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Back to the future: Testing different scenarios for the next supercontinent gathering
Abstract
The theory of plate tectonics and the discovery of large scale, deep-time cycles, such as the Supercontinent cycle and Wilson cycle, has contributed to the identification of several supercontinents in Earth's history. Using the rules of plate tectonic theory, and the dynamics of subduction zones and mantle convection, it is possible to envisage scenarios for the formation of the next supercontinent, which is believed to occur around 200–300 Ma into the future. Here, we explore the four main proposed scenarios for the formation of the next supercontinent by constructing them, using GPlates, in a novel and standardised way. Each scenario undergoes different modes of Wilson and Supercontinent cycles (i.e., introversion, extroversion, orthoversion, and combination), illustrating that the relationship between them is not trivial and suggesting that these modes should be treated as end-members of a spectrum of possibilities. While modelling the future has limitations and assumptions, the construction of the four future supercontinents here has led to new insights into the mechanisms behind Wilson and Supercontinent cycles. For example, their relationship can be complex (in terms of being of the same or different order, or being in or out of phase with each other) and the different ways they can interact may led to different outcomes of large-scale mantle reorganization. This work, when combined with geodynamical reconstructions since the Mesozoic allows the simulation of the entire present-day Supercontinent cycle and the respectively involved Wilson cycles. This work has the potential to be used as the background for a number of studies, it was just recently used in tidal modelling experiments to test the existence of a Supertidal cycle associated with the Supercontinent cycle.
Supplementary resource (1)
... The continents have coalesced into supercontinents and then dispersed several times in Earth's history in a process known as the supercontinent cycle (Nance et al., 1988). While the cycle has an irregular period (Bradley, 2011), the breakup and reformation typically occurs over 500 -600 Ma (Nance et al., 2013;Davies et al., 2018;Yoshida and Santosh, 2017;2018). Pangea was the latest supercontinent to exist on Earth, forming ~300 Ma ago, and breaking up around 180 Ma ago, thus initiating the current supercontinent cycle (Scotese, 1991;Golonka, 2007). ...
... Pangea was the latest supercontinent to exist on Earth, forming ~300 Ma ago, and breaking up around 180 Ma ago, thus initiating the current supercontinent cycle (Scotese, 1991;Golonka, 2007). Another supercontinent should therefore form 25 within the next 200 -300 Ma (e.g., Scotese 2003;Yoshida, 2016;Yoshida andSantosh, 2011 and2017;Duarte et al., 2018;Davies et al., 2018). ...
... The life cycle of each ocean basin is known as the Wilson cycle. A supercontinent cycle may comprise more than one Wilson cycle since several oceans may open and close between the breakup and reformation of a supercontinent (e.g., Hatton, 1997;Murphy and Nance, 2003;Burke, 2011;Duarte et al., 2018;Davies et al., 2018). As ocean basins evolve during the progression of the Wilson cycle (and associated supercontinent cycle), the energetics of the tides within the basins also change (Kagan, 1997;Green et al., 2017). ...
Abstract. The Earth is currently 180 Ma into a supercontinent cycle that began with the breakup of Pangea, and will end in around 200–250 Ma (Mega-annum) in the future, as the next supercontinent forms. As the continents move around the planet, they change the geometry of ocean basins, and thereby modify their resonant properties. In doing so oceans move through tidal resonance, causing the global tides to be profoundly affected. Here, we use a dedicated and established global tidal model to simulate the evolution of tides during four future supercontinent scenarios. We show that the number of tidal resonances on Earth vary between 1 and 5 in a supercontinent cycle, and that they last for no longer than 20 Ma. They occur in opening basins after about 140–180 Ma, an age equivalent to the Present-Day Atlantic Ocean, which is near resonance for the dominating semi-diurnal tide. They also occur when an ocean basin is closing, highlighting that in its lifetime, a large ocean basin – its history described by the Wilson cycle – may go through two resonances: one when opening and one when closing. The results further support the existence of a super-tidal cycle associated with the supercontinent cycle, and gives a deep-time proxy for global tidal energetics.
... We refer to this simulation as 'control' in the following. The assumption for this choice of depths is that the period of study is at a similar point in the supercontinent cycle as PD, so the age of the oceanic plates would be comparable between the Devonian and PD [58,62]. This means that the mean depths of the abyssal plain and continental shelfs should be similar for both; this underpins our control bathymetry set (see figure 1 for the 420 Ma and 400 Ma control bathymetries). ...
... The earlier time slices for our period of study (420-400 Ma) and PD are believed to be at roughly similar central points in their respective super-continental cycles [58,62], whereas the 380 Ma slice is closer to the formation of a supercontinent (Pangea in this case) than we currently are [62]. This central position in the cycle is associated with multiple ocean basins, and thus an increased chance of ocean resonances in one or multiple basins which would lead to the tides becoming more energetic [65]. ...
... The earlier time slices for our period of study (420-400 Ma) and PD are believed to be at roughly similar central points in their respective super-continental cycles [58,62], whereas the 380 Ma slice is closer to the formation of a supercontinent (Pangea in this case) than we currently are [62]. This central position in the cycle is associated with multiple ocean basins, and thus an increased chance of ocean resonances in one or multiple basins which would lead to the tides becoming more energetic [65]. ...
Tides are a major component of the interaction between the marine and terrestrial environments, and thus play an important part in shaping the environmental context for the evolution of shallow marine and coastal organisms. Here, we use a dedicated tidal model and palaeogeographic reconstructions from the Late Silurian to early Late Devonian (420 Ma, 400 Ma and 380 Ma, Ma = millions of years ago) to explore the potential significance of tides for the evolution of osteichthyans (bony fish) and tetrapods (land vertebrates). The earliest members of the osteichthyan crown-group date to the Late Silurian, approximately 425 Ma, while the earliest evidence for tetrapods is provided by trackways from the Middle Devonian, dated to approximately 393 Ma, and the oldest tetrapod body fossils are Late Devonian, approximately 373 Ma. Large tidal ranges could have fostered both the evolution of air-breathing organs in osteichthyans to facilitate breathing in oxygen-depleted tidal pools, and the development of weight-bearing tetrapod limbs to aid navigation within the intertidal zones. We find that tidal ranges over 4 m were present around areas of evolutionary significance for the origin of osteichthyans and the fish-tetrapod transition, highlighting the possible importance of tidal dynamics as a driver for these evolutionary processes.
... Therefore, it is possible that they also record the collision of accretionary orogens during the amalgamation of Rodinia. Wright et al. (2013) and 0.0 to +0.3 Ga forward models were created approximating the scissor-like closure of the extra-Nuna Ocean and the configuration of Novopangea after Davies et al. (2018). Spencer et al. (2017) The development of peripheral subduction systems along the margins of supercontinents is well documented (Cawood and Buchan, 2007;Li and Zhong, 2009;Murphy and Nance, 1991) and has been attributed as a necessary driver for supercontinent breakup (Dal Zilio et al., 2018;Heron and Lowman, 2011). ...
... The two opposed accretionary orogens that were once along strike along the Gondwanan and Pangean margins (Nelson and Cottle, 2018) are presently located on opposite sides of the intervening Pacific ocean basin in a dual subduction tectonic regime ( Figure 9B). One of the results of forward modeling this scenario 300 Ma into the future, yields the assembly of a supercontinent similar to the formation Rodinia (Figure 9), the proposed supercontinent Novopangea (Davies et al., 2018;Duarte et al., 2016). ...
Long-lived (800 Ma) Paleo– to Mesoproterozoic accretionary orogens on the margins of Laurentia, Baltica, Amazonia, and Kalahari collided to form the core of the supercontinent, Rodinia. Accretionary orogens in Laurentia and Baltica record predominately radiogenic zircon εHf(t) and whole-rock Pb isotopic compositions, short crustal residence times (ca. 0.5 Ga), and the development of arc-backarc complexes. The accretionary orogenic record of Laurentia and Baltica is consistent with a retreating accretionary orogen and analogous to the Phanerozoic western Pacific orogenic system. In contrast, the Mesoproterozoic orogens of Amazon and Kalahari cratons record unradiogenic zircon εHf(t) values, ca. 0.8 Ga crustal residence times, and more ancient whole-rock Pb isotopic signatures. The accretionary orogenic record of Amazonia and Kalahari indicates the preferential incorporation of cratonic material in continental arcs of advancing accretionary orogens comparable to the Phanerozoic eastern Pacific orogenic system. Based on similarities in the geodynamic evolution of the Phanerozoic circum-Pacific orogens peripheral to Gondwana/Pangea, we suggest that the Mesoproterozoic accretionary orogens formed as peripheral subduction zones along the margin of the supercontinent Nuna (ca. 1.8–1.6 Ga). The eventual collapse of this peripheral subduction zone onto itself and closure of the external ocean around Nuna to form Rodinia is equivalent to the projected future collapse of the circum-Pacific subduction system and juxtaposition of Australia-Asia with South America. The juxtaposition of advancing and retreating accretionary orogens at the core of the supercontinent Rodinia demonstrates that supercontinent assembly can occur by the closure of external oceans and indicates that future closure of the Pacific Ocean is plausible.
... Present day validation. The core model set-up used here is the same as in other deep-time tidal simulations 17,48 , and it is briefly described here. A present day control simulation 17 gives a root-mean-square error of about 11 cm for the M2 tidal amplitudes when compared to the data in TPXO8 (http://www.tpxo.net). ...
The severe "Snowball Earth" glaciations proposed to have existed during the Cryogenian period (720 to 635 million years ago) coincided with the breakup of one supercontinent and assembly of another. Whereas the presence of extensive continental ice sheets predicts a tidally energetic Snowball ocean due to the reduced ocean depth, the supercontinent palaeogeography predicts weak tides because the surrounding ocean is too large to host tidal resonances. Here we show, using an established numerical global tidal model and paleo-geographic reconstructions, that the Cryogenian ocean hosted diminished tidal amplitudes and associated energy dissipation rates, reaching 10-50% of today's rates, during the Snowball glaciations. We argue that the near-absence of Cryogenian tidal processes may have been one contributor to the prolonged glaciations if these were near-global. These results also constrain lunar distance and orbital evolution throughout the Cryogenian, and highlight that simulations of past oceans should include explicit tidally driven mixing processes.
... Pangea is only the most recent such a supercontinent (Fig. 4), existing between about 350 and 200 Ma. However, geologic reconstructions indicate supercontinents during approximately 900-750 and 1550-1400 Ma, which have been named Rodinia and Nuna, respectively (Li et al., 2019), and geoscientists have predicted various future supercontinents based on different extrapolations of current tectonic motions (Davies et al., 2018). Such cycles of supercontinent assembly and dispersal, which are often referred to as "Wilson Cycles," are governed by changes to the distribution of heat within the Earth's mantle (Rolf et al., 2012), with supercontinents trapping heat beneath them until this heat is released by onset of ridge spreading within the supercontinent. ...
... Life on Earth will last for a very long time, with conditions supporting complex multicellular life and eukaryotes pre-dicted to continue for 0.8-1.2 billion years [1]. In 200-250 million years, tectonic plates will have rearranged landmasses into new supercontinents, fundamentally disrupting existing biogeographic patterns [9]. Thus, multicellular life will continue for hundreds of millions of years after humans -longer than all of prehistory [1]. ...
Conditions capable of supporting multicellular life are predicted to continue for another billion years, but humans will inevitably become extinct within several million years. We explore the paradox of a habitable planet devoid of people, and consider how to prioritise our actions to maximise life after we are gone.
We explore two possible Earth climate scenarios, 200 and 250 million years into the future, using projections of the evolution of plate tectonics, solar luminosity, and rotation rate. In one scenario, a supercontinent forms at low latitudes, whereas in the other it forms at high northern latitudes with an Antarctic subcontinent remaining at the south pole. The climates between these two end points are quite stark, with differences in mean surface temperatures approaching several degrees. The main factor in these differences is related to the topographic height of the high latitude supercontinents where higher elevations promote snowfall and subsequent higher planetary albedos. These results demonstrate the need to consider multiple boundary conditions when simulating Earth-like exoplanetary climates.
The Earth is currently 180 Myr into a supercontinent
cycle that began with the break-up of Pangaea and which will end around 200–250 Myr (million years) in the future, as the next supercontinent forms. As
the continents move around the planet they change the geometry of ocean
basins, and thereby modify their resonant properties. In doing so, oceans
move through tidal resonance, causing the global tides to be profoundly
affected. Here, we use a dedicated and established global tidal model to
simulate the evolution of tides during four future supercontinent scenarios.
We show that the number of tidal resonances on Earth varies between one and five in
a supercontinent cycle and that they last for no longer than 20 Myr. They
occur in opening basins after about 140–180 Myr, an age equivalent to the
present-day Atlantic Ocean, which is near resonance for the dominating
semi-diurnal tide. They also occur when an ocean basin is closing,
highlighting that within its lifetime, a large ocean basin – its history
described by the Wilson cycle – may go through two resonances: one when
opening and one when closing. The results further support the existence of a
super-tidal cycle associated with the supercontinent cycle and gives a
deep-time proxy for global tidal energetics.
Three‐dimensional spherical mantle convection was simulated to predict future continental motion and investigate the driving force of continental motion. Results show that both the time required (≥ 300 million years from the present) and the process for the next supercontinent formation are sensitive to the choice of critical rheological parameters for mantle dynamics, such as a viscosity contrast between the upper and lower mantles and a yield strength of the lithosphere. From all the numerical models studied herein, mantle drag force by horizontal mantle flow beneath the continents may mostly act as a resistance force for the continental motion in the process of forming a new supercontinent. The maximum absolute magnitude of the tensional and compressional stress acting at the base of the moving continents is in the order of 100 MPa, which is comparable to a typical value of the slab pull force.
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Abstract:
Detailed global plate motion models that provide a continuous description of plate boundaries through time are an effective tool for exploring processes both at and below the Earth’s surface. A new generation of numerical models of mantle dynamics pre- and post-Pangea timeframes requires global kinematic descriptions with full plate reconstructions extending into the Paleozoic (410 Ma). Current plate models that cover Paleozoic times are characterised by large plate speeds and trench migration rates because they assume that lowermost mantle structures are rigid and fixed through time. When used as a surface boundary constraint in geodynamic models, these plate reconstructions do not accurately reproduce the present-day structure of the lowermost mantle. Building upon previous work, we present a global plate motion model with continuously closing plate boundaries ranging from the early Devonian at 410 Ma to present day.
We analyse the model in terms of surface kinematics and predicted lower mantle structure. The magnitude of global plate speeds has been greatly reduced in our reconstruction by modifying the evolution of the synthetic Panthalassa oceanic plates, implementing a Paleozoic reference frame independent of any geodynamic assumptions, and implementing revised models for the Paleozoic evolution of North and South China and the closure of the Rheic Ocean. Paleozoic (410-250 Ma) RMS plate speeds are on average ~8 cm/yr, which is comparable to Mesozoic-Cenozoic rates of ~6 cm/yr on average. Paleozoic global median values of trench migration trend from higher speeds (~2.5 cm/yr) in the late Devonian to rates closer to 0 cm/yr at the end of the Permian (~250 Ma), and during the Mesozoic-Cenozoic (250-0 Ma) generally cluster tightly around ~1.1 cm/yr. Plate motions are best constrained over the past 130 Myr and calculations of global trench convergence rates over this period indicate median rates range between 3.2-12.4 cm/yr with a present day median rate estimated at ~5 cm/yr. For Paleozoic times (410-251 Ma) our model results in median convergence rates largely ~5 cm/yr. Globally, ~90% of subduction zones modelled in our reconstruction are determined to be in a convergent regime for the period 120-0 Ma. Over the full span of the model, from 410-0 Ma, ~93% of subduction zones are calculated to be convergent, and at least 85% of subduction zones are converging for 97% of modelled times. Our changes improve global plate and trench kinematics since the late Paleozoic and our reconstructions of the lowermost mantle structure challenge the proposed fixity of lower mantle structures, suggesting that the eastern margin of the African LLSVP margin has moved by as much as ~1450 km since late Permian times (260 Ma). The model of the plate-mantle system we present suggests that during the Permian Period, South China was proximal to the eastern margin of the African LLSVP and not the western margin of the Pacific LLSVP as previously thought.
The central Indian Ocean is considered the archetypal diffuse oceanic plate boundary. Data from seismic stratigraphy and deep-sea drilling indicate that the contractional deformation of the Indian Ocean lithosphere commenced at 15.4–13.9 Ma, but experienced a sharp increase at 8–7.5 Ma. This has been maintained through to the present day, with over 80% of the shortening accrued over the past 8 Myr. Here we build on previous efforts to refine the form, timing and magnitude of the regional plate-motion changes by mitigating the noise in reconstructed Indian and Capricorn plate motions relative to Somalia. Our noise-mitigated reconstructions tightly constrain the significant speed up of the Capricorn plate over the past 8 Myr and demonstrate that the history of the Indian Ocean floor deformation cannot be explained without this plate-motion change. We propose that the Capricorn plate-motion change is driven by an increase in the eastward-directed asthenospheric flow associated with the adjacent Reunion plume, and quantitatively demonstrate the viability of this hypothesis. Our inference is supported by volcanic age distributions along the Reunion hotspot track, the anomalously high residual bathymetry of the Central Indian Ridge, full-waveform seismic tomography of the underlying asthenosphere and geochemical observations from the Central Indian Ridge. These findings challenge the commonly accepted link between the deformation of the Indian Ocean floor and the Tibetan Plateau’s orogenic evolution and demonstrate that temporal variations in upwelling mantle flow can drive major tectonic events at the Earth’s surface.
Earth is 180 Myr into the current Supercontinent cycle and the next Supercontinent is predicted to form in 250 Myr. The continuous changes in continental configuration can move the ocean between resonant states, and the semi‐diurnal tides are currently large compared to the past 252 Myr due to tidal resonance in the Atlantic. This leads to the hypothesis that there is a “super‐tidal” cycle linked to the Supercontinent cycle. Here, this is tested using new tectonic predictions for the next 250 Myr as bathymetry in a numerical tidal model. The simulations support the hypothesis: a new tidal resonance will appear 150 Myr from now, followed by a decreasing tide as the supercontinent forms 100 Myr later. This affects the dissipation of tidal energy in the oceans, with consequences for the evolution of the Earth‐Moon system, ocean circulation and climate, and implications for the ocean's capacity of hosting and evolving life.
Tectonic plates and plate boundaries migrate substantially through time and mantle plumes are generally accepted to be mobile within the convecting mantle, but it has been proposed that large low shear velocity provinces (LLSVPs) could have been fixed and rigid for as much as 540 million years (Myr). The hypotheses of fixed and rigid LLSVPs cannot be easily tested in the absence of constraints on the past location of lowermost mantle structures. We evaluated the hypothesis9of lower mantle thermochemical structure fixity with numerical experiments. As in earlier studies, we argue that the location of lower mantle thermochemical structures has changed through time.
Recent advances in three-dimensional numerical simulations of mantle convection have aided in approximately reproducing continental movement since the Pangea breakup at 200 Ma. These have also led to a better understanding of the thermal and mechanical coupling between mantle convection and surface plate motion and predictions of the configuration of the next supercontinent. The simulations of mantle convection from 200 Ma to the present reveals that the development of large-scale cold mantle downwellings in the North Tethys Ocean at the earlier stage of the Pangea breakup triggered the northward movement of the Indian subcontinent. The model of high temperature anomaly region beneath Pangea resulting from the thermal insulation effect support the breakup of Pangea in the real Earth time scale, as also suggested in previous geological and geodynamic models. However, considering the low radioactive heat generation rate of the depleted upper mantle, the high temperature anomaly region might have been generated by upwelling plumes with contribution of deep subducted TTG (tonalite-trondhjemite-granite) materials enriched in radiogenic elements. Integrating the numerical results of mantle convection from 200 Ma to the present, and from the present to the future, it is considered that the mantle drag force acting on the base of continents may be comparable to the slab pull force, which implies that convection in the shallower part of the mantle is strongly coupled with surface plate motion. © 2017 China University of Geosciences (Beijing) and Peking University.
Neoproterozoic tectonic geography was dominated by the formation of the supercontinent Rodinia, its break-up and the subsequent amalgamation of Gondwana. The Neoproterozoic was a tumultuous time of Earth's history, with large climatic variations, the emergence of complex life and a series of continent-building orogenies of a scale not repeated until the Cenozoic. Here we synthesise available geological and palaeomagnetic data and build the first full-plate, topological model of the Neoproterozoic that maps the evolution of the tectonic plate configurations during this time. Topological models trace evolving plate boundaries and facilitate the evaluation of " plate tectonic rules " such as subduction zone migration through time when building plate models. There is a rich history of subduction zone proxies preserved in the Neoproterozoic geological record, providing good evidence for the existence of continental and intra-oceanic subduction zones through time. These are preserved either as volcanic arc protoliths accreted in continent-continent, or continent-arc, collisions, or as the detritus of these volcanic arcs preserved in successor basins. Despite this, we find that the model presented here only predicts, on average, ~90% of the total length of subduction active today, suggesting that we have produced a conservative model and are likely underestimating the amount of subduction, either due to a simplification of tectonically complex areas, or because of the absence of preservation in the geological record (e.g. ocean-ocean convergence). Furthermore, the reconstruction of plate boundary geometries provides constraints for global-scale earth system parameters, such as the role of volcanism or ridge production on the planet’s icehouse climatic excursion during the Cryogenian. Besides modelling plate boundaries, our model presents some notable departures from previous Rodinia models. We omit India and South China from Rodinia completely, due to long-lived subduction preserved on margins of India and conflicting palaeomagnetic data for the Cryogenian, such that these two cratons act as ‘lonely wanderers’ for much of the Neoproterozoic. We also introduce a Tonian-Cryogenian aged rotation of the Congo-São Francisco Craton relative to Rodinia to better fit palaeomagnetic data and account for thick passive margin sediments along its southern margin during the Tonian. The GPlates files of the model are released to the public and it is our expectation that this model can act as a foundation for future model refinements, the testing of alternative models, as well as providing constraints for both geodynamic and palaeoclimate models.
Full text available from: http://www.nature.com/articles/ncomms14164
A unique structure in the Earth’s lowermost mantle, the Perm Anomaly, was recently identified beneath Eurasia. It seismologically resembles the large low-shear velocity provinces (LLSVPs) under Africa and the Pacific, but is much smaller. This challenges the current understanding of the evolution of the plate–mantle system in which plumes rise from the edges of the two LLSVPs, spatially fixed in time. New models of mantle flow over the last 230 million years reproduce the present-day structure of the lower mantle, and show a Perm-like anomaly. The anomaly formed in isolation within a closed subduction network ∼22,000 km in circumference prior to 150 million years ago before migrating ∼1,500 km westward at an average rate of 1 cm year−1, indicating a greater mobility of deep mantle structures than previously recognized. We hypothesize that the mobile Perm Anomaly could be linked to the Emeishan volcanics, in contrast to the previously proposed Siberian Traps.
Subduction initiation is a cornerstone in the edifice of plate tectonics. It marks the turning
point of the Earth’s Wilson cycles and ultimately the supercycles as well. In this paper, we explore the
consequences of subduction zone invasion in the Atlantic Ocean, following recent discoveries at the
SW Iberia margin.We discuss a buoyancy argument based on the premise that old oceanic lithosphere
is unstable for supporting large basins, implying that it must be removed in subduction zones. As a
consequence, we propose a new conceptual model in which both the Pacific and the Atlantic oceans
close simultaneously, leading to the termination of the present Earth’s supercycle and to the formation
of a new supercontinent, which we name Aurica. Our new conceptual model also provides insights
into supercontinent formation and destruction (supercycles) proposed for past geological times (e.g.
Pangaea, Rodinia, Columbia, Kenorland).
The paper presents the detailed plate tectonic, paleogeographic, paleoenvironment and paleolithofacies maps for eight Early Paleozoic time intervals. Forty maps, generated using PLATES and PALEOMAP programs, contain information about plate tectonics, paleoenvironment, and plaeolithofacies during Cambrian, Ordovician and Silurian. Disintegration of supercontinent Pannotia and origin of Gondwana, Laurentia and Baltica occur during Early Cambrian. Oceans spreading continued during Late Cambrian and Early Ordovician; vast platform flooded by shallow seas existed on the continents. The plate tectonic reorganization happened during Middle Ordovician. Silurian was a time of Caledonian orogeny, closing of Early Paleozoic oceans and origin of supercontinent Laurussia as a result of Laurentia and Baltica collision. Information contained within global and regional papers were posted on the maps and the detailed paleoenvironment and plaeolithofacies zones were distinguished within the platforms, basins and ridges.
Subduction of oceanic crust into the mantle results in the relatively young Mesozoic-Cenozoic age of the current oceanic basins, thus, hindering our knowledge of ancient oceanic lithospheres. Believed to be an exception, the eastern Mediterranean Sea (containing the Herodotus and Levant basins) preserves the southern margin of the Neotethyan, or older, ocean. An exceptionally thick sedimentary cover and a lack of accurate magnetic anomaly data have led to contradicting views about its crustal nature and age. Here I analyse total and vector magnetic anomaly data from the Herodotus Basin. I identify a long sequence of lineated magnetic anomalies, which imply that the crust is oceanic. I use the shape, or skewness, of these magnetic anomalies to constrain the timing of crustal formation and find that it formed about 340 million years ago. I suggest that this oceanic crust formed either along the Tethys spreading system, implying the Neotethys Ocean came into being earlier than previously thought, or during the amalgamation of the Pangaea Supercontinent. Finally, the transition from the rather weak and stretched continental crust found in the Levant Basin to the relatively strong oceanic Herodotus crust seems to guide the present-day seismicity pattern as well as the plate kinematic evolution of the region. © 2016 Macmillan Publishers Limited, part of Springer Nature. All rights reserved.
Tarim Basin is the large, very complex, oil-bearing basin in China, surrounded by the Tianshan–Beishan, West Kunlun and Altyn Tagh mountain belts to the north, south, and southeast, respectively. Understanding the processes and evolution of this complex superimposed basin, especially with respect to the effects of single tectonic movements, is a difficult challenge, which concerns the tectonic and dynamic interrelationships between the basin and the orogenic belts during the different stages of the Paleozoic in the Paleo-Asian and Tethyan tectonic systems and for the evaluation of the resource potentials in Tarim Basin. In this study, we focused on 3-dimensional, basin-scale structural architecture and the properties of two regional unconformities that occur within the basin and its adjacent areas. Here, we outline the structural deformations underlying the unconformity, the structural architecture styles and the distributions of the unconformity, and the stratigraphic sequence and nature of the sedimentary rocks immediately overlying the unconformity. During the late Early and Middle Paleozoic tectonic movements, disconformities developed mainly in the northern and the central parts of the basin, and angular unconformities which beneath layers were monocline and faulted-fold deformations developed in the southern, or the southern and northern parts of the basin, respectively. Before the Silurian, the Low Hotian Uplift, the NE-trending faulted-folded belts of the Tangguzibas Depression and the southern Tazhong Uplift underwent intense deformation related to SE–NW-directed tectonic compression. In addition, the NE-trending faults in the Tangguzibas Depression developed during periods of activity on the South Altyn Tagh Fault. The structural deformation, as well as the depositional facies, formed in response to the subduction and closure of the South Altun Ocean and West Kunlun–Kudi Ocean, and the resulting collisional orogeny. Prior to deposition of Upper Devonian sediments, structural deformation and erosion occurred in two marginal parts of the basin. The extent and intensity of deformation on the NE-trending faults in the Tangguzibas Depression were also reduced, whereas the NE-trending folds developed in the Manjiaer Depression. The Tabei Uplift experienced uplift and deformation. The closure of the North Altun Ocean and the eastern part of the South Tianshan Ocean with south-subduction may be the main driving forces for the tectonic activity. Extensive areas in the northern and southern margins of the basin were uplifted and denudated by orogenic activity as a prelude to the molasse basin that developed in the early Late Devonian in the northeastern and southeastern parts of the basin. The structural architecture of the unconformities reveals the geometry and dynamics of the basin–orogen system in single tectonic movements.
The tectonics, dynamics, and biogeographic landscape of the early Paleozoic were dominated by the opening and expansion of one large ocean-the Rheic-and the diminution to terminal closure of another-Iapetus. An understanding of the evolution of these oceans is thus central to an understanding of the early Paleozoic, but their chronicle also presents a rich temporal profile of the Wilson cycle, illustrating continental-scale rifting, microcontinent formation, ocean basin development, arc accretion, and continent-continent collision. Nevertheless, contemporary paleogeographic models of the Iapetus and Rheic oceans remain mostly schematic or spatiotemporally disjointed, which limits their utility and hinders their testing. Moreover, many of the important kinematic and dynamic aspects of the evolution of these oceans are impossible to unambiguously resolve from a conceptual perspective and the existing models unsurprisingly present a host of contradictory scenarios. With the specific aim to resolve some of the uncertainties in the evolution of this early Paleozoic domain, and a broader aim to instigate the application of quantitative kinematic models to the early Paleozoic, I present a new plate tectonic model for the Iapetus and Rheic oceans. The model has realistic tectonic plates, which include oceanic lithosphere, that are defined by explicit and rigorously managed plate boundaries, the nature and kinematics of which are derived from geological evidence and plate tectonic principles. Accompanying the presentation and discussion of the plate model, an extensive review of the underlying geological and paleogeographic data is also presented.
Structural analyses in the northern part of the North Patagonia Massif, in the foliated Caita Có granite and in La Seña and Pangaré mylonites, indicate that the pluton was intruded as a sheet-like body into an opening pull-apart structure during the Gondwana Orogeny. Geochronological studies in the massif indicate a first, lower to middle Permian stage of regional deformation, related to movements during indentation tectonics, with emplacement of foliated granites in the western and central areas of the North Patagonian Massif. Between the upper Permian and lower Triassic, evidence indicates emplacement of undeformed granitic bodies in the central part of the North Patagonian Massif. A second pulse of deformation between the middle and upper Triassic is related to the emplacement of the Caita Có granite, the development of mylonitic belts, and the opening of the Los Menucos Basin. During this pulse of deformation, compression direction was from the eastern quadrant.
The paleogeographic relationship between South China and Australia during the Ordovician is important for understanding the configuration of South China in Gondwana. However, high-quality Ordovician paleomagnetic results for the Yangtze Block are scarce. Here we report the results of a new paleomagnetic study of the Late Ordovician limestones of Wangcang County in the northern Yangtze Block, performed in order to constrain the paleoposition of South China. Two magnetic components were isolated by detailed stepwise thermal demagnetization. The low-temperature component falls close to the local current Earth's field direction. The site-mean direction obtained from the high-temperature component (HTC) carried by magnetite is D/I=132.6°/-35.2° (α95=3.6°) after bedding correction, yielding a paleomagnetic pole at 45.8°S, 191.3°E (dp=2.4°, dm=4.2°). The HTC direction passed reversal and fold tests, and its corresponding pole differs from the available paleomagnetic poles since the Silurian of the South China Block. These results suggest that the remanent magnetization was probably acquired during the earliest stage of sedimentation. The high-temperature component yields a paleolatitude of 19.5°S, implying that the Yangtze Block was at tropic latitudes during the Late Ordovician. These new and reliable paleomagnetic results bridge the Ordovician data gap, and favor the proximity between South China and Australia during the Late Ordovician.
RESUMEN. Trilobites devonicos de Buill, Chile (42°S). Se describe un trilobites calmoniido (Calmoniidae gen. nov. aff. Bainella Rennie, 1930) un trilobites lacopoide indeterminado y un coral zafrentido, recolectados en rodados de pizarras en Buill, Chile (42°24'S; 72°43'W), en los Andes de Chiloe. Esta faunula es consistente con una edad de sedimentacion devonica interior, para al menos parte del complejo acrecionario del sur de Chile, que aflora extensamente al oeste de Buill y que fue metamorfizado en el Paleozoico Superior a Mesozoico inferior. La presencia de una asociacion faunistica de Gondwana meridional, que puede ser comparada con faunas devonicas de Argentina, Bolivia, Islas Malvinas (Falkland) y Sudafrica, sugiere que esta area formaba parte de una extensa plataforma somera durante el Devonico.
Detailed mapping on the Leeward Antilles islands of Aruba, Curacao, Bonaire, and La Blanquilla has led to a reassessment of their stratigraphic, magmatic, and structural evolution. In general, each island preserves its own distinct sequence of geologic events. The Cretaceous geology of Aruba and Curacao consists of a mafic igneous complex, long interpreted to represent exposures of the Caribbean-Colombian Oceanic Plateau (CCOP), intruded by 89-86 Ma arc-related plutons and dikes. The rocks on both islands that are interpreted as remnants of the CCOP underwent a period of subaerial erosion in the Late Cretaceous, but subsequently their geologic histories diverge significantly in terms of their stratigraphic and structural evolution. Mapping on Bonaire has resulted in a major revision to the Cretaceous bedrock geology. Instead of a single stratigraphic unit (Washikemba Formation) the island contains two stratigraphic units separated by a northwest-trending fault. The southwest side of the fault consists of an arc-related Early to Late Cretaceous volcaniclastic section cut by shallow level intrusions, whereas the northeast side is composed of Early to Late Cretaceous epiclastic/hemipelagic strata that are locally cut by small arc-related mafic intrusions. La Blanquilla represents the southern-most exposure of the Aves Ridge which is a remnant arc separated from the modern arc of the Lesser Antilles by the Grenada back arc basin. The bedrock geology consists of two Late Cretaceous arc-related plutons. The geologic evolution of the Leeward Antilles when combined within a broader context of Caribbean tectonics leads us to a tectonic model involving three distinct arcs rather than a single "Great Arc" of the Caribbean as an explanation for the geodynamic evolution of the CCOP and its fringing arc system.
A multidisciplinary study (U-Pb sensitive high-resolution ion microprobe geochronology, Hf and O isotopes in zircon, Sr and Nd isotopes in whole-rocks, as well as major and trace element geochemistry) has been carried out on granitoid samples from the area west of Valcheta, North Patagonian Massif, Argentina. These confirm the Cambrian age of the Tardugno Granodiorite (528 ± 4 Ma) and the Late Permian age of granites in the central part of the Yaminué complex (250 Ma). Together with petrological and structural information for the area, we consider a previously suggested idea that the Cambrian and Ordovician granites of northeastern Patagonia represent continuation of the Pampean and Famatinian orogenic belts of the Sierras Pampeanas, respectively. Our interpretation does not support the hypothesis that Patagonia was accreted in Late Palaeozoic times as a far-travelled terrane, originating in the Central Transantarctic Mountains, and the arguments for and against this idea are reviewed. A parautochthonous origin is preferred with no major ocean closure between the North Patagonian Massif and the Sierra de la Ventana fold belt.
Chinese East Tianshan is a key area for understanding the Paleozoic accretion of the southern Central Asian Orogenic Belt. A first accretion-collision stage, before the Visean, developed the Eo-Tianshan range, which exhibits north-verging structures. The geodynamic evolution included: i) Ordovician-Early Devonian southward subduction of a Central Tianshan ocean beneath a Central Tianshan arc; ii) Devonian oceanic closure and collision between Central Tianshan arc and Yili-North Tianshan block, along the Central Tianshan Suture Zone; iii) Late Devonian-earliest Carboniferous closure of a South Tianshan back-arc basin, and subsequent Central Tianshan-Tarim active margin collision along the South Tianshan Suture Zone. A second stage involved: i) Late Devonian-Carboniferous south-ward subduction of North Tianshan ocean beneath the Eo-Tianshan active margin (Yili-North Tianshan arc); ii) Late Carboniferous-Early Permian North Tianshan-Junggar collision. The Harlike range, unit of Mongolian Fold Belt, collided with Junggar at Mid-Carboniferous, ending a north-dipping subduction. The last CAOB oceanic suture is likely the North Tianshan Suture Zone, between Yili-North Tianshan and Junggar. During the Permian, all the already welded units suffered from a major wrenching, dextral in Tianshan, sinistral in Mongolian Fold Belt, due to opposite motion of Siberia and Tarim.
A detrital zircon U-Pb and Lu-Hf isotopic study was carried out on the Middle Silurian to Late Carboniferous sedimentary strata of the northwestern Tarim Craton in order to understand accretionary processes in the southern part of the Central Asian Orogenic Belt. Detrital zircons from these strata yielded U-Pb ages clustering around 2.8-2.3 Ga, 2.0-1.7 Ga, 1.3-0.9 Ga, 880-660 Ma and 500-400 Ma, with age populations and Hf isotopic signatures matching those of magmatic rocks in the Tarim Craton and the Central Tianshan Block. Abundant 500-400 Ma detrital zircons most likely reflect deposition in a retro-arc foreland basin inboard of an Andean-type magmatic arc to the north, supporting the northern Tarim-Central Tianshan connection during Early Paleozoic time. The absence of 380-310 Ma zircon population in the Carboniferous siliciclastic rocks suggests that the Central Tianshan Block may have been separated from the Tarim Craton in the Early Devonian, caused by the inter-arc/back-arc opening of the South Tianshan Ocean. We propose an accretionary orogenic model switching from advancing to retreating mode during Paleozoic time in the southwestern part of the Paleo-Asian Ocean. This transition most likely occurred coevally with the rifting of Southeast Asian blocks from the northeastern margin of Gondwana.
Subduction initiation at straight passive margins can be investigated with two-dimensional (2-D) numerical models, because the geometry is purely cylindrical. However, on Earth, straight margins rarely occur. The construction of 3-D models is therefore critical in the modeling of spontaneous subduction initiation at realistic, curved passive margins. Here we report on the results obtained from gravitationally driven, 3-D thermomechanical numerical models using a visco-plastic rheology and a passive margin with a single curved section in the middle. The models show that the curvature angle beta can control subduction initiation: the greater beta is, the more difficult subduction initiation becomes. The 3-D thermomechanical models provide an in-depth physical understanding of the processes. Specifically, we find that pressure gradients, arising from density differences between oceanic and continental rocks, drive subduction initiation, and strongly influence the timing. The main difference between straight (cylindrical) and curved margins is that the orientation of the pressure gradient in 3-D is no longer constant, thus producing a horizontal, along-margin component of flow. We thus conclude that the reason for the impedance of subduction initiation is the result of partitioning of the vertical velocity component into a horizontal component, which therefore decreases the effective slab pull. We infer that, although favorable for subduction initiation in a 2-D model, because the estimated force balance is adequate, the pronounced curvature in the southeast Brazilian margin is a likely explanation why subduction initiation is hampered there.
This book presents the first systematic English review of the tectonostratigraphic evolution of the various regional tectonic elements which comprise the central European orogens. Recent political changes have permitted authors from all central European countries, to contribute to this work, thus providing access to previously unavailable material. Each major tectonic element is systematically described in terms of its stratigraphic, structural, igneous, metamorphic and economic evolution. Each section is thoroughly referenced, thus providing an invaluable introduction to the geological complexity of the region. Introductory chapters provide the required regional geophysical and tectonic overviews.
How new subduction zones form is an emerging field of scientific research with important implications for our understanding of lithospheric strength, the driving force of plate tectonics, and Earth's tectonic history. We are making good progress towards understanding how new subduction zones form by combining field studies to identify candidates and reconstruct their timing and magmatic evolution and undertaking numerical modeling (informed by rheological constraints) to test hypotheses. Here, we review the state of the art by combining and comparing results coming from natural observations and numerical models of SI. Two modes of subduction initiation (SI) can be identified in both nature and models, spontaneous and induced. Induced SI occurs when pre-existing plate convergence causes a new subduction zone to form whereas spontaneous SI occurs without pre-existing plate motion when large lateral density contrasts occur across profound lithospheric weaknesses of various origin. We have good natural examples of 3 modes of subduction initiation, one type by induced nucleation of a subduction zone (polarity reversal) and two types of spontaneous nucleation of a subduction zone (transform collapse and plumehead margin collapse). In contrast, two proposed types of subduction initiation are not well supported by natural observations: (induced) transference and (spontaneous) passive margin collapse. Further work is therefore needed to expand on and understand the implications of these observations. Our future advancing understanding of SI will come from better geologic insights, laboratory experiments, and numerical modeling, and with improving communications between these communities.
The Liuling Group is exposed in the Northern part of the South Qinling orogenic belt. LA-ICP-MS U-Pb analysis of detrital zircons from the meta-sandstones in this Group yields ages ranging between 400 Ma to 3200 Ma, with three prominent age clusters at 500–400 Ma, 850–700 Ma and 1000–900 Ma. A few older zircon populations with U–Pb ages of 1750–1450 Ma, 2000 Ma and 2600–2400 Ma are also present. Age data integrated with cathodoluminescence, trace element data and εHf(t) values of zircon grains show that the Liuling sediments have a complex source. Source rocks mainly include Early Neoproterozoic and Early Paleozoic granitoids, together with minor ultra-high pressure/high pressure (HP-UHP) metamorphic rocks, and paragneiss in the North Qinling belt, and Middle-Late Neoproterozoic magmatic rocks in the South Qinling belt. The dominant population of detrital zircon grains with ages between 500 Ma and 400 Ma show the characteristics of both magmatic and metamorphic zircons. They show three age clusters at 497 Ma, 451 Ma, and ca. 420 Ma and show marked correlation with the three stages of Palaeozoic magmatism, as well as with the peak and retrograde HP-UHP metamorphic stages in the North Qinling belt. This correlation demonstrates that these Early Palaeozoic granitoids and HP-UHP metamorphic rocks in the North Qinling belt were already exhumed to the surface, underwent erosion prior to Middle Devonian time and were then deposited in an extensional basin. Based on the results from detrital zircon U-Pb dating, combined with geochemical data and the regional geology, the deposition of Liuling sediments is inferred to have occurred in a post-orogenic extensional basin, rather than a subduction-related fore-arc basin or a foreland basin formed during or after continental collision.
Many aspects of deep-time Earth System models, including mantle convection, paleoclimatology, paleobiogeography and the deep Earth carbon cycle, require high-resolution plate models that include the evolution of the mosaic of plate boundaries through time. We present the first continuous late Paleozoic to present-day global plate model with evolving plate boundaries, building on and extending two previously published models for the late Paleozoic (410–250 Ma) and Mesozoic-Cenozoic (230–0 Ma). We ensure continuity during the 250–230 Ma transition period between the two models, update the absolute reference frame of the Mesozoic-Cenozoic model and add a new Paleozoic reconstruction for the Baltica-derived Alexander Terrane, now accreted to western North America. This 410–0 Ma open access model provides a framework for deep-time whole Earth modelling and acts as a base for future extensions and refinement.
The oceanic Pacific Plate started forming in Early Jurassic time within the vast Panthalassa Ocean that surrounded the supercontinent Pangea, and contains the oldest lithosphere that can directly constrain the geodynamic history of the circum-Pangean Earth. We show that the geometry of the oldest marine magnetic anomalies of the Pacific Plate attests to a unique plate kinematic event that sparked the plate's birth at virtually a point location, surrounded by the Izanagi, Farallon, and Phoenix Plates. We reconstruct the unstable triple junction that caused the plate reorganization, which led to the birth of the Pacific Plate, and present a model of the plate tectonic configuration that preconditioned this event. We show that a stable but migrating triple junction involving the gradual cessation of intraoceanic Panthalassa subduction culminated in the formation of an unstable transform-transform-transform triple junction. The consequent plate boundary reorganization resulted in the formation of a stable triangular three-ridge system from which the nascent Pacific Plate expanded. We link the birth of the Pacific Plate to the regional termination of intra-Panthalassa subduction. Remnants thereof have been identified in the deep lower mantle of which the locations may provide paleolongitudinal control on the absolute location of the early Pacific Plate. Our results constitute an essential step in unraveling the plate tectonic evolution of " Thalassa Incognita " that comprises the comprehensive Panthalassa Ocean surrounding Pangea.
Series of high-resolution numerical simulations of three-dimensional mantle convection were performed to examine the interaction between the drifting continental lithospheres and the underlying mantle structure for 250 m.y. from the present, and to predict the configuration of the future supercontinent. The density anomaly of the mantle interior was determined by the seismic velocity anomaly from global seismic tomography data sets, which contain well-resolved subducting slabs. The present-day plate motion was imposed for the first stage of the simulation as a velocity boundary condition at the top surface boundary, instead of a shear stress-free condition. The switching time from the plate motion boundary to shear stress-free conditions was taken as a free parameter. The results revealed that Australia, Eurasia, North America, and Africa will merge together in the Northern Hemisphere to form a new supercontinent within ~250 m.y. from the present. The continental drift was assumed to be realized by plate-scale mantle flow, rather than large-scale upwelling plumes. That is, continuously moving plates at the surface for the first stage of the simulation are mechanically coupled with the subducting slabs in the mantle; this enhances the underlying mantle downwelling flow. As a result, persistent continental drift can be reproduced for long future time periods even though top surface boundary conditions may switch in response to shear stress-free conditions. The configuration of the numerically reproduced future supercontinent in this study is broadly consistent with the hypothetical model of Amasia as indicated by previous findings from geological correlations and a paleogeographic reconstruction.
The continental crust is commonly viewed as being formed in subduction zones, but there is no consensus on the relative roles of oceanic or continental arcs in the formation of the continental crust. The main difficulties of the oceanic arc model are how the oceanic arcs can be preserved from being subducted, how we can trace the former oceanic arcs through their high-Si products, and how the oceanic arcs can generate the high-Si, K-rich granitoid composition similar to the upper continental crust. The eastern Qinling orogen provides an optimal place to address these issues as it preserves the well-exposed Erlangping oceanic arc with large amounts of granitoids. In this study, we present an integrated investigation of zircon U–Pb ages and Hf–O isotopes for four representative granitoid plutons in the Erlangping unit. In situ zircon SIMS U–Pb dating indicated that the Zhangjiadazhuang, Xizhuanghe, and Taoyuan plutons formed at 472 ± 7, 458 ± 6 and 443 ± 5 Ma, respectively, all of which postdated the deep subduction of the Qinling microcontinent under the Erlangping oceanic arc. The Zhangjiadazhuang, Xizhuanghe, and Taoyuan plutons are sodic granitoid and have highly positive εHf(t) (+7.6 to +12.9) and relatively low δ18O (4.7–5.0‰) values, which were suggested to result from prompt remelting of hydrothermally altered lower oceanic crust of the accreted Erlangping oceanic arc. The zircon grains from the Manziying monzogranitic pluton show similar Hf–O isotopic compositions to those of the Xizhuanghe pluton, and thus the Manziying monzogranitic pluton was likely derived from the dehydration melting of previous tonalites as exemplified by the Xizhuanghe pluton. The deep subduction of Qinling microcontinent resulted in the accretion of the Erlangping oceanic arc, which implies that arc–continent collision provides an effective way for preventing oceanic arcs from being completely subducted. The highly positive εHf(t) and relatively low δ18O values of zircon grains from the granitoids in the Erlangping unit reveal that the continental crust can acquire its high-Si, K-rich nature from accreted oceanic arcs through differentiation by post-accretional magmatism, and thus highlight the significance of oceanic arcs for the generation of continental crust.
Kevin Burke’s original and thought-provoking contributions have been published steadily for the past sixty years, and more than a decade ago he set out to resolve how plate tectonics and mantle plumes interact by proposing a simple conceptual model, which we will refer to as “the Burkian Earth”. On the Burkian Earth, mantle plumes take us from the deepest mantle to sub-lithospheric depths, where partial melting occurs, and to the surface, where hotspot lavas erupt today, and where large igneous provinces and kimberlites have erupted episodically in the past. The arrival of a plume head contributes to continental break-up and punctuates plate tectonics by creating and modifying plate boundaries. Conversely, plate tectonics makes an essential contribution to the mantle through subduction. Slabs restore mass to the lowermost mantle and are the triggering mechanism for plumes that rise from the margins of the two large-scale low shear-wave velocity structures in the lowermost mantle, that Kevin christened TUZO and JASON. Situated just above the core-mantle boundary beneath Africa and the Pacific, these are stable and antipodal thermochemical piles, which Kevin reasons represent the immediate after-effect of the moon-forming event and the final magma ocean crystallization.
After careful seismic interpretation, Late Silurian-Carboniferous extensional structures were revealed in western Manjiaer Sag, central Tarim Basin. These extensional structures comprise many small normal faults which usually group sinistral en echelon and form 2 tenso-shear normal fault zones. Combinations of the normal faults in profile become negative flower structures and small horst-graben structures. On the basis of growth index calculation, the normal fault formed in Late Silurian, continue active in Devonian and Carboniferous, ceased at the end of Carboniferous. The peak-stage of normal fault activity is Late Silurian. Late Silurian-Carboniferous normal fault also developed in Tazhong and Tabei areas, implying that Tarim Basin was under regional extensional tectonic setting in Late Silurian-Carboniferous. The extensional structure is the result of post-orogeny stress relaxation of Kunlun Caledonian orogenic belt.
Abstract
The Variscides of Europe and N-Africa are the result of the convergence of the plates of Gondwana and Laurussia in the Paleozoic. This orogen is characterized by the juxtaposition of blocks of continental crust that are little affected by the Variscan orogeny. These low strain domains principally consist of Neoproterozoic/Cambrian Cadomian basement overlain by volcano-sedimentary successions of an extended peri-Gondwana shelf. These Cadomian blocks are separated by high strain zones containing the record of subduction-related processes. Traditionally the high strain zones are interpreted as sutures between one or more postulated lithospheric microplates sandwiched between the two major plates. Paleobio-geographic constraints in combination with geochemical and isotopic fingerprints of the protoliths, however, imply that the Variscides are the result of the exclusive interaction of the two plates of Gondwana and Laurussia. Here we explain the Variscan orogen in a two plate scenario, reasoning that the complexity of the Variscan orogen (multitude of high-grade metamorphic belts, compositional diversity of coeval magmatism, and arrangement of foreland basins) is the result of the distribution of crustal domains of contrasting rheological properties. Post-Cadomian rifting along the Cadomian–Avalonian belt, which culminated in the opening of the Rheic Ocean, resulted in vast coeval intracontinental extension and the formation of extended peri-Gondwana shelf areas, namely the Avalonian shelf and the Armorican Spur to the north and south of the evolving Rheic Ocean, respectively. Both shelf areas affected by heterogeneous extension consist of stable continental blocks separated by zones of thinner continental crust. During Variscan collisional tectonics the continental blocks behave as unsubductable crust, whereas the thinner continental crust was subductable and came to constitute the high strain domains of the orogen. The variable interplay between both crustal types in space and time is seen as the principal cause for the observed sequence of orogenic processes. The first collisional contact along the convergent Gondwana–Laurussia plate boundary occurred between Brittany and the Midland microcraton causing the early Devonian deformation along the Anglo-Brabant Fold Belt. This process is coeval with the initiation of continental subduction along the Armorican Spur of the Gondwana plate and the formation of back arc and transtensional basins to both sides of the Armorican Spur (e.g., Lizard, Rheno-Hercynian, Careón, Sleza) on the Laurussia plate. As further subduction along this collision zone is blocked, the plate boundary zone between the Gondwana and Laurentia plates is reorganized, leading to a flip of the subduction polarity and a subduction zone jump outboard of the already accreted blocks. The following Devonian–Early Carboniferous subduction accretion process is responsible for the juxtaposition of additional Cadomian blocks against Laurussia and a second suite of high-pressure rocks. The final collision between Gondwana and Laurussia is marked by an intracontinental subduction event affecting the entire internal zone of the orogen. Subduction stopped at 340 Ma and the following isothermal exhumation of the deeply subducted continental crust is primarily responsible for Late Variscan high-temperature metamorphism and cogenetic voluminous granitic magmatism. During this final transpressional stage the irregular shape of the Variscan orogen was established by the highly oblique motion of the decoupled lithospheric blocks (e.g. Iberia and Saxo-Thuringia). Rapid overfilling of synorogenic marine basins in the foreland and subsequent folding of these deposits along vast external fold and thrust belts finally shaped the Variscides, feigning a relatively simple architecture.
In terms of plate tectonics, the model places the opening of the Paleotethys in the Devonian with a rotational axis of the spreading center just east of the Variscan orogen. The movement of Gondwana relative to Laurussia follows small circle paths about this axis from 370 to 300 Ma. As a consequence of the incomplete closure of the Rheic Ocean after the termination of the Variscan orogeny, Gondwana decoupled from the European Variscides along the dextral Gibraltar Fault Zone. The relative motion between Gondwana and Laurussia after 300 Ma is associated with a shift of the rotational axis to a position close to the Oslo Rift, and is related to the opening of the Neotethys and the evolution of the Central European Extensional Province. The Permian convergence of Gondwana and Laurussia led to the final Permian collisional tectonics along the Mauritanides/Alleghanides. The assembly of the “Wegenerian” Pangea is complete by the end of the Paleozoic.
The Qinling Orogenic Belt (QOB) is located between the North China and South China Blocks, and has been considered to have formed by the collision between these blocks. This contribution provides an overview of the composition, nature and ages of the principal tectonic elements including ophiolitic mélanges and related volcanic rocks, gabbroic–granitic intrusions, metamorphic basement, sedimentary cover and its provenance in this orogen. The QOB represents a composite orogenic belt that witnessed four major episodes of accretion and collision between discrete continental blocks, such as the North China Block, North Qinling Block and the South China Block. The available geology, geochemistry and geochronology of these tectonic elements together with those of the adjacent regions, can be used to trace the polarity of the four stages of plate subduction, accretion, collision and the related tectonic history as follows. (1) The Grenvillian-aged orogeny along the Kuanping suture between the North Qinling Terrane and North China Block is associated with the southward subduction of Mesoproterozoic Ocean, which led to the amalgamation of the North Qinling Terrane and the North China Block at ca. 1.0 Ga. (2) The Neoproterozoic subduction/accretion as represented by the widely distributed terranes and volcanic–sedimentary rocks, resulted in a wide accretionary wedge formed by the southward accretion to the South China Block. (3) The Paleozoic orogeny along the Shangdan suture between the North and South Qinling Blocks is characterized by Early Paleozoic ocean–continent subduction and a long-lived Late Paleozoic continent–continent subduction. The polarity and detailed evolutionary process of the Early Paleozoic ocean–continent subduction have been constrained by the ophiolitic mélange, island-arc related volcanics and intrusions in the North Qinling Belt, as well as the evolutionary history of the Erlangping back-arc basin. The northward subduction and destruction of the Shangdan Ocean during Early Devonian was succeeded by continent–continent subduction beneath the North Qinling Terrane from Middle Devonian to Early Triassic. (4) The Triassic collisional orogeny occurred between the South Qinling Block and South China Block along the Mianlue suture. Silurian rifting along the present Mianlue zone marks the precursor of the eastern Mianlue Ocean, which separated the South Qinling Block from the South China Block during Late Paleozoic. The northward subduction of the ocean led to the Middle Triassic collision between the South China Block and the South Qinling Block. (5) After the collision, the whole QOB evolved into an intra-continental orogen, including Early Jurassic differential tectonics, Late Jurassic to Early Cretaceous compression and thrusting, and Late Cretaceous to Paleogene orogen collapse and depression. These multiple orogenies resulted in abundant mineralization, the genetic types, spatial distribution and metallogenic epochs which correlate well with the tectonics and evolutionary history of the QOB.
Chondrite-based models of the composition of the bulk Earth suggest that a substantial reservoir of heat-generating potassium is present in the Earth's deep mantle or core. Mantle convection is driven both by heat generated within the mantle and by heat escaping from the core. Heat from the deep mantle and core is transported by rapidly rising plumes which are not randomly distributed, but are preferentially located within an African ring centred on 0°N 10°E and a Pacific ring centred on 0°N 190°E. The rising plumes continually introduce hot lower mantle into the upper mantle within the rings, and the resultant stresses cause preferential rifting along their margins. Mid-ocean ridges develop and ocean floor grows away from the rings. On an Earth with finite radius, ocean floor cannot grow indefinitely. At present ocean floor is spreading from the African ring and overriding, by subduction, ocean floor expanding from the Pacific ring. This cycle has run for the last 160 Ma, since the breakup of Pangea. In the previous cycle, from 540 to 160 Ma, ocean floor expanding from the Pacific ring overrode ocean floor expanding from the African ring. The cycles are self-terminating and end when the expanding ocean floor has subducted all the sudductible components in its path, and must itself begin to subduct. The process is not perfectly symmetrical because the strongest plumes are located within the Pacific ring. These strong plumes are difficult to override and inhibit the development of supercontinental aggregations. Within the African ring supercontinental assemblages are more stable and are more easily recognized within the geological record. Supercontinents developed in the African ring at 160 Ma, 1 Ga, 2 Ga, and possibly 3 Ga. Supercontinents are not an essential part of the superocean cycle, and the first aggregation of non-subductible components gathered in the African ring at 4 Ga at the end of the first superocean cycle, which began with the expansion of ocean floor from the Pacific ring. Non-subductible components gathered in the Pacific ring at 0.5, 1.5, 2.5, and 3.5 Ga. Nine superocean cycles have therefore been completed.
Major late Paleozoic faults, many with documented strike-slip motion, have dissected the Ordovician - Devonian Appalachian orogen in the Maritime Provinces of Atlantic Canada. Activity alternated between east-west faults (Minas trend) and NE-SW faults (Appalachian trend). NW-SE faults (Canso trend) were probably conjugate to Minas-trend faults. Major dextral movement, on faults with Appalachian trend, in total between 200 and 300 km, began in the Late Devonian. This movement initiated the Maritimes Basin in a transtensional environment at a releasing bend formed around a promontory in the Laurentian margin, and thinned the crust, accounting for the major subsidence of the basin. Appalachian-trend strike slip continued in the Mississippian, but was accompanied by major movement on E-W Minas-trend faults culminating around the Mississippian-Pennsylvanian boundary, juxtaposing the Meguma and Avalon terranes of the Appalachians close to their present-day configuration. However, strike slip continued during the Pennsylvanian-Permian interval resulting in transpressional deformation that reactivated and inverted earlier extensional faults. A final major episode of transtension, mainly sinistral, occurred during the Mesozoic opening of the Atlantic Ocean. Restoration of movements on these faults, amounting to several hundred kilometres of slip, explains anomalies in the present-day distribution of terranes amalgamated during early Paleozoic Appalachian tectonism. In the restored geometry, the Nashoba and Ellsworth terranes of Ganderia are adjacent to one another, and the Meguma terrane lies clearly outboard of Avalonia. A restored post-Acadian paleogeography, not the present-day geometry of the orogen, should be used as a basis for reconstructions of its earlier Paleozoic history.
The Qinling Orogen separating the North China plate from the Yangtze plate is a key area for understanding the timing and process of aggregation between the two plates. Two competing and highly contrasting tectonic models currently exist to explain the timing and nature of collision; one advocates a Devonian continental collision while the other favors a Triassic collision. The Wuguan Complex, between the early Paleozoic North Qinling and the Mesozoic South Qinling terranes, can provide important constraints on the late Paleozoic evolutionary processes of the Qinling Orogen. Metamorphosed sedimentary rock of the Wuguan Complex have a detrital zircon age spectrum with two major peaks at 453 Ma and 800 Ma, several minor age populations of 350-430 Ma and 1000-2868 Ma, and a youngest weighted mean age of 358 ± 3 Ma, indicating a mixed source from the North Qinling terrane. The recrystallized zircons yield a weighted mean age of 333 ± 2 Ma, representing the metamorphic age. Geochemical analyses imply that the sedimentary rocks were originally deposited in an active continental margin dominated by an acidic-arc source with a subordinate mafic-ultramafic source. The youngest population of detrital zircons (358 Ma) suggests that the Wuguan Complex developed as forearc basin along the southern accreted margin of the North Qinling terrane during the early Carboniferous, whereas the ca. 520-460 Ma mafic rocks with E-MORB, N-MORB, OIB or island arc basalt signatures probably derived from the Danfeng Group. In combination with regional data, we suggest that the depositional age of the Wuguan Complex is ca. 389-330 Ma, but it was subsequently incorporated into tectonic mélange by the northward subduction of the Paleo-Qinling Ocean. A long-lived southward-facing subduction-accretionary system in front of the North Qinling terrane probably lasted until at least the early Carboniferous.
Several lines of evidence suggest that subduction zones are weak and that the unique availability of water on Earth is a critical factor in the weakening process. We have evaluated the strength of subduction zone interfaces using two approaches: i) from empirical relationships between shear stress at the interface and subduction velocity, deduced from laboratory experiments; and ii) from a parametric study of natural subduction zones that provides new insights on subduction zone interface strength. Our results suggest that subduction is only mechanically feasible when shear stresses along the plate interface are relatively low (less than ~35 MPa). To account for this requirement, we propose that there is a feedback mechanism between subduction velocity, water released from the subducting plate and weakening of the forearc mantle that may explain how relatively low shear stresses are maintained at subduction interfaces globally.
The radially anisotropic shear velocity structure of the Earth's mantle provides a critical window on the interior dynamics of the planet, with isotropic variations that are interpreted in terms of thermal and compositional heterogeneity and anisotropy in terms of flow. While significant progress has been made in the more than 30 yr since the advent of global seismic tomography, many open questions remain regarding the dual roles of temperature and composition in shaping mantle convection, as well as interactions between different dominant scales of convective phenomena. We believe that advanced seismic imaging techniques, such as waveform inversion using accurate numerical simulations of the seismic wavefield, represent a clear path forwards towards addressing these open questions through application to whole-mantle imaging. To this end, we employ a `hybrid' waveform-inversion approach, which combines the accuracy and generality of the spectral finite element method (SEM) for forward modelling of the global wavefield, with non-linear asymptotic coupling theory for efficient inverse modelling. The resulting whole-mantle model (SEMUCB-WM1) builds on the earlier successful application of these techniques for global modelling at upper mantle and transition-zone depths (≤800 km) which delivered the models SEMum and SEMum2. Indeed, SEMUCB-WM1 is the first whole-mantle model derived from fully numerical SEM-based forward modelling. Here, we detail the technical aspects of the development of our whole-mantle model, as well as provide a broad discussion of isotropic and radially anisotropic model structure. We also include an extensive discussion of model uncertainties, specifically focused on assessing our results at transition-zone and lower-mantle depths.
Shear wave splitting of SK(K)S phases is often used to examine upper mantle anisotropy. In specific cases, however, splitting of these phases may reflect anisotropy in the lowermost mantle. Here we present SKS and SKKS splitting measurements for 233 event-station pairs at 34 seismic stations that sample D” beneath Africa. Of these, 36 pairs show significantly different splitting between the two phases, which likely reflects a contribution from lowermost mantle anisotropy. The vast majority of discrepant pairs sample the boundary of the African large low shear velocity province (LLSVP), which dominates the lower mantle structure beneath this region. In general, we observe little or no splitting of phases that have passed through the LLSVP itself and significant splitting for phases that have sampled the boundary of the LLSVP. We infer that the D” region just outside the LLSVP boundary is strongly deformed, while its interior remains undeformed (or weakly deformed).
The Southern Tianshan Orogenic Belt (STOB) is an important region to understand the prolonged evolution of the Central Asian Orogenic Belt (CAOB). A comprehensive study of zircon U-Pb ages and Lu-Hf isotopes, whole-rock elements and Sr-Nd isotopes was carried out for two intermediate-mafic intrusions in the Baicheng and Heiyingshan areas of Southwest Tianshan. LA-ICP-MS zircon U-Pb dating yielded crystallization ages of 431 ± 3 Ma (MSWD = 0.073, n = 31) and 430 ± 3 Ma (MSWD = 0.104, n = 30) for the Baicheng diorites, and 423.9 ± 2.1 Ma (MSWD = 0.5, n = 12) for the Heiyingshan gabbros. These data, in combination with other Late Silurian ages previously reported for the intrusive suites from the STOB, indicate an Early Paleozoic magmatic event in the region. In-situ zircon Hf isotope data on two samples from the Baicheng diorites show εHf(t) values of − 2.4 to 4.9 and 0.2 to 6, and the Heiyingshan gabbros exhibit similar εHf(t) values from − 1 to 1.8. The dioritic pluton exhibits Na-rich, arc-type geochemical and calc-alkaline affinity, and shows trace element patterns characterized by enrichment in large ion lithophile elements (Rb, Ba, K and Sr) and depletion in high strength field elements (Nb, Ta, Ti, Zr and Hf). These features resemble those of arc-type igneous rocks. Combined with initial 87Sr/86Sr ratios of 0.7047 to 0.7064 and εNd(t) of − 1.6 to − 5.06, we infer that the magma from which the Baicheng dioritic pluton formed was sourced from a mantle wedge which was subjected to metasomatism by fluids and melts related to subducted oceanic slab and the overlying sediments. The Heiyingshan gabbro shows a Na-rich nature and tholeiitic affinity. They not only display supra-subduction zone affinity, but also are geochemically similar to E-MORB, possible implying a back-arc basin setting. The initial 87Sr/86Sr ratios of the Heiyingshan samples range from 0.7046 to 0.7051 and εNd(t) values vary from 1.76 to 5.19. We speculate that the upwelling of the asthenospheric mantle to the supra-subduction wedge, facilitated by subduction roll-back, provided the source for the Heiyingshan gabbroic rocks. Together with other geologic evidence, we correlate the generation of the Baicheng diorite with the southward subduction of the South-Tianshan Ocean during the Early Paleozoic. The Heiyingshan gabbroic plutons were probably formed in a back-arc basin in response to the southward subduction of the Paleo-Tianshan Ocean. In combination with coeval arc-type magmatic rocks widely exposed in the southern margin of the Central Tianshan Block, a double-sided subduction model is proposed to explain the evolution of the South-Tianshan Ocean in the Silurian.