Plate Tectonic Processes Research Articles

Two Effective Degrees of Freedom Can Represent the Dominant Features of Global Rayleigh Wave Dispersion Maps

AbstractObjectively exploring global variations in crust and upper mantle structure helps constrain fundamental aspects of Earth's plate tectonic and convective processes. Here we adopted a Variational Auto‐Encoder to explore the degrees of freedom of global Rayleigh wave dispersion maps at 4–40 mHz. We found that two latent variables sufficiently represent the global variations, suggesting inherent coupling between crustal and mantle seismic properties. We propose that the two extracted latent variables mostly correspond with crustal thickness and upper mantle thermal structure. The first variable shows low values for continental mountain belts and ocean spreading ridges, contrasted by high values for abyssal plains. The second variable shows low values for most oceanic lithosphere and Phanerozoic continental areas, contrasted by high values for Archean cratons. Latent space correlations indicate that continental lithosphere has more strongly coupled depth features than beneath the oceans, which might be a consequence of its longevity.

The legacy of African kimberlite and diamond research and exploration for the earth sciences

Kimberlite is a rare and volumetrically insignificant magmatic rock, which nevertheless has an outsized economic importance as the world's principal source for natural diamonds. Discovered in 1870, in South Africa, it was named after Kimberley, the town that grew around the first mines developed in the pipe-shaped bodies of the newly discovered, ultramafic diamond source rock. Apart from their economic significance, kimberlites are of enormous scientific interest. Originating at great depth, they have entrained a variety of ultramafic and ultrabasic rock fragments (xenoliths) and their disaggregated mineral grains (xenocrysts) that provide a direct window into the upper mantle, making it possible to identify regional variations in the nature of the upper mantle and to understand mantle processes. After a flurry of studies on the heels of the initial kimberlite discoveries, there was a long hiatus in publications between the 1930's and mid-1960's. Kimberlite, diamond, and upper mantle studies received a lasting stimulus in the early 1970's, largely as the result of the seminal First International Kimberlite Conference (1 IKC), held at the University of Cape Town in 1973, and 10 follow-up conferences held since on every continent except Antarctica. This review explores examples of advances and controversies resulting from African-centered research on the subject. The definition and nomenclature of “kimberlite” progressed from applying the term to all igneous rocks with diamond, to distinguishing two types of diamond-bearing kimberlite (Groups I and II), and eventually recognizing that Group I (“archetypal” kimberlite) and Group II (formerly orangeite, now assigned to the olivine lamproite clan) are entirely different rock types, originating below and within the lower part of the lithosphere, respectively. As to the shape of kimberlite pipes, the classic southern African model of a carrot-shaped magmatic system, consisting of a root zone, a steep-sided diatreme, and a crater zone, was based on a composite of the highly eroded Kimberley and Jagersfontein pipes, for the lower part, and the significantly less eroded Orapa pipe in Botswana, for the upper part. Restriction of diamond-bearing igneous rocks to the ancient cratons of Africa led to the formulation of “Clifford's Rule”, an important tool in diamond exploration worldwide. Studies of peridotite xenoliths and Mesoarchean ages determined on peridotitic garnet inclusions in diamond xenocrysts from the Kaapvaal craton revealed that the craton is underlain by a deep diamondiferous lithospheric root which is older, cooler, and more refractory (harzburgitic) than the shallower lithosphere under adjacent Proterozoic orogenic belts, in fact providing a petrologic explanation for Clifford's rule. The correlation between harzburgitic garnet and diamond was expanded into a powerful exploration tool, developed, and tested in Africa, that was successfully applied in many other parts of the world. Interpretations of eclogite xenoliths changed from being cognate nodules in kimberlite, to pieces of an eclogite layer, to high-pressure cumulates from eclogitic melts in a peridotitic mantle, to fragments of subducted oceanic crust. Combining the latter interpretation with Archean ages for eclogitic mineral inclusions in diamond xenocrysts, it was recognized that plate tectonic processes began as early as ca. 2.9 Ga and were involved in the formation of the Kaapvaal craton, the oldest major Archean craton in the world. Based on the model developed for typical kimberlite pipes in southern Africa, it has been possible to quantify erosion processes across the southern part of the continent, particularly since the disruption of Gondwana. Dispersion of “pathfinder” kimberlitic indicator minerals (KIMs), including diamond, into drainage networks provide tracers to reconstruct past river systems. Being well endowed with kimberlite pipes, Africa has also been central to the important debate about the relevance of plumes as triggers for kimberlite and other alkaline magmatism. Nevertheless, despite the importance of resolving this geological conundrum, it remains highly controversial. Many kimberlites are capped by well-preserved crater-lake facies sediments, most of which are dated as Cretaceous to early Cenozoic, within the time of the proposed Angiosperm Terrestrial Revolution. As the associated plant fossil records are the only ones for the interior of southern Africa during this time interval, they may contribute to solving what has been coined as “Darwin's Abominable Mystery”, the poorly understood origin of angiosperms (flowering plants).

Strontium isotope proxy of sedimentological records reveals uplift and erosion in the Southeastern Neo-Tethys ocean during the late Cretaceous

The mutual interplays between plate tectonic processes, orogenesis and continental uplift, high-flux magmatism, and high sedimentation rates can affect the geochemical signatures and composition of marine sediments. Here, we examine two major disconformities, the Cenomanian–Turonian boundary (CT-ES) and the middle Turonian (mT-ES) exposure surfaces, from the Upper Cretaceous sedimentary successions of the southeastern Neo-Tethys Ocean in the Zagros Basin (Iran). The disconformities are expressed as distinct positive peaks in rubidium (Rb) contents and 87Sr/86Sr isotopic ratios of carbonates. The 87Sr/86Sr values of samples bracketing the disconformities in seven well cores give average age ranges of 94.4–93.6 Ma for the CT-ES and 91–86 Ma for the mT-ES. These ages fall in the timespan of forearc/ophiolite formation through subduction initiation in the Neo-Tethys realm (southern margin of Eurasia), high convergence velocities between Arabia and Eurasia, and blueschist exhumation. All these processes involved buckling of the Neo-Tethyan lithosphere, initiating the Neo-Tethys closure and a high erosion rate on the Eurasian margin. The first two mechanisms exerted considerable compressional forces on the adjacent carbonate platforms, reactivated basement faults, and led to the uplift and erosion of the Arabian Plate, which provided the high contents of Rb and the high 87Sr/86Sr ratios in the carbonates.

Sublithospheric Diamonds: Plate Tectonics from Earth's Deepest Mantle Samples

Sublithospheric diamonds and the inclusions they may carry crystallize in the asthenosphere, transition zone, or uppermost lower mantle (from 300 to ∼800 km), and are the deepest minerals so far recognized to form by plate tectonics. These diamonds are distinctive in their deformation features, low nitrogen content, and inclusions of these major mantle minerals: majoritic garnet, clinopyroxene, ringwoodite, CaSi perovskite, ferropericlase, and bridgmanite or their retrograde equivalents. The stable isotopic compositions of elements within these diamonds (δ11B, δ13C, δ15N) and their inclusions (δ18O, δ56Fe) are typically well outside normal mantle ranges, showing that these elements were either organic (C) or modified by seawater alteration (B, O, Fe) at relatively low temperatures. Metamorphic minerals in cold slabs are effective hosts that transport C as CO3 and H as H2O, OH, or CH4 below the island arc and mantle wedge. Warming of the slab generates carbonatitic melts, supercritical aqueous fluids, or metallic liquids, forming three types of sublithospheric diamonds. Diamond crystallization occurs by movement and reduction of mobile fluids as they pass through host mantle via fractures—a process that creates chemical heterogeneity and may promote deep focus earthquakes. Geobarometry of majoritic garnet inclusions and diamond ages suggest upward transport, perhaps to the base of mantle lithosphere. From there, diamonds are carried to Earth's surface by eruptions of kimberlite magma. Mineral assemblages in sublithospheric diamonds directly trace Earth's deep volatile cycle, demonstrating how the hydrosphere of a rocky planet can connect to its solid interior. ▪Sublithospheric diamonds from the deep upper mantle, transition zone, and lower mantle host Earth's deepest obtainable mineral samples.▪Low-temperature seawater alteration of the ocean floor captures organic and inorganic carbon at the surface eventually to become some of the most precious gem diamonds.▪Subduction transports fluids in metamorphic minerals to great depth. Fluids released by slab heating migrate, react with host mantle to induce diamond crystallization, and may trigger earthquakes.▪Sublithospheric diamonds are powerful tracers of subduction—a plate tectonic process that deeply recycles part of Earth's planetary volatile budget.

Paleomagnetic Constraint on the Age of the Shyok Suture Zone

AbstractThe India‐Eurasia collision is a key case study for understanding the influence of plate tectonic processes on Earth's crust, atmosphere, hydrosphere, and biosphere. However, the timing of the final India‐Eurasia continental collision is debated due to significant uncertainty in the age of the collision between the Kohistan‐Ladakh arc (KLA) and Eurasia along the Shyok suture zone. Here we present paleomagnetic results that constrain the Karakoram terrane in northwest India to a paleolatitude of 19.9 ± 8.9°N between 93 and 75 million years ago (Ma). Our results show that the Karakoram terrane was situated on the southern margin of Eurasia in the Late‐Cretaceous. Our results indicate that the KLA and Eurasian continent had a not converged until <61.6 Ma, placing a Paleocene older limit on the age of final closure of the Shyok suture zone. This suggests that the India‐Eurasia collision in northwestern India likely occurred after the closure of the oceanic basin between the KLA and Eurasia. The Paleocene collision event affecting India that has been widely interpreted to represent final India‐Eurasia collision instead records the arc‐continent collision between the KLA and the northern edge of India prior to final India‐Eurasia collision. Final India‐Eurasia collision in northwest India most likely occurred after the closure of the oceanic basin between the KLA and Eurasia.

Heavy silicon and oxygen isotope signatures of TTGs formed in distinct tectonic settings

Tonalite–trondhjemite–granodiorite suites (TTGs) are key components of Archean continental crust and are therefore crucial to understanding the evolution of Archean Earth. However, the tectonic settings of their formation remain unresolved. Here we present new in situ zircon Si–O–Hf isotopes and quartz Si–O isotopes from 2.95 to 2.54 Ga TTGs of the Jiaodong terrane in the North China Craton. Through first-principles calculations, the corresponding TTG melt δ30Simelt and δ18Omelt values are determined as –0.03 ‰ to 0.06 ‰ and 7.12 ‰ to 7.60 ‰, respectively, indicating heavier isotope signatures than those of mantle-derived juvenile rocks. The 2.95–2.93 Ga and 2.56–2.54 Ga TTGs have depleted Hf isotopes [εHf(t) = +3.7 to +4.9, and +2.5 to +5.5, respectively] and higher (La/Yb)N ratios compared with the 2.75–2.71 Ga TTGs, which have relatively less depleted Hf isotopes [εHf(t) = +0.9 to +2.0] and lower (La/Yb)N ratios. Although the heavy Si–O isotope compositions indicate the involvement of supracrustal materials in the melting sources of all studied TTGs, the variable zircon Hf isotopes and (La/Yb)N ratios suggest that these rocks formed in distinct tectonic settings. The 2.95–2.93 Ga and 2.56–2.54 Ga TTGs are inferred to have been generated in a subduction setting involving the melting of juvenile oceanic crust under high-pressure conditions, whereas the 2.75–2.71 Ga TTGs are inferred to have been derived from the melting of buried ancient oceanic crust under a mantle-plume-induced lower-pressure environment. We conclude that the heavy Si–O isotope signatures of Archean TTGs may not be directly related to plate-tectonic processes.

Impact of tidal friction on the motion of lithospheric plates and earthquake activity

AbstractThis paper draws attention to the contribution of tidal friction (TF) to plate tectonic processes. Our proposed model supports recent geodetic observations and geodynamic models highlighting exogenous local contributions to plate motions. The TF has a westward effect on lithospheric plates and the lithosphere as a whole. To determine the extent of this effect, we use the asthenospheric viscosity estimated using the Eötvös force. We also show that TF has a detectable impact on the latitudinal distribution of earthquake activity.

The lithosphere–asthenosphere boundary structure at 11–21 Ma from wide-angle seismic data in the equatorial Atlantic Ocean

SUMMARYThe lithosphere–asthenosphere boundary (LAB) separates the rigid lithospheric plate above with the ductile and convective asthenosphere below and plays a fundamental role in plate tectonic processes. The LAB has been imaged using passive geophysical methods, but these methods only provide low-resolution images. Recently, seismic reflection imaging method has provided high-resolution images of the LAB, but imaging of the LAB at younger ages has been difficult. Here, we present the image of the LAB using wide-angle seismic reflection data covering 11–21 Ma old lithosphere in the equatorial Atlantic Ocean. Using ocean bottom seismometers (OBSs), we have observed wide-angle reflections between 150 and 400 km offsets along with crustal and mantle refraction arrivals. We first performed traveltime tomography to obtain the velocity in the crust and upper mantle. The Pn arrivals provide the information about P-wave velocity down to 4 km below the Moho. The disappearance of Pn arrivals beyond 130 km offset suggests that vertical P-wave velocity gradient is negligible or negative below this depth. We extended these velocities down to 90 km depth and then applied two imaging techniques to wide-angle reflection data, namely traveltime mapping of picked reflection arrivals and pre-stack depth migration of full wavefield data. We find that these reflections originate between 34 and 67 km depth, possibly from the LAB system. We have carried out extensive modelling to show that these reflections are real and not artefacts of imaging. Comparison of our results with coincident passive seismological and magnetotelluric results suggests that wide-angle imaging technique can be successfully used to study the lithosphere and the LAB system. We find that the LAB gradually deepens with age, but becomes very deep at 17–19 Ma, which we interpret to be due to the anomalous geology along this part of the profile.

Reconstructing the Paleoproterozoic Heart of Nuna, from Fennoscandia to Northeastern Laurentia

Abstract Accurate palaeogeographical reconstructions of past supercontinents are necessary to test models of supercontinent cyclicity, secular variation in plate tectonics and mantle geodynamics. However, numerous factors limit our ability to reconstruct past supercontinents as is evident in the ongoing debates regarding the construction phase and geometry of Earth's most recent supercontinent, late Paleozoic to early Mesozoic Pangaea. An important factor in these debates, and a focus of our study is how best to palinspastically restore, in section and in plan-view, the orogenic belts along which supercontinents were stitched together. We utilize the orogenic belts spanning Baffin Island, Greenland and Fennoscandia that are inferred to record the assembly of the Paleoproterozoic supercontinent Columbia to test the accepted reconstruction of the Nuna core of Columbia. We show that as reconstructed in the Nuna model, each of Baffin Island, Greenland (with some complications) and Fennoscandia are characterized by an older cratonic backstop that gives way to younger accretionary complexes toward the inferred oceanic domain that lay to the south and which is inferred to have closed by subduction beneath northeastern Laurentia–Fennoscandia during Columbia assembly. This southward transition from cratonic backstop to accretionary orogen is a hallmark of upper plates in modern convergent plate margins and is consistent with the construction of the Columbia supercontinent, including its Nuna core, through plate tectonic processes, and provides a broad validation of the Nuna reconstruction and hence for Columbia as a whole. Map view curvature of the cratonic backstops is restricted to long wavelength, open bends consistent with the Archean crust having been characterized by significant lithospheric strength. The more accretionary southern portions of Nuna are, however, characterized by sinuous orogens that developed by oroclinal bending of formerly more linear belts, significantly complicating their palinspastic restoration and rendering detailed correlation of juvenile orogenic belts across Nuna problematic.

The significance of Nd isotope mapping in Precambrian orogenic belts: A case study from the Saguenay region of the Quebec Grenville Province

Fifty new Nd isotope analyses are presented for the Saguenay region of the Quebec Grenville Province. Most samples yield juvenile Pinwarian model ages, with an average depleted mantle model age (TDM) of 1.55 Ga, typical of the Quebecia accreted arc terrane. However, ten samples yield older model ages, averaging 1.78 Ga. These samples have an approximately linear geographical distribution, and are interpreted as melts of old basement panels trapped between juvenile Pinwarian crustal blocks during the accretion of the Quebecia terrane as a composite arc belt. These panels are the most tangible remnants of pre-Pinwarian crust in the Saguenay region, and their mapping is an important step in understanding the geometry of Mesoproterozoic crustal accretion on the SE margin of Laurentia. This study illustrates the usefulness of Nd isotope mapping for interpolating crustal formation ages between U-Pb dated samples in deeply exhumed orogenic belts. The data confirm that the Quebecia composite arc belt consists almost entirely of accreted juvenile Pinwarian arc crust, with less than 5% of older Paleoproterozoic crustal slivers. A comparison with the modern analogue of the Japan Arc suggests that normal plate tectonic processes can insert juvenile arc terranes behind rifted slivers of old crust to create the observed geometrical pattern of crustal formation ages.

The role of recycled oceanic crust in the alkaline basalts: Evidence from Zn isotopic compositions of alkaline basalts from the Madeira hotspot (eastern North Atlantic)

Plate tectonic processes introduce oceanic crust (as eclogite) into the sources of oceanic island basalts (OIB). The fate of this recycled material in the deep mantle is still poorly understood. Here we present a systematic study of Zn isotopes on well-characterized alkaline basalts (<5 Ma) from the Madeira Islands in the eastern North Atlantic. Our analyses show that the δ66Zn values of alkaline basalts range from 0.25‰ to 0.34‰, with an average of 0.30 ± 0.05‰ (2SD, N = 15), which is similar to the average of mid-ocean ridge basalts (MORB) (0.28 ± 0.03‰, 2SD), and ∼ 0.14‰ higher than that of the asthenospheric mantle (0.16 ± 0.06‰, 2SD). However, these alkaline basalts have higher Zn/Fe ratios (up to 14.20) than MORB (generally less than 12). Model calculations show that the partial melting of mantle peridotite cannot simultaneously produce the observed MORB-like δ66Zn values and trace element ratios. After excluding the effects of post-eruption alteration, crystal fractionation, and the assimilation of crustal materials during magma upwelling on Zn isotopic compositions of the studied samples, we suggest that the MORB-like δ66Zn values reflect the mixing of peridotite melt and eclogite/pyroxenite-derived melt in the magma source of the Madeira hotspot. This is supported by trace element ratios and radiogenic isotopes, such as high Zn/Fe and Dy/Yb ratios and 206Pb/204Pb values. Our new data provide independent evidence in support of the important role of recycled oceanic crust in the source and generation of alkaline OIBs, and also highlight that such sources are not ubiquitously carbonated.

An extended Neoarchaean to Neoproterozoic history of the Sandmata Complex (Aravalli Craton, northwestern India): Insights from metamorphic evolution and zircon-monazite geochronology of high-grade quartzofeldspathic gneisses

The Sandmata Complex in the Aravalli Craton of northwestern India experienced syn- to post-collisional high-grade metamorphism and migmatization related to the 1.9–1.7 Ga Aravalli orogeny. Garnet-bearing quartzofeldspathic gneisses are an important rock type in the Sandmata Complex. Their prograde assemblage is represented by garnet cores and inclusion minerals Bt + Pl + Qz, whereas the assemblage Pl + Kfs + Qz + Btmatrix + Sil + Ilm, along with chemically homogeneous garnet porphyroblasts, represents peak metamorphism. Post-peak retrograde metamorphism is reflected in the growth of muscovite, chlorite and biotite, and diffusion adjustments in garnet rims. Mineral paragenesis, mineral chemistry, conventional thermobarometry, and phase equilibrium modelling indicate P–T conditions of 6.7–10.2 kbar at 725–830 °C for peak metamorphism, and 4.8–6.9 kbar at 450–610 °C for post-peak retrogression. The P–T conditions determined from the quartzofeldspathic gneisses define a near-isobaric cooling P–T path. In contrast, previously studied high-grade pelitic gneisses and metagranitoids from the Sandmata Complex define an isothermal decompression P–T path. U–Pb dating of magmatic zircon cores in the quartzofeldspathic gneisses provides the crystallization age of their felsic protoliths as ∼ 2.52 Ga. Zircon and monazite geochronology show that peak metamorphism occurred at ∼ 1.90–1.78 Ga and thus shortly preceded the emplacement of the Gyangarh-Asind igneous suite and Anjana granite, previously dated at ∼ 1.72–1.64 Ga. A model of oceanic crust subduction followed by continent–continent collision during the Aravalli orogeny can explain granulite facies metamorphism in the Sandmata Complex. The combined results of this and previous studies support the operation of modern-style plate tectonic processes involving subduction and continental collision during Palaeoproterozoic time. We also identify a younger (∼0.90–0.78 Ga) metamorphic episode, preserved mostly in recrystallized monazite grains elongated parallel to the final deformation fabric, likely related to the South Delhi orogeny.

Earthquake hotspot and coldspot: Where, why and how?

• Global earthquake hotspot and coldspot were identified. • Statistical methods of hotspot analysis and optimized hotspot analysis. • Benioff/subduction zones are the most potential hotspot zone. • Optimized hotspot analysis emerges as the most probabilistic method. Global tectonic activities are playing an important role in the occurrences of devastating earthquakes and related long-term changes in the earth's system surface. However, the plate tectonics processes and their interaction with the earth's crust are very much complex, and it is a subject of unending debate. Therefore, tectonism-induced landslide, tsunami, liquefaction, and fire are significant earthquake-related hazards, which have a larger potential and overwhelming impact on life and infrastructural properties throughout the world. In this study, we have emphasized the identification of earthquake hotspot and coldspot zones considering historical earthquake data across the plate boundary of the world. Here, a total of 7773 historical earthquake points were collected as input parameters with three-moment magnitude (M w ) classes (<4.5, 4.5-6.0, and >6.0). Two statistical methods namely hotspot analysis (Getis-Ord GI*) and optimized hotspot analysis were used in the detection of global earthquake hotspot and coldspot zones using the geographic information system (GIS) platform. Hotspot and coldspot zone are identified under 99%, 95%, and 90% confidence levels. Alongside, here we have also discussed the paradigm, evidence of tectonic, and historical earthquakes, and how and why they are formed with the help of the existing theoretical constraints. The result indicates that the Pacific ring of fire, Peru-Chile Trench, and the mid-Atlantic oceanic ridge is fall in the hotspot zones of 99%, 95%, and 90% confidence levels.

Plate tectonics in action in the Mesoarchean: Implication from the Olondo greenstone belt on the Aldan Shield of Siberian Craton

The Archean Olondo greenstone belt (OGB) is located on the Aldan shield, the largest basement of the Siberia craton. With well-preserved abundant mafic-ultramafic rocks, ≥30% in volume, the OGB is unique among other greenstone belts in the world. In this study, we present the most up-to-date geochemical and isotopic data for the ultramafic-mafic rocks of the OGB, in order to better constrain their mantle sources and the plate tectonic process involved in the formation of OGB at ca. 3 Ga. The ultramafic rocks vary from fresh to serpentinized dunites, and are highly refractory as residual mantle phase as indicated by depletion in P-Platinum Group Elements (PGE) relative to I-PGEs for highly siderophile elements (HSE). Fresh dunites show U-shaped rare earth element (REE) patterns, with positive to negative Nb anomalies, indicative of late metasomatism in their mantle source. Rhenium-Osmium isotopic compositions of these dunites yield mantle model age (TMA) of 2960–3020 Ma, comparable to the formation age of the OGB at ca. 3 Ga. Together, the data suggest that, unlike mantle cumulate origin for most of the Archean ultramafic rocks, the OGB dunites were mantle residuals after a high degree of partial melting (>30%), which subsequently interacted with the subduction-related melt/fluid. On the other hand, the OGB mafic rocks including komatiitic and tholeiitic basalts show geochemical characteristics relative to the ultramafic residuals that reinforce a subduction-related regime as their formation setting, despite extra mid-ocean ridge and plume settings. Tholeiitic basalts yield variable REE patterns from depleted, chondritic, to enriched light rare earth elements (LREE) patterns, with variable Nb-Ta anomalies, indicating their similarities with modern N-MORB and boninites, comparable to mafic rocks in typical supra-subduction zone (SSZ) ophiolites. Such mafic rocks with combined lower εNd(t) and negative Nb-Ta anomalies were most likely the result of mixing with subducted components, consistent with the observed Nb depletion in the residual dunites. The Al-depleted komatiitic basalts may have originated from deep mantle source, corresponding to garnet stability field, confirmed by their depletion in HREE and requiring a mantle plume to transport and melt at such a depth. The OGB ultramafic-mafic rocks could be a record to witness plume-induced subduction initiation processes such that mantle plume, sea-floor spreading and subduction were all in operation in the Mesoarchean time.

Intra-oceanic subduction in the southeastern Paleo-Asian ocean: Evidence from early Permian gabbro of the Diyanmiao ophiolite in central Inner Mongolia, North China

Investigation of intra-oceanic subduction can improve our understanding of plate tectonic processes and the history of continental growth. Evidence for intra-oceanic subduction in the Paleo-Asian Ocean has recently become an important focus of research, including the Diyanmiao ophiolite in central Inner Mongolia, North China. Here, we report a newly discovered occurrence of early Permian gabbro in the Diyanmiao ophiolite zone. The gabbro yields a weighted mean zircon U–Pb age of 294.4 ± 2.2 Ma. The gabbro samples are characterized by moderate SiO2 (47.32–50.51 wt%), low TiO2 (0.26–0.54 wt%) and K2O (0.04–0.75 wt%), and high Na2O (1.84–4.52 wt%) contents, high Na2O/K2O ratios (2.92–58.29), and depleted chondrite-normalized light rare Earth element patterns that are similar to N-MORB. The gabbros show slightly lower contents of high-field-strength elements (e.g., Nb, Ta, and Ti) and slightly higher contents of large-ion lithophile elements (e.g., K, Rb, Ba, and U) relative to N-MORB. In addition, the gabbros show high εNd (t) values (8.0–9.8) that are similar to those of forearc basalt in the Diyanmiao ophiolite and N-MORB. Integrating these new data with available results for ophiolite and arc-magmatic rocks of central Inner Mongolia, we propose that the studied gabbro was formed during the initial stage of intra-oceanic subduction and that the Paleo-Asian Ocean was still in a subduction setting during the early Permian.

The Mesozoic West and Central Africa Rift System (WCARS) and the older Kandi Shear Zone (KSZ): Rifting and tectonics of North Africa and South America and fragmentation of Gondwana based on geophysical investigations

Intra- and trans-continental tectonic lineaments and tectonic structures cutting Africa and Gondwana have long been identified by many authors. These structures were initially identified from topography, coast lines and geological data but more recently from remote sensing and geophysical data. Here gravity and aeromagnetic data for Africa and South America have been used to identify, track and model these structures at depth within the crust/upper mantle. The two structures investigated are the West and Central African Rift System (WCARS) and the Kandi Shear Zone (KSZ). WCARSthe nature and spatial extent of the WCARS have been identified using gravity data. In its simplest form, the WCARS consists of a set of strike-slip shear zones linked to extensional rift basins that can be traced as continuous structures from the Atlantic coast of Nigeria to southern Algeria and from the coast of Cameroon to Kenya. The rift system developed as a set of discrete tectonic pulses throughout the Mesozoic at the same time as the Central, South and Equatorial Atlantic Oceans developed into a single unified spreading centre. Each tectonic pulse had a different basin response due to the changing orientation of the plate tectonic controlled displacement field. The African rift basins formed by crustal extension involving brittle failure and ductile flow of the upper and lower crust respectively, resulting in the isostatic subsidence of the surface and rapid infilling of the basins with mainly continental derived sediments. Gravity and seismological studies support this crustal basin model originally proposed by McKenzie. This contribution shows that the evolution of the WCARS is clearly linked to the oceanic plate tectonic processes. New analysis of the fracture zone geometry in the Central Atlantic shows that these rift pulses involving crustal extension, strike-slip faulting, stratigraphic unconformities and short episodes of crustal contraction e.g. Santonian and Maastrichtian events, which can be identified and correlated with changes in the azimuth of the fracture zone geometry. KSZThe second, shorter part of the paper uses predominantly aeromagnetic data to track the KSZ from the Atlantic coast of Benin north into central Algeria before splaying out beneath the Saharan Platform. To the south the KSZ is shown, prior to the formation of the Equatorial Atlantic, to link with the Trans-Brazilian Lineament but its continuation south of Brazil still remain highly speculative.Finally, this review raises new insights and questions that are ideal for future research topics, both in developing the analytic methodologies and their application to other continental scale datasets.

Tectonic setting of metamorphism and exhumation of eclogite-facies rocks in the South Beishan orogen, northwestern China

AbstractHigh-pressure metamorphic rocks occur as distinct belts along subduction zones and collisional orogens or as isolated blocks within orogens or mélanges and represent continental materials that were subducted to deep depths and subsequently exhumed to the shallow crust. Understanding the burial and exhumation processes and the sizes and shapes of the high-pressure blocks is important for providing insight into global geodynamics and plate tectonic processes. The South Beishan orogen of northwestern China is notable for the exposure of early Paleozoic high-pressure (HP), eclogite-facies metamorphic rocks, yet the tectonism associated with the HP metamorphism and mechanism of exhumation are poorly understood despite being key to understanding the tectonic evolution of the larger Central Asian Orogenic System. To address this issue, we examined the geometries, kinematics, and overprinting relationships of structures and determined the temperatures and timings of deformation and metamorphism of the HP rocks of the South Beishan orogen. Geochronological results show that the South Beishan orogen contains ca. 1.55–1.35 Ga basement metamorphic rocks and ca. 970–866 Ma granitoids generated during a regional tectono-magmatic event. Ca. 500–450 Ma crustal thickening and HP metamorphism may have been related to regional contraction in the South Beishan orogen. Ca. 900–800 Ma protoliths experienced eclogite-facies metamorphism (~1.2–2.1 GPa and ~700–800 °C) in thickened lower crust. These HP rocks were subsequently exhumed after ca. 450 Ma to mid-crustal depths in the footwall of a regional detachment fault during southeast-northwest–oriented crustal extension, possibly as the result of rollback of a subducted oceanic slab. Prior to ca. 438 Ma, north-south–oriented contraction resulted in isoclinal folding of the detachment fault and HP rocks. Following this contractional phase in the middle Mesozoic, the South Beishan orogen experienced thrusting interpreted to be the response to the closure of the Tethyan and Paleo-Asian Ocean domains. This contractional phase was followed by late Mesozoic extension and subsequent surface erosion that controlled exhumation of the HP rocks.

Did the Neoproterozoic Revolution Extend to the Deep Mantle?

Did the Neoproterozoic Revolution Extend to the Deep Mantle?

Western structural closing of the Pripyat–Dneprov–Donetsk aulacogen

In segmented paleorift systems such as the Pripyat–Dnieper–Donetsk aulacogen, which penetrate deep into the continent from the side of the paleoocean margin and most often close here, each segment represents an autonomous structure of the lithosphere. The presented article shows that the tectonic position of the Pripyat paleorift as the western closing segment of the indicated aulacogen was due to a change in the intensity of syn-rift and plate-tectonic processes in the extended rift system from east to west. In the band of the Paleoproterozoic Fenno-Sarmatian collision, the Central Belarusian suture zone of the Central Russian transpression belt isolated by S. V. Bogdanova in 2018 was a blocking transverse barrier to relative development of Hercynian synrift processes in the western direction.

Paleoproterozoic Plate Tectonics Recorded in the Northern Margin Orogen, North China Craton

AbstractThe occurrence of plate tectonic processes on Earth during the Paleoproterozoic is supported by ca. 2.2–1.8 Ga subduction‐collision orogens associated with the assembly of the Columbia‐Nuna supercontinent. Subsequent supercontinent breakup is evidence by global ca. 1.8–1.6 Ga large igneous provinces. The North China craton is notable for containing Paleoproterozoic orogens along its margins, herein named the Northern Margin orogen, yet the nature and timing of orogenic and extensional processes of these orogens and their role in the supercontinent cycle remain unclear. In this contribution, we present new field observations, U‐Pb zircon and baddeleyite geochronology dates, and major/trace‐element and isotope geochemical analyses from the northern margin of the North China craton that detail its Paleoproterozoic tectonic and magmatic history. Specifically, we record the occurrence of ca. 2.2–2.0 Ga magmatic arc rocks, ca. 1.9–1.88 Ga tectonic mélange and mylonitic shear zones, and folded lower Paleoproterozoic strata. These rocks were affected by ca. 1.9–1.8 Ga granulite‐facies metamorphism and ca. 1.87–1.78 Ga post‐collisional, extension‐related magmatism along the cratonal northern margin. We interpret that the generation and emplacement of these rocks, and the coupled metamorphic and magmatic processes, were related to oceanic subduction and subsequent continent‐continent collision during the Paleoproterozoic. The occurrence of ca. 1.77–1.73 Ga mafic dykes and ca. 1.75 Ga mylonitic shear zones along the northern margin of the North China craton may have been related to a regional mantle plume event. Our results are consistent with modern style plate tectonics, including oceanic subduction‐related plate convergence and continent‐continent collision, operating in the Paleoproterozoic.