land’s End and Tregonning granites show aplite-pegmatite dikes and tourmaline + quartz + muscovite veins (greisen) that are frequently mineralized. Synchronous intrusions of lamprophyre dikes suggest an additional heat source for crustal melting may have been from underplating of alkaline magmas. The lack of significant erosion means that the source region is not exposed. In an accompanying paper (Part 2; Watts et al., 2024), gravity modeling reveals possible solutions for the shape and depth of the granite and the structure of the lower crust. We present a new model for the Land’s End, Tregonning, and Carnmenellis granites showing a mid-crustal source composed of amphibolite facies migmatites bounded by prominent seismic reflectors, with upward expanding dikes feeding inter-connected granite laccoliths that show inflated cupolas with shallow contact metamorphism. The Cornubian granites intruded >90 m.y. after obduction of the Lizard ophiolite complex, and after Upper Devonian−Carboniferous Variscan compressional, and later extensional, deformation of the surrounding Devonian country rocks. Comparisons are made between the Cornubian batholith and the Patagonian batholith in Chile, the Himalayan leucogranites, and the Baltoro granite batholith along the Karakoram range in northern Pakistan.
... The emplacement of granitic magmas in collisional orogenic belts is commonly thought to be related to extensional structures formed during post orogenic collapse (Searle et al., 1997;Sylvester, 1998). Several studies have attributed the formation and segregation of granitic melts in the crust, and the subsequent emplacement of these melts, to compression (Collins and Sawyer, 1996;Brown and Solar, 1998). These studies have shown that shear zones and faults formed during deformation are important in providing pathways for magma extraction and ascent in the crust (Hutton, 1988;Grocott et al., 1994;Sawyer, 1994;Brown and Solar, 1998). ...
... Several studies have attributed the formation and segregation of granitic melts in the crust, and the subsequent emplacement of these melts, to compression (Collins and Sawyer, 1996;Brown and Solar, 1998). These studies have shown that shear zones and faults formed during deformation are important in providing pathways for magma extraction and ascent in the crust (Hutton, 1988;Grocott et al., 1994;Sawyer, 1994;Brown and Solar, 1998). Therefore, granites in many convergent orogens may be spatially and temporally related to regional-scale structures that formed during compression rather than extension . ...
Brittle tectonic analysis of the Younger Granites of Gabal (G.) Abu Hamr has been carried out on 1121 field measurements in 21 sites, using win tensor computer program. The analysis of fault slip data revealed that 413 faults (36.84 %) are of extensional (normal) faults, and 708 faults (63.16 %) belong to compressional strike slip faults systems and allowed computation of 138 paleostress tensors. These tensors are distinguished as 52 tensors corresponding to extensional faulting and 86 tensors corresponding to compressional faulting. The structural elements of Younger Granite of G. Abu Hamr were statistically treated and stress analyses were carried out on theses structural data to delineate the paleostresses, which affected the granite. These granites are subjected to five compressional and five extensional phases. Five compressional phases can be grouped into five main events as the following: E-W, WNW-ESE, NE-SW to ENE-WSW, NW-SE to NNW-SSE and N-S to NNE-SSW compressional events. On other hand five extensional phases can be grouped into five main events as the following: N-S to NNE-SSW, NW-SE to NNW-SSE, WNW-ESE, NE-SW to ENE-WSW and E-W extensional event. One uranium occurrence is distinguished along the contact between G. Abu Hamr Younger Granite and metavolcanics with other radioactive anomaly investigated near from the uranium occurrence. Beta-uranophane and minor pitchblende are the principal uranium minerals, identified within the altered metavolcanics. The structural studies at this uranium occurrence indicated that the WNW-ESE extensional event is the main phase responsible for the generation of uranium mineralization in the investigated area.
... Shear zones in tangential, transcurrent, or transpressional systems (sensu Twiss and Moore, 2007) act as important conduits in the transport and accumulation of anatectic melt along the crust (Brown and Rushmer, 1997;Brown and Solar 1998a, 1998b, 1999Brown, 2004). In contact with regions of anatexis, these zones operate as anatectic melt migration engines, creating dilational spaces that act as traps for the expulsion and accommodation of molten material (Weinberg et al., 2009(Weinberg et al., , 2010. ...
... Field evidence and experimental studies carried out by several authors suggest that dilation is a common feature in existing shear zones of varied kinematics in high-grade metamorphic areas, thus representing an important structural control for the emplacement of neosomes (Mogk, 1992, Brown andSolar 1998a;Kärki, 2015). According to these authors, the shear zones can give rise to favorable domains for accumulating molten material, highlighting in this sense the transtensional systems. ...
The Minas-Bahia Orogen, which is Siderian-Orosirian age, is exposed in the northern sector of the Sao Francisco Craton and in its African counterpart, the Congo Craton. In this sector, two orogenic domains outcrops with geological evolution and terrains with different trends: Western and Eastern Bahia. The advance of scientific knowledge in the northeast of the West Bahia Orogenic Domain has revealed the existence of more tectonic terrains than those traditionally delimited, and reveal the presence of juvenile terranes and cratonic crust separated complex zones where they interacted. The region of interaction between the Gaviao and Bom Jesus da Lapa Paleoplate has been the subject of debates about the tectonic significance of granulitic rocks with Meso-
Neoarchean protoliths, which are intruded by Rhyacian - Orosirian granitoids. The set of geological data presented in this article demonstrates that the metatexite migmatites with felsic granulitic paleosome and charnockitic neosome were generated during the initial phases of the collision between these two paleoplates, between 2068 and 2058 Ma. The structural mass transport is from NW to SE. The youngest migmatization event, in high amphibolite facies, occurred during NNW-SSE trending sinistral strike-slip tectonics as suggested by the emplacement of Dn+3 neosomes controlled by S/C/C’ structures, in between 2049–2000 Ma. The interpreted tectonic model is complex and involves the accretion and collision among Bom Jesus da Lapa, Gaviao and Jequie paleoplates during the Rhyacian-Orosirian, contributing to the advancement of knowledge of tectonic pieces and their interactions that contributed to the formation of Columbia supercontinent.
... However, large tectonic differential stresses cannot exist in the ductile lower crust (Brown and Solar, 1998). The buoyant ascent of magma via dykes, driven by its density contrast with surrounding rocks, has been proposed by numerical models as the principal method of magma ascent through the crust (Petford, 1996). ...
... Nevertheless, magmatic overpressure (imparted by tectonic compression) may be important in the more brittle middle and upper crust, and thus control the ascent of granitic magma to its emplacement depth, including aiding space creation during emplacement (Hutton, 1997). Furthermore, increased pore-fluid pressure enables brittle behaviour at lower differential stresses, enabling brittle behaviour to extend into the lower crust through melt-enhanced embrittlement (Brown and Solar, 1998). ...
This paper applies magnetic fabric analyses to plutons of the East Pacific continental arc. Continental arc magmatism is strongly episodic, with voluminous granitic magma addition occurring during discrete high-flux events (‘flare-ups’). The cause of these flare-ups is debated, variously invoking tectonic, mantle, or crustal controls. To understand how the syn-magmatic strain history changes during a flare-up, we compare granitic magnetic fabric (Anisotropy of Magnetic Susceptibility, AMS) and geochronological data from the Antarctic Peninsula (Lassiter Coast), Sierra Nevada, and Chile. This comparison indicates a common pattern in orientation and magnitude of syn-magmatic deformation, showing flare-up events occur during increased tectonic compression driven by enhanced interplate coupling and fast subduction. Flare-ups terminate as tectonic compression reduces or the regime becomes extensional, even if convergence rates remain high. As with enhanced seismicity, magmatic flare-ups result from high tectonic compression, during discrete periods of enhanced interplate coupling within broader periods of increased subduction rates. The enhanced magmatic flux results either from crustal thickening leading to partial melting of a newly accreted, hydrous mafic underplate, enhanced melt segregation in the source, or in response to high tectonic compression rendering lithostatic compression the weakest compressive force, enhancing magma extraction and ascent from the mantle.
... The coupling of deformation and magmatism received the attention of the structural geology community since the seventies of the last century, yet an exhaustive grasping of the nature of the spatial and temporal relationships between two processes remains elusive (e.g., Pitcher, 1979;Pitcher and Bussell, 1977;Hutton, 1982Hutton, , 1988Wadge and Cross, 1988;Hutton and Reavy, 1992;Vigneresse, 1995;Brown et al., 1997;Roman-Berdiel et al., 1997;Brown and Solar, 1998;Musumeci et al., 2005;Žá k et al., 2013). Evidence of the pairing of plutons and shear zones have been widely reported in different tectonic settings including extensional (e.g., Hutton, 1988), transcurrent (e.g., Hutton, 1982;McCaffrey, 1992;Neves et al., 1996;Weinberg et al., 2004) and compressional regimes (e.g., Davidson et al., 1992;D'lemos et al., 1992;Musumeci et al., 2005;Ferré et al., 2012). ...
... Evidence of the pairing of plutons and shear zones have been widely reported in different tectonic settings including extensional (e.g., Hutton, 1988), transcurrent (e.g., Hutton, 1982;McCaffrey, 1992;Neves et al., 1996;Weinberg et al., 2004) and compressional regimes (e.g., Davidson et al., 1992;D'lemos et al., 1992;Musumeci et al., 2005;Ferré et al., 2012). The accumulated data provoked a growing perception that a causative link exists between the two processes, either shear zones control the ascent and emplacement of magma at different crustal levels (Brown and Solar, 1998;Petford and Koenders, 1998;Weinberg et al., 2004Weinberg et al., , 2005, or the magma emplacement affects the rheology of host rocks and triggers strain localization and nucleation of shear zones on pluton margins (e.g., Neves and Vauchez, 1995;Neves et al., 1996;Neves and Mariano, 1999;Cao and Neubauer, 2016). Rosenberg (2004) compiled data from Alpine granite plutons, and concluded that, with few exceptions, there is a clear spatial and temporal relationship between shear zones and emplacement and distribution of plutons. ...
Coupling of deformation and magmatism has been widely reported in several old orogenic belts, particularly along faults and shear zones. The syn-kinematic plutons
in exhumed shear zones offer the best opportunity to understand the complex relationship between magmatism and regional deformation. The present paper investigates
the geometry and internal structure of granite plutons emplaced in thrust faults and shear zones, and structural control on their emplacement mechanism.
Abu Ziran pluton is an example of the intrusions emplaced in an active brittle-ductile thrust shear zone in the Nubian shield, and documents clear evidence on the
interaction between magmatism and deformation during melt ascent and emplacement. Results from detailed geological mapping, remote sensing and structural
analysis of the pluton permitted the constraining of the pluton’s geometry, emplacement mechanism, and spatio-temporal evolution. Structural analysis indicates that
pluton emplacement was syn-to-late-tectonic. The brittle-ductile fabrics in the wall rock are consistent with a sub-horizontal thrust shear zone with a top-to-NW shear
sense. The activity of the shear zone was accompanied by an episode of a calc-alkaline magmatic pulse. Granitic magma ascended upward via non-exposed feeder
dykes, or through ramps and flats in the thrust system and emplaced laterally along the shear zone, forming complex sub-horizontal sheet-shaped intrusion. The
geometry and extent of pluton emphasize that inherited heterogeneities and regional stress states played important role in the emplacement processes. In addition,
localization of pluton along or near the contact between ophiolitic nappes and mylonitic metasediments suggests that the rheological boundaries act as barriers that
impede the rise of ascending magma, causing magma arrest and trigger lateral spreading and emplacement. The outcomes of this study allowed the reconstruction of
the geometry and internal structure of Abu Ziran pluton and grasping its evolution in space and time.
... The coupling of deformation and magmatism received the attention of the structural geology community since the seventies of the last century, yet an exhaustive grasping of the nature of the spatial and temporal relationships between two processes remains elusive (e.g., Pitcher, 1979;Pitcher and Bussell, 1977;Hutton, 1982Hutton, , 1988Wadge and Cross, 1988;Hutton and Reavy, 1992;Vigneresse, 1995;Brown et al., 1997;Roman-Berdiel et al., 1997;Brown and Solar, 1998;Musumeci et al., 2005;Žá k et al., 2013). Evidence of the pairing of plutons and shear zones have been widely reported in different tectonic settings including extensional (e.g., Hutton, 1988), transcurrent (e.g., Hutton, 1982;McCaffrey, 1992;Neves et al., 1996;Weinberg et al., 2004) and compressional regimes (e.g., Davidson et al., 1992;D'lemos et al., 1992;Musumeci et al., 2005;Ferré et al., 2012). ...
... Evidence of the pairing of plutons and shear zones have been widely reported in different tectonic settings including extensional (e.g., Hutton, 1988), transcurrent (e.g., Hutton, 1982;McCaffrey, 1992;Neves et al., 1996;Weinberg et al., 2004) and compressional regimes (e.g., Davidson et al., 1992;D'lemos et al., 1992;Musumeci et al., 2005;Ferré et al., 2012). The accumulated data provoked a growing perception that a causative link exists between the two processes, either shear zones control the ascent and emplacement of magma at different crustal levels (Brown and Solar, 1998;Petford and Koenders, 1998;Weinberg et al., 2004Weinberg et al., , 2005, or the magma emplacement affects the rheology of host rocks and triggers strain localization and nucleation of shear zones on pluton margins (e.g., Neves and Vauchez, 1995;Neves et al., 1996;Neves and Mariano, 1999;Cao and Neubauer, 2016). Rosenberg (2004) compiled data from Alpine granite plutons, and concluded that, with few exceptions, there is a clear spatial and temporal relationship between shear zones and emplacement and distribution of plutons. ...
Coupling of deformation and magmatism has been widely reported in several old orogenic belts, particularly along faults and shear zones. The syn-kinematic plutons in exhumed shear zones offer the best opportunity to understand the complex relationship between magmatism and regional deformation. The present paper investigates the geometry and internal structure of granite plutons emplaced in thrust faults and shear zones, and structural control on their emplacement mechanism. Abu Ziran pluton is an example of the intrusions emplaced in an active brittle-ductile thrust shear zone in the Nubian shield, and documents clear evidence on the interaction between magmatism and deformation during melt ascent and emplacement. Results from detailed geological mapping, remote sensing and structural analysis of the pluton permitted the constraining of the pluton's geometry, emplacement mechanism, and spatio-temporal evolution. Structural analysis indicates that pluton emplacement was syn-to-late-tectonic. The brittle-ductile fabrics in the wall rock are consistent with a sub-horizontal thrust shear zone with a top-to-NW shear sense. The activity of the shear zone was accompanied by an episode of a calc-alkaline magmatic pulse. Granitic magma ascended upward via non-exposed feeder dykes, or through ramps and flats in the thrust system and emplaced laterally along the shear zone, forming complex sub-horizontal sheet-shaped intrusion. The geometry and extent of pluton emphasize that inherited heterogeneities and regional stress states played important role in the emplacement processes. In addition, localization of pluton along or near the contact between ophiolitic nappes and mylonitic metasediments suggests that the rheological boundaries act as barriers that impede the rise of ascending magma, causing magma arrest and trigger lateral spreading and emplacement. The outcomes of this study allowed the reconstruction of the geometry and internal structure of Abu Ziran pluton and grasping its evolution in space and time.
... Such orogens form when the overriding plate advances towards the downgoing plate causing repeated amalgamation of crustal fragments, rift/oceanic basins, volcano-sedimentary wedges, magmatic arcs, high-and low-pressure metamorphic rocks, and juvenile granitoid magmas, requiring strong mechanical coupling, localized strain in shear zones, and thermal rejuvenation (Cawood et al., 2009). Most petrological studies indicate that the primary source of granitoid melts in such orogens is from melting of mafic upper mantle rocks below magmatic arcs and/or back-arcs, or residual para-and ortho-gneisses and granulite/migmatites of the lower continental crust, and most likely melting occurred in a syn-to post-orogenic setting (Brown and Solar, 1998a;Sawyer et al., 2011). The processes however, by which continental margin crust is enriched in juvenile felsic magmas, i.e., the nature of segregation, extraction, and ascent of melt triggered by accretionary tectonism, are still uncertain (Brown and Rushmer, 2006;Brown, 2013). ...
... settings (Wickham, 1987;Lucas and St. Onge, 1995;Brown and Solar, 1998a;Vernon and Paterson, 2001;Weinberg et al., 2013). For example, where tabular granites that link deeper migmatite zones to shallower plutons are transported in dykes oriented at a high angle to the maximum shortening direction (Reichhardt and Weinberg, 2012;Brown, 2013). ...
The Ersfjord Granite is part of a suite of c.1.80–1.75 Ga syeno-granites in the West Troms Basement Complex, northern Norway, presumed to belong to the Transscandinavian Igneous Belt (TIB-1) in the Fennoscandian Shield. Previous data suggest the granite formed post-collisional and ascended as a batholith pluton from a source generated by delamination of mafic-intermediate lower crust. We argue that the Ersfjord Granite was emplaced initially (c. 1.80 Ga) as multiple tabular sills in an extensional setting, then as successive melt injections (c. 1.78–1.75 Ga) in an evolving Andean/Cordilleran type accretionary orogen at the waning stages of the Svecofennian orogen. Field observations indicate melt ascent initiated as successive sills (EG-I) into well-foliated Meso/Neoarchaean TTG gneisses. Some sills preserved a magmatic layering, others injected and assimilated the host rock gneisses leaving pendants of mafic gneiss/migmatite residuum in between the granite sills. The first tectonic patches of melts (EG-II) ascended into the middle/upper crust along regional shear zones and injected into ductile imbricate thrust stacks (D1 event) during NE-SW directed crustal shortening and medium grade P-T conditions, using the sills and ancestor migmatite pendants as melt pathways. Then the tabular EG-I and II granite sheets and adjacent gneisses were coaxially folded by upright macro-folds (D2 event) and steep, granitic pegmatite dyke swarms (EG-III) intruded parallel to the fold axial surface and in related D2 thrusts, at low grade metamorphic conditions. The final melt emplacement (D3 event) included granite pegmatite dykes and sills (EG-IV) along subvertical D3 fold limbs and steep strike-slip shear zones. Our provisional extension, and successive advancing accretionary orogenic emplacement model for the Ersfjord Granite may explain ascent of many other TIB-1 magmas in the Fennoscandian Shield.
... Metamorphism and migmatitization are related to a major regional metamorphic event coupled with plutonism during the Acadian orogeny. Some authors (e.g., [21]) suggested that some of these plutonic intrusions were post-tectonic. Peak metamorphism is located at CMB's core [22] where the studied pegmatites are located, and include upper amphibolite facies with migmatites to the SW of the Oxford field, and green-schist facies to the NE [23,24]. ...
... (a) Geographic location of the Oxford and Grafton pegmatite fields. Modified from[21]; (b) Lithological map of the Oxford pegmatite field modified from[20,[31][32][33]. ...
Almandine-spessartine garnets, from the Oxford County pegmatites and the Palermo No. 1 pegmatite, record significant compositional variations according to the degree of evolution of their hosting rock. Garnets from the most fractionated pegmatites (Mt. Mica, Berry-Havey, and Emmons) show the highest Mn, Nb, Ta, Zr, and Hf values, followed by those from the intermediate grade pegmatites (Palermo No. 1) and, finally, garnets from the barren pegmatites show the lowest values (Perham and Stop-35). Iron, Ca, and Mg contents follow an inverse order, with the highest contents in the latter pegmatites. Major element zoning shows increasing Mn values from core to rim in most garnet samples, while trace element zoning is not systematic except for some crystals which show a core to rim depletion for most of these elements. Chondrite normalized HREE (Heavy Rare Earth Elements) spectra show positive slopes for garnets from barren pegmatites, both positive and negative slopes for those associated with the intermediate pegmatite, and negative or flat slopes in garnets from the highly fractionated pegmatites. Ion exchange mechanisms, including Fe2+−1Mn2+1, (Fe2+, Mn2+)−1Si−1Li1P1; and, (Y, Ho3+)2(vac)1(Fe2+, Mn2+)−3, could explain most of the compositional variations observed in these garnets. These compositional variations are the reflection of the composition of the pegmatitic magma (barren pegmatites originate from a more ferromagnesian magma than fractionated pegmatites); and of the coexisting mineral phases competing with garnets to host certain chemical elements, such as biotite, schorl, plagioclase, apatite, Fe-Mn phosphates, Nb-Ta oxides, zircon, xenotime, and monazite.
... Lehmann et al., 2020). It is possible to describe the commonly encountered temporal and geographic relation between deformation and granite emplacement as either strain localization caused by the injection of magma along shear zones or shear-zone-aided melt transfer (Brown and Solar, 1998). According to our combined analyses, the studied deformed granites may have been syntectonically formed as tabular bodies related in timing to the (NSZ). ...
... The leucosomes in these migmatites show evidence for "petrographic continuity" with larger, often discordant, intrusive granite dykes and plutons. The concept of petrographic continuity is routinely interpreted as robust evidence that two or more melt-bearing structures once formed a continuous melt-bearing network in the crust (e.g., Brown, 2007;Brown, 2013), and detailed observations of these features in the field has allowed the pathways of magma migration through the crust to be mapped out in detail (e.g., Brown, 1994;Collins and Sawyer, 1996;Brown and Solar 1998a;Brown and Solar, 1998b;Searle, 1999;Searle et al., 2009;Reichardt and Weinberg, 2012;Martini et al., 2019). The observation of petrographic continuity in this case strongly implies that the metatexite and diatexite migmatites of the Abbabis Complex observed along the lower Swakop River are approximately the same age as the larger leucogranite dykes and sills, and that these features all represent part of a magma migration network which accommodated the ascent of granitic melt through the crust. ...
The Abbabis Complex outcrops at the deepest exposed level of the southern Central Zone of the Damara Orogen and consists primarily of granitic gneisses, including augen gneisses, along with extensive granitic intrusions and some subordinate metasedimentary and metavolcanic horizons. Most previous studies have interpreted the Abbabis Complex as a c. 1 or 2 Ga granitic basement complex onto which the overlying Damara metasedimentary Supergroup was deposited unconformably at c. 870-590 Ma. However, in this study, we present extensive new field evidence from the lower Swakop River, which shows clearly and consistently that granitic rocks of the underlying Abbabis Complex are universally intrusive into the overlying Damara Supergroup at all locations visited. These findings do not support the interpretation that the Damara metasedimentary Supergroup was deposited unconformably onto the Abbabis Complex. We propose that the Abbabis Complex instead represents a syn-orogenic granite-migmatite complex which formed exclusively from granite intrusion and accumulation at c. 550-500 Ma in the mid-crust of the Damara Orogen. This was facilitated by high-temperature granulite-facies regional metamorphic conditions which peaked at c. 520 Ma in the southern Central Zone. The timing of this high-temperature metamorphism and coeval granite intrusion, lagging the onset of collision in the Central Zone of the Damara Orogen by several tens of millions of years, is consistent with numerical simulations of heat produced from long-lived radioactive decay in thickened orogenic crust. The interpretation of the Abbabis Complex as a syn-orogenic granite-migmatite terrain in the mid-crust of the Damara Orogen, rather than as an older basement complex, is therefore consistent with 1) the detailed description of field relationships reported in this study, 2) the spatial and temporal evolution of metamorphism and magmatism within the orogen, and 3) increasingly sophisticated tectonic and geodynamic models presented in the wider scientific literature to explain the evolution of orogenic belts.
... These dunite bodies are thought to have formed by the reaction of peridotite and melt (Kelemen et al. 1995;Suhr et al. 1998;Suhr 1999;Xu et al. 2003). In contrast, previous studies on the mechanisms of non-reactive fluid and melt transport through the crust underscore the importance of fluid accumulation and channelization in the chemical segregation of the crust, which reduces the degree of fluid-rock interaction with increasing transport distance (Walther and Orville 1982;Brown 1994;Holness 1997;Brown and Solar 1998;Scaillet and Searle 2006). ...
Leucogranite bodies are ubiquitous in the upper structural levels of the Himalayan metamorphic slab. Their formation has ramifications for myriad processes including the generation of crustal melts and orogenic heat budgets. One particularly enigmatic variety of the Himalayan leucogranites, abundant in the Langtang region of Nepal, are the banded tourmaline leucogranites; typified by centimetre-scale compositional banding between tourmaline-rich and quartzo-feldspathic domains. Here, we use in-situ Rb–Sr isotopic chemistry and microstructural analysis to show these banded tourmaline leucogranites do not represent a direct product of melt crystallization but instead were formed by metasomatism of psammitic country-rock. This metasomatism was likely driven by the release of boron-rich volatiles during the crystallization of neighboring muscovite–biotite leucogranite bodies. Rb–Sr isochrons based on biotite–plagioclase ± white mica and K-feldspar data define overlapping dates of ca. 17.5 Ma from both the banded tourmaline leucogranite and its paired muscovite–biotite leucogranite. The characteristic banding appearance of these rocks is a product of heteroepitaxial nucleation of tourmaline on biotite folia and the replacement of pre-existing biotite and plagioclase with tourmaline and K-feldspar. The heteroepitaxy relationship of tourmaline on biotite is characterized by the {10–10} face of tourmaline parallel to biotite (001), with the tourmaline c-axis parallel to either the biotite [110] or [010] direction. One of the broader implications of our findings is that field estimates based on the volume of coarse-grained leucocratic outcrop overestimates the amount of melt generated at the top of the Himalayan slab.
... The syntectonic nature of the granite is also supported by its association with main strike-slip shear zones, in particular the CTSZ toward which there is a steepening of the magmatic foliation (Figure 13a). It cannot be excluded, moreover, that the shear zones controlled the ascent and emplacement of the granitic magmas (Brown & Solar, 1998;Hutton, 1988;D'Lemos et al., 1992;Ferré et al., 1997;Gébelin et al., 2006). The E-W elongation of the Tourraque body and its termination against ca. ...
The Camarat Granitic Complex (CGC), emplaced in the migmatitic Internal Zone of the Maures–Tanneron Massif (MTM), SE Variscides, consists of the Gigaro granodiorite and the composite Camarat granite. U‐Pb dating of the latter gives crystallization ages of 304.5 ± 3.3 Ma (zircon date) and 303.5 ± 4.0 Ma (monazite date). Two representatives of late felsic dykes cutting across the MTM Internal Zone have ³⁹Ar/⁴⁰Ar muscovite ages of 302.43 ± 2.62 Ma and 298.11 ± 2.38. Magmatic lineations revealed by anisotropy of magnetic susceptibility (AMS) measurements and image analysis, aplite dykes, dyke‐like bodies of cordierite microgranite and joints in the Camarat granite, as well as the late dykes all have orientations consistent with subhorizontal, NNE‐SSW‐trending lineations in the migmatitic country rocks representing the stretching direction of a late‐Variscan transpression phase (D3). The CGC and the late dykes are therefore witness to a thermal event that affected the MTM between ∼305 and ∼298 Ma (late Pennsylvanian–earliest Permian times), at the end of D3 which initiated at ∼325 Ma. Grabens related to post‐Variscan, Permian rifting (D4 phase), which cut across the MTM are WNW‐ESE‐trending, indicating a NNE‐SSW direction of extension, parallel to the previous (D3) lateral horizontal flow. The present results and a comparison with AMS data published for the Corsica–Sardinia Batholith reveal that evolution in the SE Variscides from Devonian–early Carboniferous contraction to Permian extension, through late Carboniferous transpression is characterized by the persistence of a ca. N‐S stretching direction, supporting a strong horizontal, orogen‐parallel crustal flow.
... Hacker et al., 2015;Huang et al., 2013;Rudnick & Gao, 2014). It is generally accepted that partial melting of crustal rocks can effectively differentiate their protolith composition by generation, segregation and migration of felsic magmas from their sources, giving rise to granites in the shallow level and granulites in the deep level (Brown, 2013;Brown & Solari, 1998;Cipar et al., 2020;Clemens, 1990;Sawyer, 1994;Sawyer et al., 2011;Zheng & Chen, 2017, 2021. Although the geodynamic mechanism of intracrustal differentiation can be deciphered by investigating the structure and composition of granites such as autochthonous granites, granitic veins and leucosomes in well-exposed granulite and migmatite sections (e.g. ...
The cover image is based on the Original Article Significance of selective crystal entrainment and differential crystal‐melt separation in petrogenesis of granites from the Tongbai orogen by Qiang‐Qiang Zhang et al., https://doi.org/10.1111/jmg.12691
... This scenario, which invokes a "wide rift", can explain the ascent of epidote-bearing granitic magma. Viscosity is usually a key factor in the velocity of rising granitic magmas (Brown & Solar 1998;Giordano et al. 2008) and depends on the melt composition and particularly SiO 2 content, temperature, and H 2 O content (Cruden & Weinberg 2018). However, the thermal and mechanical weakening of a former continental domain at a "wide-rift" margin might allow for relatively viscous acidic magmas to ascend. ...
... Magmatic fabrics of granite plutons constitute robust tools in order to decipher pluton emplacement mechanisms and its relationships with tectonic processes in both active and ancient orogens (Brown and Solar, 1998;Paterson et al., 1998;Benn et al., 2001;Žá k et al., 2005a;Paterson et al., 2019). When magmatic flow occurs in non-active tectonic environments, magmatic fabrics are assumed to record buoyancy-driven, forceful strain imposed by the intrusion during migration and transfer at different levels along the continental crust (Paterson and Fowler, 1993;Moyen et al., 2002;Stevenson, 2009). ...
The Itapetim pluton is a sigmoid-shaped intrusion bounded by the NE-SW, sinistral Itapetim and Taperoá shear zones. The Anisotropy of Magnetic Susceptibility (AMS) and Anhysteretic Remanent Magnetization (AARM) techniques and microstructural analysis were employed in order to characterize the pluton's emplacement mechanism and its relationships with regional tectonics. Microstructures are mainly sub-magmatic/incipient solid-state. Magnetic susceptibilities are typically paramagnetic (k ≤ 0.5 mSI). Thermomagnetic curves and hysteresis loops further indicate that negligible ferrimagnetic susceptibilities may be the result of fine-grained, oxidized magnetite inclusions. AARM fabrics do not show direct correlations with AMS fabrics. A WNW to E-W trending, shallowly-plunging magnetic lineation is associated with an intermediate-to steep-dipping magnetic foliation. The magnetic lineation locally displays intermediate plunges that may represent possible feeder zones for magma ascent during the initial stages of emplacement. The proposed emplacement model for the Itapetim pluton combines bulk NNW-SSE shortening with NE-SW trending simple shear along the Itapetim and Taperoá shear zones, and a WNW-ESE/E-W dextral stretching in the splay between both shear zones. The crystallizing magma mush flowed laterally along the splay during bulk NNW-SSE transpression, which is consistent with continental collision and subsequent extrusion of the Central Domain of the Borborema Province during the Neoproterozoic.
... The rise of magma from the lower crust to the middle and upper crust can be entirely driven by melt withdrawal, and heat from lower lower-crustal sources, following a variety of models of plumbing system geometry (see review in Cruden & Weinberg, 2018). Yet many studies have also noted links between magma ascent and regional deformation (Pitcher, 1979;Castro, 1986;Hutton, 1988;Vigneresse 1995;Brown & Solar, 1998;de Saint Blanquat et al., 1998;Brown, 2013;Le Corvec et al., 2013;Biggs & Annen, 2019). Magma tends to follow pathways normal to the minimum compressive stress, and pre-existing structures can act stress barriers to magma migration (Marti et al., 2017) as well as being sites of migration. ...
Structure-magma interactions in rifts, particularly those associated with mantle plumes, contain examples where structure predominantly controls magmatism and conversely examples where magmatism strongly controls structure. Contrasting examples of these cases from East Africa and SE Asia are discussed. These two regions illustrate that the starting conditions of the lithosphere are very important for how subsequent rift-plume interactions progress. In Thailand the subduction zone setting, and thin, young, hot crust of SE Asia minimised the impact of contemporaneous mantle plume activity on passive rifting, particularly in terms of upper crustal
magmatic activity, and surface uplift. Rift structure exerts significant control on the timing and location of Neogene magmatic activity in the upper crust. At the largest scale this control varies according to rift mode (narrow vs wide) which in turn is related to large-scale pre-existing fabrics. Magma has been extensively emplaced
in the lower crust since c. 24 Ma, yet only reached the surface after 6 Ma. Stress rotation, and a later change to strike-slip activity probably created dilatant pathways for the late (<6 Ma) magma emplacement. The late magmatism includes alkaline basalts bearing xenoliths with economic quantities of sapphires and rubies. The East African Rift system (EARS) is the classic example of an active mode rift, where plume activity acting on cold,
thick lithosphere has resulted in magma-assisted rifting. However, the early rifting history in the Turkana area of the EARS progressed from a passive rift affected by a plume (more akin to the Thailand setting) during the Eocene-Oligocene, and only during the Miocene did widespread propagation of the rift rely on magma-assisted rifting, and development of a truly active rift mode. While the two examples are in many ways end-members of rift-plume interaction, each can still offer clearer examples of certain processes that may, to a greater or lesser degree, be operating in the other system.
... Except for Songpan-Ganzi terranes and Qin-Qi-Kun Orogens, the cobalt abundance of all cratons was higher than that of orogens. It is mainly related to the uplift emplacement of granites induced by orogenic processes [55,56]. Among the five orogens, the cobalt abundance of the Precambirian Cathaysia Foldbelt is the lowest, and that of the Phanerozoic Altay-Mongolia-Hinggan Orogens is the highest. ...
Geochemical background is the basis of anomaly evaluation in mineral exploration and environmental investigation. However, the background value obtained from secondary media is inevitably affected by weathering, transportation, and sorting, which leads to secondary depletion or enrichment of chemical elements. This problem can be avoided by the geochemical mapping of the exposed crust. In this paper, more than 38,000 samples of rocks data were collected, and an original method was presented for the first time to produce the cobalt geochemical map of exposed crust across the whole of China. Using a geological map grid of 1:200,000 as the basic calculation unit, the area and content of rock types in each unit were calculated, and then the grid was assigned using the area-weighted average method. Based on this, an geochemical contour map was drawn in ArcGIS. The results show that the median of the exposed crust is 9.74 mg/kg, which is lower than that of the whole crust and soils. This value is explained by the vertical heterogeneity of the crust and the secondary enrichment, respectively. Co anomalies and anomalous centers of exposed crust are distributed in southwest, north, and northwest China, where they are primarily related to Permian Emeishan Large Igneous Province (ELIP), Pacific plate, and the big mantle wedge (BMW), Archean metamorphic basic rocks, and ophiolite belts.
... Hacker et al., 2015;Huang et al., 2013;Rudnick & Gao, 2014). It is generally accepted that partial melting of crustal rocks can effectively differentiate their protolith composition by generation, segregation and migration of felsic magmas from their sources, giving rise to granites in the shallow level and granulites in the deep level (Brown, 2013;Brown & Solari, 1998;Cipar et al., 2020;Clemens, 1990;Sawyer, 1994;Sawyer et al., 2011;Zheng & Chen, 2017, 2021. Although the geodynamic mechanism of intracrustal differentiation can be deciphered by investigating the structure and composition of granites such as autochthonous granites, granitic veins and leucosomes in well-exposed granulite and migmatite sections (e.g. ...
Partial melting has been shown to be an important mechanism for intracrustal differentiation and granite petrogenesis. However, a series of compositional differences between granitic melt from experiments and natural granites indicate that the processes of crustal differentiation are complex. To shed light on factors that control the processes of crustal differentiation, and then the compositions of granitic magma, a combined study of petrology and geochemistry was carried out for granites (in the forms of granitic veins and parautochthonous granite) from a granulite terrane in the Tongbai orogen, China. These granites are characterized by high SiO2 (>72 wt.%) and low FeO and MgO (<4 wt.%) with low Na2O/K2O ratios (<0.7). Minerals in these granites show variable microstructures and compositions. Phase equilibrium modelling using P‐T pseudosections shows that neither anatectic melts nor fractionated melts match the compositions of the target granites, challenging the conventional paradigm that granites are the crystallized product of pure granitic melts. Based on the microstructural features of minerals in the granites, and a comparison of their compositions with crystallized minerals from anatectic melts and minerals in granulites, the minerals in these granitoids are considered to have three origins. The first is entrained garnets, which show comparable compositions with those in host granulites. The second is early crystallized mineral from melts, which include large plagioclase and K‐feldspar (with high Ca contents) crystals as well as a part of biotite whose composition can be reproduced by crystallization of the anatectic melt. The compositions of other minerals such as small grained plagioclase, K‐feldspar and anorthoclase in the granites with low Ca contents are not well reconstructed, so they are considered as the third origin of crystallized products of fractionated melts. The results of mass balance calculation show that the compositions of these granites can be produced by mixing between different proportions of crystallized minerals and fractionated melts with variable amounts of entrained minerals. However, the calculated modal proportions of different crystallized minerals (plagioclase, K‐feldspar, biotite, and quartz) in the granites are significantly different from those predicted by melt crystallization modelling. Specifically, some rocks have lower modes of biotite and plagioclase, whereas others show lower K‐feldspar modes than those produced by melt crystallization. This indicates that the crystallized minerals would be differentially separated from the primary magmas to form the evolved magmas that produce these granites. Therefore, the crystal entrainment and differential melt‐crystal separation make important contributions to the composition of the target granites. Compared with leucogranites worldwide, the target granites show comparable compositions. As such, the leucogranites may form through the crystal fractionation of primary granitic magmas at different extents in addition to variable degrees of partial melting.
... The effect of the thermal metamorphism occurs to about 1-2 km from the pluton contact. Additionally, the occurrence of both angular paleosome and rounded and/ or deformed fragments from the host rocks along the pluton contact may imply complex processes of emplacement and could involve superposition of emplacement mechanisms (Barton et al. 1991, Brown and Solar, 1998. ...
The Borborema Province, NE Brazil, is marked by several Ediacaran granitic plutons that generated high-temperature metamorphic aureoles
in the country rock. However, information about magma emplacement, age of plutonism, and metamorphic conditions are necessary
to understand this scenario. To this end, we present field, petrographic, and zircon U-Pb geochronological data for migmatized hornfels
and intrusive Umarizal and Tourão-Caraúbas plutons. The field features allowed the construction of a structural evolution model starting
with high-temperature sinistral strike-slip shear zones followed by dextral strike-slip movement associated with the magmatic emplacement.
Mineral paragenesis of andalusite/sillimanite, garnet, scapolite, and phlogopite in country rocks within the metamorphic aureole indicate
temperatures of at least 700-800°C and pressures lower than 4.5 kbar corresponding to the pyroxene hornfels facies. Zircon U-Pb ages of
563.7 ± 6.2 for the Umarizal granite, 589 ± 4.4 Ma for the Tourão-Caraúbas granite, and 580.5 ± 4 Ma for the neosome from contact aureole were obtained. The results show that magma emplacement and HT/LP (high-T/low-P) contact metamorphism were synchronous with a
transtensional event. These features suggest a late- to post-tectonic context, following the collapse of the Brasiliano/Pan-African orogenetic
chain, that favored Late Ediacaran plutonism and synchronous HT/LP metamorphism.
... Our correlations among multiple geologic features reinforce two general findings from previous work. One is that igneous rocks commonly intrude along preexisting foliations (e.g., Brown & Solar, 1998;Hutton, 1988;Mahan et al., 2003;Paterson et al., 1989). Another is that preexisting mechanical heterogeneities influence subsequent deformation as demonstrated in mechanical theory (e.g., Griffith, 1924), laboratory experiments (e.g., Byerlee, 1978;Everall & Sanislav, 2018), and regional geologic studies (e.g., Burchfiel & Davis, 1975;Dunbar & Sawyer, 1988;Nadin & Saleeby, 2010). ...
Deep continental crustal structures are enigmatic due to lack of direct exposures and limited tools to investigate them remotely. Seismic waves can sample these rocks, but most seismic methods focus on coarse crustal structures while laboratory measurements concentrate on crystal‐scale rock properties, and little work has been conducted to bridge this interpretation gap. In some places, geologic maps of crystalline basement provide samples of the intermediate‐scale fabrics and structures that may represent in situ deep crust. However, previous research has not considered natural geometric variations from map data, nor is this heterogeneity typically included in map‐scale seismic property calculations. Here, we test how map‐scale fabrics influence crustal seismic anisotropy in Colorado by analyzing structural data from geologic maps, combining those data with bulk rock elastic tensors to calculate map‐scale seismic properties, and evaluating the resulting comparisons with observed receiver function A1 (360° periodic) arrivals. Crystalline fabrics, predicted seismic properties, and tectonic structures positively correlate with shallow and deep crustal A1 arrivals. Additionally, widespread correlations occur between mapped fault traces and regional foliations, implying that preexisting mechanical heterogeneity may have strongly influenced subsequent reactivation. We interpret that various mapped geologic contact types (e.g., lithologic and structural) generate A1 arrivals and that multiple parallel features (e.g., faults, foliations, and intrusions) contribute to a seismically visible tectonic grain. Therefore, Colorado's exhumed basement, as expressed in outcrops and maps, offers insight into modern deep crustal geological and geophysical structure.
... Tectonic controls related to the emplacement and exhumation of granitic bodies along strike-slip zones have been largely studied for more than three decades (Hutton 1988, Paterson and Tobisch 1992, Vigneresse 1995, Vauchez et al. 1997, Rosenberg 2004). In addition to the record of final exhumation stages of large-scale structures, it is recognized that magma emplacement mechanisms play an important role in the chronology of deformational events in orogenic regions (e.g., Brown and Solar 1998, Paterson et al. 1998, Schmidt and Paterson 2000, Weinberg et al. 2004. ...
The southern boundary of the Patos Shear Zone (PSZ) is characterized by mylonites
esulting from the deformation of Paleoproterozoic basement gneisses, Neoproterozoic metavolcanic and metasedimentary rocks, granitic and granodioritic intrusions. Among the latter, the Santa Terezinha and Catingueira plutons show stretched shapes in agreement with the the shear zone’s regional trend. Recrystallized quartz grains in these plutons accommodate deformation by dislocation creep. The host banded gneisses show similar deformation structures to the plutons. Quartz crystallographic fabrics mainly record the activity of basal and prism slip planes with local contribution of rhomb planes, whilst feldspar clasts show evidence of intracrystalline deformation in the larger porphyroclasts. These characteristics suggest that shearing occurred at lower- amphibolite facies conditions. U-Pb zircon data for the Santa Terezinha pluton yields an age of 625 ± 7 Ma which, coupled with the pluton elongated shape and microstructure, suggest that its emplacement was pre-kinematic in relation to the Patos shear zone. Post-emplacement elongation of the granite body occurred along the strike-slip trend. Such structural and geochronological constraints highlight the close association between magma emplacement and shear zone deformation during distinct stages of the Brasiliano-Pan-African orogeny in the Borborema Province.
... At the same time, movement of magma creates a thermal antiform providing a low-viscosity zone where strain rate increases and ductile deformation is enhanced; the thermal corridor propagates upward leading to melt-enhanced embrittlement at its top Solar, 1998a, b, 1999;Gébelin et al., 2009). The tectonic overpressuring related to transpression, adding to the buoyancy-induced overpressure, facilitates differential rates of flow for melt and its solid host-rock, favoring melt segregation and consequent anisotropic volume loss (Brown and Solar, 1998b). Such a type of steep pressure gradients explains the ability of transpressional systems to transport magmas upwards. ...
Oblique convergent margins, like the subduction of Phoenix beneath the South American plate during Jurassic and Early Cretaceous times, are characterized by strain partitioning and a positive feedback loop between strike-slip deformation and magma ascent along the magmatic arc.
Located in the Coastal batholith of northern Chile, the Flamenco pluton is one of the youngest Andean intrusives emplaced in the western active margin of South America. Besides an older SW domain of granodioritic rocks (c.a. 213 Ma), the NW and E domains of the Flamenco pluton were emplaced between 194 and 186 Ma. They present a normally zoned structure constituted by external gabbroic to Qtz-dioritic magmatic facies and tonalites and granodiorites located in inner areas of the intrusive body. These domains are separated by a central strip of stretched coarse-grained Crd-schists that presents ductile asymmetrical folding and S–C structures that point to the NW-directed displacement of the E domain of the pluton. This syn-emplacement shear zone shows kinematic compatibility and continuity to the north and south with folded and mylonitic metasediments out of the contact aureole of the pluton. Together, these segments constitute a large, steeply dipping sigmoidal structure of average N–S direction; the called here Chañaral transcurrent shear zone.
Contemporary to the emplacement of the Flamenco pluton, slight variations in the trend of the crustal-scale structure generated strike-slip and transpressive sectors along the Chañaral shear zone, which favored the access of intruding magmas to the final emplacement level. As a paradigmatic example, the curviplanar Flamenco shear zone, an internal, magmatic branch of the main structure that traverses the E domain of the pluton, is defined by the sinistral and reverse shearing under magmatic conditions of the previously mingled mafic and felsic batches. Consequently, the transpressive Flamenco shear zone is interpreted as an ascent conduit where gabbroic and granodioritic liquids interacted during the building of the intrusive body. In addition, these sheared rocks were affected by late textural coarsening processes that evidence the slow and cyclical cooling of the growing magma reservoir. In contrast with the steeply dipping contacts and structures found to the east, the NW domain of the pluton shows sharp and gently dipping contacts between almost horizontal magmatic layers. We suggest that the western block of the Chañaral shear zone was a relatively passive footwall dominated by horizontal flow trajectories and lower replenishment rates according to the inverse emplacement sequence, i.e., late external mafic batches intruded along the margins of the felsic core. The variable structural arrangement of the crustal rocks that hosted the Flamenco pluton was the result of the complex interaction between far-field and local, magmatic forces during the emplacement process, besides the interference with pre-Andean structures.
The presence of the Chañaral shear zone favoring the emplacement of the Flamenco pluton demonstrates that the Late Jurassic to latest Early Cretaceous Atacama Fault System had earlier precursors and both the magmatic arc axis and the transcurrent shear zones migrated landward during Jurassic times.
... The host rock metamorphic foliation probably guided magma ascent and imposed a structural control during the early stages of pluton assembly. Zones of anisotropy are more prone to localize strain, become fractured and faulted, and facilitate magmatic flow (Brown and Solar, 1998;Das et al., 2014;Gudmunsson, 1984;Wickham, 1987). Conditions that favour the opening of cracks along the pre-existing host rock anisotropy are a favourable orientation with respect to the direction of the least compressive regional stress (Delaney et al., 1986) or a combination of high melt pressure and small deviatoric stresses (Cosgrove, 1997). ...
The Lower Carboniferous Guandacolinos pluton of northwestern Argentina (Western Sierras Pampeanas) preserves field, structural, and petrological evidence of sheet-like transport and assembly of granitic magmas in the upper crust. The pluton is a relatively small (~24 km2) subduction-related granitic body, elongated in map view, and hosted in Neoproterozoic metamorphic rocks. Exceptional exposure records a subparallel array of steep NNE-SSW trending structures, including steep contacts partly concordant with host rock structure, numerous sheets of granite separated by host rock rafts, abundant xenoliths, and magmatic and solid-state foliations. Along the eastern half of the pluton, the granite is massive and host rock inclusions are less abundant. Regional markers of the host rock are deflected along a concordant bulged contact in the northeastern region of the pluton. Field relations indicate emplacement by multiple material transfer processes including fracture propagation, magma wedging, stoping, and lateral shortening. Contrasting mechanisms imply a changing mechanical response of host rock and multiple stages of intrusion. Emplacement began with dominant brittle fracturing and intrusion of sheets influenced by host rock anisotropies, followed by a viscoelastic phase were larger batches of magma caused downward transfer of stoped blocks, lateral expansion, and minor ductile deformation of the host rock. Thermal modelling indicates that the construction of the pluton required lateral accretion rates in the order of dm/years and less than a few tens of thousands of years to form. This case study documents the ability of incrementally assembled sheeted intrusions to efficiently heat rocks of the upper crust and trigger conditions favourable for transfer and storage of magma.
... Collisional processes can be accompanied by the multistage deformation and metamorphic events, both compression and extension deformation, with appearance of diverse metamorphic types, including zonal distribution of metamorphic lithology and strongly contrasting P-T conditions (Korhonen et al., 2012;He et al., 2014;Yang et al., 2018;Zhang et al., 2018;Liu et al., 2020). High pressure and ultrahigh pressure eclogitic and glaucophane-schist metamorphism types, medium-and low-pressure high-grade metamorphic types and associating granitic formations are widespread (Wells, 1980;England and Thompson, 1984;Harley, 1989;De Yoreo et al., 1989;Thompson, 1990;Spear, 1993;Korikovsky, 1995;Brown and Solar, 1998;Rusin, 2004). The increase in the PT conditions is due to the thickening of the earth's crust during thrusts, an increase in fluid-heat flows at the early collisional stage, and thermal relaxation of the crust occurs at the late collisional stage (Likhanov et al, 2004;Rusin, 2007;Reverdatto et al., 2019). ...
Accretionary-collisional events occurred at the western margin of the Tuva-Mongolian microcontinent are used to explain the largely contradictory Cambrian geodynamic history of the western Central Asian Orogenic Belt (CAOB).
This study presents new data for the Erzin metamorphic complex in order to constrain the petrogenesis and tectonic implications of the metamorphic rocks in this complex. The Erzin complex in the tectonic Erzin zone is composed of high-grade metamorphic rocks that have formed under variable metamorphic conditions (T = 730–835 °C, P = 5.3–7.5 kbar). In the Erzin complex, metamorphic rocks have undergone multiple stages of ductile deformation with subvertical mineral lineation and were then superimposed by sub-horizontal ductile deformation. The stages of tectonic deformation are close in time and mark collisional stages characterized by a sequential change from the compression regime to the extension regime in the period of 495 ± 5 Ma. The determination of the time interval is based on structural and petrological data, previously published materials, as well as the dating of granite dikes (U-Pb, zircon) sealing the Erzin complex. P-T conditions and structural-petrological studies analysed in this paper suggest that the formation of the Erzin metamorphic complex occurred as a result of a collisional event between the Tannuola island arc and Tuva-Mongolian microcontinent. We suggest that the Erzin metamorphic complex is important for study of transitional regime between the collisional event and initial orogenic collapse in the ancient fold belts. Metamorphic record of the Tuva-Mongolian microcontinent provides important information about the processes that occurred at the Early Paleozoic geodynamic evolution in the western CAOB.
... In such contexts, the newly developed fabrics may modify or erase the earlier ones, depending on temperature and strain intensity. In many convergent orogenic belts, spatial and temporal relationships between granite and regional tectonic structures are generally interpreted in terms of syn-late-to post-tectonic granite emplacement during contraction rather than extension (Brown, 1978(Brown, , 1995Hutton, 1988;Brown and Solar, 1998a, 1998b, 1999Solar et al., 1998). However, according to Paterson and Tobisch (1988), Paterson et al. (1989) and Vanderhaeghe (2009), a structural analysis of a granitic rock and its surroundings can Caby et al., 1991). ...
The Pan-African (578-563 Ma, U-Pb zircon age) Dschang granitic pluton belongs to the edge of the central domain of the Central African Fold Belt in Cameroon. It is made up of biotite granites that intruded pre-Pan-African biotite-hornblende orthogneiss basement. A new detailed structural study testifies that this area recorded four deformation phases (D1-D4). The D1-related structures described in the basement rocks are folded. They were progressively transposed into the main S2 foliation during D2 deformation which is synchronous with the emplacement of E-W granitic dykes. The D2 strikes NNW-SSE to NW-SE. It is characterized by main S2 metamorphic foliation (mean pole of the whole data at 269°/66°), NW–SE low to moderate plunging lineations (mean lines of the whole data at 159°/15°), F2 folds with NW plunging axes, C2 sinistral E-W ductile shear indicators (marked by injection of granite dykes into the biotite-hornblende orthogneiss). The D3 phase is marked in the biotite-hornblende orthogneiss by C3 dextral shear planes, followed by dextral rotated boudins (ß3). In the granites, it is characterized by S3 magmatic foliation (mean pole of the whole data at 323°/61°) due to the reorientation of platy megacrystals of K-feldspar, F3 folds profile with moderate plunging axes (mean lines of the whole data at 145°/28°) and sinistral vergence. The D4 phase is usually ductile. It is characterized by minor faults with C4 sinistral shear planes which crosscut the S3 magmatic foliation. The D2 phase is synchronous with the early emplacement of the Dschang granites at 578 Ma whereas D3 marks its end at 563 Ma. Mesoscopic (phenocrysts alignment, elongated enclaves, schlierens) and microscopic features (quartz, plagioclase and alkali feldspar deformation microstructures) are consistent with a continuum of deformation from magmatic to submagmatic states in the Dschang granites, the later state being more common. We conclude that the history of the Dschang area began with the emplacement of biotite-hornblende orthogneiss before 578 Ma; it was followed by the emplacement of granite that started towards the end of the D2 phase at 578 Ma (probably aided by the E-W sinistral shear fractures) and ended during the D3 phase at 563 Ma. This is consistent with the emplacement of the Dschang granitic pluton during the transition period between the sinistral left lateral wrench movement and the dextral right lateral wrench movement of the history of western Gondwana. However, the scarcity of typical structures of a synkinematic intrusion or a post-magmatic ductile shearing (mylonites, S/C-structures) in the study granite suggests its emplacement into a low strain crustal domain within an anastomosing pattern of shear zones.
... The migmatite outcrops are very much restricted in dimension (ranging from 5 m  1 m to 50 m  20 m) and often diverse in nature. Regional-scale litho-structural mapping (Brown & Solar, 1998;Solar & Brown, 2001) is conventionally difficult in this area because the structure and rock types show too much variation and outcrops are too small, too numerous and discontinuous. However, a few zones are demarcated as static and dynamic mode migmatite-rich domains ( Figure 3). ...
The southern basement of the Cuddapah Basin comprises the Dharwar Batholith and greenstone belt complex. Granitoids of the batholith exhibit extensive variation in terms of geomorphology, age, mineralogy, and micro/meso scale structures. The eastern part of Dharwar Craton along 13°50′ to 14°8′N latitude and 78°45′ to 79°05′E longitude was studied to enlighten the rheological influence on crustal evolution. Frequent occurrences of migmatites of restricted dimension are observed in the south of 14°10′N latitude. The granite‐migmatite contacts are not sharp in general. Different types of migmatite complex and their relationships with granitoids as well as older country rocks represent an exhumed segment of the crustal catazone. The widespread group of migmatitic rocks are classified in a composite manner on the basis of morphology and structure. Furthermore, genetic implication vis‐a‐vis anatexis history is also evaluated. Static and dynamic modes of migmatites are recognized with reference to geothermal gradient and tectonics. Based on the degree of anatexis, two categories of migmatites are identified in the field, that is, metatexites and diatexites. In addition, metatexites are classified into four sub‐types (viz, patch, dilatant, net, and stromatic) and diatexites are also sub‐divided into two categories (viz, schollen or raft and schlieren). The hybrid nature of migmatitic rocks with both metamorphic and igneous characteristics are used to analyse pre‐ and post‐anatectic events. The preserved evidences of partial melting are marked as leucocratic patches. In situ stagnation of the melt or subsequent separation from the remaining solid provides different morphology of static mode. Importance of dihedral angle at solid–liquid contacts is also considered in the present context to describe the grain boundary penetration by partial melt. Folds, veins, and boudins of different styles and generations played significant role in dynamic mode migmatitization. The syn‐ and post‐metamorphic deformation events and granite melt generation from migmatites are schematically defined. Spatial and temporal relationships of schist‐gneiss‐migmatites of both static as well as dynamic mode reveal initiation of the crustal development by vertical accretion of ultramafic‐mafic lava and TTG. Cyclic partial remelting of the metabasic lava and TTG and underplating led to development of the lithospheric plate. Later upwelling material at convergent plate and associated heat transfer led to generation of granitic magma. The established prograde and retrograde cycle of metamorphism were possibly interrupted by crustal reworking events. This study confirms about the crustal catazone segment (with >15 km depth and >500°C) in which physical processes control generation, segregation, ascent, and emplacement of juvenile granite from migmatites. Migmatites from East Dharwar Craton is classified for the first time with proper genetic model Structure and metamorphism are correlated with tectonics and genetic link among different types of migmatite are established with schematic model Significance of migmatites are explained in the formation of granite with respect to crustal reworking Crustal evolution of East Dharwar Craton vis‐à‐vis schist‐gneiss‐migmatite is explained with reference to global frame.
... It has been recognized that deformation can have a significant impact on the migration of melt through the crust (e.g., Rosenberg & Handy, 2005;Sawyer et al., 2011;Vielzeuf et al., 1990). At larger scales, compressive stresses can drive melt migration by promoting permeability and creating structures that can act as pathways, promoting magma transfer and emplacement (e.g., Brown, 1994;Brown & Rushmer, 1997;Brown & Solar, 1998;Collins & Sawyer, 1996;Weinberg et al., 2015). Partial melting itself can also actively influence deformation by introducing a liquid phase that locally reduces the viscosity, concentrating deformation (e.g., Davidson , 1994;Vanderhaege & Teyssier, 2001). ...
The Coompana Province, located between the three cratonic blocks of Australia, is significant for understanding the Proterozoic assembly of the continent. However, the basement rocks are completely covered and poorly understood, and as a result reconstructions are poorly constrained. Recent collection of detailed geophysical data sets and a stratigraphic drilling campaign in the eastern Coompana Province is used to understand the crustal architecture and tectonomagmatic history of the region. A north to northeast trending grain represents a series of amalgamated 1618–1526 Ma tectonic belts between the Western and South Australian cratons. These belts are cut by a northeast trending, shear‐bounded corridor of thinned crust, intruded by voluminous circa 1150–1140 Ma granite plutons. The new data indicate two main stages of deformation between circa 1200 and 1140 Ma. Circa 1200–1170 Ma east‐west extension was pervasive, causing widespread partial melting, shallowly dipping layering, recumbent folding, and minor shoshonitic magmatism. Circa 1160–1140 Ma deformation was partitioned into the northeast trending corridor, which focused transfer and emplacement of voluminous A‐type magmas. The two stages represent a switch from pervasive to localized deformation, with accompanying changes in magmatic style. Early pervasive extension caused progressive melt extraction, producing small sheets. Subsequent shearing locally promoted melt reorganization in the lower and middle crust. This caused local changes in rock strength with the continued partitioning of deformation into the oblique corridor. The crustal‐scale structures disrupted the lower crustal MASH zones, mobilizing and focusing the voluminous magmatism.
Structural and tectono-metamorphic analysis of strain partitioning
during late-orogenic oblique deformation : insights from the Variscan
Tanneron Massif and the Terre Adélie Craton
Exploration and mining of lithium pegmatites has increased in recent years to meet the growing demand for critical metals, which are required for electric-powered transportation as well as sustainable energy production and storage. Pegmatite deposits produced approximately 60% of global lithium production in 2018, the vast majority of which was from spodumene pegmatites. However, there is lack of comprehensive models linking mineral system evolution and structural controls of lithium pegmatites. The majority of studies on lithium pegmatites have focused on chemical fractionation and assume that pegmatites have evolved from a parental granitic source, yet problems emerge when the connection between the granitic source and the pegmatite bodies is not obvious. This lack of connection has given rise to an alternative model of pegmatite formation, the anatectic model. In the anatectic model, granitic pegmatites form from granitic melts produced by partial melting of a suitable lithology, typically pelitic metasediments, without forming a large granitic body followed by fractional crystallization.
Here, we first examine the formation of granitic melts related to anatexis, as well as how regional crustal structures influence chemical composition, crustal migration, and melt accumulation within the crust. We next examine the Wekusko Lake pegmatite field located in Manitoba, Canada, in terms of the probable melt source migration, the relationship between crustal-scale strain-zones and spatial pegmatite emplacement, and the chemical fractionation trend that records the formation of lithium-enriched pegmatites.
The Alto Maranhão batholith (SE Brazil) comprises granitic juvenile arc-related rocks within the Paleoproterozoic Mineiro Belt. The pluton is bounded by the NE-SW sinistral Jeceaba-Bom Sucesso (JBSZ) and NW-SE dextral Congonhas-Itaverava (CISZ) shear zones. Field, microstructural and magnetic fabric studies were employed in order to constrain its emplacement model and the spatial and temporal relationships between plutons and shear zones during Paleoproterozoic orogenic events. Combined meso- and microscale fabric analyses allowed us to individualize two different domains or lobes across the intrusion. The magmatic foliation, defined by biotite and hornblende, strikes NE-SW in the western lobe and rotates to NW-SE in the eastern lobe, moderately to steeply dipping, trending accordingly with the solid-state foliation exhibited by the country rocks. Field observations show that magmatic structures are preserved, such as elongated to rounded syn-magmatic enclaves and magmatic layering. The microstructures described for both domains are a direct record that deformation occurs locally and under magmatic conditions, with limited features of solid-state deformation such as undulose extinction in quartz and plagioclase on both tonalite and dioritic enclaves. AMS data exhibits dominantly steep magnetic lineations in the eastern lobe of the intrusion, except for sampling sites close to the CISZ, whose lineations are shallowly plunging. In both cases, the magnetic lineation trends NW-SE. Towards the western lobe, the magnetic lineation becomes horizontal and randomly oriented close to its westernmost tip. The magnetic and magmatic foliations follow the same orientation, therefore striking NW-SE in the eastern lobe and NE-SW in the west. Considering the batholith's heterogeneities highlighted by the AMS measurements, we suggest that the eastern lobe of the intrusion is characterized as a feeder zone in which magma was injected along the CISZ, being subsequently spread into the pluton's western lobe, until being stopped by a rheological barrier defined by JBSZ and country rocks, controlling its boomerang-like form. The proposed emplacement model suggests that the Alto Maranhão batholith was emplaced in a contractional site, by arc-normal dextral simple shear revealing that transcurrent deformation was taking place concurrently with the growth of the São Francisco paleocontinent, and addition of mantle-derived magmas to the continental crust.
Continental‐scale strike‐slip shear zones often record significant tectono‐magmatism and dynamic deformation processes of the crustal lithosphere. However, the genetic relationships and timing among the anatexis, deformation, and initial shearing along a strike‐slip shear zone have not been well defined. Here, we carried out detailed field, microstructural, zircon U–Pb geochronology, geochemical and EBSD texture analyses of leucogranites and migmatites in the Chongshan shear zone (CS‐SZ). The results indicate that most migmatites and leucogranites exhibit strong shear deformation and well‐developed high‐temperature mylonitic microstructures. The quartz aggregated from foliated leucogranites developed dominant high‐temperature prism and prism slip systems. The pre‐ and syn‐kinematic crustal anatexis and localized weak zone mainly occurred from 35 to 29 Ma along the CS‐SZ, which is closely related to the post‐collisional extension and collapse of overthickened crust. Biotite dehydration melting formed pre‐ and syn‐kinematic melts and leucogranites that further experienced fractional crystallization of plagioclase and K‐feldspar during melting and subsequent melt migration and emplacement upward along the tectonic weak zone. The thinning and weakening of lithospheric crust further facilitated the initial and formation strike‐slip displacement along the CS‐SZ, which occurred from 29 to 20 Ma or much later to 18 Ma. Finally, we propose that crustal anatexis and upward migrating melts play a key role in controlling the thermal state and rheological strength of the crust, resulting in nucleation and initiation of the localized deep‐seated shear zone that accommodates significant displacement for the India–Asia continental forward collision and intracontinental deformation.
Magmatic and high-to low-temperature sub-solidus fabrics in intrusive rocks are frequently used to infer the relative timing of deformation with respect to magma emplacement and cooling. Here we describe the relationships between strain and fabric development in leucogranite sheets (pegmatite, aplite) emplaced into shear zones that localized post-peak metamorphic deformation in the contact metamorphic aureole of an upper crustal pluton (<0.2 GPa) on the Island of Elba, Italy. The leucogranite sheets contain igneous, mylonitic, and cataclastic fabrics. Detailed meso- and microscopic structural analysis suggests that dykes emplaced into shear zones behaved as strong, competent rigid bodies during mylonitic deformation of the surrounding host rocks. Thermal modelling indicates that the emplacement and cooling of sheets occurred very rapidly (a few days to years) compared to typical tectonic strain rates and strain accumulation timescales in the host rocks, which would have inhibited melt or magma-induced thermal softening of the host rocks during deformation. The local development of mylonitic and cataclastic fabrics in the dykes was controlled by the local activation of fluid-controlled reaction softening mechanisms (mylonitic fabric) and embrittlement during cooling in sites of high-strain (cataclastic fabric). We show that a broad spectrum of fabric types can form in igneous sheet intrusions emplaced at the same time and crustal level. The coexistence of isotropic (non-foliated igneous) versus anisotropic (mylonitic and cataclastic) fabrics in igneous sheet intrusions should therefore be evaluated in terms of tectonic strain rates, cooling rates, the thermal state of the host, the distribution of heterogeneous strain and the activation of strain softening mechanisms. Our observations highlight that the concepts of pre-, syn-, late- and post-tectonic fabrics in intrusive igneous rocks should therefore be used with caution when interpreting relative timing relationships between deformation and magmatism.
In the Rio Maria and Sapucaia Domains of the Carajás Province, during the Mesoarchean, tonalite-trondhjemite-granodiorite (TTG) series (2.98–2.92 Ga; Colorado Trondhjemite) and sanukitoid rocks (∼2.87 Ga; Rio Maria Suite of Ourilândia do Norte) were formed. They were followed, in the Neoarchean (2.75–2.73 Ga), by numerous stocks of granitoids similar to A-type granites (Vila Jussara Suite). Despite the compositional differences between these granitoids, epidote is a mineral phase common to all of them, with pistacite contents of 26–29 mol% in TTGs, 22–33 mol% in sanukitoids and 25–30 mol% in Neoarchean granitoids. The study of the dissolution kinetics of Archean epidotes of the Carajás Province reveals that the partial dissolution time of their crystals was ∼4–10 years, with corresponding magma ascent rates of 4–8 km/year. Magma viscosities at liquidus temperature were estimated at 105.3 Pa s for TTG magma and 102.5 Pa s for sanukitoid magma, whereas monzogranitic magmas of the Vila Jussara Suite exhibited a viscosity of 104.4 Pa s. In contrast, the viscosity of tonalitic magma of the Vila Jussara Suite was 103.5 Pa s. The preservation of magmatic epidote in Archean granitoids requires that the plutons grew in an incremental way, similar to their Phanerozoic counterparts, with the stacking of small sill-dykes of about 100 m thickness each intruded in a rather cold crust, allowing fast cooling rates so as to prevent complete dissolution of epidote at the final emplacement level.
Partial melting is the fundamental process by which juvenile crust was produced from the mantle and subsequently reworked to become the stable, compositionally-differentiated continents on which we live and which host most of the elemental resources required by our modern technological society. Irreversible differentiation of the continental crust occurs principally through the production, segregation and migration of silica-rich (felsic) melts from deeper source rocks to shallower sinks where they erupt or, more commonly, crystallise as granite sensu lato. Here we provide for both novices and professionals a comprehensive, but accessible, account of the processes involved in crustal melting and suprasolidus phase equilibria from first principles to the forefront of modern research. To begin we introduce the tectono-metamorphic context for crustal melting before considering the evidence at outcrop and in thin section for partial melting, and briefly reviewing melt extraction from crustal rocks. As a prerequisite to understanding the physicochemical basis for crustal melting, we summarize the essential thermodynamics that underpin calculated phase equilibria, distinguish different types of melting reaction, and review the requirements for, methodology behind, and limitations of a phase equilibrium modelling approach based on equilibrium thermodynamics. We explain the various types of phase diagram used to investigate partial melting and assess open versus closed system processes, including internal and external buffering of H2O. Those crustal sources that partially melt to produce granite are considered in detail, namely basic rocks such as basalts and gabbros, clastic sedimentary rocks such as greywackes, siltstones and mudstones (pelites), and granites themselves. We concentrate mainly on intracrustal partial melting in convergent-margin settings, and on anatexis during exhumation of deeply-subducted continental crust from mantle depths. We discuss the behaviour of trace elements and accessory minerals during melting, and consider the implications for isotope geochemistry. To close we include a brief summary of some of the important points and offer a few suggestions for future lines of research on crustal melting.
The Amritpur Granite, sandwiched between the two intensely deformed rock packages, occurs adjacent to the Main Boundary Thrust (MBT) in the outer part of the Kumaun Lesser Himalaya. Characteristically, the granite lacks any penetrative tectonic fabric at the microscopic or mesoscopic scales. Using high-resolution mapping, litholog, and outcrop-scale overprinting relationships, we first revisit the tectonic setting of the Amritpur Granite and then attempt to detect the magnetic fabric, if any, in it. For this, we investigate the anisotropy of magnetic susceptibility in the Amritpur Granite and the adjacent lithounits deformed presumably during the Himalayan orogeny. High-resolution mapping demonstrates that the MBT traces the boundary between the Neogene Siwalik sediments and the Proterozoic amphibolite of the Nagthat Formation in the study area. Field relationships and structural setting imply that the granite occurs either as a probable klippe or a basement sliver.
The orientations of the mesoscopic scale foliations and lineations are similar to the respective magnetic fabric orientations in the deformed lithounits occurring adjacent to the Amritpur Granite. The tectonically induced magnetic fabric orientations in the deformed lithounits also correlate well with the magnetic fabric orientations in the Amritpur Granite. Several lines of evidence suggest that the magnetic fabrics in the Amritpur Granite and those in the adjacent deformed lithounits were induced during a common deformation phase.
Field, microstructural and mineral compositional evidence from the Mesoproterozoic K-feldspar megacrystic Red Granite at Oncócua Platform (southwestern Angola) is consistent with crystal transfer from magma to wallrock during syntectonic intrusion. K-feldspar megacrystic Red Granite intruded during folding of wallrock tonalite. Enclaves of the wallrock tonalite are elongated parallel to Red Granite intrusive contacts, a K-feldspar megacryst and hornblende defined magmatic foliation, and a gneissosity in the Red Granite. Stromatic layering present in the tonalite is crosscut by the Red Granite intrusive contacts or is isoclinally folded with fold axial planes and hinges filled with Red Granite. K-feldspar megacryst clusters and curved grain boundaries (i.e., contact melting), as well as thin Red Granite fold axial planar sheets containing K-feldspar megacrysts that are typically wider than the sheets themselves, are all consistent with melt loss and crystal accumulation during solidification. The wallrock tonalite also hosts K-feldspar megacrysts and hornblende phenocrysts that are interpreted to be the same population to those in Red Granite, on the basis of their size, shape, nature of inclusions, compositions, and compositional zoning. We propose that these crystals were transferred from the intrusive Red Granite magma to the wallrock tonalite via magmatic conduits that subsequently collapsed due to external stresses, leaving behind the larger crystals. The pristine preservation of intrusive relations at Oncócua Platform may mean that crystal transfer from magma to wallrock is more common in incrementally assembled granitoid plutons than previously thought.
Supplementary material: [Mineral chemical data] available at https://doi.org/10.6084/m9.figshare.c.5448664
In South India, the WNW-striking ~ 130-km-long Achankovil Shear Zone (AKSZ) separating the Trivandrum Block and the Madurai Block is speculated to continue within southern Madagascar in the East Gondwanaland. Two aspects continue to be debated, whether the AKSZ is a terrain boundary shear zone, and if so, when the accretion occurred. We present, for the first time, a detailed field-based analysis of contrasting mesoscale structures and deformation kinematics in the granulite facies rocks across the AKSZ. The mineralogical segregation layering (S2T) in the Trivandrum Block is sub-horizontal to gently inclined and describes F3T regional-scale open asymmetric WNW-trending sub-horizontal folds. By contrast, the S2M granulite facies layering in the Madurai Block describes regional-scale steeply plunging F3M folds. Both blocks share the NNW/NNE-trending sub-parallel sinistral and dextral S3 shears of the AKSZ. Deformation microstructures and garnet-bearing melt-hosted syn-S3 extensional shears attest to the persistence of high temperature during D2–D3 deformations. Monazite chemical dates and existing Pb–Pb zircon dates are overwhelmingly Pan-African in and across the AKSZ, but the Paleoproterozoic (2.1–1.8 Ga) dates in the Trivandrum Block, equivalent to those reported in Anosyan and Androyan domains of southern Madagascar, are lacking in the Madurai Block. By contrast, the Mid Neoproterozoic (800–750 Ma) Pb–Pb zircon dates in the Madurai Block, equivalent to the Antananarivo domain (Central Madagascar), are lacking in the Trivandrum Block. The structural–chronological contrasts between the two blocks suggest the AKSZ to be a Pan-African terrain boundary shear zone system that is continuous with the Ranotsara shear zone in Madagascar. [248]
Low-pressure metamorphic assemblages are very common in many orogenic belts. Characteristically, they are related to the thermal metamorphism caused by post-orogenic rift-related plutonism. However, it has proven correct that magmatic activity at any stage of the orogen can provide the required temperature to initiate and drive the metamorphism. The Arabian–Nubian Shield (ANS), the northern part of the Neoproterozoic East African Orogen (EAO), is characterized by widespread exposures of the pelites-dominated metasedimentary terranes originated in different tectonic settings. Low-pressure metamorphic belts constitute important components, controversial in places, of some of these terranes. The current chapter documents the field characterization of four multiply deformed and metamorphosed low-pressure/medium- to high-temperature metamorphic belts located in the Eastern Desert and Midyan terranes in the extreme north-eastern tip of the ANS.
The deformation of sheet-like igneous intrusions in multiple orientations can give detailed insights into the kinematics of ductile flow in shear zones. Using the Torrisdale Vein Complex in the northern Scottish Caledonides as a case study, we examine the relationships between folded and boudinaged pegmatitic intrusions deformed by strike-slip dominated shear. Within the shear zone, intrusions that trend clockwise of the adjacent regional foliation were commonly (75%) folded, whereas >80% of pegmatites at anticlockwise angles were asymmetrically boudinaged, giving dextral senses of shear parallel to the dominant and gently-plunging mineral lineations. Pegmatite fold trains at high angles to foliation (Sn) display the greatest % shortening, marked by steeply plunging ‘Z’ shaped folds when viewed down plunge. Pegmatites typically form Class 1C folds and were therefore more competent than their host gneisses at the time of deformation, i.e. they were not in the magmatic state. Such fold geometries are consistent with flattened parallel folds and suggest that up to 0.5 homogenous flattening was superimposed on the folds. Fold styles also indicate viscosity contrasts between the pegmatites and host gneisses of between 50 and 250 at the time of folding. Fold hinges are dispersed about an arc within the steeply-dipping foliation, suggesting a component of hinge rotation towards the mineral lineation during non-coaxial deformation. Folded pegmatites may also display boudinage on their limbs indicating that fold limbs had rotated into the extensional field during progressive deformation. Pegmatite boudin trains are typically developed <45° anticlockwise of Sn and display the greatest % extension when trending sub-parallel to the shear zone foliation. Within such trains, individual shearband boudins are more anticlockwise than the overall train and show right-stepping relative to neighbouring bodies. More equant boudins have rotated less than elongate boudins (aspect ratios >3) and preserve the largest anticlockwise angles with the overall boudin train. Conversely, domino boudins are clockwise of overall boudin trains and have undergone clockwise rotation marked by left-stepping of adjacent boudins. Domino boudins display flanking folds which progressively open as the deformable boudin rotates towards the foliation and margins of the boudin lozenges locally become clockwise of flow. This ‘unfolding’ of deflected foliation, together with local reversals in the sense of shear around rotating boudins, is consistent with a clockwise vorticity associated with bulk dextral shear. These observations collectively indicate that bulk dextral shear with an additional component of pure shear operated across the shear zone and provides further insights into the kinematics of mid-crustal deformation.
Analogue experiments were used to investigate pluton emplacement during transpression in a layered crust. Models consisted of (1) a silicone gum-PbO suspension as analogue magma, (2) a silicone gum-Pb suspension as a basal ductile layer, and (3) an overlying sand pack representing brittle crust. The models were transpressed at 3 mm/hr causing the extrusion of the analogue magma from a progressively closing slot, and its emplacement into the ductile layer. The thicknesses of the layers were critical in controlling the shapes of intrusions and the structures that developed in the brittle overburden. Thicker sand packs led to flattened, symmetrical laccolith-shaped intrusions and the nucleation of one oblique thrust in the sand pack above the extremity of the intrusion. Thinner sand packs led to thicker, asymmetrical laccolith-like intrusions with uplift of the overburden on an oblique thrust, and the formation of a shallow graben in the extrados of a bending fold. Reducing the thickness of the basal ductile layer resulted in a larger number of shear zones in the sand pack, and structural geometries approaching those produced in experiments involving only a brittle analogue crust and no ductile layer. Shear zones in the sand pack were localised by intrusions, and also played a key role in displacing analogue brittle crust to make space for intrusions. The results suggest that tectonic forces may play an important role in displacing blocks of crust during pluton emplacement in transpressional belts. They also suggest that pluton shapes, and the geometries and kinematics of emplacement-related shear zones and faults, may depend on the depth of emplacement. In nature, depending on the structural level exposed in the map plane, faults and shear zones that helped make space for emplacement may not appear to be spatially associated with the pluton.
Discusses the changes in the tectonic regime in the Peruvian and Bolivian Andes that have occurred since the upper Miocene when the present-day elevation of the Cordillera above sea level has been almost reached. The stress patterns are deduced essentially from a field study of fault kinematics and a numerical inversion of the slip vector data measured on the fault planes. The Cuzco fault system in southern Peru is chosen as an example to illustrate the methodology used. Extension in the High Andes is of small magnitude, of the order of 1% during the last 1-2 m.y.; in a few basins, it may have attained 40% during the Pliocene (~5-3 m.y.). -from Authors
The Lexington batholith in western Maine intruded upper crustal rocks in three pulses, now exposed as the north, central, and south lobes. A study of the metamorphosed pelitic rocks surrounding the batholith has shown that this terrane was subjected to three metamorphic subevents of the major event, M2 over a short time interval at about 400 Ma. Following M1, a low-grade event, cordierite, andalusite, and sillimanite + K-feldspar were produced by the contact metamorphic subevent (M2n) associated with the north lobe. Then a metamorphic subevent of more regional extent (M2) produced garnet, staurolite, and andalusite in the southern half of the study area. Isograd patterns suggest that this subevent was largely concentric with, and broadly related to, the central and south lobes of the batholith. Staurolite, andalusite, and sillimanite were produced by slightly later contact metamorphic subevent (M2s) produced by the central and south lobes. -from Authors
The rheological and chemical behaviour of the lower crust during anatexis has been a major focus of geological investigations for many years. Modern studies of crustal evolution require significant knowledge, not only of the potential source regions for granites, but also of the transport paths and emplacement mechanisms operating during granite genesis. We have gained significant insights into the segregation and transport of granitoid melts from the results of experimental studies on rock behaviour during partial melting. Experiments performed on crustal rock cores under both hydrostatic conditions and during deformation have led, in part, to two conclusions. (1) The interfacial energy controlling melt distribution is anisotropic and, as a result, the textures deviate significantly from those predicted for ideal systems—planar solid-melt interfaces are developed in addition to triple junction melt pockets. The ideal dihedral angle model for melt distribution cannot be used as a constraint to predict melt migration in the lower crust. (2) The ‘critical melt fraction’ model, which requires viscous, granitic melt to remain in the source until melt fractions reach >25 vol%, is not a reliable model for melt segregation. The most recent experimental results on crustal rock cores which have helped advance our understanding of melt segregation processes have shown that melt segregation is controlled by several variables, including the depth of melting, the type of reaction and the volume change associated with that reaction. Larger scale processes such as tectonic environment determine the rate at which the lower crust heats and deforms, thus the tectonic setting controls the melt fraction at which segregation takes place, in addition to the pressure and temperature of the potential melting reactions. Melt migration therefore can occur at a variety of different melt fractions depending on the tectonic environment; these results have significant implications for the predicted geochemistry of the magmas themselves.
The permeability of a partially molten rock is one of the primary factors controlling the melt segregation rate. With decreasing melt percentage, permeability becomes increasingly sensitive to the grain-scale geometry of partial melt. At low melt percentages, the ratio of grain-boundary energy to solid-melt interfacial energy, [γ ss ]/[γ sl ], is the fundamental physical property that determines the equilibrium melt geometry, including the wetting angle θ at a solid-solid-melt triple junction, the area-tovolume ratio s/v of melt pockets at grain corners, and the permeability threshold φ c (φ c , is the volume melt percentage at which melt interconnection is established). The trends of increasing θ and φ c or decreasing s/v with decreasing [γ ss ]/[γ sl ] are well established in the case of a monomineralic system with isotropic interfacial energies. We argue that these general trends must apply as well in natural systems where solid-melt interfacial energies are essentially anisotropic. Measurements of wetting angles at quartz-quartz-melt, feldspar-feldspar-melt and amphibole-amphibole-melt triple junctions consistently yielded low to very low median values, in the range 10°–60°. These low angles result from high values of [γ ss ]/[γ sl ], up to 2.0 for the lowest angles. They indicate that the permeability thresholds of partially molten crustal protoliths should be very low: The major implication of the experimental studies is to show that melt segregation may potentially operate at very low degrees of melting (theoretically, any value ≥ φ c ). Because of the high viscosities of granitic melts, melt segregation is presumed to be inefficient at such low degrees of melting. There may exist therefore a range of melt percentages above (φ c over which the partial melt is interconnected but nearly stagnant.
Compaction driven fluid flow is inherently unstable such that an obstruction to upward fluid flow (i.e. a shock) may induce fluid-filled waves of porosity, propagated by dilational deformation due to an effective pressure gradient within the wave. Viscous porosity waves have attracted attention as a mechanism for melt transport, but are also a mechanism for both the transport and trapping of fluids released by diagenetic and metamorphic reactions. We introduce a mathematical formulation applicable to compaction driven flow for the entire range of rheological behaviors realized in the lithosphere. We then examine three first-order factors that influence the character of fluid flow: (1) thermally activated creep, (2) dependence of bulk viscosity on porosity, and (3) fluid flow in the limit of zero initial connected porosity. For normal geothermal gradients, thermally activated creep stabilizes horizontal waves, a geometry that was thought to be unstable on the basis of constant viscosity models. Implications of this stabilization are that: (1) the vertical length scale for compaction driven flow is generally constrained by the activation energy for viscous deformation rather than the viscous compaction length, and (2) lateral fluid flow in viscous regimes may occur on greater length scales than anticipated from earlier estimates of compaction length scales. In viscous rock, inverted geothermal gradients stabilize vertically elongated waves or vertical channels. Decreasing temperature toward the earth's surface can induce an abrupt transition from viscous to elastic deformation-propagated fluid flow. Below the transition, fluid flow is accomplished by short wavelength, large amplitude waves; above the transition flow is by high velocity, low amplitude surges. The resulting transient flow patterns vary strongly in space and time. Solitary porosity waves may nucleate in viscous, viscoplastic, and viscoelastic rheologies. The amplitude of these waves is effectively unlimited for physically realistic models with dependence of bulk viscosity on porosity. In the limit of zero initial connected porosity, arguably the only model relevant for melt extraction, travelling waves are only possible in a viscoelastic matrix. Such waves are truly self-propagating in that the fluid and the wave phase velocities are identical; thus, if no chemical processes occur during propagation, the waves have the capacity to transmit geochemical signatures indefinitely. In addition to solitary waves, we find that periodic solutions to the compaction equations are common though previously unrecognized. The transition between the solutions depends on the pore volume carried by the wave and the Darcyian velocity of the background fluid flux. Periodic solutions are possible for all velocities, whereas solitary solutions require large volumes and low velocities.
Laboratory measurements of rock strength provide limiting values of lithospheric stress, provided that one effective principal stress is known. Fracture strengths are too variable to be useful; however, rocks at shallow depth are probably fractured so that frictional strength may apply. A single linear friction law, termed Byerlee's law, holds for all materials except clays, to pressures of more than 1 GPa, to temperatures of 500°C, and over a wide range of strain rates. Byerlee's law, converted to maximum or minimum stress, is a good upper or lower bound to observed in situ stresses to 5 km, for pore pressure hydrostatic or subhydrostatic. Byerlee's law combined with the quartz or olivine flow law provides a maximum stress profile to about 25 or 50 km, respectively. For temperature gradient of 15°K/km, stress will be close to zero at the surface and at 25 km (quartz) or 50 km (olivine) and reach a maximum of 600 MPa (quartz) or 1100 MPa (olivine) for hydrostatic pore pressure. Some new permeabiltiy studies of crystalline rocks suggest that pore pressure will be low in the absence of a thick argillaceous cover.
The obstruction to fluid flow formed by the rocks overlying a metamorphic devolatilization front causes the fluid pressure gradient in the reacting rocks to diverge from lithostatic. This drives deformation in tandem with the fluid pressure anomaly generated by the volume change of the reaction. Numerical simulations show that once the vertical extent of the reacted rocks is comparable to the compaction length, compaction processes caused by the difference between confining and fluid pressure gradients generate a positive fluid pressure anomaly (effective pressure
This report attempts to synthesize the results of recent geologic mapping and petrologic studies in western Maine, an area that lies on the transition zone between the sillimanitic portion of the Merrimack synclinorium in New Hampshire and the greenschist-facies portion in central Maine. Results of two fundamentally different deformations are recognized in the transition zone. The first or early deformation produced northeast-trending tight, upright passive-flow folds; longitudinal premetamorphic faults of large displacement; and slaty cleavage formed also by the first metamorphism (M-l). These features were produced throughout the whole geosyncline that was ancestral to the Merrimack synclinorium; they characterize the structure of the greenschist terrane. But southwestward across the transition zone, they are increasingly blurred by younger superposed structural and metamorphic features. Inferred origins are geosynclinally controlled slump deformation and tectonic dewatering culminating in low-grade metamorphism. The younger features collectively define the late deformation and at least two late metamorphic events (M-2 and M-3). The late deformation produced a complex pattern of slip cleavages; schistosity derived from slip cleavage, small recumbent folds, large flexural folds, and domes of varied shapes and trends; and conspicuous northwest-trending flexural cross folds. In the northeast part of the transition zone, M-2 and M-3 metamorphic events produced overlapping metamorphic zones approximately concentric to granitic plutons. To the southwest, where metamorphic grade is regionally in the sillimanite or potassium feldspar plus sillimanite zones, M-2 and M-3 zones coalesce and become indistinguishable. Many structural features are directly magma-generated, but the northwest-trending cross folds and related cleavages probably express regional strain that is only temporally magma-related. Nine samples of granite and aplite have yielded an isotopic age of 379 ± 6 m.y. by the Rb-Sr whole-rock method. This age dates the youngest intrusive rocks of the New Hampshire Plutonic Series in the report area, M-3 metamorphism, and the late deformations that accompanied M-3. Because all the plutons intrude previously deformed Devonian and older strata, the isotopic age narrowly restricts the duration of the early and late deformations, plutonism, and M-l to M-3 metamorphisms. We speculate that the Mooselookmeguntic pluton and the larger Sebago pluton farther south are irregular but broadly arched subhorizontal sheets only a few kilometres thick that were emplaced at depths of 11 to 15 km. The transition zone is where these and possibly other sheetlike bodies dive monoclinelike to the northeast. Northeast of the transition zone, the tops of the sheets are now a few kilometres below the surface. Associated with them are many of the late structural and metamorphic features that characterize the transition zone and the sillimanite terrane. Above them is a partly eroded thick slate layer. Numerous plutons now exposed northeast of the transition zone are cupolas and possibly isolated bodies that rose above the sheets. In the sillimanite terrane southwest of the transition zone, a subhorizontal granitic sheet complex is exposed. The thick slate layer that is now exposed only in the northeast once extended over the sillimanite terrane.
A sequence of distinctive stratigraphic units has been defined in a succession of sillimanite-grade metasedimentary schists, gneisses, and granulites in eastern and central New Hampshire north of the 44th parallel. The units are correlated and continuous with the formations of the Rangeley area, Maine, to the northeast. The same sequence of units has been mapped immediately south of latitude 44 degree and is firmly correlated southward with a succession of units in south-central New Hampshire.
Syntectonic granite emplacement in dip slip and strike slip contractional shear zones is now well documented by a number of case histories including the spectacular 1200 km long, 20 km wide Great Tonalite Sill of North America. These examples show fundamentally that magma driving forces can overcome compressional tectonic stresses and suggest that in a contractional orogen it is not in general a necessity to make space for plutons by localised dilation along faults and shear zones. There are a number of other magma driving forces that are available, in addition to buoyancy, which can combine to exceed the tectonic compression, including one which is derived from the compression itself. These extra forces are optimised when ascent and emplacement is achieved along major transcrustal faults and lineaments. The principle of effective stress is applied to the general case of granitic magmas in crust undergoing tectonic horizontal compressive stress and it is argued that the magma pressure is an indistinguishable part of the regional (effective) stress field. This allows a general solution to the space problem in granite emplacement since in the lower crust, where the host rocks are weak, the emplacement strain will also become an indistinguishable part of the regional deformation field. This principle is also likely to underlie space creation mechanisms in the often highly heterogeneous middle crust, where it may be obscured by the observed opportunistic exploitation of dilation along tectonic structures and other weaknesses.
In this monograph I have attempted to set out, in as elementary a form as possible, the basic mathematics of the theories of elasticity, plasticity, viscosity, and rheology, together with a discussion of the properties of the materials involved and the way in which they are idealized to form a basis for the mathematical theory. There are many mathematical textbooks on these subjects, but they are largely devoted to methods for the solution of special problems, and, while the present book may be regarded as an introduction to these, it is also intended for the large class of readers such as engineers and geologists who are more interested in the detailed analysis of stress and strain, the properties of some of the materials they use, criteria for flow and fracture, and so on, and whose interest in the theory is rather in the assumptions involved in it and the way in which they affect the solutions than in the study of special problems.
The systematic use of gravity data inversion over various granitic intrusions helps in defining the shape and extent of plutonic bodies. Petrographic and structural studies allow a better understanding of the emplacement process within a regional tectonical context. These methods were applied to several massifs selected in respect of their tectonic environment and their geochemical character. The paper covers granites ranging from alkaline type of an anorogenic setting to peraluminous type related to a collision tectonic regime. Morphologies vary from the classical intrusive type (ballooning diapir) to a more complex, tectonically related types (sheared or laminated plutons). The selected typology takes into account the regional tectonic environment as this affects the shape of the pluton. Anorogenic granitic-plutons appear as vertical cylindrical intrusive bodies, the diameter of which is about 14 km, with steep walls and moderate depth (10-12 km). Synkinematically emplaced granite-plutons are mineralogically more differentiated and often appear as flattened bodies. Their thickness (about 6 to 10 km) is quite lower than their breadth (about 100-300 km2). This character is more pronounced in instances involving thrust tectonics. However, some plutons are anomalously large, with a huge surface area (some thousand of km2) and thickness up to 20 km. Such plutons are not easily explained in a simple tectonic context where they are observed, as in Cornwall (UK) or in Nova Scotia (Canada), they are postorogenic. -English summary
This project is the first of its kind to attempt to characterize a volume of the earth's crust the dimensions of a transect (880 km long, 100 km wide, and up to 45 km deep) using 3-dimensional, digital modeling techniques. The transect is divided into discrete blocks approximately 50 km by 70 km in areal extent and extending down to the base of the crust, or MOHO. These blocks are being used to construct 3-dimensional geologic models. Five principal geological/geophysical data sets were combined with a digital elevation model and used to construct 3-dimensional models. -from Authors
Reviews the laboratory work on materials appropriate to the oceanic lithosphere with emphasis on contributions during the quadrennial period and the need for future work. The important results of flexure models incorporating realistic material properties are then summarized. Reviews the experimental rock mechanics studies of crustal rocks with emphasis on the important role of chemically-reactive aqueous fluids in controlling inelastic processes in the continental crust.-from Author
Diapirism has been discredited as a transport mechanism for magmas partly because diapirs seem to be unable to bring magmas to shallow crustal levels (< 10 km) and partly because recent developments in the theory of dyke propagation have shown that sufficiently wide dykes are able to efficiently transport felsic magmas through the crust. However, it is still unclear how felsic dykes grow to widths that allow them to propagate faster than they close by magma freezing. Ultimately, it may be the ability of felsic dykes to grow within the source that controls which mechanism dominates ascent. The ability of dykes to propagate from the top of rising diapirs depends among other factors on the changing temperature gradient of the wall rocks. The steep gradient around rapidly rising diapirs in the low viscosity lower crust will cause dykes to freeze. As diapirs rise to colder stiffer crust and decelerate, heat diffuses further from the diapir, resulting in shallower temperature gradients that favour dyke propagation. The mechanism may thus swap, during ascent, from diapirism to dyking. Calculations of the thermal evolution of diapirs and their surroundings show that basaltic diapirs may never form because they would be drained by dykes at a very early stage; felsic diapirs may be unable to give rise to successful dykes, whereas diapirs of intermediate magmas may propagate dykes during ascent.
Metamorphism is part of a continuum of processes that involve migration of heat and fluid. While the generation of metamorphic and metasomatic rocks, ore deposits, geothermal systems and magmatic intrusions all involve heat and fluid mass transfer, it is not simple to relate one of these processes to another in a cycle of crustal orogenesis.
Granites in both crystalline terranes and continental magmatic arcs tend to be circular to elliptical in map view and vary in width from about 3 to 100 km. Available gravity and structural data suggests that many of these plutons are tabular in shape with an average thickness of about 3 km. Ductile structures observed around mesozonal granites indicate that space is created by a combination of lateral and vertical displacements of wall rocks, whereas contact relationships of epizonal plutons imply that only vertical displacements are involved during emplacement. In both settings magma arrives at the emplacement site via one or more vertical feeder zones and flows laterally. With the exception of very high-level epizonal plutons, structural studies suggest that space for many tabular intrusions must be provided mainly by floor-depression (lopolith emplacement) rather than roof-lifting (laccolith emplacement). An emplacement model for this type of tabular granite is proposed which involves progressive depression of the floor of an initially horizontal chamber as it is filled by one or more vertical conduits. A crustal-scale balance in the rates of melt extraction, magma ascent and pluton-filling is required by the model, and transfer of material from the source to the pluton is accommodated by broadly distributed deformation of low strain magnitude. The process is evaluated with end-member cantilever and piston sinking mechanisms. The models predict that large (10-100 km wide), tabular plutons (less than or similar to 3 km thick) can be emplaced quickly (100 a to 1 Ma) with floor-depression and related wall-rock strain rates similar those expected during tectonic deformation (10(-10) to 10(-15) s(-1)). Bulk strains in the intervening crustal column rarely exceed a strain ratio of 1.5, which is likely to remain undetected in the geological record unless the required deformation is accommodated on discrete structures such as normal faults or shear zones at the base of the pluton.
Gilbert (1877) proposed that the level of emplacement of laccoliths is controlled by the density contrast between rising magma and the weighted mean density of the overburden. For felsic laccoliths, his hypothesis is strongly supported by gravity surveys of a number of laccolith groups. Epizonal felsic laccoliths are consistently found to have zero density contrast with the host rocks. Constraining the emplacement level provides a basis for analysis of the growth of laccoliths.
Mechanical analysis suggests that the diverse shapes of laccolithic intrusions observed in the field can be represented by a continuous series of intrusion modes between two distinct end members. The simplest end member is an epizonal intrusion formed by a single sill that acts mechanically as a vertical punch. Punched laccoliths are characterized by flat tops, peripheral faults, and steep or vertical sides.
The other end member results from the intrusion of multiple sills stacked vertically in a fashion suggestive of a Christmas tree. The multiple-level loading results in plastic deformation of the country rock. Christmas-tree laccoliths lack peripheral faults and have a characteristic rounded dome appearance on the surface. The floor of these laccoliths may, or may not, sag. Gilbert’s (1877) ideal laccolith falls between these two end members.
The end members of the laccolith growth series are treated as boundary value problems in continuum mechanics. Geometrically and materially nonlinear finite element analysis is used to solve the boundary value problems. Field observation, a physical model, and the theoretical models provide convergent answers to the mechanical analysis of the growth of laccoliths.
As a check on the theoretical models, a gazetteer of the dimensions and locations of approximately 900 laccoliths is included. Of these, approximately 600 are located in the United States. If North America represents a statistically valid sample, then there must be between 5,000 and 10,000 laccoliths around the world.
Monazite and sphene, separated from sillimanite-grade schists, gneisses and two-mica granites in the Central Maine Terrane and Merrimack Trough, have been dated using the U-Pb system. The ages constrain the timing of peak high-grade metamorphism and plutonism in northern New England. In the Central Maine Terrane metamorphic ages are Acadian (early Devonian); in the Merrimack Trough, across the Campbell Hill-Nonesuch River-Norumbega fault zone, the age of metamorphism is Alleghenian(?) (Permian). The granite ages outline a distinct pulse of Devonian magmatism, characteristic of the Central Maine Terrane. The high-grade terrane of this part of the Appalachian Orogen is composite. It is made up of crustal blocks that experienced discrete pulses of high-grade metamorphism beginning perhaps as long ago as the pre-Middle Ordovician and extending into the Permian. Ages of peak metamorphism support the hypothesis that the Central Maine and Merrimack teiranes had different tectono-metamorphic histories and are coincidentally juxtaposed at the same metamorphic grade. The Campbell Hill-Nonesuch River-Norumbega fault zone has had an active and complex history, beginning approximately 360 Ma and lasting at least to 250 Ma. This boundary is a likely candidate for the western Alleghenian (Variscan) Front in New England. The final juxtaposition of the Central Maine and Merrimack terranes may have occurred during the Mesozoic along extensional, terrane-bounding faults, possibly the reactivated Acadian compressional and/or Alleghenian transpressional structures. RÉSUMÉ L'application de la systènatique U-Pb a permis la datation de la monazite et du sphène extraits de schistes métamorphisés au grade de la sillimanite, de gneiss et de granites à deux micas provenant de la Lanière de Central Maine et de la Fosse de Merrimack. Les èges ainsi obtenus imposent des contraintes précises sur l’époque du paroxysme métamorphique et du plutonisme en Nouvelle-Angleterre septentrionale. Dans la Lanière de Central Maine, le métamorphisme est d'âge acadien (éodévonien). Dans la Fosse de Merrimack, d'un côté à l’autre de la zone de failles de Campbell Hill-Nonesuch River-Norumbega, le métamorphisme date de l’Alléghanien(?) (Permien). Les âges des granites mettent en évidence un épisode distinct de magmaiisme dévonien qui caractérise la Lanière de Central Maine. Dans cette portion de l'Orogène appalachien, la lanière présentant un haut degré de métamoiphisme est composite. Elle comprend des blocs crustaux ayant subi des épisodes distincts de métamorphisme de haut degré qui débutent possiblement à l’Ordovicien moyen pour s'étaler jusqu'au Permien. Les âges du paroxysme métamorphique confortent l’hypothèse voulanique les lanières de Central Maine et de Merrimack aienl eudes histoires techono-métamorphiques différentes et que leur juxtaposition à un même grade métamorphique ne soit que coincidence. L'hisloire de la zone de failles de Campbell Hill-Nonesuch River-Norumbega est active, complexe et s'échelonne d'environ 360 Ma jusqu'à au moins 250 Ma. Cette frontière formé un candidal plausible pour le Front alléhanien (varisque) occidental en Nouvelle-angleterre. La juxtaposition finale des lanières de Central Maine et de Merrimack s'effectua peut-être au Mésozoique le long de failles en extension bordant les lanières, possiblement par réactivation des accidents compressifs acadiens et/ou des structures transpressives alléghaniennes. [Traduit par le journal]
Coupled thermal-mechanical models of convergent orogens offer a novel way to investigate the interactions between heat and tectonics that lead to regional metamorphism. In this study, the effects of different distributions of heat-producing material in the crust and upper mantle on crustal thermal histories and deformation fields are investigated. The models involve subduction-driven collision with moderate convergence and erosion rates. For models involving standard continental crust, where heat production is initially concentrated in the upper crust, P-T-t paths do not intersect the held of typical Barrovian P-T conditions. However, heat-producing material can be tectonically redistributed, for example, by subduction of crustal rocks to upper mantle depths, or by formation of thick accretionary wedges or continental margin sequences during convergence. Models that include a wedge of heat-producing material in the upper mantle generate high temperatures in the lower crust and upper mantle that lead to a change in orogenic style; radioactive heating of partially subducted crustal material on time scales of 10-30 Ma yields temperatures high enough for partial melting. However, crustal P-T-t paths are unlikely to intersect the Barrovian field unless erosion or convergence rates change. Models that include a crustal-scale region with moderate, uniform heat production, simulating a large accretionary wedge or tectonically thickened continental margin sequence, generate P-T-t paths that intersect the Barrovian field. However, as convergence proceeds, the heat-producing region is deformed, eroded, and reduced in volume, so that the model orogen begins to cool down after about 20 Ma. The model results provide an explanation for many first-order tectonic and metamorphic features of small orogens, including metamorphic styles ranging from blueschists to the Barrovian series to granulites, late-orogenic granitoid magmatism, and the crustal-scale tectonic features associated with regional metamorphic belts. We conclude that the thermal state of an orogen is controlled by the evolving competition between cooling by subduction and radioactive heating within the deforming orogen.
A theoretical framework is developed for defining the ascent of a magmatic mass in terms of the dynamics of drag, heat transfer and other factors controlling the velocity of ascent and exerting an influence on the emplacement process. -M.S.
Field, petrologic, and isotopic data suggest that pervasive fluid flow associated with prograde mineral reactions in Maine and Vermont was mostly parallel to lithologic layering, subhorizontal, and in the direction of increasing temperature. Calculated amounts of fluid flow are similar in both areas and lie in the following ranges: pelitic schists, (1-26)×105 cm3 fluid/cm2 rock; micaceous sandstones, (1-42)×105 cm3/cm2; micaceous limestones, (0.02-24)×105 cm3/cm2. Substantial reactive fluid flow appears to be a fundamental aspect of regional metamorphism to depths of at least 30 km in northern New England at grades ranging from those of the biotite zone to those of the sillimanite zone. Although flow was not excluded from any particular rock type, geometry was greatly controlled at the outcrop scale by enhanced flow in more permeable layers and restricted flow in other layers. -from Author
Granitic magmas commonly ascend tens of kilometres from their source terranes to upper crustal emplacement levels, or to the Earth's surface. Apart from its obvious bearing on the interpretation of the geology and geochemistry of felsic igneous rocks, the magma ascent mechanism critically affects any modelling of the metamorphic evolution of the upper lithosphere as well as its rheology. We propose that, in general, granitic magmas ascend via propagating fractures, as dykes, in extremely short time periods. Long-distance diapiric transport of granitoid magmas, through crustal sections, is not viable on thermal or mechanical grounds, and there is an apparent total absence of field evidence for diapiric rise of such magmas, even in supposed “type” localities. Neither the shapes nor internal or external characteristics of granitic plutons necessarily reveal anything about the transport of their precursor magmas; these are purely arrival phenomena dictated by local structure, kinematics, and stress states. Based on existing numerical treatments of the problem, we show that granitic magmas are apparently sufficiently inviscid to travel through fractures, to high crustal levels, without suffering thermal death by freezing. For example, a 2000-km3 granitic batholith can be inflated by a 1 km × 3m × 20 km-deep dyke system in less than 900 years. The model proposed has numerous, far-reaching, petrological and rheological consequences, some of which are outlined.
The Early Proterozoic (1715 Ma) Harney Peak Granite (Black Hills, SD, U.S.A.) is a complex of hundreds of dykes and sills. Earlier studies of Nd, O and Pb isotope variations demonstrated that the complex was not derived from a single source, or even different sources of a single age. Instead, the granites can be divided into a group with sources probably dominated by Early Proterozoic sediments and a group with sources probably dominated by Archean sediments. New results on the Nd isotopic variations of many additional samples indicate that there is considerable overlap between Nd isotopic compositions within the complex. Values of εNd (1715 Ma) of the Harney Peak Granite suite (n = 20) range from −2·0, indicating an Early Proterozoic (2300-2200 Ma) crustal source, to −13·4, indicating a Middle to Late Archean (3200-3100 Ma) protolith. These results suggest that the Early Proterozoic source may have included rocks such as the c. 2200-1900 Ma metasedimentary rocks that occur in the southern Black Hills. The Archean sources might have included rocks such as those exposed on the periphery of the Black Hills. The range in Nd model ages negates the usefulness of the concept of the ‘average’ age of the crust in this part of the craton. Because such heterogeneity is present in the magmatic compositions of the Harney Peak Granite, it can be inferred that at least as much heterogeneity was present in the sources. In this granite system, melts were evidently derived from isolated, heterogeneous zones and did not have the opportunity to coalesce into large magma bodies. In systems where coalescence does occur, the evidence for such highly heterogeneous sources may be lost. These results emphasise that inferences drawn from a few samples of plutonic rocks in which magma mixing and homogenisation occurred can lead to erroneous conclusions about the age and nature of protoliths and, consequently, the development of continental crust.
Buddington (1959) pointed out that the construction of large crustal magma chambers involves complex internal processes as well as multiple country rock material transfer processes (MTPs), which reflect large horizontal, vertical and temporal gradients in physical conditions. Thus, we have attempted to determine the relative importance of different magmatic and country rock MTPs at various crustal depths, and whether country rock MTPs largely transport material vertically or horizontally, rather than seeking a single model of magma ascent and emplacement.
Partially preserved roofs of nine plutons and in some cases roof–wall transitions with roof emplacement depths of 1·5–11 km were mapped. During emplacement, these roofs were not deformed in a ductile manner, detached or extended by faults, or significantly uplifted. Instead, sharp, irregular, discordant contacts are the rule with stoped blocks often preserved immediately below the roof, even at depths of 10 km. The upper portions of these magma chambers are varied, sometimes preserving the crests of more evolved magmas or local zones of volatile-rich phases and complex zones of dyking and magma mingling. Magmatic structures near roofs display a wide variety of patterns and generally formed after emplacement. Transitions from gently dipping roofs to steep walls are abrupt. At shallow crustal levels, steep wall contacts have sharp, discordant, stepped patterns with locally preserved stoped blocks indicating that the chamber grew sideways in part by stoping. Around deeper plutons, an abrupt transition (sometimes within hundreds of metres) occurs in the country rock from discordant, brittle roofs to moderately concordant, walls deformed in a ductile manner defining narrow structural aureoles. Brittle or ductile faults are not present at roof–wall joins.
Near steep wall contacts at shallow to mid-crustal depths (5–15 km), vertical and horizontal deflections of pre-emplacement markers (e.g. bedding, faults, dykes), and ductile strains in narrow aureoles (0·1–0·3 body radii) give a complete range of bulk strain values that account for 0–100% of the needed space, but average around 30%, or less, particularly for larger batholiths. A lack of far-field deflection of these same markers rules out significant horizontal displacement outside the aureoles and requires that any near-field lateral shortening is accommodated by vertical flow. Lateral variations from ductile (inner aureole) to brittle (outer aureole) MTPs are typically observed. Compositional zoning is widespread within these magma bodies and is thought to represent separately evolved pulses that travelled up the same magma plumbing system. Magmatic foliations and lineations commonly cross-cut contacts between pulses and reflect the strain caused either by the late flow of melt or regional deformation.
Country rocks near the few examined mid- to deep crustal walls (10–30 km) are extensively deformed, with both discordant and concordant contacts present; however, the distinction between regional and emplacement-related deformation is less clear than for shallower plutons. Internal sheeting is more common, although elliptical masses are present. Lateral compositional variations are as large as vertical variations at shallower depths and occur over shorter distances. Magmatic foliations and lineations often reflect regional deformation rather than emplacement processes.
The lack of evidence for horizontal displacement outside the narrow, shallow to mid-crustal aureoles and the lack of lateral or upwards displacement of pluton roofs indicate that during emplacement most country rock is transported downwards in the region now occupied by the magma body and its aureole. The internal sheeting and zoning indicate that during the downwards flow of country rock, multiple pulses of magma travelled up the same magma system. If these relationships are widespread in arcs, magma emplacement is the driving mechanism for a huge crustal-scale exchange process.
Diapirism has been discredited as a transport mechanism for magmas partly because diapirs seem to be unable to bring magmas to shallow crustal levels (<10km) and partly because recent developments in the theory of dyke propagation have shown that sufficiently wide dykes are able to efficiently transport felsic magmas through the crust. However, it is still unclear how felsic dykes grow to widths that allow them to propagate faster than they close by magma freezing. Ultimately, it may be the ability of felsic dykes to grow within the source that controls which mechanism dominates ascent.
The ability of dykes to propagate, from the top of rising diapirs depends among other factors on the changing temperature gradient of the wall rocks. The steep gradient around rapidly rising diapirs in the low viscosity lower crust will cause dykes to freeze. As diapirs rise to colder stiffer crust and decelerate, heat diffuses further from the diapir, resulting in shallower temperature gradients that favour dyke propagation. The mechanism may thus swap, during ascent, from diapirism to dyking. Calculations of the thermal evolution of diapirs and their surroundings show that basaltic diapirs may never form because they would be drained by dykes at a very early stage; felsic diapirs may be unable to give rise to successful dykes, whereas diapirs of intermediate magmas may propagate dykes during ascent.
To form a granite pluton, the felsic melt produced by partial melting of the middle and lower continental crust must separate from its source and residuum. This can happen in three ways: (1) simple melt segregation, where only the melt fraction moves; (2) magma mobility, in which all the melt and residuum move together; and (3) magma mobility with melt segregation, in which the melt and residuum move together as a magma, but become separated during flow. The first mechanism applies to metatexite migmatites and the other two to diatexite migmatites, but the primary driving forces for each are deviatoric stresses related to regional-scale deformation. Neither of the first two mechanisms generates parental granite magmas. In the first mechanism segregation is so effective that the resulting magmas are too depleted in FeOT, MgO, Rb, Zr, Th and the REEs, and in the second no segregation occurs. Only the third mechanism produces magmas with compositions comparable with parental granites, and occurs at a large enough scale in the highest grade parts of migmatite terranes, to be considered representative of the segregation processes occurring in the source regions of granites.
There is 'too much' water in a variety of crustal environments,
including sedimentary basins and regional metamorphic belts. Regional
and contact metamorphic rocks have time-integrated fluxes of 10(exp 6)
cu cm water/sq cm for pervasive flow and up to 10(exp 9) cu cm water/sq
cm in fracture flow, amounts far greater than can be supplied by
dehydration of immediately subjacent rocks. The types of evidence
supporting high fluxes are so numerous and varied, including petralogic,
mineralogic, textural, microstructural, and stable isotopic data, that
it is unlikely that all of them could have been interpreted erroneously.
Thus the question arises as to what mechanisms of permeability
enhancement might operate to facilitate large fluid fluxes. Field
evidence suggests the operation of reaction enhancement of permeability,
grain-scale dilatancy, and hydraulic fracturing during metamorphism. All
these mechanisms are inherently episodic and transient. The transport
properties of dynamic permeability enhancement are poorly known in
relation to the thermal and deformational cycles of metamorphism.
Davis cites four areas of low-angle faulting in which he believes that high fluid pressures can have played no important part in the development and movement of the thrust plates, but it seems to us that the concept or some variant of it may help to explain the observed field relationships in the three out of these four areas with which we have had some first-hand experience. In the areas of the Heart Mountain thrust of Wyoming, the Muddy Mountain thrust of Nevada, and the structurally higher, crystalline thrust sheets of the Swiss Alps, field relationships which include evidence of dehydration reactions during metamorphism of evaporites and of pelitic rocks suggest that interstitial fluid pressures may have been high and thus have played an essential part in the development of the thrust faults. In the fourth area cited by Davis, that of his own studies in the Klamath Mountains of California, we have had no first-hand experience and thus are not competent to answer his criticisms.