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This paper reports two types of basalt that have different ages (Carboniferous and Permian) but occur next to each other in the northern part of the Changning–Menglian accretionary wedge, southwestern China. We use the geochronology and geochemical data to evaluate the tectonic evolution of the Palaeo‐Tethys during this period. Zircon grains from the mafic‐ultramafic rocks associated with the Permian basalts in the Xiaomengtai area yield a U–Pb age of 281 Ma and εHf(t) values from +9.2 to +12.8. The Permian mafic‐ultramafic rocks and the associated basalts are all characterized by normal mid‐ocean ridge basalt (N‐MORB)‐like chondrite‐normalized rare earth elements patterns, moderately negative Nb‐Ta anomalies in the mantle‐normalized immobile incompatible trace element patterns, and positive εNd(t) values from +4.2 to +6.5, which are consistent with the geochemical characteristics of mafic‐ultramafic rocks in supra‐subduction zone (SSZ)‐type ophiolite. On the contrary, the Carboniferous basalts, which are associated with marine carbonates, are characterized by light REE enrichments, slightly positive Nb‐Ta anomalies, and εNd(t) values from +2.8 to +4.0. These features are similar to those of typical oceanic island basalts (OIBs) worldwide. Modelling results using REEs show that the parental magmas for the Carboniferous OIBs and the Permian basalts were likely derived from mantle peridotites at the depths of garnet and spinel stability, respectively, consistent with the formation depth of these two different types of basalt globally. The occurrence of these two different types of mafic‐ultramafic rocks with significantly different ages in the same area supports the view that they are the remnants of the accreted Palaeo‐Tethys oceanic crust. The Carboniferous OIBs are considered to be parts of an OIB‐carbonate seamount chain that originally formed in the southern part of the Palaeo‐Tethys. The Permian mafic‐ultramafic rocks are regarded as fragments of SSZ‐type ophiolites that were present in the northern part of the Palaeo‐Tethys. These different pieces of oceanic crust were accreted to the Simao–Indochina continental Block by subduction between the Late Permian and the Triassic. [ABSTRACT FROM AUTHOR]

Accreted basaltic rocks are expected to provide information on intraplate volcanism in the oceans. Basaltic rocks, originating from mid‐ocean ridges, plateaus, and seamounts, have been reported from exhumed accretionary complexes. However, basaltic rocks related to petit‐spot volcanism remain poorly documented. We examined basaltic intrusive rocks in an exhumed accretionary complex on Amami‐Oshima Island, Ryukyu Arc. Basaltic sills intruded mélange composed of seamount‐derived micritic limestone and basaltic rocks, which deformed together with the mudstone‐dominated mélange in the subduction zone. A separate basaltic sill intruded pelagic chert before being incorporated as a mélange block in the mudstone matrix during deformation in the subduction zone. Geochemical discrimination diagrams and trace element patterns indicate that the accreted dolerites and nonintrusive basalts likely originate from mid‐ocean ridges and hotspots, respectively. On the other hand, the geochemical characteristics of basaltic sills are compatible with alkaline basalt with enriched in incompatible trace elements relative to other basaltic rocks of hotspot origin and may represent a low degree of partial melting from a deeper mantle source (>90 km). A higher ratio of gadolinium to ytterbium in the basaltic sills relative to hotspot or mid‐ocean ridge‐related basalt indicates both a deeper melting depth, and a magma source which upwelled through thick oceanic lithosphere far from the mid‐ocean ridge. Based on the timing of basaltic intrusion and geochemical features, we suggest that the basaltic intrusive rocks could record past petit‐spot volcanism in the oceanic plate. Plain Language Summary: Basaltic rocks in exhumed accretionary complexes are crucial for understanding volcanism in oceanic plates. Petit‐spot volcanoes on modern seafloor have been found in the region seaward of the trench, derived from rising basaltic magma from the deep mantle along fractures from the bending of oceanic plates. However, past petit‐spot volcanism on already subducted oceanic plates remains poorly understood. This paper reports on the geological features and geochemical element analyses used to determine that basaltic rocks found in the Chichibu accretionary complex on Amami‐Oshima Island, Ryukyu Arc, originated from the deep mantle and intruded oceanic sedimentary rocks and seamount‐related rocks. We suggest that the basaltic intrusion resulted from petit‐spot volcanism. Our results highlight that basaltic rocks found in outcrops exhumed from subduction zones could contribute to understanding past petit‐spot volcanism in the oceanic plate that has completely subducted to the mantle. Key Points: Alkaline basaltic rocks intruded mélange in an exhumed accretionary complex on Amami‐Oshima IslandGeochemical features suggest that alkaline basaltic intrusive rocks originated from low‐degree partial melts generated in the deep mantleTiming and geochemical signatures of the alkaline basalt intrusions suggest they represent past petit‐spot volcanism [ABSTRACT FROM AUTHOR]

The Central Asian Orogenic Belt (CAOB) is the largest accretionary orogenic belt in the world and its formation involved a complicated process of arc–continent collision. Large‐scale Mesozoic volcanic activity in the Great Xing'an Range (GXAR) in northeastern China, which constitutes part of the eastern CAOB, has received much recent research attention. However, the petrogenesis and formation mechanism of the late Mesozoic igneous rocks remain enigmatic. This paper investigates the geochronology, petrogenesis, and formation mechanism of late Mesozoic magmatic rocks in the Kelihe area of the GXAR based on zircon U–Pb dating, petrology, and geochemistry. Late Mesozoic magmatism in the Kelihe area occurred during the Late Jurassic (165–155 Ma) and Early Cretaceous (130–120 Ma). These igneous rocks generated by this magmatism are characterized by high SiO2 (69.97–75.49 wt%) and low MgO (0.11–0.68 wt%), Cr (10–30 ppm), Co (0.2–2.6 ppm), and Ni (0.5–1.6 ppm) contents. Trace elements show enrichment in Rb, Th, U, Zr, and K, and depletion in Ba, P, Nb, and Ti. These features indicate that the rocks were derived from partial melting of the crust. Furthermore, the rhyolites have high Na2O (5.02–5.44 wt%) and K2O (3.94–4.88 wt%) contents, similar to those of melts formed by the partial melting of basaltic crust. Combining the present results with the findings of previous studies, it is inferred that the Mesozoic igneous rocks formed in a post‐collisional extensional setting related to lithospheric thinning and asthenospheric upwelling after the closure of the Mongol–Okhotsk Ocean. [ABSTRACT FROM AUTHOR]

The Kelameili Ocean was one branch of the Palaeo‐Asian Ocean, the initiation and closure history of which remains elusive. Consequently, they hamper our understanding of the accretionary evolution of the southern Altaids. In this paper, we carried out field mapping and also geochronological and geochemical analyses on the Huangcaopo Group in the northern Harlik Mountains, NW China. Field investigation revealed that "block‐in‐matrix" structure develops with blocks of andesite, limestone, sandstone, and gabbro in matrices of turbidites. Combined with our recent work, our field investigation indicates that the Huangcaopo Group is the southern extension of the Kelameili accretionary complex. A gabbro block (19HAM057) was dated at 500 ± 4 Ma, its geochemical affinity was defined to be similar to arc magmatic rocks. Magmatisms represented by two gabbroic intrusions (19HAM057‐2 and 19HAM048) were dated at the latest Devonian (360 ± 5 Ma) and the latest Carboniferous (299 ± 4 Ma), respectively. Their geochemistry displays enrichments in light rare earth elements (LREE) and large ion lithophile elements (LILE), but depletions in high field strength elements (HFSE, especially Nb and Ta). And a younger gabbroic intrusion (19HAM052) yielded crystallization age of the latest Permian (254 ± 3 Ma), and its geochemistry also shows affinity to arc magmatism. The magmatism represented by the gabbro block (19HAM057) was coeval with the Zhaheba‐Aermantai ophiolite, indicating that it was the product of incipient arc magmatism following subduction initiation in the Kelameili branch of the Palaeo‐Asian Ocean. However, two episodes of gabbroic magmas (19HAM057‐2 and 19HAM048) that intruded the Kelameili accretionary complex from the latest Devonian to latest Carboniferous indicate southward rollback of the subducting Kelameili oceanic lithosphere and resulted in the development of a growing arc. Meanwhile, the youngest gabbroic intrusion (19HAM052) suggests that northward subduction of the Kelameili oceanic lithosphere may have at least lasted to the latest Permian, indicating that the closure of the Kelameili Ocean might have been later than the latest Permian, most likely in the early Triassic. Our new data provide further constraints on the initiation and closure history of the Kelameili Ocean, which shed light on the accretionary evolution of the Altaids. [ABSTRACT FROM AUTHOR]

The southern West Junggar terrane in the south‐western Central Asian Orogenic Belt (CAOB) is mainly composed of accretionary complexes and characterized by widespread outcrops of ophiolitic mélanges in the Barleik, Mayile, and Tangbale (BMT) areas, representing the site of consumption of the Junggar–Balkhash Ocean. However, due to a lack of comprehensive synthesis of the ophiolitic mélanges, the early evolution of the Junggar–Balkhash Ocean remains poorly understood. This paper first focuses on the little‐known Barleik ophiolitic blocks and associated pillow basalts and then integrates the Barleik, Mayile, and Tangbale ophiolitic mélanges into one model. Mineral and whole‐rock chemistry suggests that the Barleik ophiolitic blocks, including serpentinized dunite, clinopyroxenite, and gabbro, show supra‐subduction zone (SSZ) affinities, as indicated by high Cr# values (76–78) in spinels and strong depletion in Nb, Ta, Zr, and Hf, whereas the Barleik pillow basalts are alkaline and have high TiO2 contents (2.82–3.42 wt.%) and Nb–Ta enrichment, similar to ocean‐island basalt (OIB) lavas. Combined with published chronological and chemical data from ophiolites, metamorphic rocks, and arc plutons, we propose that the BMT Ediacaran to middle Cambrian ophiolitic gabbros were formed in a SSZ environment, implying a single Mariana‐type arc system within the Junggar–Balkhash Ocean, which was coeval with an Andean‐type arc system in the Kyrgyz North Tianshan, supporting an archipelagic model for the south‐western CAOB. By contrast, the BMT OIB‐type basalts are early Silurian in age, much younger than the SSZ‐type ophiolites, and can be interpreted as relicts of seamounts. Thus, a model involving the Ediacaran to Ordovician intra‐oceanic subduction and late Silurian to Early Devonian seamount accretion is helpful for clarifying the different components of the accretionary complexes in the southern West Junggar terrane. [ABSTRACT FROM AUTHOR]

The Altai‐Junggar‐Tianshan collage in southern Altaids is an important metallogenic domain in Central Asia that contain world‐class copper‐iron‐nickel deposits. As an accretionary‐type metallogenic system, the metallogenic processes of the Altai‐Junggar‐Tianshan collage is essential in understanding the genetic mechanism of ore deposits in general. Here in this paper we present a brief introduction to the project on the western part of the Southern Altaids, entitled "The deep structure and metallegenic processes of the North China accretionary metallogenic systems". This project mainly focuses on the deep structure and metallogenic background of the Altai‐Junggar‐Tianshan collage by integrated studies from field geology, structural mapping, geochemistry and geophysical exploration. Multiple new geological and geophysical methods will be applied to make transparency of the Kalatongke and Kalatage ore clusters. This will update our understanding of the geodynamic processes of metallogenesis and lead to the development and foundation of new metallogenic theories in accretionary orogens. [ABSTRACT FROM AUTHOR]

In the eastern part of the Karoo Basin of South Africa, the sedimentary record of the Late Palaeozoic Ice Age, the Dwyka Group, consists of an up to 200 m thick accumulation of massive to crudely stratified diamictite occasionally interstratified with siltstone, sandstone and conglomerate horizons. Three distinct sedimentary units, separated by intervening glacial erosion surfaces, are viewed as ice‐margin fluctuation sequences. The lowermost one, resting on highly uneven, glacially abraded Archaean basement, has been interpreted as a grounding zone wedge deposited after the retreat and stabilization of the ice margin after the inundation of the Karoo Basin. The grounding zone wedge interpretation is based on its thickness (up to 100 m), the dominance of diamictite, and its facies assemblage and inferred depositional processes (rain‐out of debris, dropstone dumping, mass and debris flow, till). Overlying the grounding zone wedge, deposits are sedimentary units interpreted as glaciofluvial or ice‐contact delta and grounding zone wedges, respectively. By analogy with Quaternary sedimentary sequences, deposition of the Dwyka Group in the study area might have been very rapid (tens to hundreds of thousand years) and may hence correlate with the ultimate deglacial sequence of the Western Karoo Basin, as both successions are covered by the postglacial Ecca Group. Although commonly observed and imaged on modern, high‐latitude continental shelves, grounding zone wedges have never been interpreted in the ancient geological record. This paper therefore outlines a model defining criteria necessary for their identification. [ABSTRACT FROM AUTHOR]

The Kaiyuan tectonic mélange makes up the western part of the Kaiyuan–Yanji Accretionary Complex Belt, NE China, and there has been a long‐standing controversy about its age and tectonic setting. In this paper, we present new zircon U–Pb ages and geochemical data for the mafic rocks of the Lujiapuzi Formation in the Kaiyuan tectonic mélange, and these data provide insights into the tectonic setting during the late Paleozoic and the evolutionary history of the Paleo‐Asian Ocean in the eastern segment of the Central Asian Orogenic Belt (CAOB). SHRIMP U–Pb dating of zircons from samples of albite–actinolite schist and amphibolite yielded a crystallization age of 258 ± 5.5 Ma for the protoliths, suggesting that the protoliths formed during the late Permian. The albite–actinolite schists and amphibolites have similar geochemical characteristics, and they represent metamorphosed basaltic volcanic rocks. Major, rare earth and trace element data show that the mafic rocks have a close affinity with mid‐ocean ridge basalt. REE petrogenetic modelling indicates that the primary magma was generated through the partial melting of a mixed garnet‐ and spinel‐bearing mantle source, and assimilated crustal material and underwent crystal fractionation during ascent. The geochemical features and inherited/xenocrystic zircons in the mafic samples indicate that they formed in a tectonic setting similar to the back‐arc basin. These new findings, together with other previously published data, indicate that the Paleo‐Asian Ocean still existed in the eastern segment of the CAOB during the late Permian to Early Triassic. The Kaiyuan tectonic mélange formed during the Middle Triassic, and its formation may represent the timing of final closure of the Paleo‐Asian Ocean. [ABSTRACT FROM AUTHOR]

The detrital zircon records significant information in the 'source‐sink' system. With the application of in situ laser ablation technology, a large number of high‐quality detrital zircon data have been published since 2000. In this study, a total of 41,342 detrital zircon U–Pb ages and 6,129 Hf isotopes were compiled from the published literatures of the Middle East (Iranian and Arabian plates). Through data filtering and recalculation, valid data were employed for further analysis. The detrital zircons from the Middle East show a Cambrian–Precambrian age population of 500–1,000 Ma, with a major age peak of ~620 Ma and dispersed εHf(t) values of −35 to +20. The Alborz Mountains and central Iran terrane show a Permo–Triassic age range of 200–300 Ma. The Mesozoic–Cenozoic detrital zircons are mostly occurred in the Zagros orogenic belt and Makran accretionary complex, with three obvious age ranges of 145–180 Ma, 80–110 Ma and 15–65 Ma. The Mesozoic zircons yield positive εHf(t) values, while Cenozoic zircons have varied εHf(t) values. This database allows for the further exploration of the provenance analysis and application in constraining the timing of the major tectonic events in the Middle East, and may also help to explore the affinities of plates, thus guiding future palaeogeographic research efforts. [ABSTRACT FROM AUTHOR]

The recognition of unconformities is important for stratigraphic subdivision and correlation and for determining the timing of tectonic activity and the ocean‐continent transition process. Here, we have identified a Late Palaeozoic disconformity in the Moqinwula area of the northern Junggar Block, NW China. Integrated field relationship observations and sedimentological, biostratigraphical, geochemical and detrital zircon geochronological data, suggest that the underlying Hongliugou Formation is of Devonian age and was deposited in a passive continental margin setting. The overlying Tamugang Formation formed in the Early Carboniferous and was deposited in a foreland basin setting. Integrating the published data and our new work, we suggest a new tectonic evolution model related to the Karamaili ocean‐continent transition process. During the Early Devonian, northward subduction of the Karamaili Ocean plate beneath the East Junggar Belt formed the Yemaquan arc. At this time, a passive margin sequence was deposited on the northern Junggar Block. During the Early Carboniferous, the closure of the Karamaili Ocean resulted from the collision between the Junggar Block and the East Junggar Belt. Foreland basins subsequently developed, forming an extensive unconformity in the accretionary wedge to the north and a disconformity in the relatively rigid and stable Junggar Block to the south. This significant difference in stratigraphic contact relationships confirms that regional convergence‐induced shortening deformation had no significant influence on the northern margin of the Junggar Block, suggesting the existence of a relatively rigid basement, probably an oceanic plateau, under the cover sediments of the Junggar Block. [ABSTRACT FROM AUTHOR]

The Tertiary‐Piedmont Basin (NW Italy) is an episutural basin that developed from the late Eocene on the Alps–Apennines tectonic junction. Several coeval geodynamic processes, including the loading and exhumation of the Western Alps, the outward migration of the Apennine accretionary wedge and the opening of the Liguro‐Provençal rift basin, controlled the basin evolution. We integrate fluid‐inclusion microthermometry, low‐temperature thermochronology and burial history with numerical modelling to constrain the palaeo‐geothermal gradients required and evaluate the mechanisms that governed the basin thermal history. Apatite fission‐track and (U‐Th‐Sm)/He analyses of the basal late Eocene turbidites show reset ages of ca. 25 and 20 Ma, respectively, which require temperatures to be >120°C. Homogenization temperatures up to ca. 130°C from fluid inclusion analyses from authigenic minerals confirm the thermochronometric data, supporting a significant post‐depositional heating in the lower sequence of the basin. Stratigraphic reconstructions and decompaction of the basin fill indicate that the maximum burial experienced by the basal strata at 25 Ma is 2.3 ± 0.1 km, which is not sufficient to reset the AFT thermochronometric system when applying a typical geothermal gradient (ca. 20–30°C/km). An elevated geothermal gradient of 45 ± 5°C/km is thus necessary to explain the thermochronometric dates and the elevated thermal signature at shallow depths. 2D numerical simulations indicate that such an elevated palaeo‐geothermal gradient can be best explained by mantle upwelling, consistent with crustal thinning caused by the inception of the Liguro‐Provençal rift basin and related outward migration of the Alpine and Apennine fronts during the Oligocene. [ABSTRACT FROM AUTHOR]

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]

Accretionary processes at mid‐ocean ridge segments with low magma input have seldom been investigated over the long term. The evolution of such magma‐starved segments over time is still largely unknown. We present a study on the structure and evolution of the southernmost intra‐transform ridge segment of the St. Paul Transform Fault System in the Equatorial Mid‐Atlantic Ridge, based on new bathymetry, gravity, and rock sampling data. We show that this area evolves differently from previously described tectonics along ridge segments of similar spreading rate. On the flanks of the axial ridge segment, we observe a succession of structures exhumed by detachment faulting, evolving from east‐facing, long‐lived, corrugated oceanic core complexes (∼6 Ma ago), to short‐lived detachment faults exposing lower crust and mantle rocks and facing alternatively east and west in the more recent part of the segment. The oldest detachment faults have been repeatedly split and partially transferred to the opposite flank through the formation of new detachments into the footwall. The terminations of three old, east‐facing detachments are observed on the east flank of the segment. The westward relocations of the plate boundary appear to compensate for the asymmetry of accretion through detachment faulting, overall creating the same amount of lithosphere on both flanks of the ridge. We interpret the observed changes in the time of the accretionary processes to reflect a decrease of the melt supply over the last ∼6 Myr. Plain Language Summary: The generation of new seafloor at mid‐ocean ridges where cold underlying mantle delivers low magma supply has not been investigated over the long term. Here we present the analysis of new bathymetry, gravity, and rock sampling data over such a ridge segment located within the St. Paul Transform Fault system in the Equatorial Mid‐Atlantic Ridge, which allowed us to bring constraints on its structure and evolution over the last ∼6 Myr. We show that this area evolves differently from previously described ridge segments of similar spreading rate. We observe the remnants of very large normal faults called detachment faults, which have been active for very long times, forming domes called oceanic core complexes. The fault surfaces have been dissected by further extensional deformation, until the plate motion became accommodated along a new detachment fault formed west of the previous plate boundary. The emergence lines of the detachment faults are observed on the eastern flank of the ridge segment. We also observe a change in time of the structures, from typical oceanic core complexes to shorter ridges formed by the exhumation of mantle rocks. We interpret these changes to possibly reflect a decrease in the melt supply in the last 6 Myr. Key Points: The southern intra‐transform ridge segment in the St. Paul transform fault system, over the last ∼6 Myr is dominated by detachment faultingSeveral detachments have been split and partly transferred to the opposite ridge flank through ridge axis relocationThe change in the accretionary processes, from Oceanic Core Complexes to mantle exhumation, suggests a decrease in the time of the melt supply [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]

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]

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]

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]

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]

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]

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]

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]

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]

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]

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]

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]

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 Kitakami Massif of the Tohoku district, Northeast Japan, consists mainly of the South Kitakami Belt (Silurian-Cretaceous forearc shallow-marine sediments, granitoids, and forearc ophiolite) and the North Kitakami Belt (a Jurassic accretionary complex). The Nedamo Belt (a Carboniferous accretionary complex) occurs as a small unit between those two belts. An accretionary unit in the Nedamo Belt is lithologically divided into the Early Carboniferous Tsunatori Unit and the age-unknown Takinosawa Unit. In order to constrain the accretionary age of the Takinosawa Unit, detrital zircon U-Pb dating was conducted. The new data revealed that the youngest cluster ages from sandstone and tuffaceous rock are 257-248 Ma and 288-281 Ma, respectively. The Early Triassic depositional age of the sandstone may correspond to a period of intense magmatic activity in the eastern margin of the paleo-Asian continent. A 30-40 my interval between the youngest cluster ages of the sandstone and the tuffaceous rock can be explained by the absence of syn-sedimentary zircon in the tuffaceous rock. The new detrital zircon data suggest that the Takinosawa Unit can be distinguished as an Early Triassic accretionary complex distinct from the Early Carboniferous Tsunatori Unit. This recognition establishes a long-duration northeastward younging polarity of accretionary units, from the Carboniferous to Early Cretaceous, in the northern Kitakami Massif. Lithological features and detrital zircon spectra suggest that the Early Triassic Takinosawa Unit in the Nedamo Belt is comparable with the Hisone and Shingai units in the Kurosegawa Belt in Shikoku. The existence of this Early Triassic accretionary complex strongly supports a pre-Jurassic geotectonic correlation and similarity between Southwest and Northeast Japan. [ABSTRACT FROM AUTHOR]

Cherts, radiolarites, and radiolarian cherts are one of the most important components of accretionary complexes, and much used in the reconstruction of imbricated sections of ocean plate stratigraphy, and of the palaeogeographic and tectonic settings of ophiolitic mélanges. There has been little research on radiolarian cherts in the Kyrgyz South Tianshan, and the tectonic setting and temporal framework of the South Tianshan ocean are controversial. Here, we report well‐preserved radiolarians in ribbon‐bedded cherts in an ophiolitic mélange of the Atbashi accretionary complex (South Tianshan, Kyrgyzstan), which provide key evidence of ambient Palaeozoic oceanic conditions. The hydrofluoric acid technique was used to extract from the cherty rocks siliceous radiolarian, which yielded an age of late Early Carboniferous. Analyses of major, trace, and rare‐earth elements of four cherts indicate they are biogenic and deposited with little hydrothermal influence in a hemipelagic to continental margin environment. Our fossil ages and geochemical results of the radiolarian cherts from the Atbashi accretionary complex indicate that the South Tianshan ocean was still open in the Late Carboniferous, implying that it was a wide, long‐lived, major ocean that closed by northward subduction. [ABSTRACT FROM AUTHOR]

Late Palaeozoic–Mesozoic volcano‐sedimentary rocks within the Rushan–Pshart Suture zone in the Pamir contain critical information on the subduction–accretion history of the Rushan–Pshart Ocean (Meso‐Tethys) prior to the Central–South Pamir collision. In this article, we report new field, petrographic, geochronological, and geochemical data of the Permian to Triassic basic volcanic and sedimentary rocks from the Pshart area. Our field study unravels block‐in‐matrix components within some sedimentary mélanges, which, together with some previously defined ophiolitic mélanges, enables us to define the Pshart accretionary complex (AC), and thus for the first time to discuss the subduction–accretion history of the Rushan–Pshart Ocean and the growth of the Pshart AC. The youngest detrital U–Pb zircons suggest that deposition of sediments was in the Late Triassic (212 Ma). Detritus of Triassic age was primarily derived from the Triassic Karakul‐Mazar Arc‐AC (Northern Pamir) and the Bashgumbaz Magmatic Arc, which developed along the northern margin of South Pamir. The evidence of north‐directed thrusts along the Kara Djilga‐2 and Ken Djilga transverses confirms previous interpretations of southward subduction of the Rushan–Pshart oceanic lithosphere beneath the South Pamir. Geochemical OIB‐type data of the Pshart alkaline basaltic rocks suggest formation in seamounts incorporated into the Pshart AC during the southward subduction of the Rushan–Pshart Ocean. The youngest Late Triassic deposition age is consistent with the coeval time of closure of the Palaeo‐Tethys and Meso‐Tethys oceans in the Pamir. The Pshart AC formed by subduction–accretion processes during the southward subduction of the Meso‐Tethys Ocean along the northern South Pamir and the final docking of the Central and South Pamir (Cimmerian Blocks) may have occurred after the Late Triassic. [ABSTRACT FROM AUTHOR]

We examined the lithostratigraphy and deformation structures of the accreted pelagic rocks in Lake Hamana area, Central Japan, based on a detailed field mapping and the radiolarian ages. The constructed lithostratigraphy is composed of Lopingian (Upper Permian) grey chert, black chert, black claystone with a high carbon content of 4.85–6.78 wt%, siliceous claystone, black chert, and Anisian (Middle Triassic) grey chert, in the ascending order. The symmetry in the lithostratigraphy and the change in the radiolarian ages with respect to black carbonaceous claystone are presumed to represent a deep‐sea anoxic event that occurred across the Permian–Triassic (P–Tr) boundary. The P–Tr boundary black carbonaceous claystone suffered from an intense shear, resulting in blocks of siliceous claystone in the scaly black claystone matrix. In places, the sheared terrigenous rocks composed of sandstone blocks in the mudstone matrix are intercalated into the pelagic rocks, possibly representing complex mélange forming processes during the development of the Jurassic accretionary complex. [ABSTRACT FROM AUTHOR]

We carried out an integrated study of zircon U–Pb geochronology, Hf isotopes and geochemistry on the paragneisses and amphibolites from the Chencai area, South China. The detrital zircon ages of four paragneisses range from 2974 to 501 Ma with the main peak at ca. 820 Ma. The detrital materials with Neoproterozoic age may derive from the Jiangnan Orogen and those of Palaeo–Mesoproterozoic age may come from the Wuyi terrane. The youngest age indicates that the protoliths of the paragneisses were formed after ca. 501 Ma. The εHf(t) values (−20.6 to +20.9) of the detrital zircons suggest that the detrital materials of the protoliths are composed of old crustal and juvenile materials. The geochemical characteristics suggest that the protoliths of the paragneisses are greywackes, which formed in an active continental margin or continental island arc environment. The metamorphic ages of the three amphibolites are 428 ± 4, 430 ± 4, and 438 ± 3 Ma. The protoliths of the amphibolites are tholeiites, which can be divided into two groups. Group 1 is characterized by low ∑REE contents and (La/Yb)N ratios showing left‐inclined REE patterns and Nb, Ta depletion which resembles N‐MORB. Group 2 has higher ∑REE contents and (La/Yb)N ratios, with Eu negative anomalies showing affinity to E‐MORB. The data show that the Chencai Group is composed of oceanic crust remnants and continental crust materials, indicating it should be a subduction accretionary complex rather than a metasedimentary sequence and thus it cannot be regarded as the Precambrian basement of the Cathaysia Block. [ABSTRACT FROM AUTHOR]

The Neoproterozoic Jiangnan orogen plays an important role in the study of the Precambrian tectonic evolution of South China. The tectonic nature of the Neoproterozoic sedimentary basins is still controversial, due to poor understanding of the sedimentary sequences and the lack of geochronological data. Here, we present sedimentological, provenance and geochronological data from the Heshangzhen Group in the eastern Jiangnan orogen. Sedimentological analysis shows that the Luojiamen Formation was deposited in a submarine fan, and the overlying Hongchicun Formation was deposited in front of a fan delta. The youngest detrital zircons constrain the lower Luojiamen and Hongchicun formations with ages of 827.3 ± 8.4 Ma and 825 ± 12 Ma, respectively. The sandstones of the Luojiamen Formation are characterized by a large number of intermediate to felsic volcanic grains, suggesting a volcanic arc source. In contrast, quartz and sedimentary lithic grains increase in the Hongchicun Formation, showing a new input from a collisional orogenic source. Detrital zircon from six sandstone samples in the Luojiamen and Hongchicun formations yield similar age spectra of 930–820 Ma with a peak at ca. 845–860 Ma, with one main cluster at 930–820 Ma. Detrital zircons of 930–845 Ma show a positive value of ∊Hf (t) (+2.4 to +11, mean +7.6), which is similar to the volcanic arc of the nearby Shuangxiwu Group. There are a minor group of zircons with U‐Pb ages ranging from 820 Ma to 845 Ma from the middle part of the Luojiamen Formation and Hongchicun Formation, with ∊Hf (t) values between –20 to +2.4, which are consistent with the characteristics of the Shuangqiaoshan Group. within light of the bidirectional paleocurrents in the Luojiamen Formation, it is speculated that the zircons of 820–845 Ma were recycled from the Shuangqiaoshan Group, which is derived from a continental arc to the northwest. Our data suggests that the Luojiamen Formation was formed in an inter‐arc basin, while the Hongchicun Formation was formed in an accretionary wedge‐top basin. When juxtaposed with the conglomeratic characteristics at the bottom of the Luojiamen Formation, it is believed that the unconformity represented by the 'Shen Gong Movement' reflects the rapid erosion and accumulation process of island arc volcanic material. The disconformity between the Luojiamen and Hongchicun formations is the imprint of transition from inter‐arc basin to accretionary wedge‐top basin, which represents the collision between the Shuangxiwu arc and the Yangtze Plate. [ABSTRACT FROM AUTHOR]

The Beishan orogenic zone is a key area to understand evolution of the Central Asian Orogenic Belt that is an accretionary factory well‐enough preserved in the Paleozoic. In early Paleozoic, the tectonic mélange zone containing the coherent unit and mélange unit is triggered by the complicated accretionary process of the Beishan area. The early Paleozoic tectonic evolution of the Beishan orogenic zone is investigated in this study using sedimentology and stratigraphic correlations of the lowe Paleozoic deposits. From the Cambrian to the middle Ordovician, this region was characterized by geographically extensive, flat‐bedded siliceous mudstone, indicating the existence of a large ocean basin. The oceanic plate entered the convergence phase in terms of the Wilson Circle during the Middle Ordovician, when numerous magmatic arcs formed along two opposite sides of the ocean. The magmatic arcs became the widest during the Silurian, suggesting that the Hongliuhe‐Niujuanzi‐Xichangjing Ocean (HNX; a southern branch of the Paleo Asian Ocean) was reduced to a small residual ocean in the central Beishan region by that time, and probably lasted till the Carboniferous or later by newly published data. [ABSTRACT FROM AUTHOR]

The South Qilian belt mainly comprises an early Paleozoic arc‐ophiolite complex, accretionary prism, microcontinental block, and foreland basin. These elements represent accretion‐collision during Cambrian to Silurian time in response to closure of the Proto‐Tethyan Ocean in the NE of the present‐day Tibet Plateau. Closure of the Proto‐Tethyan Ocean between the Central Qilian block and the Oulongbuluke block and the associated collision took place from NE to SW in a zipper‐like style. Sediment would have been dispersed longitudinally SW‐ward with a progressive facies migration from marginal alluvial sediments toward slope deep‐water and deep‐sea turbidites. This migration path indicates an ocean basin that shrank toward the SW. The Balonggongga'er Formation in the western South Qilian belt represents the fill of a latest Ordovician‐Silurian remnant ocean basin that separated the Oulongbuluke block from the Central Qilian block, and records Silurian closure of the Proto‐Tethyan Ocean and subduction beneath the Central Qilian block. However, alluvial deposits in the Lajishan area were accumulated in a retro‐foreland basin, indicating that continent‐continent collision in the eastern South Qilian belt occurred at c. 450–440 Ma. These results demonstrate that the Proto‐Tethyan Ocean closed diachronously during early Paleozoic time. [ABSTRACT FROM AUTHOR]

Three‐dimensional (3‐D) seismic reflection data and sediment cores record ~2.87 million years of structural and depositional history of a trench slope basin along the outer Nankai accretionary prism, southwest Japan. Numerous mass transport deposits (MTDs) and fault structures are present in the basin. Here, we investigate the links between slope failures, slope basin development, and movement along a prominent out‐of‐sequence thrust (OOST) fault and development of a large anticline. Three two‐dimensional (2‐D) cross sections are restored to ~2.2 Ma using stratigraphic and structural relationships interpreted in the 3‐D data. The restorations are then compared and combined to provide a 3‐D perspective of basin development. We find that total shortening across all faults and folds was accommodated by different displacement styles along multiple branches, with 5.3, 5.5, and 7.3 km of shortening from northeast to southwest over the past ~2.2 Ma. We believe that the majority of this displacement occurred prior to ~1.7 Ma, followed by a dramatic decrease in slip rate within the study area as slip was transferred to the more seaward portions of the prism. In the northeast, deformation is primarily accommodated by the main branch of the OOST and anticlinal faulting, while deformation in the southwest is primarily along deeper branches of the OOST. This differential motion explains the occurrences of various mass wasting events, and lateral differences in trench slope basin geometry within the study area. Plain Language Summary: Accretionary prisms form at subduction zones where one tectonic plate dives beneath another. Sediments from the downgoing plate are scraped off and added to the overriding plate over millions of years to form the prism. Here, we use three‐dimensional (3‐D) seismic reflection data and sediment cores recovered during drilling into the prism to restore the movement along the faults back through time. Numerous submarine landslide deposits, faults, and other geologic structures are present in the data. To understand the links between submarine landslides, sediment basin development, and fault and geologic structure development, three two‐dimensional (2‐D) cross sections within the 3‐D data are restored back through time ~2.2 million years. The restorations are then compared and combined to provide a 3‐D perspective of sediment basin development. We find more displacement along faults near the seafloor in the northeastern part of the study area while fault displacement is much deeper under the seafloor in the southwest. Total shortening across the faults is partitioned differently across the area, with total displacement being the greatest in the southwest and gradually decreasing to the northeast. These differential motions explain the occurrences of various landslide events and seafloor variations within the study area. Key Points: Faulting that generated a slope basin on the Nankai prism evolved in the context of an accretionary prism maintaining critical taperTrench slope basin development was influenced by an underlying regional anticline and local slope failuresShortening in the midslope region slowed by ~1.7 Ma and uplift of the outer fore arc was accommodated on faults in the upper slope region [ABSTRACT FROM AUTHOR]

Underthrusting is a typical process at compressive margins responsible for nappe stacking and sediment subduction. In nature, underthrusting is often associated with weak basal faults, although static mechanical analysis (critical taper theory) suggests that weak basal faults promote accretion while strong basal faults promote underthrusting. We perform mathematical analyses and numerical simulations to determine whether permanent fault weakening promotes or inhibits underthrusting. We investigate the control of permanent fault weakening on the dynamics of a strong‐based ((1−λb*)μb≈(1−λ*)μ) non‐cohesive wedge (μ and μb are internal and basal friction, respectively). We control the wedge material strength by a spatially constant fluid overpressure factor (λ*), and fault strength by a plastic strain weakening factor (χ). First, we use the critical taper theory to determine a mechanical mode diagram that predicts structural styles. Then, we perform numerical simulations of accretionary wedge formation to establish their dynamical structural characteristics. We determine a continuum of structural styles between three end‐members which correspond to the theoretical mechanical mode transitions. Style 1 is characterized by thin tectonic slices and little to no underthrusting. Style 2 shows thick slices, nappe stacking, and shallow gravity‐driven tectonics. Style 3 displays the complete underthrusting of the incoming sediments, that are exhumed when they reach the backstop. We conclude that in the condition of an initially strong wedge base, permanent fault weakening promotes underthrusting. Thus, this contribution enlightens the control of the dynamic evolution of material properties on the formation of subduction channels, slope instabilities, and antiformal nappe stacks. Key Points: We use theory and numerical models to study internal and basal strength control on wedge dynamicsSimulations reveal three structural styles that correspond to theoretical mechanical modesThese styles exhibit (1) pure accretion, (2) accretion + underthrusting, and (3) pure underthrusting [ABSTRACT FROM AUTHOR]

The Tekturmas ophiolite belt of central Kazakhstan is located in the western Central Asian Orogenic Belt. It consists of three structural‐formational zones: Tekturmas (centre), Bazarbai (north), and Sarysui (south). The Tekturmas zone includes igneous and sedimentary rocks of Karamurun, Tekturmas, and Sarytau formations, which are dominated by volcanogenic, deep sea, and clastic deposits, respectively. The Bazarbai zone consists of Kuzek and Basarbai formations dominated, respectively, by basaltic and sedimentary rocks. The Sarysui zone includes terrigenous Airtau and olistostromic Ermek formations. The age of sedimentary rocks was constrained by microfossils. The igneous rocks of the Karamurun and Bazarbai formations are alkaline and subalkaline basalt/dolerite, andesibasalt, and andesite. There are three main groups of rocks: high‐Ti, mid‐Ti, and low‐Ti. The high‐Ti Karamurun volcanics and low‐Ti Bazarbai are variably enriched in light rare earth element (LREE) showing LREE enriched (high‐Ti) and LREE flat (low‐Ti) REE patterns. The high‐Ti group shows enrichment in Nb, Th, Zr, and Sm compared with the low‐Ti group. The low‐Ti group is special for the Nb troughs in primitive mantle‐normalized multi‐element diagrams, which are typical of supra‐subduction settings. Both groups yielded positive εNd values, although the averages are 4.6 and 7.5 for the high‐ and low‐Ti rocks, respectively. The geochemical features suggest formation of the igneous rocks in oceanic and supra‐subduction (intra‐oceanic arc) settings. The geological and geochemical features of the Tekturmas ophiolite belt accord well with the tectonic emplacement of ophiolites and formation of accretionary complex at a Pacific‐type convergent margin similar to those of the western Pacific. [ABSTRACT FROM AUTHOR]

Three tectonic units have been recognized in the Chifeng area, Inner Mongolia, from north to south, including the Qiganmiao accretionary prism, Jiefangyingzi arc belt and Sidaozhangpeng molasse basin, which formed an Andean‐type active continent margin during the early to middle Paleozoic. The Qiganmiao accretionary prism is characterized by a mélange that consists of gabbro, two‐mica quartz schist and basic volcanic rock blocks and heterogeneously deformed marble matrix. Two zircon U‐Pb ages of 446.0±6.3 Ma and 1104±27 Ma have been acquired and been interpreted as the metamorphic and forming ages for the gabbro and two‐mica quartz schist, respectively. The prism formed during the early to middle Paleozoic southward subduction of the Paleo Asian Ocean (PAO) and represents a suture between the North China craton (NCC) and Central Asian Orogenic Belt (CAOB). The Jiefangyingzi arc belt consists of pluton complex and volcanic rocks of the Xibiehe and Badangshan Formations, and Geochronology analysis indicates that the development of it can be divided into two stages. The first stage is represented by the Xibiehe Formation volcanic rocks, which belong to the subalkaline series, enriched LREE and LILE and depleted HFSE, with negative Eu anomalies, and plot in the volcanic arc field in discrimination diagrams. These characters indicate that the Xibiehe Formation results from to the continental arc magmatic activity related to the subduction of the PAO during 400‐420 Ma. Magmatism of the second stage in 380‐390 Ma consists of the Badangshan Formation volcanic rocks. Geochemistry analysis reveals that rhyolite, basaltic andesite and basalt of the Badangshan Formation were developed in continental margin arc setting. Moreover, the basaltic andesite and basalt display positive Sr anomalies, and the basalt have very low Nb/La values, suggesting that fluid is involved in magma evolution and the basalts were contaminated by continental crust. The sequence of Sidaozhangpeng molasse basin is characterized by proximity, coarseness and large thickness, similar to the proximity molasses basin. According to our field investigation, geochronological and geochemical data, combined with previous research in this area, a tectonic evolutionary model for Andes‐type active continental margin of the CAOB has been proposed, including a development of the subduction‐free PAO before 446 Ma, a subduction of the PAO and arc‐related magmatism during 446‐380 Ma, and formation of a molasse basin during 380‐360 Ma. [ABSTRACT FROM AUTHOR]

There is a broad consensus that the final closure of the Paleo‐Asian Ocean lead to the formation of the southern segment of the Central Asian Orogenic Belt. However, the final closure of the Paleo‐Asian Ocean remains controversial. To address this issue, a systematical study of geochemical and zircon U–Pb on the Kawabulak ophiolite in the southern Eastern Tianshan was carried out. The Kawabulak ophiolite is an accretionary prism that consists of different lithological tectonic slices and marks the suture zone that welded the Central Tianshan composite arc and the northern passive margin of the Tarim Craton. Zircon U–Pb data show that gabbro blocks of the Kawabulak ophiolite formed at 360.9 ± 3.3 Ma in the northern part and 326.0 ± 3.3 Ma in the southern part, respectively. The geochemistry features of basalts and gabbros show OIB‐type, arc/subduction‐related affinities, indicating different tectonic setting origins. A granitic gneiss with an arc‐related geochemical feature yielded a zircon U–Pb age of 477.2 Ma, indicating the earliest subduction event. The youngest deformed arc‐related granitic intrusion (295 Ma) in the Kawabulak prism indicates that the final closure timing of the Paleo‐Asian Ocean in Eastern Tianshan is after the Early Permian. The results of our study shed light to the temporal and spatial evolution of the southern Central Asian Orogenic Belt. [ABSTRACT FROM AUTHOR]

We present new meteoric 10Be concentration data from marine sediment cores recovered during Integrated Ocean Drilling Program Expedition 343 (JFAST) that help constrain the age, origin, and internal structure of the frontal prism at the Japan trench in the vicinity of the 2011 Tohoku‐oki M9 earthquake rupture. 10Be sediment ages from the lower portion of the frontal prism range from ~0–10 Ma, with >60% of analyzed samples above the décollement yielding young ages <2 Ma. Repetition and inversion of high‐over‐low 10Be‐concentration sediments indicate the presence of stratigraphic inversions that correspond to faulting and imbrication of late Miocene and Quaternary sediments. The density of faults inferred indicates that the frontal prism has a fault spacing on the order of 10 s of meters, and the identification of faults in the underthrust section suggests that the plate boundary décollement may be a zone with multiple slip surfaces. Comparison of 10Be concentrations in the frontal prism with those of the incoming and forearc slope sediments indicates that the majority of the prism is sourced from accretion of Pacific Plate sediments, rather than from reworked frontal prism or slope sediments. These data suggest that over at least the past ~1–2 Ma, the décollement preferentially has localized at or near the base of the incoming sediment section, with relatively efficient sediment accretion occurring even in the presence of subducted horst‐and‐graben topography. Plain Language Summary: We help constrain the physical and mechanical properties of the materials near the trench that hosted the 2011 Tohoku M9 earthquake and tsunami and test whether sediments from the Pacific Plate are scraped off or subducted as they enter the subduction zone. We use sediments from cores recovered during the Integrated Ocean Drilling Program Expedition 343. Because traditional sedimentologic and biostratigraphic techniques alone are not sufficient to determine sediment source region, we apply a new approach using the radioactive decay of the 10Be isotope. We use 10Be to date sediments, to identify sediments that have been faulted out of chronologic order, and to correlate sediments between sites. Our results show numerous faults in the system, likely resulting from multiple stages of deformation during slip over rough seafloor topography. Our data also indicate that 50–90% of the sediments on the incoming Pacific Plate are scraped off and 10–50% of the incoming sediments are subducted. The subducted sediments consist of muds and frictionally weak clays that might promote slip during subduction earthquakes. Similar approaches may provide insight on the composition and evolution of faults that host large earthquakes and tsunami at other plate boundaries. Key Points: Meteoric 10Be provides high‐resolution age control in the frontal prism of the Japan trench at IODP Exp 343 (JFAST) Site C0019The frontal prism and plate boundary contain faults at decameter scale spacing, possibly due to subduction of horst‐and‐graben topographyThe majority of incoming sediments at this location have been accreted to the overriding plate during at least the Quaternary period [ABSTRACT FROM AUTHOR]

The strength of accretionary prisms is important for understanding their formation and the generation of earthquakes along the underlying plate boundary faults. We establish a continuous depth profile of rock strength and possible ranges of the minimum and maximum horizontal stresses (Shmin and SHmax) based on indentation tests on cuttings samples at the Integrated Ocean Drilling Program (IODP) Site C0002 in the Nankai accretionary prism. The rock strength (i.e., differential stress) increases with depth from 5.1 and 11.0 MPa at 975 meters below the seafloor (mbsf) to 22.8 and 59.6 MPa at 3005 mbsf in normal and reverse faulting regimes, respectively. The minimum value of Shmin and the maximum value of SHmax range from 33.2 and 49.4 MPa at 975 mbsf to 57.1 and 139.6 MPa at 3005 mbsf, respectively. Our estimated strength, in combination with Coulomb wedge model, may improve understanding of the evolution of accretionary prisms. Key Points: Strength of the inner Nankai accretionary prism is estimated from indentation tests on cuttingsOur estimation constrains the ranges of possible maximum and minimum principal stresses, which are limited by rock strength and loading stress regimesDifferential stress reaches 57.1 and 139.6 MPa at ~3 km below seafloor for the normal and reverse faulting stress regimes, respectively [ABSTRACT FROM AUTHOR]

Sand‐shale mélanges from the Kodiak accretionary complex and Shimanto belt of Japan record deformation during underthrusting along a paleosubduction interface in the range 150 to 350 °C. We use observations from these mélanges to construct a simple kinetic model that estimates the maximum time required to seal a single fracture as a measure of the rate of fault zone healing. Crack sealing involves diffusive redistribution of Si from mudstones with scaly fabric to undersaturated fluid‐filled cracks in sandstone blocks. Two driving forces are considered for the chemical potential gradient that drives crack sealing: (1) a transient drop in fluid pressure ∆Pf, and (2) a difference in mean stress between scaly slip surfaces in mudstones and cracks in stronger sandstone blocks. Sealing times are more sensitive to mean stress than ∆Pf, with up to four orders of magnitude faster sealing. Sealing durations are dependent on crack spacing, silica diffusion kinetics, and magnitude of the strength contrast between block and matrix, each of which is loosely constrained for conditions relevant to the seismogenic zone. We apply the model to three active subduction zones and find that sealing rates are fastest along Cascadia and several orders of magnitude slower for a given depth along Nicaragua and Tohoku slab‐top geotherms. The model provides (1) a framework for geochemical processes that influence subduction mechanics via crack sealing and shear fabric development and (2) demonstration that kinetically driven mass redistribution during the interseismic period is a plausible mechanism for creating asperities along smooth, sediment‐dominated convergent margins. Plain Language Summary: Geophysical monitoring of active subduction zones has revealed plate boundary slip behaviors such as creep, slow slip events, and earthquakes that vary spatially and temporally for different plate boundaries and downdip along a given boundary. Fault rocks exposed on land from paleosubduction plate boundaries provide a record of the deformation processes that likely occur during slip along active boundaries, so we review the characteristics of these ancient rocks to develop insight into slip behavior in subduction zones. We find that plate boundary deformation in these cases occurs within a wide fault zone through processes that involve redistribution of silica from shearing mudstones to cracking sandstone blocks. We use a geochemical model to calculate how long it would take to seal a crack by this process and conclude that cracks seal at rates that could influence the earthquake cycle, with rates of crack healing dependent on the temperature structure and the depth where slip occurs. Our results suggest that processes of frictional failure and geochemical healing in downgoing sediments may influence the slip stability along the subduction interface. Key Points: Microstructural observations of subduction mélange record dissolution along a scaly fabric and diffusion to cracks in sandstone blocksA silica kinetics model depicts crack healing that is highly dependent on temperature and slab‐top geothermKinetic healing of cracks could modulate fault zone strength during the interseismic period [ABSTRACT FROM AUTHOR]

The Datong area is located in the western part of the Western Kunlun Orogen (WKO), occupying a key area of the Tethyan domain. Our new SHRIMP U–Pb zircon ages from the Datong area provide new information about the Palaeozoic evolution of the Western Kunlun Orogen. A 473.0 ± 2.8 Ma granite with 493.6 ± 4.8 Ma inherited zircons from the Kusilafu extrusive rocks is interpretation as an A‐type granite formed in a fore‐arc tectonic setting. The 485.5 ± 1.9 Ma granitoid of the Datongdong pluton combined with the 452.7 ± 7 Ma granitoid of the Datong pluton indicate that the Datong pluton is an intra‐oceanic arc complex formed by the southern subduction of the Proto‐Tethyan ocean crust. Zircons from gabbros of the Datong mafic–ultramafic complex have a 499.2 ± 2.1 Ma U–Pb crystallization age. Combined with its complicated whole‐rock geochemistry, we propose that there may have existed a Datong seamount in the early Palaeozoic. The detrital zircon results of Silurian–Devonian sandstones reveal that the Late Silurian sandstones have the characteristic zircons both from the Tarim Block and Datong Arc, suggesting that the Datong arc collided with the Tarim Block in the Late Silurian. The tectonic history of the WKO was characterized by a southern subduction of Proto‐Tethys terminated in the Late Silurian. [ABSTRACT FROM AUTHOR]

Copyright of Island Arc is the property of Wiley-Blackwell and its content may not be copied or emailed to multiple sites or posted to a listserv without the copyright holder's express written permission. However, users may print, download, or email articles for individual use. This abstract may be abridged. No warranty is given about the accuracy of the copy. Users should refer to the original published version of the material for the full abstract. (Copyright applies to all Abstracts.)

Copyright of Island Arc is the property of Wiley-Blackwell and its content may not be copied or emailed to multiple sites or posted to a listserv without the copyright holder's express written permission. However, users may print, download, or email articles for individual use. This abstract may be abridged. No warranty is given about the accuracy of the copy. Users should refer to the original published version of the material for the full abstract. (Copyright applies to all Abstracts.)

Copyright of Island Arc is the property of Wiley-Blackwell and its content may not be copied or emailed to multiple sites or posted to a listserv without the copyright holder's express written permission. However, users may print, download, or email articles for individual use. This abstract may be abridged. No warranty is given about the accuracy of the copy. Users should refer to the original published version of the material for the full abstract. (Copyright applies to all Abstracts.)

The Borneo Accretionary Orogen (BAO) is located in a central position of SE Asia, surrounded by the long-lived westward subduction of the Pacific Plate, northward subduction of the Indian Plate beneath the Eurasian Plate and a passive continental margin of the South China Sea. Based on the previous studies in stratigraphy, structural geology and geophysics, a number of tectonic models on the BAO have been proposed, but these models do not perfectly match all of the geological facts. A field-based structural analysis of the rocks of Sabah in Malaysia has recognized that the deformation in Sabah can be divided into four episodes (D1-D4). The D1 deformation developed complete displacement foliation (S1) and NNE-trending thrusts. Then D1 structures were reworked by the subsequent D2 deformation. D2 deformation resulted in WNW- or NW-striking thrusts, followed by NNW-SSE-trending thrusts and folds (D3). Some NNE-trending sinistral strike-slip faults and WNW-trending dextral tension-shear faults (D4) offset the D3 structures. D1 is related to the collisional event in the Sabah Orogeny during 23-16 Ma, and D2 corresponds to the formation of the Deep Regional Uncomformity (DRU) at 16 Ma. D3 is related to the Shallow Regional Uncomformity (SRU) at 10 Ma. D4 is associated to the NW-SE-trending extension after the multi-stage collisional events. Analysis of the sedimentary and magmatic data reveals that the BAO is a Mesozoic and Cenozoic accretionary orogeny, represented by the intensely deformed Rajang-Crocker Group Accretionary prism, ophiolites and calc-alkaline igneous rocks. The accretionary orogen is related to the evolution of the Proto-South China Sea Ocean (PSCS). The PSCS continuously subducted under the Borneo Block and closed in the Late Eocene-Early Miocene time. The subduction zone of the PSCS retreated to the north in the Mesozoic and Cenozoic and migrated to the present-day Nansha Trough. The BAO is still active, as represented by a series of thrusting and subduction of the Dangerous Grounds under the Borneo Block. The NNE-trending subduction-related faults are considered as transform faults, rotating to the present-day NW-trending faults due to the anti-clockwise rotation of the entire Borneo Block. In particular, the previous NNE-trending Tinjar Fault is regarded as a major boundary in sedimentation and magmatism. To the NE of the fault, the strata and igneous rocks are Oligocene-Early Miocene, but to the SW of the fault, the strata and igneous rocks developed during the Cretaceous-Late Eocene interval. Copyright © 2016 John Wiley & Sons, Ltd. [ABSTRACT FROM AUTHOR]