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Deforming belts in the Arabian‐Nubian Shield (ANS) are classified into (1) suture‐related belts, including arc–arc and arc‐continental, and (2) post‐accretionary systems, including N‐trending compression zones and NW‐trending strike‐slip faults. Terrane accretion took place in the ANS between 800 and 700 Ma, along arc–arc sutures. Such sutures are directed from E to NE in the northern part of the ANS, and from N to NE in the south, and are aligned in the north and east with N‐ or S‐verging ophiolitic nappes, or in the south with W‐verging nappes. The Asir, Hijaz, and Midyan terranes formed the Western Arabian shield by 715 Ma. The Afif terrane collided with the Hijaz and Asir terranes between 680 and 640 Ma, terminating the subduction along the Nabitah suture. Subduction began west of the Al Amar arc near the margin of the Ar Rayn terrane at 670 Ma. Afif and Ar Rayn terranes collided along the Al Amar‐Idsas suture about 640 Ma, producing the Idsas orogeny that initiated the major faulting and folding. Strike‐slip faults and upright folds related to oblique convergence between terranes and/or post‐accretionary systems deform the southern sutures. The eastern and western boundaries of the ANS are marked by arc‐continental sutures and characterized by N‐trending deformation belts that formed at 750–650 Ma when the ANS collided with East and West Gondwana. [ABSTRACT FROM AUTHOR]

The International Ocean Discovery Program cored five sites during Expeditions 372 and 375 to investigate the Hikurangi margin, offshore Poverty Bay, North Island, New Zealand. Trench‐wedge deposits were recovered at Site U1520, and the frontal accretionary prism was cored at Site U1518. Based on X‐ray diffraction analyses of the clay‐sized fraction, illite and smectite are the dominant clay minerals. Compositional scatter is unusually large, and a well‐defined mixing trend is evident between illite‐rich and smectite‐rich end members. The proportion of smectite ranges from 9 to 59 wt.% at Site U1518 and from 12 to 62 wt.% at Site U1520. Nearby slope deposits at Sites U1517 and U1519 are more homogeneous, with consistently higher proportions of smectite. The East Cape Current is probably responsible for transporting smectite‐rich suspensions towards the south‐west from sources that include the Taupo Volcanic Zone. Illite‐rich muds require different sources and different routing paths to the trench. Some suspended sediment probably originated in the Waipaoa watershed on the North Island, entering the trench via Māhia Canyon. Other gravity flows probably started in Kaikōura Canyon or Cook Strait and moved towards the north‐east, down the trench's axial channel. The influence of turbidity currents on clay composition changes with water depth, as do the effects of reworking by contour currents and the role of passive hemipelagic settling. The broad continuum of clay compositions in the trench is enhanced by variable amounts of mixing among multiple ocean currents and fluctuating durations of suspension within a bottom nepheloid layer. That complexity deviates from the paradigm of turbidite‐dominated trench wedges. The margin's compositional diversity also complicates the goal of predicting or modelling three‐dimensional variations in frictional, geotechnical and hydrogeological properties. Those lithological properties are important because they affect slope stability along strike and changes in fault‐slip behaviour along the subduction interface. [ABSTRACT FROM AUTHOR]

During orogenesis, the forming structures vary according to several factors such as the nature of rocks, geological environment, P-T conditions, and type of collision. Deformational styles play a significant role in orogenic processes. While thin-skinned tectonics comprise only supracrustal deformation, in thick-skinned tectonics both supracrustal rocks and the basement are deformed reaching Moho depths. Both styles may coexist in a single orogenic system and the recognition of different sectors may help to unravel the nature of the orogen. In this paper, we apply the definition of tectonic styles to the São Miguel do Aleixo Shear Zone, a major transpressive shear zone in the Sergipano Belt, NE Brazil. It separates domains with different structural and metamorphic grades and provides evidence for understanding the tectonic implications of this structure and its role in the evolution of the orogen. We suggest that the São Miguel do Aleixo Shear Zone is a major crustal boundary formed during the oblique collision with and a possible transitional region between areas with dominant thin-skinned and thick-skinned tectonics. The presence of both tectonic styles together with metamorphic grade and geological environment lead to suggest that accretionary processes had a key role in the evolution of the Sergipano Belt. • Accretionary process in Western Gondwana revealed by aeromagnetic data. • Main suture zone at the southern limit of the Pernambuco-Alagoas Superterrane. • Aeromagnetic characterization of Sergipano Belt, Borborema Province, NE Brazil. [ABSTRACT FROM AUTHOR]

The evolution of orogenic wedges can be determined through stratigraphic and thermochronological analysis. We used apatite fission-track (AFT) and apatite and zircon (U–Th–Sm) / He (AHe and ZHe) low-temperature thermochronology to assess the thermal evolution of the Ukrainian Carpathians, a prime example of an orogenic wedge forming in a retreating subduction zone setting. Whereas most of our AHe ages are reset by burial heating, 8 out of 10 of our AFT ages are partially reset, and none of the ZHe ages are reset. We inverse-modeled our thermochronology data to determine the time–temperature paths of six of the eight nappes composing the wedge. The models were integrated with burial diagrams derived from the stratigraphy of the individual nappes, which allowed us to distinguish sedimentary from tectonic burial. This analysis reveals that accretion of successive nappes and their subsequent exhumation mostly occurred sequentially, with an apparent increase in exhumation rate towards the external nappes. Following a phase of tectonic burial, the nappes were generally exhumed when a new nappe was accreted, whereas, in one case, duplexing resulted in prolonged burial. An early orogenic wedge formed with the accretion of the innermost nappe at 34 Ma, leading to an increase in sediment supply to the remnant basin. Most of the other nappes were accreted between 28 and 18 Ma. Modeled exhumation of the outermost nappe started at 12 Ma and was accompanied by out-of-sequence thrusting. The latter was linked to emplacement of the wedge onto the European platform and consequent slab detachment. The distribution of thermochronological ages across the wedge, showing non-reset ages in both the inner and outer part of the belt, suggests that the wedge was unable to reach dynamic equilibrium for a period long enough to fully reset all thermochronometers. Non-reset ZHe ages indicate that sediments in the inner part of the Carpathian embayment were mostly supplied by the Inner Carpathians, while sediments in the outer part of the basin were derived mostly from the Teisseyre–Tornquist Zone (TTZ) or the southwestern margin of the East European Platform. Our results suggest that during the accretionary phase, few sediments were recycled from the wedge to the foredeep. Most of the sediments derived from the Ukrainian Carpathian wedge were likely transported directly to the present pro- and retro-foreland basins. [ABSTRACT FROM AUTHOR]

The Indochina Block is considered as a part of the Indian and Australian margins of Gondwana, and the Tam Ky‐Phuoc Son Suture Zone (TPSZ) lies in its central region as the boundary between the Truong Son Belt and Kon Tum Massif (KTM). However, the early Palaeozoic tectonic nature and timing of magmatism within the TPSZ are still controversial. To tackle the issue, we present herein the zircon U–Pb ages with their trace‐element compositions of five dioritic–granitic granitoids in the TPSZ. U–Pb age dating of magmatic zircons from them suggests emplacement ages of ca. 447–450 Ma. Whole‐rock geochemistry of the samples shows low TiO2 contents (0.36–1.18 wt%) and Mg# (42.04–53.07). They display enrichment in light rare earth elements ([La/Yb]N = 4.10–15.19), pronounced negative Nb, Ta and Ti anomalies relative to the chondrite‐normalized rare earth element patterns and primitive mantle‐normalized trace‐element distribution diagrams. The whole‐rock geochemical characteristics together with zircon trace‐element compositions from the same samples revealed their petrogenesis of a continental arc tectonic setting. Combining the results from this study together with available data from the TPSZ and its neighbouring units, we suggest accretionary orogenesis was responsible for generation of the Ordovician magmatic arc in the KTM, which will provide better understanding of the Early Palaeozoic evolution of the eastern Indochina Block. [ABSTRACT FROM AUTHOR]

We compare and contrast the materials and mechanisms of continental crustal growth in the largest preserved regions of Phanerozoic and Archean juvenile additions to the crust, to test for similarities or differences in the formation of continents through time. We accomplish this through a comparison of map patterns, lithological contents, and structural and metamorphic evolution of the Phanerozoic Altaid orogenic system of Asia, with the Archean Superior Province of the North American Craton, using a method termed comparative orotomy. Both orogenic systems consist of collages of curvilinear belts of eroded arcs, some older continental slivers, and vast tracts of former subduction/accretionary complexes. These contain numerous shreds of portions of the ophiolite suite, slivers of island and continental arcs, and accreted oceanic plateau, all intruded by multiple magmatic suites during or between multiple deformation events, then sliced by large transcurrent fault systems and bent into large oroclinal structures. We make this comparison because the Superior Province is a typical Archean craton that was later, in the Paleoproterozoic, incorporated into the larger North American Craton, and has occupied a central position in several supercontinents (e.g. Kenorland and Nuna, which then formed the core of Columbia, Rodinia, Laurentia and Pangea) during its longevity. Since it is the largest single fragment of Archean continental cratonic lithosphere preserved on Earth, the Superior Province is widely regarded as a testing ground for how Earth's continental crust was formed. Likewise, the Altaids encompass the largest region of crustal growth for the Phanerozoic. Our comparison with the Altaids is needed, as in recent years many myths about how the planet may have responded to higher heat production and flow in the Archean have emerged, because of trends in the science where regional geology is ignored in favor of numerical models, isotopic proxies for assumed models of chemical behavior for crust-forming or tectonic processes, or comparisons with other-worldly bodies that bear little resemblance to our hydrous Earth. Thus, we return to the geological record, and here describe the map patterns, lithological associations, structural patterns and evolution of both the Altaids and Superior Province, showing how comparative tectonics, orotomy, is useful in the absence of meaningful paleomagnetic or biostratigraphic data. We pay particular attention to the style of preservation of disaggregated members of the ophiolite suite (ophirags) and their relationships with other tectonic units, and to the widespread but largely overlooked role of late-stage major transcurrent motions and structural slicing of both Archean and Phanerozoic orogenic systems in defining the present-day architecture of both orogenic systems. [ABSTRACT FROM AUTHOR]

The Gamburtsev Subglacial Mountains (GSMs) in the interior East Antarctic Craton are entirely buried under the massive East Antarctic ice sheet, with a ~50–60 km thick crust and ~200 km thick lithosphere, but little is known of the crustal structure and uplift mechanism. Here, we use airborne gravity and aeromagnetic anomalies for characteristic analysis and inverse calculations. The gravity and magnetic images show three distinct geophysical domains. Based on the gravity anomalies, a dense lower crustal root is modelled to underlie the GSMs, which may have formed by underplating during the continental collision of Antarctica and India. The high frequency linear magnetic characteristics parallel to the suture zone suggest that the upper crustal architecture is dominated by thrusts, consisting of a large transpressional fault system with a trailing contractional imbricate fan. A 2D model along the seismic profile is created to investigate the crustal architecture of the GSMs with the aid of depth to magnetic source estimates. Combined with the calculated crustal geometry and physical properties and the geological background of East Antarctica, a new evolutionary model is proposed, suggesting that the GSMs are underlain by part of a Pan-African age advancing accretionary orogen superimposed on Precambrian basement. [ABSTRACT FROM AUTHOR]

Conceptual models of orogenic accretionary prisms assume that peak temperatures (Tmax⁡) increase towards the internal domains as crustal rocks are accreted from the lower to the upper plate. However, the recognition of pre-orogenic heating events in mountain belts questions the magnitude of thermal overprint during nappe stacking. Using Raman spectroscopy on carbonaceous material (RSCM) to calculate Tmax⁡ , we have investigated the thermal record of Lower Jurassic to Eocene strata exposed along six stratigraphic sections at the front of the Digne Nappe (SW Alps), from the Devoluy Massif to the Castellane Arc. Our results highlight two groups of depth-dependent temperatures: (1) a regionally extensive and constant Tmax⁡ up to 300–330 ∘ C measured in the Jurassic succession and (2) regionally variable lower temperatures (<150 ∘ C) recorded either in the Upper Mesozoic or the syn-orogenic sequence. Modelling shows that the highest palaeotemperatures were achieved during the Early Cretaceous (∼ 130 Ma), associated with the Valaisan–Vocontian rifting, while the lowest Tmax⁡ reflect post-rift thermal relaxation in the Alpine foreland basin. This study provides a striking new example where mid-crustal palaeotemperatures measured in sediments accreted from the downgoing plate are inherited. An estimated peak thermal gradient of 80–90 ∘ C km -1 requires a crustal thickness of 8–10 km during the Early Cretaceous, hence placing constraints for tectonic reconstruction of rift domains and geophysical interpretation of current crustal thickness in the SW Alps. These results call for the careful interpretation of palaeothermal data when they are used to identify past collisional thermal events. Where details of basin evolution are lacking, high-temperature records may be misinterpreted as syn-orogenic, which can in turn lead to an overestimation of both orogenic thickening and horizontal displacement in mountain belts. [ABSTRACT FROM AUTHOR]

The NE Jiangxi ophiolitic mélange belt is the key area for understanding the evolution of the Paleo‐South China Ocean and the assembly of the Yangtze and Cathaysia blocks. The age of the deformed matrix is closer to the tectonic emplacement time of the ophiolitic mélange than that of the blocks. However, the rock types, ages and structural deformation of the matrix in the NE Jiangxi ophiolitic mélange belt lack comprehensive understanding. Based on the zircon U–Pb geochronology, Hf isotope and trace element analyses of the deformed matrix in the NE Jiangxi ophiolitic mélange belt, we report the ages of meta‐rhyolite and tuffaceous phyllite to be 800–760 Ma, and the maximum depositional age of the clastic matrix is ~760 Ma. The youngest deformed matrix ages of 800–760 Ma constrain the tectonic emplacement age of the NE Jiangxi ophiolitic mélange belt to the late Qingbaikou period after 800–760 Ma and before the deposition of the Xiuning Formation (765–732 Ma). Zircon ages, trace element and Hf isotope compositions indicate that the 800–760 Ma matrix was formed in a back‐arc basin environment with obvious addition of 1000–800 Ma arc materials and recycled Paleoproterozoic crustal materials. The detrital material source areas were the 800–760 Ma arc and earlier accretionary wedge as juvenile crust. The multi‐stage arc magmatism, metamorphism and deformation in the NE Jiangxi ophiolitic mélange belt suggest that multi‐stage subduction of the Paleo‐South China Ocean and Neoproterozoic accretionary orogeny occurred during 1–0.76 Ga at the southeast margin of the Yangtze Block. Combined with the regional geological data, the Neoproterozoic back‐arc basin or the foreland basin around the Yangtze Block closed after ~760 Ma. The collision and amalgamation of the Yangtze and Cathaysia blocks resulted in the final closure of the Paleo‐South China Ocean, which have occurred in the early Paleozoic. [ABSTRACT FROM AUTHOR]

This study attempts to evaluate the effects of two important structural design parameters on the dynamic behavior of back-to-back mechanically stabilized earth walls (BBMSEWs). For this purpose, a series of shake table tests were carried out on seven BBMSEW models with different overlap lengths of reinforcements (LR) and arrangements of connecting two opposing walls to each other. Based on the results of the particle image velocimetry (PIV), a combination of a concave surface and an inclined plane was observed as the failure surface in models and it was concluded that the complete connection of two opposing walls to each other could prevent the formation of a failure wedge in BBMSEWs. Findings indicated that although separating two opposing walls from each other and reducing the overlap LR increase lateral deformations, these could be two effective solutions for reducing the lateral earth pressure in BBMSEWs. On the other hand, comparisons have showed that Seed-Whitman and Mononobe-Okabe methods are conservative for calculating dynamic lateral pressures (σAE) in BBMSEWs at HPGA < 0.6g (where HPGA is horizontal peak ground acceleration) and that this conservatism fades gradually by increasing HPGA, and finally at HPGA ≥ 0.9g, the obtained values of σAE becomes almost greater than those predicted by these methods. [ABSTRACT FROM AUTHOR]

To determine the metamorphic ages of the accretionary complexes in the Northern Chichibu Belt in SW Japan, K–Ar dating was conducted using weakly metamorphosed sedimentary rocks collected from the Kanto Mountains, Central Japan. Whole-rock ages were obtained for chert and red shale samples, and the mineral ages of fine-grained illite with a grain size of less than 4 μ m were obtained for chert, red shale, mudstone, acidic tuff, and basic tuff. The K–Ar ages of chert and red shale presented large variations, with systematically older ages compared to those of mudstone and tuff in the same strata. The influence of submarine hydrothermal activities on chert and red shale before subduction is a possible cause of this deviation. The illite samples, which were fractionated into four grain-size classes using a suspension method, yielded older ages and higher illite crystallinity (i.e., smaller values of Kübler's crystallinity index) for larger grain-size classes. The peak metamorphic ages were determined from the K–Ar ages of the 3–4 μ m class illite in mudstone and tuff. The Late Jurassic to the Earliest Cretaceous accretionary complex of the lowest structural unit (Kashiwagi Unit) was dated within a small range between 117–110 Ma, which is distinctly older than the K–Ar ages of white mica reported from the Sanbagawa Belt. The peak metamorphic age of acidic tuff (113 Ma) at the type locality of the Mikabu Greenstones indicates that the subducted Mikabu seamount is a constituent of the Kashiwagi Unit. The peak metamorphic ages of the Manba and Kamiyoshida Units were obtained as 132–107 Ma and 163–144 Ma, respectively. Major structural discontinuity is suggested within the Middle Jurassic accretionary complexes. [ABSTRACT FROM AUTHOR]

The Polish Geophysical Expedition to West Antarctica in 1979-1980 was carried out by the Institute of Geophysics, Polish Academy of Sciences. Beside deep seismic soundings, 12 multi-channel seismic profiles, with a total length of ca 1000 km have been recorded north and east of the South Shetland Islands and in the Bransfield Strait, but they have never before been completely interpreted and published. All profiles have been processed with modern processing flow including time migration. Profiles crossing the South Shetland Trench revealed distinct reflector inside continental slope, which has been interpreted as border between buried accretionary prism and overlying slope sediments of glacial-marine origin. Profiles in the Bransfield Strait show traces of the Last Glacial Maximum (LGM) in the form of glacial foreground valleys, with some of them used as weak spots for young age volcanic intrusions. This paper is the first comprehensive geological interpretation of collected dataset and differences between results from other expeditions are discussed. [ABSTRACT FROM AUTHOR]

The displacement-based design methods for bridge abutments, essentially cantilever retaining walls, require an input of expected ground motion on-site. For major projects, these motions are obtained based on site-specific response studies. In other projects, it is merely a guess based on the PGA values prescribed in the design codes. Further, the seismic displacements in retaining walls are usually estimated using simplified rigid–plastic analytical models. These models do not account for the effect of soil nonlinearity, alteration of input signal due to soil–structure interaction, amplification effects, or material damping. The currently followed seismic load factor of 1 on the prescribed PGA does not account for the induced uncertainties for the reasons given here, which can often be unconservative and lead to failure. This study statistically quantifies the effect of these uncertainties and proposes scaling factors on the design PGA of the input motion to estimate more reliable seismic displacements accounting for the assumptions in the simplified rigid–plastic model. The nonlinear finite-element analysis of retaining wall in OpenSees has been compared with the analytical double-wedge model considering a realistic V-shaped mechanism in backfill. It includes analysis for 83 different cases of earthquake motions scaled to four different PGAs. Finally, the scaling factors are proposed to estimate residual as well as peak sliding and in-plane rotational displacements of cantilever retaining walls with and without shear key. [ABSTRACT FROM AUTHOR]

The southern Altaids is of critical significance for a better understanding of the orogenic architecture and continental growth in Central Asia. The tectonic setting of the South Tianshan, a typical Paleozoic accretionary orogen within the southern Altaids, is controversial. This study reports new structural relations, geochemistry, and geochronology of previously assigned Late Silurian–Early Devonian strata in the Karaghol area of the South Tianshan. The constituent rocks are pillow basalts, ribbon cherts, limestones, siliceous mudstones, and turbiditic sandstones. Field relations show that these rocks are characterized by fault-bound tectonic slices and mélanges with typical block-in-matrix structures. The pillow basalts show N-MORB, E-MORB, and OIB geochemical fingerprints, and our new LA-ICP-MS zircon U–Pb dates indicate that the maximum eruption age of the OIB-type basalts was ca. 386 Ma, whereas the Maximum Depositional Ages (MDAs) of the turbiditic sandstones were ca. 369 Ma and 355 Ma. The age spectra of the detrital zircons, which define prominent peaks at ca. 370 Ma, ca. 420 Ma, ca. 770 Ma, ca. 960 Ma with subordinate Proterozoic ages, are a good match with the igneous ages of intermediate to felsic magmatism in the Yili–Central Tianshan arc. We conclude that the previously assigned Late Silurian–Early Devonian strata in the Karaghol area are mostly dismembered structural components and mélanges that belonged to an accretionary complex of Late Devonian to Early Carboniferous age, into which fragments of a Late Devonian seamount along with South Tianshan oceanic crustal rocks were likely emplaced during north-dipping subduction in the Early Carboniferous. This well-documented accretionary orogen in the Chinese South Tianshan is a key constraint on the accretionary orogenesis of the southern Altaids. [ABSTRACT FROM AUTHOR]

Bottom simulating reflections (BSRs) interpreted in seismic data are one of the most used indicators for the presence of gas hydrates. In numerous hydrate provinces, including the Hikurangi margin, east of New Zealand, distinct and anomalous gaps in reflectivity punctuate otherwise continuous BSRs. We undertake a seismic stratigraphic and structural interpretation of a dense grid of two‐dimensional reflection seismic data to investigate possible causes of widespread BSR gaps that occur near the hinge area of synclines. We explain these BSR gaps with a tectono‐sedimentary model where sedimentation into accretionary wedge synclines leads to an upward migration of the base of gas hydrate stability with respect to stratigraphy, causing dissociation of gas hydrates to water and free gas. A trough‐shaped syncline's radially dipping beds promotes along‐strata, upward migration of gas away from the hinge area, incrementally depleting synclinal hinges of gas and resulting in a BSR gap or muted BSR. This model may be applicable to observed gaps in BSRs in tectonically active slope basins worldwide. [ABSTRACT FROM AUTHOR]

In addition to well established properties that control the presence or absence of the hydrate stability zone, such as pressure, temperature, and salinity, additional parameters appear to influence the concentration of gas hydrate in host sediments. The stratigraphic record at Site 17A in the Andaman Sea, eastern Indian Ocean, illustrates the need to better understand the role pore-scale phenomena play in the distribution and presence of marine gas hydrates in a variety of subsurface settings. In this paper we integrate field-generated datasets with newly acquired sedimentology, physical property, imaging and geochemical data with mineral saturation and ion activity products of key mineral phases such as amorphous silica and calcite, to document the presence and nature of secondary precipitates that contributed to anomalous porosity preservation at Site 17A in the Andaman Sea. This study demonstrates the importance of grain-scale subsurface heterogeneities in controlling the occurrence and distribution of concentrated gas hydrate accumulations in marine sediments, and document the importance that increased permeability and enhanced porosity play in supporting gas concentrations sufficient to support gas hydrate formation. The grain scale relationships between porosity, permeability, and gas hydrate saturation documented at Site 17A likely offer insights into what may control the occurrence and distribution of gas hydrate in other sedimentary settings. [ABSTRACT FROM AUTHOR]

The Sierra Nevada de Santa Marta Massif (SNSM) in the Northern Andes affords an insight into lower crustal section structural relations. This polymetamorphic basement comprises high-grade gneisses, which alternate with imbricate slivers of medium-to high-grade schists. New field observations coupled with LA-ICP-MS U-Pb zircon geochronology and trace element geochemistry reveal some hallmarks on the evolution of this strongly remobilized crustal section. The Proterozoic Rodinia-type basement (1100 − 880 Ma) recorded at least two separated tectono-metamorphic events (M0 and M1) and was subsequently affected by a magmatic episode during the Ordovician (~450 Ma; M2). During the Permian, subduction of Panthalassian oceanic crust beneath Pangaea led to the installation of a magmatic-arc and probably coeval metamorphism (~288-266 Ma; M3). Middle-Late Triassic roll-back extension of the upper plate led to anatexis and metamorphism (M4) reaching peak conditions at ca. 239 Ma with concurrent crystallization of garnet and sillimanite, followed by a decreasing geothermal gradient at ca. 202 Ma. Early Jurassic arc build-up under a rejuvenated convergence was contemporaneous to metamorphism on an accretionary wedge (M5) at ca. 185 Ma. The SNSM Massif basement units yield metamorphic ages that compare to similar tectono-metamorphic provinces in NW-Gondwana, including the Central Cordillera of Colombia, the Guajira Peninsula, the Mérida Andes, the Perijá Range, the Santander Massif, and the Central American and Mexican terranes to the south of the Ouachita-Marathon Suture (Oaxaquia, Mixteca, and Maya blocks). Age equivalence of these events is best explained by the existence of a conjugate margin with Laurentia until the Late Triassic ~216 Ma, which preceded the inception of a Colombian marginal seaway during the break-up of western Pangaea. Furthermore, the presence of Ordovician metamorphic rocks in the SNSM Massif confirms the extent of a Famatinian arc along NW-Gondwana during the Early Palaeozoic. [ABSTRACT FROM AUTHOR]

Zircon U-Pb-Hf isotopes, integrated with bulk-rock major, trace element, and isotopic compositions of amphibolites from the Makran ophiolitic mélange complex (OMC), carry implications on the evolution of the Neo-Tethys Ocean. Field and petrographical-geochemical data confirm these amphibolites as products of the metamorphism of a mafic protolith. Zircons from the amphibolites have U-Pb ages of Late Early Permian (273 Ma) to Late Triassic (210 Ma), and their εHf(t) values vary from +6.32 to +15.4. The Makran amphibolites are geochemically similar to transitional- or enriched-MORBs. The 143Nd/144Nd(t) ratios for amphibolites range from 0.51260 to 0.512551, and 87Sr/86Sr(t) from 0.704433 to 0.706244. The Pb-isotope composition of these rocks is radiogenic, ranging from 15.61-15.64 for 207Pb/204Pb(t), 18.98-19.21 for 206Pb/204Pb(t) and 38.88-39.36 for 208Pb/204Pb(t). The isotopic and geochemical signatures of these rocks suggest that they were most likely formed during continental rifting and originated from a relatively enriched mantle source. This is also attested by their high zircon Ce/Ce* ratios. We interpret these amphibolites related to the Permian-Triassic plumes that triggered Gondwana rifting to open the Neo-Tethyan Ocean before metamorphosed, fragmented, and accreted into the Makran accretionary complex probably during Early to Late Cretaceous time. [ABSTRACT FROM AUTHOR]

The Western Kunlun orogenic belt (WKOB) located south of the Tarim Basin and the north-western margin of the Tibetan Plateau, was previously considered a complex orogenic belt that closed along the Kudi-Qimanyute suture zone (KQSZ) and Mazar–Kangxiwar suture zone (MKSZ) from north to south during Proto- and Paleo-Tethys. The MKSZ between the South Kunlun and Tianshuihai terranes was interpreted as the southern boundary of the WKOB that formed during the subduction of the Paleo-Tethys oceanic crust. However, the evolution of the MKSZ in the Proto-Tethys Ocean remains controversial. We newly recognized seamount formation with pillow basalts and carbonate cap from Dongguashan group on Tianshuihai terrane. The pillow basalts had geochemical features of typical oceanic island basalts (OIBs). Zircon U–Pb dating revealed that this basalt had a crystallization age of 465 ± 6.6 Ma, with a gap of more than 10 Ma between the pillow basalts and fossils in the seamount. This implied that the basalt base reached the carbonate compensation depth. Accordingly, the seamount depositional age was restricted to the Late Ordovician. Detrital zircon showed that part of the clastic unit at the top of the Dongguashan group originated from the South Kunlun and Tianshuihai terranes, suggesting that the analysed sediments probably formed in the remnant of the Proto-Tethys Ocean and were deposited on the top of or accreted into the seamount during oceanic crust subduction. This discovery provides robust evidence of the MKSZ undergoing an evolution with Proto-Tethys. Moreover, our results supported the approach that an accretionary wedge, including the Late Ordovician seamount in the southern MKSZ, should be considered part the WKOB. [ABSTRACT FROM AUTHOR]

Deciphering the tectono‐metamorphic evolution of Precambrian terranes can be difficult due to reworking by later superimposed events. Whole‐rock elemental and isotopic geochemistry and zircon U–Pb geochronology are often employed in those studies, but these approaches are often not sensitive to the presence of multiple events and medium‐grade metamorphic episodes. The Rio Apa Terrane (RAT), an allochthonous fragment of the Amazonian Craton, is a crustal block with a well‐characterized crustal evolution but with no detailed thermal constraints for its tectono‐metamorphic evolution. In contrast to previous studies, we show the existence of four tectono‐metamorphic events at c. 1,780, c. 1,625, c. 1,420–1,340, and c. 1,300–1,200 Ma on the basis of apatite, titanite, and rutile U–Pb, in situ white‐mica Rb–Sr, and in situ garnet Lu–Hf geochronology combined with mineral chemistry and phase‐equilibria modelling. The c. 1,780 Ma event is recorded in the basement of the Western domain, representing an extensional event coeval with the development of its Eastern domain in response to the retreat stage of the accretionary system. This is followed by juxtaposition of the Western and Eastern domains along a major crustal boundary at c. 1,625 Ma, which is defined by the magnetic profiles and zircon U–Pb–Hf data across the boundary. The third and fourth events correspond to progressive high‐pressure/medium‐temperature (HP/MT) metamorphism, characterized by an anticlockwise P–T path, suggesting a convergent‐to‐collisional tectonic setting. The RAT was accreted to the adjoining Paraguá Terrane at c. 1,420–1,340 Ma under an isobaric P–T evolution spanning ~530°C to 600°C and ~10.0 kbar. Subsequently, the combined Rio Apa and Paraguá terranes collided with the SW Amazonian Craton at c. 1,300–1,200 Ma, reaching P–T conditions of ~560–580°C and ~10.9–11.7 kbar during crustal thickening. This study reveals for the first time the existence of a HP/MT metamorphic evolution related to the growth of the SW Amazonian Craton as part of an accretionary orogenic system during Rodinia assembly in the Palaeoproterozoic to Mesoproterozoic. [ABSTRACT FROM AUTHOR]

In contrast to well‐documented (U)HP rocks of the Erzgebirge crystalline complex (Saxothuringian Domain, Bohemian Massif), the surrounding medium‐ to low‐grade metasediments are scarcely studied, although they provide an important link between deep subduction and mid‐crustal processes. To constrain the Variscan evolution from continental subduction to orogenic wedge, the transition from the low‐grade phyllites to the footwall medium‐grade micaschists was investigated. We used thermodynamic modelling to constrain the P‐T conditions of four deformation fabrics (D1–D4) identified by the structural analysis. The studied area was first affected by the M1‐D1 event characterized by HP‐LT minerals (garnet, chloritoid, phengite, paragonite, and rutile) defining the S1 foliation. The calculated peak P‐T conditions for M1 increase from 13 kbar and 520°C in phyllites to 25 kbar and 560°C in micaschists. The corresponding apparent geothermal gradient 6–11°C/km is typical for a subduction environment. The M2‐D2 event corresponds to deformation and metamorphic overprint of the previous fabric during partial decompression. The M3‐D3 event is mainly developed in micaschists, and the M3 overprint intensifies towards the footwall. It is accompanied by a subhorizontal S3 cleavage characterized by MP‐MT assemblage containing biotite, staurolite and ilmenite. The inferred peak P‐T conditions for M3 are 5–9 kbar and 595°C representing a barrovian‐type geothermal gradient from 17–30°C/km. Finally, all metamorphic fabrics were heterogeneously affected by the low‐grade M4‐D4 upright folding. The D1‐D2 events are interpreted to record the growth and evolution of the Saxothuringian orogenic wedge while its present‐day architecture resulted from significant vertical shortening D3 associated with barrovian type M3‐metamorphism. This stage corresponds to the ductile thinning of the wedge. A new model is proposed, in which the Erzgebirge part of the Saxothuringian Domain reveals a spectacular example of active margin evolution characterized by (i) formation of accretionary prism, (ii) building of the orogenic wedge by accretion of subducted continental crust, and (iii) its extensional collapse. [ABSTRACT FROM AUTHOR]

The southern Ryukyu subduction zone is one of the potential sources for tsunamigenic earthquakes. Despite a great seismic risk, the deformation pattern remains poorly known, primarily due to the absence of seafloor constraints. With GNSS‐acoustic measurements over years, we characterize the convergence rate across this margin growing from 92 mm/yr offshore eastern Taiwan to 123 mm/yr near the Gagua Ridge. The new data suggest the subduction interface is capable of hosting Mw 7.5–8.4 earthquakes. The orientations of seafloor movement and P‐axes in the Nanao Basin are both subnormal to the trench, notably deviate from the direction of plate convergence. By considering the combined effect of plate convergence and backarc rifting, different trends between the forearc convergence, P‐axes, and seafloor movement may indicate some degree of slip‐partitioning. The trench‐parallel component is likely accommodated in part by earthquakes near Taiwan, lower plate deformation, and strike‐slip faults within the accretionary wedge. Plain Language Summary: The southern Ryukyu margin near Taiwan accommodates the relative plate motion and backarc rifting. This region has suffered from tsunamis and large earthquakes with magnitudes of about eight in the history, yet insufficient offshore data still hamper our ability to fully characterize fault slip behaviors. We demonstrate how a seafloor geodetic technique, with the combination of the Global Navigation Satellite System and acoustic ranging, can help us constraining fault slip behaviors and assessing seismic risks. We infer a fraction of oblique plate motion is accommodated by the subduction interface capable of hosting earthquakes with moment magnitudes ranging from 7.5 to 8.4. Incorporating seismological observations, we find different trends of the compressive axes of earthquakes, seafloor movement, and the forearc convergence, which may indicate some degree of strain partitioning. The trench‐parallel motion is partially taken up by seismic slip near Taiwan, subducting plate deformation, and strike‐slip faults within the forearc. More near‐trench observations are crucial to assessing seismic hazard and evaluating strain partitioning along the southern Ryukyu Trench. Key Points: Rapid convergence across the southern Ryukyu margin increases eastward from 92 mm/yr offshore Taiwan to 123 mm/yr near the Gagua RidgeOblique convergence is accommodated in part by the plate interface fault capable of hosting earthquakes with Mw 7.5–8.4Different trends between the forearc convergence, P‐axes, and seafloor movement may indicate some degree of slip partitioning [ABSTRACT FROM AUTHOR]

The ~1 200 to 1 000 Ma Namaqua-Natal Metamorphic Province (NNMP) is part of a major orogenic belt in the westcentral parts of South Africa and most likely formed during the convergence of the Laurentia and Kalahari cratons during the assembly of Rodinia. Controversy exists regarding the mode of formation of the Namaqua sector of the NNMP, with some supporting the idea of accretion of different crustal fragments while others argue that it formed in a continental back-arc setting. Here we address this controversy and provide information regarding the provenance of two metasedimentary successions that are preserved to the east and west of the Trooilapspan shear zone, which represents the Eastern Namaqua Front. 207Pb/206Pb SHRIMP ages for detrital zircon grains sampled from the Keis supergroup, which is preserved in the Kheis Terrane and situated on the Kalahari Craton, support derivation from Archaean to Paleoproterozoic source areas on the Kaapvaal Craton. In contrast, SHRIMP ages from detrital zircon grains of the Korannaland Group, which forms the metamorphosed supracrustal sequence of the Kakamas Domain, show derivation from late-Paleoproterozoic to Mesoproterozoic source areas like the Bushmanland and Richtersveld domains as well as the gneissic basement of the Kakamas Domain. The zircon age distribution data show that the metasedimentary rocks of the Keis supergroup and Korannaland Group did not obtain detrital material from the same source areas during their deposition, and that the Keis supergroup never served as a source for the detrital material in the Korannaland Group. This indicates that these successions had different geological histories prior to the formation of the NNMP, which supports the accretionary model for the development of the Namaqua Sector of the NNMP. [ABSTRACT FROM AUTHOR]

Abstract: According to theoretical studies, the foreland basin consists of: accretionary wedge (including wedge top or piggyback basin), foredeep, forebulge and backbulge depozones. All of them are parallel to the orogenic belts of the overlying and underlying plates. The closure of the southern branch of the Neotethys during the Late Cretaceous led to an oblique collision of the Arabian Plate and the Anatolide–Taurides Platform, leading to the development of the Miocene Kahramanmaraş Foreland Basin (KFB). Thus, the promontory shape of the Arabian Plate prevented the development of an accretionary wedge parallel to the orogenic belt. The accretionary wedge of the KFB includes blocks of various sizes and age (mainly Mesozoic limestone) scattered within an Early Tertiary matrix (mass wasting deposits and shallow to deep marine sediments). At the beginning of the Miocene, transtensional tectonism led to the development of half-graben basins on top of the accretionary wedge. These basins (namely; the Tekir and Çukurhisar) also cut the foredeep of the KFB obliquely (in contrast with the theoretical study). This paper focuses on the evolution and fillings of those basins. Initially, claystone and basin margin reef deposits filled the half-graben basins as a consequence of the Lower Miocene sea invasion. Then, long and narrow conglomeratic channels starting from the northern edge of the basins (fan-delta) progressed southwards, passing into sandy lobes, then into claystones. An activation of the boundary faults of the wedge top basin stopped the progression of the Lower–Middle Miocene sediments and led to their deformation. Then, the sedimentation of the KFB shifted towards the basin centre during the Middle Miocene. [Copyright &y& Elsevier]

Southern South America hosts a vast geologic record of Precambrian to Paleozoic geodynamic and tectonic processes, particularly related to the assembly and crustal growth of Western Gondwana. This special volume comprises 24 novel contributions focused on the Precambrian to Paleozoic tectonic evolution of southern South America, focused on different tectonostratigraphic domains of southeastern Brazil, Uruguay, Argentina and Chile (Figs. 2, 3). Papers also cover a wide temporal frame, including Archean to Tonian processes of pre-Brasiliano basement inliers; the tectonometamorphic, magmatic and sedimentary record of the late Neoproterozoic Brasiliano Orogeny, particularly related to the Dom Feliciano Belt; and Paleozoic accretionary systems of the proto-Pacific margin (Fig. 4). [ABSTRACT FROM AUTHOR]

Sedimentology, petrography, and geochemistry of the Ispikan Conglomerate of southwest Makran have been studied to establish its stratigraphic position, age, provenance, and depositional environment. Very thin to massive beds, poor sorting, no fabric, and poor grading are the common features. It is composed of mostly reworked, medium- to coarse-grained pebbly sandstone, siltstone, and discontinuous lenses of matrix- and clast-supported conglomerate. The sandstone is composed of angular to sub-angular, poorly sorted, immature grains having Q72F13L15 as average composition. Pebbles are of mostly metamorphic and acidic igneous origin. Large angular boulders (~1.5m) of sandstone indicate quick debris flow conditions. Geochemical discriminants suggest the derivation of Ispikan sediments from an active continental margin. The Nb and Zr/Th and Ba/Y values indicate the continental island arc setting, whereas a high K/Rb ratio points to the derivation of detritus from an acid and intermediate source. Ispikan Conglomerate is interpreted as an Eocene olistostrome formed after a localized submarine debris flow triggered by slope failure. [ABSTRACT FROM AUTHOR]

Understanding sedimentary routing systems at continental margins can be challenging in tectonically active regions such as forearc basins. Here, we combine sand modal composition and detrital zircon U‐Pb data from the Kumano Basin and surrounding Nankai Trough region of Japan to explore sedimentation in an active forearc and its linkages to the regional tectonic history. Our analyses are on Miocene‐to‐Pleistocene samples collected by the Integrated Ocean Drilling Program (IODP) at inner and outer forearc basin sites C0009 and C0002, river samples from onshore Honshu, and published results from other Nankai IODP sites. Results show the sediments of many IODP samples are similar to that of the modern rivers. Data from C0009 demonstrate that the early accretionary prism already contained slope basins fed by sediments similar to those in the modern Kumano River by 7.1 Ma. The more distal C0002 section was predominantly fed by a system similar to the modern Yodo River, located to the west‐southwest. We conclude that the section was therefore tectonically transported, in agreement with the 'mobile model' of oblique tectonic convergence across SW Japan during late Miocene time. Deposition resumed after a ca. 5.6–3.8 Ma hiatus, with sediments primarily fed from the paleo‐Kumano River, with some input from the paleo‐Tenryu and Kiso/Nagara rivers located to the northeast. Input from the paleo‐Tenryu and Kiso/Nagara rivers was likely re‐routed towards the trench by ca. 1.3 Ma. Sediments similar to those of the distal Yodo River also make up part of the Kumano basin fill and are best explained by erosion of uplifted older accretionary prism sediments along the Megasplay Fault that bounds the forearc basin. Despite the temporal variability in sedimentation routed to the basin, basin‐fill dynamics appear to have been governed primarily by the critical‐wedge style dynamics driven by variations in frontal accretion and in‐ or out‐of‐sequence thrusting response. [ABSTRACT FROM AUTHOR]

• The source of granitoids varies from different origins of the crust to mantle-derived magmas. • The compositional diversity of granitoids is indicated not only in major and trace elements but also in stable and radiogenic isotopes. • Granitoids may have their sources of either ancient or juvenile crust, or their mixing at great depths. • Partial melting and fractional crystallization are the two fundamental processes of granitic magmatism. • Actively and previously convergent plate boundaries are common sites of granitic magmatism. The origin and classification of granites and related rocks are the two fundamental aspects of igneous petrology. This special volume is based on the 9th Hutton symposium that was held at Nanjing University in China in 2019. It focuses on micro- to macro-scale aspects that are temporally and spatially linked to sources and processes of granitic magmatism. These aspects are a key to understanding a wide spectrum of phenomena in continental geology, involving not only partial melting of ancient and juvenile crustal rocks but also fractional crystallization of mantle-derived mafic magmas at convergent plate boundaries. Papers in this volume report progresses in mineralogical, petrological, geochronological and geochemical studies of granites and related rocks, with tectonic settings varying from accretionary to collisional orogens. Microbeam in-situ analyses of various minerals are successfully utilized to solve petrogenetic problems involving crustal anataxis and magma mixing. In addition to their geochronological applications to dating of magmatic processes in different stages of partial melting and fractional crystallization, microbeam techniques have also served as an efficient means to trace different generations of mineral growth during granitic magmatism. Furthermore, dehydration and melting of the crustal rocks at different depths are emphasized with respect to their operation in continental interiors. These have provided new insights into the formation and evolution of continental crust at convergent plate boundaries. [ABSTRACT FROM AUTHOR]

[Display omitted] • Collision created two metamorphic types (Grt + Ky + St and Grt + Crd + Sil) at ~515 Ma. • Grt + Crd + Sil metamorphism is associated with the formation of a shear zone in CAOB. • Grt + Crd + Sil metamorphism is nonzonal, characterized by basic and felsic magmatism. Accretionary-collisional events occurred at the western margin of the Tuva-Mongolian microcontinent are used to explain the largely contradictory Cambrian geodynamic history of the western Central Asian Orogenic Belt (CAOB). This study presents new data for the Erzin metamorphic complex in order to constrain the petrogenesis and tectonic implications of the metamorphic rocks in this complex. The Erzin complex in the tectonic Erzin zone is composed of high-grade metamorphic rocks that have formed under variable metamorphic conditions (T = 730–835 °C, P = 5.3–7.5 kbar). In the Erzin complex, metamorphic rocks have undergone multiple stages of ductile deformation with subvertical mineral lineation and were then superimposed by sub-horizontal ductile deformation. The stages of tectonic deformation are close in time and mark collisional stages characterized by a sequential change from the compression regime to the extension regime in the period of 495 ± 5 Ma. The determination of the time interval is based on structural and petrological data, previously published materials, as well as the dating of granite dikes (U-Pb, zircon) sealing the Erzin complex. P-T conditions and structural-petrological studies analysed in this paper suggest that the formation of the Erzin metamorphic complex occurred as a result of a collisional event between the Tannuola island arc and Tuva-Mongolian microcontinent. We suggest that the Erzin metamorphic complex is important for study of transitional regime between the collisional event and initial orogenic collapse in the ancient fold belts. Metamorphic record of the Tuva-Mongolian microcontinent provides important information about the processes that occurred at the Early Paleozoic geodynamic evolution in the western CAOB. [ABSTRACT FROM AUTHOR]

Early Palaeozoic evolution of the Kazakhstan orocline in the Central Asian Orogenic Belt (CAOB) is key to reconstructing the Palaeozoic tectonic framework in Central Asia. This study presents field mapping, geochemistry, and geochronological data from the Wulasitai area of northern West Junggar to constrain the subduction polarity and major tectonic events in the Chingiz arc of the Kazakhstan orocline during the Early Palaeozoic. The mapping area outcrops imbricated coherent turbidite slices and mélanges that consist of chert, limestone, silt, sandstone, and conglomerate blocks in mudstone matrix, representing an OPS (Ocean Plate Stratigraphy) mélange in an accretionary complex. Existing fossil ages from these blocks range from the Ordovician to Silurian, while our new detrital zircon U-Pb samples from the turbiditic matrix yield maximum depositional ages (MDTs) from 428 ± 2 Ma to 450 ± 1 Ma. Detrital zircon ages of ~510 to 430 Ma in the mélange suggest the Chingiz arc to the south of the study area as a major sediment source, providing an indirect constraint on the polarity of subduction. We suggest that the northern part of the Chingiz arc may be underlain by southward subduction, with the accretionary complex located on the Wulasitai area of the northern West Junggar. The Wulasitai mélange is overlain depostionally by volcanic rocks carrying depleted HFSE and enriched LILE, LREE. The andesite and tuff are dated to 415 ± 5 Ma, 414 ± 3 Ma, and 402 ± 6 Ma, which we interpret as reflecting the development of the volcanic arc onto the previous subduction complex. This model implies the generation of the arc (accretion arc) as the slab rolling back and trench retreating, which may play an important role in the evolutionary history of CAOB. [ABSTRACT FROM AUTHOR]

The Changning–Menglian Belt in western Yunnan, China has been widely considered as the remnants of Paleo-Tethys. However, its geological history remains waiting for further clarification. An efficient approach to this goal is to identify the tectono-sedimentary environment of different stratigraphic successions within the belt. This study examines the Nanpihe Formation, a Devonian to Early Carboniferous siliciclastics-dominated succession in the central zone of the belt. Our sedimentologic and petrographic results reveal that this formation comprises largely turbidity current deposits. Geochemical results indicate that its source area was rich in continental island arc rocks. Detrital zircon age spectra of this formation exhibit remarkable age clusters of ca. 435 Ma and ca. 950 Ma. The former is well-correlated with Silurian magmatism in the Simao and South China blocks. We thus consider that this formation was deposited in a continental slope environment along the west margin of the Simao Block, instead of the eastern margin of the Baoshan-Shan Block. We further consider that sometime in the Permian or Early Triassic, as the eastward subduction of the Paleo-Tethys progressed, the Nanpihe Formation was sheared off or collapsed from the overriding margin of the Simao Block and entered the accretionary wedge, which contained a significant amount of rock slices scraped off the subducting oceanic plate. During the closure of Paleo-Tethys in the Late Triassic, this formation and some other slices of the accretionary wedge were thrust onto the east flank of the Baoshan–Shan block. [ABSTRACT FROM AUTHOR]

Subducted sediment plays a key role in modulating pore pressure and seismic behavior at subduction zones. We investigated the seismic character of incoming sediments to test how sediment and basement variations relate to the along‐strike changes within the accretionary prism and plate boundary conditions at the Nankai Trough. High‐resolution seismic data reveal for the first time the presence of countourite mounded drifts in the Shikoku Basin. These features have probably introduced permeability heterogeneities into an otherwise homogenous mud‐dominant unit. Additionally, we found that normal faults in this unit are more extensive than previously documented, which probably enhances along‐strike fluid transport. The wedge taper is more correlative with the thickness of the mud‐dominant facies than the turbidite thickness. This may be due to the permeability heterogeneities associated with contourite deposits and normal faults, or due to the absence of thick turbidite deposits. Turbidite deposits can either aid the drainage of the margin where they are not confined by basement topography, or contribute to high pore fluid pressures where they are confined by less permeable mudstone or basement topography. When confined turbidite deposits and contourite mounded drifts are subducted, they may contribute to localized compartments of excess pore pressure which provide the necessary conditions for slow slip behavior. We determined that along‐strike variations in seismic behavior are likely related to the subducting basement topographic and sediment characteristics, which vary on a local (<10 km) scale. Plain Language Summary: Subduction zone faults, such as those off the coast of southwest Japan, have been shown to slip at a range of speeds. The ruptures range from typical earthquakes, which occur within seconds or minutes, to slow earthquakes, which can rupture over the course of days or weeks. The sediment characteristics of the incoming plate play a key role in determining the type of slip behavior along a margin. When sediments are subducted, they are compressed and expel the fluid trapped in the sediment. If the fluid is unable to escape, the fluid pressure builds up, and can reduce the strength of the plate boundary fault. The type of sediment, as well as whether it has been faulted or modified by the ocean currents, can control how well fluid can escape. The sediment characteristics and corresponding plate boundary strength vary greatly on a sub‐10 km scale. Slow earthquakes in our study area are co‐located with subducted seamounts. We suggest that this is because the sediments commonly deposited on the sides of large seamounts are conducive to forming high pore fluid pressures upon subduction. This result in a locally weaker plate boundary fault, which may be a necessary condition for slow earthquakes. Key Points: High‐resolution seismic reflection data reveal for the first time asymmetrical contourite mounded drifts in the Shikoku BasinBasement topography and paleocurrents influenced the location of turbidite and contourite deposits, as well as how well sealed the deposits areContourites, turbidites, and normal faults result in localized heterogeneities in the hydrogeologic properties of the subducting sediment [ABSTRACT FROM AUTHOR]

Characterization of the porosity evolution across the sedimentary section entering subduction zones and accreted sediments provide valuable information for understanding the deformation history at accretionary margins and the physico‐chemical processes in and around fault zones. International Ocean Discovery Program Expeditions 372 and 375 drilled, logged, and cored the reference section on the incoming plate (Site U1520) and across the active Pāpaku thrust (Site U1518), which is a <30° westward‐dipping splay fault in the frontal accretionary wedge at the north Hikurangi margin in a region where tsunami earthquakes and recurrent slow slip events are documented. We observe strong variations of physical properties across the thrust fault. In particular, interstitial porosity increases by ∼10% through the fault zone while it exponentially decreases with depth, showing lower values in the hanging‐wall (HW; on average, ∼36%) than in the footwall (∼42%). Based on comparisons of porosity with the compaction curve at Site U1518 with that of reference Site U1520, we infer an overcompaction in the HW and a nearly normal compaction in the footwall of the Pāpaku thrust. We suggest that the porosity pattern across the thrust infers differences in the maximum burial depth of sediments, characterized by an overcompacted HW that has been uplifted, thrusted, and concomitantly tectonically eroded above the footwall. Porosity data indicate hydrostatic conditions around the thrust fault contrasting with commonly assumed excess pore pressure at the plate interface. Key Points: Strong contrasts of physical properties including porosity occur across the Pāpaku thrust fault at IODP Site U1518The hanging‐wall (HW) is overcompacted and shows lower interstitial porosity and higher P‐wave velocity and resistivity than the footwallPorosity suggests differences in maximum burial depth with an uplifted, thrusted, and concomitantly eroded HW above the footwall [ABSTRACT FROM AUTHOR]

Dating the onset of continental collision is fundamental in defining orogenic cycles and their effects on regional tectonics and geodynamic processes through time. Part of the Palaeoproterozoic Trans-Hudson Orogen, the Southeastern Churchill Province (SECP) is interpreted to result from the amalgamation of Archean to Palaeoproterozoic crustal blocks (amalgamated as the central Core Zone) that diachronically collided with the margins of the North Atlantic and Superior cratons, resulting in two bounding transpressive orogens: the Torngat and New Quebec Orogens. The SECP exposes mainly gneissic middle to lower orogenic crust in which deformation and amphibolite to granulite facies metamorphism and anatexis overprinted the early geological features classically used to constrain the timing of collisional events. To enable improved tectonic models for the development of the SECP, and the Trans-Hudson as a whole, we investigated granulite facies supracrustal sequences from the Tasiuyak Complex (TC) accretionary prism and the western margin of the North Atlantic Craton--that is, Saglek Block (upper plate)--using a multi-chronometer approach coupled with trace element geochemistry. In particular, the use of garnet Lu-Hf geochronology provides an important minimal time constraint for crustal thickening and collision. Garnet growth in the TC is constrained at 1885 ± 12 Ma (Lu-Hf), indistinguishable from U-Pb age of prograde monazite at 1873 ± 5 Ma. Zircon growth during melt crystallization occurred at 1848 ± 12 Ma. Garnet from the overriding Saglek Block is dated at 2567 ± 4.4 Ma (Lu-Hf) and indicates that gneissic rocks from the upper plate did not record the metamorphic imprint of the Torngat Orogeny. The diachronicity of the integrated metamorphic record across the strike of the SECP is explained by the location of terrane boundaries, consistent with the westward growth of the Churchill plate margin through sequential amalgamation of narrow crustal blocks during accretionary tectonics from c. 1.9 to 1.8 Ga. [ABSTRACT FROM AUTHOR]

We propose a scheme to subdivide the Samarka terrane, a Jurassic accretionary prism fragment, into tectonostratigraphic complexes. This subdivision provides a basis to study these formations and map them on a medium- to large-scale. Each complex corresponds to a certain stage in the accretionary prism formation. Thus, the complexes composed of subduction mélange and olistostromes (in our case, Ust-Zhuravlevka and Sebuchar complexes), can be correlated to episodes when the underthrusting of seamounts hampered subduction, as evidenced by seamount fragments contained in the complexes. Episodes of relatively quiet subduction have also been identified, resulting in complexes composed mainly of normally bedded terrigenous and biogenic formations (Tudovaka and Udeka and, partially, Ariadnoe complexes). Particularly considered is the Okrainka-Sergeevka allochthonous complex – a fragment of continental plate overhanging a subduction zone. It was included in the accretionary prism during gravitational sliding on the internal slope of the paleotrench. All volcanic rocks in the accretionary prism are allochthonous fragments of the accreted oceanic crust. The absence of the Jurassic-Berriasian volcanic belt related to this prism, as well as synchronous autochthonous volcanism, indicates that the Samarka terrane accretionary prism formed under conditions of flat-slab subduction, similar to modern examples along the Andean margin. [ABSTRACT FROM AUTHOR]

Basaltic and gabbroic rocks that define the seafloor have been suggested to act as sources of rheological heterogeneity during subduction, with the capacity to enhance or dampen seismicity. Despite this, relatively little is known from the rock record regarding the progression and conditions of mafic oceanic crust deformation during subduction, particularly in the shallow megathrust region of the seismogenic zone. We describe subduction‐related deformation structures and characterize deformation conditions from an exhumed, basalt‐hosted megathrust in the Chugach accretionary complex of south‐central Alaska. Rocks in the Chugach preserve a record of seafloor mineralogical changes from pre‐subduction, hydrothermal circulation that produced sheet silicates with a lower frictional strength than intact basalt. Pre‐subduction alteration also served to introduce hydrous phases that can expel water during deformation and raise the pore fluid pressure. Once strain localized within basalts onto a megathrust fault plane at lithostatic pore fluid pressures, the basalt weakened further through a combination of cataclasis, dilatational shear fracturing, and slip on chlorite‐rich shear bands. This process occurred in a narrower fault zone, and at higher maximum differential stress and greater pore fluid pressure fluctuations than recorded in some sediment‐hosted megathrusts at similar pressure and temperature conditions. Our data indicate that when the lower plate contains basalt bathymetric features, basalt dismembers during subduction into a chlorite‐rich fault gouge that surrounds lenses or slices of intact, less‐altered basalt. Plain Language Summary: When one tectonic plate slides beneath another (the process of subduction), the surface roughness and the composition of the lower plate can change how easily and how rapidly this convergence occurs. Typically, the subduction process is neither smooth nor steady, and this results in seismic activity that varies in magnitude—small to large—and character—from fast, infrequent earthquakes to slow and frequently recurring earthquakes. It is critical to determine the properties of the rocks that make up the lower plate, so that we can better understand which features lead to various types of earthquake activity. We describe the characteristics of volcanic rocks that once occupied the lower plate and that are now exposed in south‐central Alaska. These rocks record a pattern of breakdown that is dependent on compositional changes from chemical alteration on the seafloor prior to subduction and high fluid pressures during subduction. Our data indicate that volcanic topographic features on the seafloor do break apart and undergo significant compositional and mechanical changes, and the strength of the resulting rock is controlled primarily by the strength of the mineral chlorite. Key Points: Seafloor alteration produces hydrous phases and phyllosilicates that impact Pf and frictional strength of basaltBasalt weakens through a consistent pattern of deformation during subduction that depends on Pf and phyllosilicate contentSubducted basalt hosts larger shear stresses than sediment megathrusts and deforms at a narrower width, suggesting faster strain rates [ABSTRACT FROM AUTHOR]

New geologic mapping of the Franciscan Complex in the western Mt. Tamalpais area northwest of San Francisco, California, reveals a tectonostratigraphic stack of accretionary units that young westward and structurally downward. Serpentinite-matrix mélange at the top of the section, beneath a possible basal fragment of Coast Range Ophiolite, and a structurally low black-rock-bearing, mudrock-matrix Ocean Plate Stratigraphy (OPS) mélange located near the base of the section, bracket lithic and feldspathic metasandstone-dominated accretionary units. The ages of the metasedimentary accretionary units are constrained by U-Pb age dating of detrital zircons. Maximum depositional ages fall between 124 Ma and 88 Ma. Sediment provenance is dominantly in the Sierra Nevada arc to the east, but minor numbers of detrital zircons were derived from Paleozoic and Precambrian sources. The provenance age profiles (cumulative distribution plots) reveal changing age components of zircon populations over time, reflecting the history of unroofing in the Sierra Nevada arc. An unusual post-accretion thrust fault and atypical greenschist facies metamorphism uncharacteristic of the Franciscan Complex, plus probable Late Cenozoic faults, complicate the character of the geology of the Mt. Tamalpais area. [ABSTRACT FROM AUTHOR]

Eastern Bangladesh sits on the seismically active Chittagong‐Myanmar fold and thrust belt (CMFB), a north‐trending accretionary wedge on the eastern side of the India‐Eurasia collision. Earthquakes on the basal décollement and associated thrusts within the CMFB present a hazard to this densely populated region. In this study, we interpret 28 seismic reflection profiles from both published and unpublished sources to constrain the depth of the basal décollement. To convert profiles from the time domain to the depth domain, we integrate sonic log and seismic stacking velocity data to generate time‐velocity relationships for different parts of the CMFB. Our analysis reveals that the décollement is ∼9 km deep in northeast and southeast Bangladesh, but shallows to ∼5 km in east‐central Bangladesh. The décollement has an area of 7.25 × 104 km2 (∼150 × 450 km), making it capable of an Mw 8.5 earthquake. However, the warped geometry of this fault might act as a rupture barrier were a large earthquake to occur on the décollement. Our combined velocity and fault model lay the groundwork for future studies to address seismic segmentation, ground shaking, and rupture modeling in the CMFB. Finally, we use our compiled data set to analyze the evolution of fold kinematics in the CMFB. We observe that folding style and failure mode varies, from mainly ductile deformation in the foreland to mainly brittle in the hinterland. The dual‐failure modes within the CMFB support the hypothesis that a region with ductile deformation may still be capable of seismic behavior. Plain Language Summary: In Bangladesh, the interface between two tectonic plates has created a large, nearly flat, earthquake‐producing fault called a décollement. Above the décollement, sedimentary layers are compressed to create a series of north‐south trending folds and thrust faults that extend for hundreds of kilometers. This study uses 28 seismic reflection data sets (a technology similar to ultrasounds), originally collected by the oil and gas industry, and reinterprets the subsurface structure of the folds as a way to determine the décollement depth. We find that the décollement has a curved shape, with a depth of ∼9 km in northeast and southeast Bangladesh, and ∼5 km in east‐central Bangladesh. We hypothesize that the greater amount of sediment deposited in north and south Bangladesh has weighed down the surface of the earth and warped the décollement. This is the first study to constrain the geometry of this décollement, and we find that it has the potential to host a magnitude 8.5+ earthquake. The fault model presented here can be incorporated into studies of the rupture patterns of earthquakes on this décollement to better understand the earthquake hazard in a region inhabited by over 160 million people. Key Points: We present a data‐based seismic velocity model for Bangladesh to 10 km depthThe décollement underlying the Chittagong‐Myanmar fold and thrust belt forms a broad arch at depths of 5–9 kmThis morphology is likely caused by lithospheric flexure due to sediment deposition and may impact rupture propagation [ABSTRACT FROM AUTHOR]

Early Carboniferous magmatism of the southern West Junggar (SWJ) region is essential for understanding the accretionary processes and crustal growth in the southwestern segment of the Central Asian Orogenic Belt. A comprehensive study of geochronological, geochemical, and Sr-Nd-Pb-Hf isotopic compositions was carried out in this study for basalts from the Baogutu Formation in Sulushuoke region, aiming to provide constraints on the Early Carboniferous tectonic evolution of the SWJ, NW China. LA-ICP-MS zircon U-Pb dating of four basalt samples yielded crystallization ages ranging from 339 Ma to 347 Ma, indicating the presence of early Carboniferous volcanic rocks in West Junggar. The studied tholeiitic basalts are geochemically characterized by nearly flat REE patterns, enriched large iron lithosphere elements (e.g. K) and depleted high field strength elements (e.g. Nb, Ta, Zr, Hf) relative to those in N-MORB, resembling those of modern Mariana Trough back-arc basin basalts. In addition, they have high positive εNd(t) values (+5.86 to +6.59) with initial 87Sr/86Sr ratios in the range of 0.704783–0.705315 and variable εHf(t) values (+3.99 to +15.6). The rocks cover restricted ranges in initial Pb isotope ratios with (206Pb/204Pb)i from 17.529 to 18.289, (207Pb/204Pb)i from 15.509 to 15.627, and (208Pb/204Pb)i from 37.628 to 38.397. These signatures indicate that the tholeiitic basalts were derived from 10% to 20% partial melting of a spinel-lherzolitic N-MORB-like depleted mantle source modified by fluids released from the subducted lithosphere in a back-arc extensional setting. From our new data presented in this study and in conjunction with those from previous works, we, therefore, suggest that a back-arc basin related to the northwestward subduction of the Junggar oceanic crust was developed in the Hatu-Baogutu region during the early Carboniferous time. [ABSTRACT FROM AUTHOR]

The Eastern Mediterranean is the most seismically active region in Europe due to the complex interactions of the Arabian, African, and Eurasian tectonic plates. Deformation is achieved by faulting in the brittle crust, distributed flow in the viscoelastic lower-crust and mantle, and Hellenic subduction, but the long-term partitioning of these mechanisms is still unknown. We exploit an extensive suite of geodetic observations to build a kinematic model connecting strike-slip deformation, extension, subduction, and shear localization across Anatolia and the Aegean Sea by mapping the distribution of slip and strain accumulation on major active geological structures. We find that tectonic escape is facilitated by a plate-boundary-like, trans-lithospheric shear zone extending from the Gulf of Evia to the Turkish-Iranian Plateau that underlies the surface trace of the North Anatolian Fault. Additional deformation in Anatolia is taken up by a series of smaller-scale conjugate shear zones that reach the upper mantle, the largest of which is located beneath the East Anatolian Fault. Rapid north–south extension in the western part of the system, driven primarily by Hellenic Trench retreat, is accommodated by rotation and broadening of the North Anatolian mantle shear zone from the Sea of Marmara across the north Aegean Sea, and by a system of distributed transform faults and rifts including the rapidly extending Gulf of Corinth in central Greece and the active grabens of western Turkey. Africa–Eurasia convergence along the Hellenic Arc occurs at a median rate of 49.8 mm yr–1 in a largely trench-normal direction except near eastern Crete where variably oriented slip on the megathrust coincides with mixed-mode and strike-slip deformation in the overlying accretionary wedge near the Ptolemy–Pliny–Strabo trenches. Our kinematic model illustrates the competing roles the North Anatolian mantle shear zone, Hellenic Trench, overlying mantle wedge, and active crustal faults play in accommodating tectonic indentation, slab rollback and associated Aegean extension. Viscoelastic flow in the lower crust and upper mantle dominate the surface velocity field across much of Anatolia and a clear transition to megathrust-related slab pull occurs in western Turkey, the Aegean Sea and Greece. Crustal scale faults and the Hellenic wedge contribute only a minor amount to the large-scale, regional pattern of Eastern Mediterranean interseismic surface deformation. [ABSTRACT FROM AUTHOR]

The allochthonous complexes of Galicia–Trás-os-Montes Zone (NW Iberia) are part of a rootless tectonic stack which preserves part of a Variscan accretionary prism. They are formed by individual tectonic slices marked by specific tectonometamorphic evolutions, which were piled up in a piggy-back mode onto its relative autochthon, the Central Iberian Zone (CIZ). Allochthony decreases from the structurally upper thrust sheets towards the lower ones. The lowermost unit of the stack is known as the Parautochthon or Schistose Domain. It is characterized by a low metamorphic grade in contrast with higher temperatures and/or pressures estimated for the overlying allochthonous units and shares the stratigraphic sequence with the underlying autochthon. The Parautochthon is divided in two structural and stratigraphic sub-units: (i) the Lower Parautochthon (LPa) is made of synorogenic flysch-type sediments with varied turbiditic units and olistostrome bodies, showing Upper Devonian–lower Carboniferous age according to the youngest zircon populations and fossiliferous content; (ii) the Upper Parautochthon (UPa) is composed of highly deformed preorogenic upper Cambrian–Silurian volcano-sedimentary sequence comparable with the nearby autochthon and to some extent, also with the high-P and low-T Lower Allochthon laying structurally above. The UPa was emplaced onto the LPa along the Main-Trás-os-Montes Thrust, and the LPa became detached from the CIZ relative autochthon by a regional-scale structure, the Basal Lower Parautochthon Detachment, which follows a weak horizon of Silurian carbonaceous slates. A review on the detrital zircon studies on the synorogenic LPa complemented by zircon dating of 17 new samples is presented here. The results support the extension of the LPa underneath the NW Iberian allochthonous complexes, from Cabo Ortegal, to Bragança and Morais massifs. Its current exposure follows the lowermost tectonic boundary between the Galicia–Trás-os-Montes (allochthon) and Central Iberian (autochthon) zones. The youngest zircon age populations point to a maximum sedimentation age for the LPa formations ranging from Famennian to Serpukhovian and supports the piggy-back mode of emplacement of the Galicia–Trás-os-Montes Zone, of which it represents the latest imbricate. The zircon age populations in the LPa allow the sedimentary provenance areas to be constrained, showing the intervention of nearby sources (mostly the UPa) and/or multiply recycled and long-transport sediments with a typically north-central Gondwana age fingerprint, also found in the Lower Allochthon, UPa and Autochthon. Complementary geochronology of volcanic olistoliths trapped in the LPa sediments and of late Cambrian to Upper Ordovician rhyolites from the UPa is also presented. It shows a direct relationship between the major blocks source area (UPa) and the setting place (LPa). Old zircon age patterns show that the LPa sedimentary rocks were recycled from detrital rocks of the allochthon (advancing wedge) and the nearby autochthon (peripheral bulge). [ABSTRACT FROM AUTHOR]

Ground‐motion amplification in the outer Cascadia subduction zone accretionary prism has been documented previously by comparing earthquake and microseismic vertical ground‐motion and pressure records from an ONC/NEPTUNE Canada cabled observatory site on the outer prism to those from a site on the flank of the Juan de Fuca Ridge. Since then, four additional instruments became operational, and data, now spanning 10 years, include more than 100 large (Mw > 7) distant earthquakes. Well‐tuned response at the outer prism sites is observed in both vertical and horizontal components, with peaks in the spectral ratio of vertical velocity relative to nearby ocean crustal and continental bedrock sites (V/Vref) at 9 s, and in the intra‐site horizontal to vertical spectral ratio horizontal‐to‐vertical spectral ratio at 14 s, both with band widths of ±10% to 15% at half amplitude. The vertical response is consistent with ¼ wavelength compressional wave reinforcement, while the horizontal motion tuning most likely reflects the effects of the low velocity prism sediments on surface‐wave propagation. At the periods of maximum relative motion, outer prism surface‐wave vertical and horizontal accelerations are enhanced by factors of up to 15 and 25, respectively. Similar behaviour is seen in microseismic records from these sites, and, to a lesser extent, in earthquake records from temporarily deployed Cascadia Initiative seismometers along the outer Cascadia prism to the south. Such tuning and amplification must be accounted for when assessing the dynamic behavior of the prism and its basal fault at the time of large local earthquakes. Plain Language Summary: Sedimentary basins are known to amplify ground motions, a behavior that must be considered in the assessment of seismic hazards. The same is true for subduction zone accretionary prisms, where enhanced seismic shaking can lead to enhanced dynamic stressing at the plate interface of the subduction zone, weakening of the accretionary prism sediments, and triggering of submarine slope failures. Here we examine data from seafloor seismometers deployed on, seaward of, and landward of the outer Cascadia accretionary prism, spanning up to 10 years and including signals from more than 100 large earthquakes. Response of the outer prism to seismic waves shows amplification and tuning, with peaks in anomalous vertical‐ and horizontal‐motion centered at 9 and 14 s periods. At these wave periods, accelerations at the outer prism can be over 20 times those at nearby bedrock sites. Vertical and horizontal seafloor motions caused by oceanographic pressure variations are similarly enhanced. Key Points: Narrow frequency band seismic response is observed in horizontal‐ and vertical‐component motions at multiple sites on the outer Cascadia accretionary prismPeak seismic and microseismic accelerations of the outer prism are commonly 10 and up to 25 times those at nearby continental and ocean crustal reference sitesThe periods of maximum relative vertical and horizontal motion are inferred to be the consequences of modifications to body and surface waves propagating through the low velocity prism [ABSTRACT FROM AUTHOR]

Detrital zircon U–Pb ages for sediments in and around the Palaeo-Tethyan convergence zone in northern Thailand provide constraints for tectonic interpretations of the Indochina Block, the Sibumasu Block, the Inthanon Zone accretionary complex and the Nan Back-arc Basin during the Triassic. In sedimentary rocks of the Indochina Block, almost all of the Palaeozoic and Triassic zircons were sourced from the collision zone between the Indochina and South China blocks, and an active continental margin in the western Indochina Block. Sediments of the Sibumasu Block were supplied by erosion of Archaean basement and from the Grenville and the Pan African orogenies, but show no record of Permian to Triassic igneous activity. Accretionary complex sediments have provenances of both the Sukhothai Arc and the Indochina and South China blocks, with detrital zircons of various ages being supplied from crustal uplift and erosion related to the Indosinian I orogeny. Sedimentary rocks of the Nan Back-arc Basin are widely distributed not only in the Nan–Uttaradit but also in northern Sukhothai areas. The origin of the Pha Som Metamorphic Complex and associated formations can be traced to basin-filling sediments in the Nan Back-arc Basin. These detrital zircon U–Pb ages have also allowed identification of the changing tectonic setting in the Palaeo-Tethys convergence zone from the 'erosion of Proterozoic continental basement' to 'Palaeozoic active continental margin in the western Indochina Block' and 'Palaeozoic, Permian to Triassic collision zone between the South China and Indochina blocks' through to 'Triassic active Sukhothai Arc'. [ABSTRACT FROM AUTHOR]

Crustal rheology and surface processes strongly influence strain distribution and shape of orogenic wedges at their front but how they influence the wedge rear is still unclear. Here, we analyse the coupled control of viscosity and sedimentation on forearc high growth during advanced stages of subduction accretion. We use 2D thermo-mechanical finite element models constrained with data of the south Anatolian margin. Our simulations show that forearc highs grow as a thermallyactivated viscosity drop in the lower crust induces ductile deformation and viscous flow. Initial viscosity and the amount of sediments in the forearc basin control nonlinearly the occurrence and timing of the thermally-activated viscosity drop, and thus of the growth of the forearc high. High sedimentation rates result in thicker forearc basins that stabilise the subduction wedge and delay the onset of uplift in the forearc high. Low viscosities promote earlier onset of forearc high uplift and lead to larger morphological variability along the subduction margin. Increasing either the sedimentation rate or viscosity may prevent forearc high formation entirely. The thermo-viscous forearc highs grow at an age set by wedge thermal state as a function of accretionary flux, wedge viscosity, and synorogenic sedimentation. Our models explain vertical motions in south Anatolia and potentially in other accretionary margins, like the Lesser Antilles or Cascadia, during the formation of their broad forearc highs. [ABSTRACT FROM AUTHOR]

The Altaids accreted around, and grew southward, from the Siberian craton, but the time of final amalgamation of this orogen is still controversial. The Eastern Tianshan in the southernmost Altaids is characterized by multiple, late, accreted arcs and thus is an ideal tectonic environment to answer the time of final amalgamation of the Altaids. In this study we report the results of new field-based lithological mapping and structural analysis on the Kanguer mélange in the Eastern Tianshan, which is composed of blocks of basalt, chert, limestone, and other rocks within a strongly deformed and cleaved matrix of sandstone and schist. Our geochemical and isotopic data of basaltic blocks from several parts of the Kanguer mélange show they are relics of Normal-Mid-Ocean-Ridge (N-MORB)-type oceanic lithosphere, and U–Pb ages and Hf isotopes of detrital zircons from the matrix sandstones indicate they were derived only from the Dananhu arc to the north. Accordingly, our interpretation is that the Kanguer mélange was part of an accretionary complex that fringed the Dananhu arc, and therefore the subduction polarity of the Kanguer Ocean was to the north (present coordinates). The maximum depositional ages (MDAs) of our three sandstone samples (08K01, 08K02, and 08K03) from the mélange matrix were 234 ± 14 Ma, 242.5 ± 1.3 Ma, and 236 ± 2.0 Ma respectively, indicating that the Kanguer Ocean was still being subducted at ca. 234 Ma, and the accretion of the Kanguer mélange must have lasted until that time, when the accretionary complex was still located opposite to the Yamansu-CTS accretionary complex to the south. Thus, the final amalgamation of the Dananhu and Yanmansu-CTS arcs took place by the welding of two accretionary complexes in the late Middle Triassic (Ladinian) in this part of the southern Altaids. Integration with relevant amalgamation histories throughout the Tianshan indicates that the time of terminal amalgamation in the southern Altaids was probably in the Middle-Late Triassic, which is much younger than previously envisaged. [ABSTRACT FROM AUTHOR]

The Iberian Massif of the Variscan Belt is a big area of exposed Palaeozoic rocks holding an S-shaped regional bend. Profuse data and genetic models support the existence of the northern arch (Ibero-Armorican Orocline: IAO) created by Late-Variscan buckling. The southern arch (Central-Iberian Orocline: CIO) was proposed near a century ago but it remains controversial. Tectono-metamorphic studies in the core of the CIO constrain its geometry and kinematics and provide crucial data to unravel the geological evolution of the region. The CIO is here envisaged as produced by the lateral thin-skinned extrusion of a fan-like wedge (with fragments of the NW Iberia allochthonous complexes) onto the Iberian autochthon, using the parautochthon as the lower tectonic sheet. The progression of a roughly E-W linear orogen towards the W (today's NW Iberia) controlled by the obliquity of the continental collision of Gondwana and Laurussia created an escape way for the collapsing materials coming from the accretionary prism. The extrusion wedge expanded westwards along the orogenic extensional vector and progressively curved areas to the west, dragging the previous structures and the stratigraphy while incorporating the Variscan foreland basins at the base. We propose the French Massif Central (FMC) as a provenance area for the extrusion. In this sector, evidences of Eo-Variscan (~410-360Ma) subduction-obduction stages match to those described in the NW Iberia allochthonous complexes. The synorogenic extension towards NW and gneiss-dome formation in the FMC at 370-355 Ma fits with the known ages and kinematics (after restoring the IAO) of the first compressive stages in the parautochthon/autochthon of NW Iberia (360-340Ma) evidencing the relationship between regions. Tightening of the CIO occurred in the late-Variscan compressive stage, due to the formation of the IAO that wrapped the entire CIO thus rotating the extrusion fan more than 90ºcounter-clockwise in the latest Palaeozoic. [ABSTRACT FROM AUTHOR]

In an effort to improve our understanding of the seismic character of the crust beneath southeast Australia and how it relates to the tectonic evolution of the region, we analyse teleseismic earthquakes recorded by 24 temporary and 8 permanent broadband stations using the receiver function method. Due to the proximity of the temporary stations to Bass Strait, only 13 of these stations yielded usable receiver functions, whereas seven permanent stations produced receiver functions for subsequent analysis. Crustal thickness, bulk seismic velocity properties, and internal crustal structure of the southern Tasmanides – an assemblage of Palaeozoic accretionary orogens that occupy eastern Australia – are constrained by H – κ stacking and receiver function inversion, which point to the following: a ∼ 39.0 km thick crust; an intermediate–high Vp/Vs ratio (∼ 1.70–1.76), relative to ak135; and a broad (> 10 km) crust–mantle transition beneath the Lachlan Fold Belt. These results are interpreted to represent magmatic underplating of mafic materials at the base of the crust. a complex crustal structure beneath VanDieland, a putative Precambrian continental fragment embedded in the southernmost Tasmanides, that features strong variability in the crustal thickness (23–37 km) and Vp/Vs ratio (1.65–193), the latter of which likely represents compositional variability and the presence of melt. The complex origins of VanDieland, which comprises multiple continental ribbons, coupled with recent failed rifting and intraplate volcanism, likely contributes to these observations. stations located in the East Tasmania Terrane and eastern Bass Strait (ETT + EB) collectively indicate a crust of uniform thickness (31–32 km), which clearly distinguishes it from VanDieland to the west. Moho depths are also compared with the continent-wide AusMoho model in southeast Australia and are shown to be largely consistent, except in regions where AusMoho has few constraints (e.g. Flinders Island). A joint interpretation of the new results with ambient noise, teleseismic tomography, and teleseismic shear wave splitting anisotropy helps provide new insight into the way that the crust has been shaped by recent events, including failed rifting during the break-up of Australia and Antarctica and recent intraplate volcanism. [ABSTRACT FROM AUTHOR]

The metabasites of the Tim Formation occur in the Paleoproterozoic Orel, Tim–Yastrebovka, and Avilsky synforms in the Archean Kursk block of Eastern Sarmatia. The volcanics are picrites, picrobasalts, basalts, and basaltic andesites metamorphosed to the greenschist and epidote-amphibolite facies. Their zircon (SIMS) age is 2099 ± 8 Ma. Based on their (Gd/Yb)n and Ti/Y ratios, the volcanics of the Tim Formation are subdivided into two types. The first one is OIB-type metabasites with (Gd/Yb)n > 2 and Ti/Y > 500. The parental melts of these rocks corresponded to ferropicrite and were derived by melting a garnet-bearing mantle source. The rocks of the second type are MORB-type volcanics with (Gd/Yb)n < 2 and Ti/Y < 500, which implies that the magma was derived from a garnet-free source at a relatively shallow depth. Their parental melts could be variously enriched MORB-type picrites. These magmas, which are contrastingly different in petrogenesis but had approximately equal age, were likely generated at various interaction stages between the asthenospheric mantle and eclogitized oceanic crust, subcontinental lithospheric mantle, and continental crust in the destruction environment of a subducted oceanic slab at the accretionary–collisional interaction between the Kursk domain and Volga–Don orogen. [ABSTRACT FROM AUTHOR]

To understand the crustal growth of the Qinling orogenic belt and the Mesozoic continental interaction between the North China and Yangtze blocks, it is critical to study the South Qinling accretionary complex belt, which is an important part of the Qinling orogen. Based on detailed zircon U–Pb geochronology and Hf isotopic compositional analyses of the Madao complex, this study presents constraints on the migmatization and tectonic evolution of the South Qinling orogenic belt during the Mesozoic. Zircons from leucosome samples of migmatites were found to have weak oscillatory zoning, and one sample yielded a weighted mean age that reflects the time at which the regional tectono‐thermal event initiated the anatexis (210–217 ± 2 Ma). The Hf isotopic compositions indicate that the Madao migmatites were derived from the ancient middle–Late Palaeoproterozoic crust. Zircons from mesosome samples of the migmatites that varied in age from the Palaeozoic to Neoproterozoic exhibited core‐rim textures with magmatic cores (256–1,329 Ma), indicating that they were derived from multiple sources with sedimentary protoliths. Of the 55 analysis spots on the overgrowth rims that yielded 206Pb/238U ages of 200–233 Ma, 40 analyses yielded a weighted age of 211.5 ± 1.0 Ma, indicating the time of anatexis due to the regional tectono‐thermal event. Combined with previously reported Mesozoic data from the South Qinling belt, these results suggest that the migmatites were probably controlled by post‐orogenic extension driven by Mesozoic orogenesis and a prolonged melting episode (from ~200 to 233 Ma). This study reinforces the interpretation that the continental crust in the South Qinling belt may have experienced complex interactions with asthenospheric mantle material during the Mesozoic. [ABSTRACT FROM AUTHOR]

The Indochina block is one of a number of continental fragments that lay along or outboard of the East Gondwana northern margin. Its early Paleozoic tectonic nature and the relationship with East Gondwana remain poorly-known. This paper shows the general geology of northeastern Indochina and presents a set of new geochronological and geochemical data with respect to its early Paleozoic tectonics in East Laos. These results reveal early Paleozoic (~485-410 Ma) igneous activity in the northeastern Indochina, which are divisible into four geochemical groups. The ~450-434 Ma Group 1 and ~485-434 Ma Group 2 mafic-intermediate igneous rocks were derived from MORB-like and supra-subduction zone wedge source modified by the recycled-sedimentary components, respectively. The Group 3 rhyolites at ~424 Ma represent the melting products of juvenile mafic crust. The Group 4 granitoids and equivalent volcanics formed at ~485-407 Ma and were derived from crustal materials with variable input of juvenile crust. The detrital-zircon age-spectra are marked by peaks at ~560 Ma, ~960 Ma, ~1600 Ma, ~1850 Ma and ~2450 Ma for the pre-Silurian paragneiss in the NE Indochina, reflective its geographical proximity to the India-Antarctica segment of East Gondwana until the earliest Paleozoic. The age-peaks of ~447 Ma and ~780 Ma is additionally given for the detrital-zircon from the Silurian sandstones in the Truong Son zone. An accretionary assemblage model is proposed for the early Paleozoic tectonic pattern of the SW Indochina with Yangtze fragments. Such an orogenesis formed the part of the North Indo-Australie Orogen related to subduction of the Prototethyan Ocean and the subsequent accretion of Asian micro-continental fragments along the Peripheral East Gondwana until earliest Devonian. Unlabelled Image • The 410–485 Ma igneous rocks with distinct geochemical signatures are identified. • A bi-directed detritus supply is revealed in NE Indochina at the late Ordo-Silurian. • SW Indochina might near to the India northern margin prior to the Ordovician. • An accretionary orogenesis occurred at 485–400 Ma in the northeastern Indochina [ABSTRACT FROM AUTHOR]