Accretionary Prism Research Articles

Sediment Provenance Along the Middle Miocene‐Pleistocene Nankai Subduction Zone From Sediment Transport to Accretion: Implications for Stratigraphy in the Accretionary Prism

AbstractBased on U‐Pb dating of zircon crystals and petrographic analysis, this study provides new insights into the paleogeographic and accretion evolution along SW Japan. Our data are consistent with an older submarine fan identified from drilling in the Shikoku Basin (Kyushu Fan ∼14.7–12.2 Ma), having a mixed sand provenance from the paleo‐Yangtze/Yellow rivers and the Shimanto Belt, and the younger Zenisu Fan (∼9.2–7.6 Ma), which is mainly sourced from the Shimanto Belt and the Izu‐Bonin/Honshu arc collision. Our results are in agreement with the hypothesis of very oblique subduction or strike‐slip motion between the northern Shikoku Basin and mainland Honshu from ∼12.2 to 9.2 Ma, after which essentially orthogonal subduction occurred after ∼8 Ma. The two main sandbodies drilled at IODP Site C0002 within the inner Nankai Accretionary Prism have similar petrographic signatures to those of the Zenisu and Kyushu submarine fans in the Shikoku Basin. The incorporation of the Shikoku Basin deposits most likely resulted from the seaward propagation of in‐sequence thrusts forming an outer accretionary wedge. The incorporation of the Kyushu Fan into the inner accretionary prism implies that the décollement was located in the hemipelagic interval beneath the Kyushu Fan at least until ∼2 Ma, whereas it is now located in the hemipelagic intervals below the Zenisu Fan. Such shifts in décollement location are most likely related to changes in physical properties of the hemipelagic interval due to significant compaction and diagenesis during subduction.

Late Paleozoic intra-oceanic arc and its accretionary complex in East Junggar (NW China): implications for multiple arc amalgamation in the southern Altaids

The East Junggar is a key to understanding the accretionary history of the southern Altaids. However, the tectonic setting and amalgamation history of the East Junggar during the Carboniferous–Permian remain controversial. Here we report new field, geochronological and geochemical data for the Carboniferous–Permian volcanic and sedimentary rocks from the Jiangjun unit and the Hongliugou ophiolitic complex of East Junggar to constrain its tectonic evolution. Carboniferous volcanic rocks, with zircon and apatite U–Pb ages from 337 ± 2 to 303 ± 32 Ma, display arc-related geochemical characteristics. Carboniferous–Permian sedimentary rocks predominantly contain Carboniferous–Permian detrital zircons and have maximum depositional ages from 318 ± 5 to 281 ± 6 Ma. Provenance analysis indicates that the sediments from the west Jiangjun unit were sourced from the Yemaquan arc and those from the east Jiangjun unit may be sourced solely from a new intra-oceanic arc. Integrated data indicate that Carboniferous–Permian strata in the Jiangjun unit were formed in the intra-oceanic arc setting that remained until the middle Permian. These analyses suggest an amalgamation process of multiple convergence and accretion for the southern Altaids during the formation of NE Pangaea in the late Paleozoic.

Age, sedimentology, and deformational history of the Mesozoic Franciscan accretionary complex, Angel Island, California, USA

Age, sedimentology, and deformational history of the Mesozoic Franciscan accretionary complex, Angel Island, California, USA

Structural Variation Along the Southern Hikurangi Subduction Zone, Aotearoa New Zealand, From Seismic Reflection and Retro‐Deformation Analysis

AbstractThe southern Hikurangi subduction zone exhibits significant along‐strike variation in convergence rate and obliquity, sediment thickness and, uniquely, the increasing proximity of southern Hikurangi to, and impingement on, the incoming continental Chatham Rise, an ancient Gondwana accretionary complex. There are corresponding changes in the morphology and structure of the Hikurangi accretionary prism. We combine widely spaced multichannel seismic reflection profiles with high resolution bathymetry and previous interpretations to characterize the structure and the history of the accretionary prism since 2 Ma. The southern Hikurangi margin can be divided into three segments. A northeastern segment (A) characterized by a moderately wide (∼70 km), low taper (∼5°) prism recording uninhibited outward growth in the last ∼1 Myr. Deformation resolvable in seismic reflection data accounts for ∼20 % of plate convergence, comparable with the central Hikurangi margin further North. A central segment (B) characterized by a narrow (∼30 km), moderate taper (∼8°) prism, with earlier (∼2‐∼1 Ma) shortening than segment A. Outward prism growth ceased coincidentally with development of major strike‐slip faults in the prism interior, reduced margin‐normal convergence rate, and the onset of impingement on the incoming Chatham Rise to the south. A southwestern segment (C) marks the approximate southern termination of subduction but widens to ∼50 km due to rapid outward migration of the deformation front via fault reactivation within the now‐underthrusting corner of the Chatham Rise. Segment C exhibits minimal shortening as margin‐normal subduction velocity decreases and plate motion is increasingly taken up by interior thrusts and strike‐slip faults.

Frontal Thrust Ramp‐Up and Slow Earthquakes Due To Underthrusting of Basement High in the Nankai Trough

AbstractRecently, integrated geophysical‐geological surveys in the Nankai subduction zone in Japan have revealed that slow earthquakes repeatedly occur beneath the outer wedge of the forearc. During December 2020 to February 2021, clustered slow earthquakes propagated around the frontal thrust of the accretionary wedge. The frontal thrust ramps up from the basal décollement and slips over trench‐filling sediment along the landward edge of the Nankai trough floor. Here, the Paleo‐Zenisu ridge has been subducted beneath the inner‐outer slope border. In addition, ocean floor topography and geologic structure revealed by seismic reflection surveys completed before 2022 document that the basement of the Philippine Sea Plate beneath the frontal thrust has a seamount and a horst‐like basement high. The northern edge of the basement high is located at the ramp‐up position of the frontal thrust. The 2020–2021 clustered slow earthquakes started at the Paleo‐Zenisu ridge and propagated to the topographic highs beneath the deformation front. Considering that the relative plate convergence between the upper Amurian Plate of the Nankai forearc and the subducting Philippine Sea Plate is ∼6.0 cm/year, the basement high at the deformation front has uplifted the frontal crest of the wedge at an average rate of 2.7–5.7 mm/year for several tens to hundred thousand years. These rates are among some of the highest rock uplift rates measured in the world. The slow earthquakes in the off‐Kumano Nankai Trough in 2020–2021 are a snapshot of a “living” Nankai frontal thrust during the megathrust interseismic period.

Deformation mechanisms and slip behaviors of tectonically deformed conglomerates from the Central Apennines fold-and-thrust belt: Implications for shallow aseismic and seismic slip

Fold-and-thrust belts and accretionary prisms exhibit considerable variability in slip behaviors along active faults. While numerous studies have investigated fault mechanics and slip behaviors in carbonate-, shale-, and sandstone-hosted faults as well as clay-rich portions of shallow accretionary prisms, the deformation mechanisms and slip behaviors of deformed, thrust-top molasse-type conglomerates remain poorly understood. To fill this knowledge gap, we conducted structural and microstructural analyses on exhumed and deformed conglomerates in the footwall of an out-of-sequence thrust from the Central Apennines fold-and-thrust belt (Italy). Our findings aim to constrain conglomerate deformation mechanisms and infer possible slip behaviors. We observed flattened pebbles and an intense foliation comprising densely spaced, thrust-parallel, clay-rich stylolites that indicate slow deformation attributed to carbonate dissolution during aseismic creep. In contrast, quartz clasts with calcite micro veins and micrometer-thick slip increments in slickenfibers suggest fast brittle failure and fluid overpressure in competent pebbles, while the matrix continues to deform aseismically through pressure solution. The general structure is similar to block-in-matrix textures observed in tectonic mélanges from exhumed subduction zones and accretionary prisms. Our results provide new insights into the slip behaviors of shallow (<∼1 km) portions of fold-and-thrust belts, complementing existing understandings of deformation mechanisms and slip behaviors across different depth ranges in fold-and-thrust belts and accretionary prisms.

Geochemical Study of the Osumi Granodiorite, Southwestern Japan

The Osumi Granodiorite, located on the Osumi Peninsula in southwest Japan, is an example of outer zone granites that were formed during a limited period (13–15 Ma) in response to the subduction of the Philippine Sea Plate. This event, which is linked to the separation of southwest Japan from continental Asia, resulted in unique igneous activity. The Osumi Granodiorite is the largest Miocene granite body in the region. It intrudes into the Mesozoic to Paleogene accretionary complex of the Shimanto Belt and affects contact metamorphism. Despite considerable research on the Osumi Granodiorite, limited geochemical studies, especially on trace and rare earth element (REE) analyses, have been conducted. Furthermore, there are insufficient data on the Rb–Sr isotopic system, leaving the formation process unclear. This study presents whole-rock geochemical and Rb-Sr isotopic data to investigate the petrogenesis of the Osumi Granodiorite. The results suggest a common magma origin for this pluton, as indicated by linear trends on the Harker diagrams and similar REE patterns. The presence of a relatively large Eu anomaly implies formation under a reducing environment. The AKF diagram indicates predominant contamination by pelitic rocks of the Shimanto Belt during magma formation. The Rb–Sr whole-rock isochron diagram and SrI–1000/Sr diagram suggest that the Osumi Granodiorite body was formed by heterogeneous assimilation of magma into the Shimanto Belt. Furthermore, the whole-rock isochron age is 64.3 Ma, which differs by approximately 50 My from the previously reported biotite K–Ar age (14–22 Ma). This age is considered to be a pseudo-isochron age, rather than the consolidation age. During the middle Miocene, the compressive stress field in the outer zone south of the Butsuzo Tectonic Line made it difficult for magma to rise. As a result, it reacted with the sedimentary rocks of the Shimanto Belt to various degrees. The Osumi Granodiorite underwent magma differentiation upon intrusion into the Shimanto Belt. It subsequently ascended, cooled, and interacted with pelitic rocks under stable geological conditions.

Middle Triassic and middle Permian radiolarians from the Kamitaki Complex in the Sasayama area, Hyogo Prefecture, Southwest Japan: Evidence for Triassic plate subduction along the eastern margin of Paleo-Asia

The Kamitaki Complex, situated in the Sasayama area in Southwest Japan, has long been presumed to be a Permian subduction-related accretionary complex based on correlations from previous studies. However, because of the lack of fossil evidence, the exact age of the complex remained uncertain for a long time period. To address this gap in knowledge, a geological survey and microfossil mapping were conducted in the Kamitaki Complex to determine its age and geological context.A geological survey revealed that the Kamitaki Complex mainly consists of clastic rocks, and a mixture of sandstone, basalt, and chert blocks within the mudstones. The Kamitaki Complex is tectonically intercalated into the Lopingian (Late Permian) accretionary complex of the Ultra-Tamba Terrane and Late Triassic accretionary complex of the Tamba Terrane. The lithological and structural characteristics of the Kamitaki Complex confirm that it is an accretionary complex. Microfossil mapping yielded depositional ages, with radiolarian fauna such as Eptingium nakasekoi, Pseudostylosphaera japonica, Cryptostephanidium japonicum, and Oertlispongus cf. diacanthus identified in mudstones suggesting an Anisian (early Middle Triassic) age. In contrast, radiolarian fauna found in cherts, including Pseudoalbaillella? aff. longicornis, and Follicucullus cf. porrectus, indicate an early Capitanian (late Guadalupian, middle Permian) depositional age. These findings suggest that the Kamitaki Complex records a trenchward migration of the oceanic plate in a pelagic environment from the early Capitanian and an accretion at the trench during the Anisian period.Conventionally, the plate boundary between the Panthalassa and Paleo-Asia during the latest Permian to Middle Triassic was of the transform type, primarily because no subduction-related accretionary complexes from this period have been identified in the Japanese Islands. However, the discovery of Kamitaki Complex, an Anisian accretionary complex, provides evidence of Middle Triassic subduction activity along the eastern margin of Paleo-Asia. According to the internal structure and age polarity of the Ultra-Tamba and Tamba terranes, an accretionary complex developed over a prolonged period (approximately 120 million years) in a tectonic setting that persisted along the eastern margin of Paleo-Asia from the late Guadalupian to the earliest Cretaceous period.

Role of folding-related deformation in the seismicity of shallow accretionary prisms

Role of folding-related deformation in the seismicity of shallow accretionary prisms

Unveiling Deep-Seated Gravitational Slope Deformations via Aerial Photo Interpretation and Statistical Analysis in an Accretionary Complex in Japan

The objective of this study was to identify the locations of deep-seated gravitational slope deformations (DGSDs) and define the numerical characteristics of these deformations to contribute to the sustainable management of social infrastructure in the event of an increased disaster. The topographic features of the DGSDs were quantitatively characterized based on their surface morphologies. Topographic features indicative of gravitational deformation in pre-slide topographic maps, such as terminal cliff failures, irregular undulations, and gullies, suggest that progressive deformation occurred over a prolonged period. To track the gravitational deformation over time, we interpreted aerial photographs of DGSDs from 1948 and 2012 associated with deep-seated landslides on the Kii Peninsula induced by Typhoon Talas on 2–5 August 2011. Corresponding numerical analysis of the gravitational deformations using 1 m digital elevation models reveals that landslide areas exhibit eight characteristic influencing factors, demonstrating that characteristic morphologies exist in areas that eventually experience landslides. One such morphological feature is the existence of a gently sloping area in the upper section of the deep-seated landslide mass, which comprises a catchment basin without a corresponding valley or gully. These findings suggest that rainwater penetrates the ground, and degrades and deforms the rock within the landslide mass, causing the slope to fail after torrential rainfall. This study holds great significance for advancing sustainable infrastructure development and management and mitigating environmental changes.

Permian tectonic evolution of the proto-Japan and East Asia: Insights from detrital zircon geochronology and crystal morphology

Combination analyses of detrital zircon ages and morphology from Permian sandstones in Japan provide evidence of orogenic evolution caused by collision in East Asia and reveal their paleogeographic setting. This study compares published and newly measured data from Permian fore-arc accretionary complexes (Akiyoshi Belt) and back-arc basin deposits (Maizuru Belt) in the Inner Zone of Southwest Japan. The fore-arc deposits and lower part of the back-arc deposits consist mainly of approximately 300–250 Ma zircon grains that are close to the depositional ages and are generally angular to subangular. These sediments formed around the Late Paleozoic volcanic arc isolated from the older continental crust, and their source rocks were exposed immediately prior to their erosion and transport and were transported directly to the depositional site in a relatively short time. In contrast, upper part of the back-arc deposits generally contains approximately 2,700–1,800 and 500–250 Ma zircon grains, which are commonly older than their depositional ages and are rounded. These grains were supplied from multiple sources, such as the Paleozoic volcanic arc and Precambrian continental block, and were affected by long-distance transport and a strong abrasion effect during transport. Comprehensive detrital zircon data emphasize that the oceanic arc–back-arc basin–continental margin system formed along the eastern margin of East Asia, and that the volcanic arc collided with the eastern margin of the Sino-Korean Block during the Permian. The variation in detrital zircon ages and shapes of back-arc deposits provides a constraint for modeling the East Asian Permian collision. Detritus derived from continental crust was captured by a back-arc basin and blocked by the uplift zone of the volcanic arc. However, a small amount of detritus originating from the continental crust was supplied to the fore-arc.

The Shalap Mélange of the Salairian Alambay Ophiolite Zone (Northwestern Central Asian Orogenic Belt), Geological Structure and Compositional Features of Amphibolites and Greenstone Basalts

The Alambay ophiolite zone (AOZ) is located in the axial part of the Early Paleozoic Salair orogen and includes the northern extension of the Alambay-Kaim zone, Salair and Altai Mountains. The Shalap area of the AOZ is predominantly composed of clastic mélange with occasional serpentine mélange. The geological and geochemical studies showed that in the Shalap mélange there are basalt blocks of the Alambay formation whose petrogeochemical features are similar to those of the oceanic island basalts (OIB). Metamorphic rocks of the Angurep complex, represented by garnet and non-garnet amphibolites, form a tectonic slab which is a part of the accretionary complex east of the Shalap mélange area. Metamorphic rocks also form blocks in the Shalap mélange. The amphibolites of the Angurep complex are similar in their petrogeochemical features to the basalts of intraoceanic island arcs. The Shalap mélange is a fragment of the Salairian Cambrian paleosubduction zone. The subduction and exhumation processes in this paleosubduction zone terminated by the 500 Ma time stage.

Deformation mechanisms and fluid conditions of mélange shear zones associated with seamount subduction

Lithologic heterogeneity and the presence of fluids have been linked to seamount subduction and collocated with slow earthquakes. However, the deformation mechanisms and fluid conditions associated with seamount subduction remain poorly understood. The exhumed Chichibu accretionary complex on Amami-Oshima Island preserves mélange shear zones composed of mudstone-dominated mélange and basalt–limestone mélange deformed under sub-greenschist facies metamorphism. The mudstone-dominated mélange contains sandstone, siliceous mudstone, and basalt lenses in an illitic matrix. The basalt–limestone mélange contains micritic limestone and basalt lenses in a chloritic matrix derived from the mixing of limestone and basalt at the foot of a seamount. The basalt–limestone mélange overlies the mudstone-dominated mélange, possibly representing a submarine landslide from the seamount onto trench-fill terrigenous sediments. The asymmetric S–C fabrics in both mélanges show top-to-SE shear consistent with megathrust-related shear. Quartz-filled shear and extension veins in the mudstone-dominated mélange indicate brittle failure at near-lithostatic fluid pressure and low differential stress. Microstructural observations show that deformation in the mudstone-dominated mélange was accommodated by dislocation creep of quartz and combined quartz pressure solution with frictional sliding of illite, whereas the basalt-limestone mélange was accommodated by frictional sliding of chlorite and dislocation creep of coarse-grained calcite, with possible pressure solution creep and diffusion creep of fine-grained calcite. The mélange shear zones formed in association with seamount subduction record temporal changes in deformation mechanisms, fluid pressure, and stress state during megathrust shear with brittle failure under elevated fluid pressure, potentially linking tremor generation near subducting seamounts.

Volcanic arc rigidity variations illuminated by coseismic deformation of the 2011 Tohoku-oki M9.

Rock strength has long been linked to lithospheric deformation and seismicity. However, independent constraints on the related elastic heterogeneity are missing, yet could provide key information for solid Earth dynamics. Using coseismic Global Navigation Satellite Systems (GNSS) data for the 2011 M9 Tohoku-oki earthquake in Japan, we apply an inverse method to infer elastic structure and fault slip simultaneously. We find compliant material beneath the volcanic arc and in the mantle wedge within the partial melt generation zone inferred to lie above ~100 km slab depth. We also identify low-rigidity material closer to the trench matching seismicity patterns, likely associated with accretionary wedge structure. Along with traditional seismic and electromagnetic methods, our approach opens up avenues for multiphysics inversions. Those have the potential to advance earthquake and volcano science, and in particular once expanded to InSAR type constraints, may lead to a better understanding of transient lithospheric deformation across scales.

Geochemistry and magmatic petrology of meta-ophiolites from the Bajgan Complex (Makran Accretionary Prism, SE Iran): new insights on the nature of the Early Cretaceous Middle East Neotethys

The Bajgan Complex in the North Makran Domain (Makran Accretionary Prism) comprises disrupted meta-ophiolitic sequences originating from oceanic crust protoliths. They include ultramafic and mafic cumulates, isotropic gabbros, plagiogranites and basalts. Ultramafic–mafic cumulates and plagiogranites exhibit compositions akin to rocks formed in mid-ocean ridge settings. Isotropic gabbro and basalt protoliths can be subdivided into three distinct geochemical types. Type-1 rocks are sub-alkaline (Nb/Y &lt; 0.1) with low Th, Nb and Ta contents and La N /Yb N ratios &lt;1, resembling those of normal mid-ocean ridge basalts (N-MORB). Type-2 rocks display slight enrichment in Th, Ta, Nb (Nb/Y = 0.36–0.45) and La N /Yb N = 2.12–3.20, resembling the chemistry of enriched (E-) MORB. Type-3 basalts show an alkaline nature (Nb/Y = 0.88–1.82), significant Th, Ta and Nb enrichment, and high La N /Yb N ratios (7.01–20.08), resembling the chemistry of alkaline basalts (ocean island basalts, OIB). Petrogenetic modelling indicates that N-MORB protoliths originated from a depleted MORB mantle source, whereas E-MORB and OIB protoliths were generated from partial melting of sub-oceanic depleted sources that underwent varying degrees of OIB-type enrichment. The Bajgan meta-ophiolitic protoliths were formed within a Late Jurassic to Cretaceous oceanic basin influenced by mantle plume activity and plume–ridge interaction. Supplementary material : Field photographs, micrographs, rocks sampled, analytical methods and results, and supplementary tables are available at https://doi.org/10.6084/m9.figshare.c.7193937 Thematic collection: This article is part of the Ophiolites, melanges and blueschists collection available at: https://www.lyellcollection.org/topic/collections/ophiolites-melanges-and-blueschists

Temperatures of Vein Formation Associated With Plate Interface Deformation Constrained by Oxygen and Clumped Isotope Thermometry

AbstractTectonic mélanges, characterized by conditions reflective of modern subduction fault zones, preserve mineral veins formed through mass transfer, a mechanism influencing the slip behavior of subduction megathrusts. In this study, we apply secondary ion mass spectrometry quartz‐calcite oxygen isotope thermometry and clumped isotope thermometry to examine the temperatures of vein formations in six mélange units in the Cretaceous Shimanto belt and one mélange in the Kodiak accretionary prism. Calcite in the veins exhibits δ13CPDB values ranging from −17.2‰ to −6.8‰, indicative of a carbon source mixing with sedimentary carbonate and organic matter. δ18OSMOW values of calcite range from +11.1‰ to +17.2‰; quartz yields δ18OSMOW values of +14.9‰ to +21.7‰. Oxygen isotopic signatures in minerals reveal that most vein‐forming fluids are significantly affected by rock buffering, while some retain isotopic compositions of seawater and meteoric water. Temperature estimates, derived from both thermometers, fall within the range of 100–250°C. Notably, vein temperatures remain constant across diverse vein types and mélange units with distinct maximum temperatures. The combined temperature records and fluid isotopic compositions imply vein formations at shallower depths linked to the incorporation of seawater, meteoric water, and fluid released from early dehydration reactions. At greater depths, vein formations are associated with fluid released from clay dehydration and long‐distance fluid flow. Reduced vein formations between 250 and 350°C may correlate with a shift to fluid‐unsaturated conditions resulting from clay hydration reactions. Our study highlights potential mechanical and hydraulic variations within the thermal conditions of 100–350°C along the plate boundary driven by fluid‐mineral interactions.

Ulban Terrane (Zone) as Part of the Jurassic Accretionary Complex of the Sikhote-Alin Orogenic Belt

Ulban Terrane (Zone) as Part of the Jurassic Accretionary Complex of the Sikhote-Alin Orogenic Belt

Initial sedimentation after the closure of the Sumdo Paleo-Tethys Ocean: Implications for early Mesozoic tectonics in the East Tethys domain

The closure of the Paleo-Tethys Ocean and subsequent collisional orogenesis in the early Mesozoic played an important role in the formation of the Qinghai–Tibetan Plateau. However, the final closure of the southernmost Paleo-Tethys branching ocean basin in the Qinghai–Tibetan Plateau—known as Sumdo Paleo-Tethys Ocean—and collision between Central and Southern Lhasa terranes remain controversial. In particular, the tectonic significance of early Mesozoic volcanism and sedimentation in the Lhasa terrane is unknown, but crucial for understanding the final stages of ocean closure. Here, we investigate the depositional ages and provenance of detrital zircon in clastic sedimentary rocks, as well as the petrogenesis of the interbedded andesite from the volcano-sedimentary sequence of the Late Triassic to Early Jurassic Xionglai Formation in the Tangjia-Sumdo area. Detrital zircon dates from the Xionglai Formation define five age peaks at 180–200 Ma, 230–340 Ma, 500–620 Ma, 1000–1240 Ma, and 1600–1800 Ma. Detrital zircon trace elements and Hf isotopic characteristics are interpreted to represent derivation from an accretionary wedge and later magmatism related to oceanic slab break-off as well as a minor component from coeval magmatic rocks in the Nyainrong-Nang area in northern Lhasa terrane. Interbedded andesite has a magnesian andesite signature and yields ages of 192–184 Ma with zircon εHf(t) values that range from −3 to +1. We interpret these data as the andesite being sourced from sediment-derived melts that interacted with the overlying mantle wedge in a post-collision extensional setting. Our study suggests that the closure of the Sumdo Paleo-Tethys Ocean and subsequent collision between Central Lhasa and Southern Lhasa terrane may have happened in Late Triassic-Early Jurassic. The final closure of the multi-Paleo-Tethys Ocean and subsequent collision in the Qinghai–Tibetan Plateau may have induced initial subduction of the Meso-/Neo-Tethys Ocean, representing the tectonic transition from collision/post-collision to subduction/accretion in the Qinghai–Tibetan Plateau during the Late Triassic to Early Jurassic.

Reconstruction of the Cretaceous continental arc–trench system of the Japanese Islands: a basis for Cretaceous palaeoenvironmental studies

Abstract Spatiotemporal distributions of Cretaceous rocks differ markedly between the SW and NE Japan arcs. However, four parallel zonal arrangements of rocks are recognized broadly throughout both arcs: mostly non-marine sedimentary rocks in back-arc/intra-arc basins; granitic and volcanic rocks in magmatic arcs; predominantly marine and subordinately fluvial sedimentary rocks in forearc basins; and sedimentary rocks of turbiditic and mélange facies in accretionary complexes. These zones constituted a palaeo-Japan continental arc–trench system during the Cretaceous. We describe and correlate 71 Cretaceous back-arc/intra-arc and forearc basinal successions from Kyushu (south) to Hokkaido (north) islands, including a southern Sakhalin and two Kuril Arc (eastern Hokkaido) successions. Stratigraphic ranges and major sedimentary facies are generally similar between the SW and NE Japan arcs, except for the pre-Aptian Lower Cretaceous in Hokkaido of NE Japan, suggesting continuity throughout the two arcs during the Cretaceous. Although Cretaceous strata are sporadically exposed in northern Honshu, NE Japan, interpretation of seismic sections suggests that Cretaceous forearc sedimentary rock measuring several tens of kilometres laterally are developed offshore beneath the present Pacific forearc. In contrast, Cretaceous forearc strata in southern SW Japan are distributed along two narrow belts that may have been deformed by post-Early Miocene tectonism.

Tectonic influence on the evolutionary dynamics of deep-water channel systems along the active accretionary prism margin in the Rakhine Basin, Myanmar: A high-resolution 3d seismic analysis

This study reveals the morphological transformations of a deep-water channel system within the Rakhine Basin in the northeastern Bay of Bengal through a comprehensive analysis of high-resolution 3D seismic data integrated with drilling logs. Employing the “source-channel-sink” paradigm, this investigation examined the impact of various factors, including tectonic transformations, sediment influx, slope dynamics, climatic changes, and relative sea-level fluctuations, on the evolutionary trajectory of these channel systems. Applying precise time-slicing techniques integrating root-mean-square amplitude attributes, coherent amplitude attributes, and horizontal amplitudes on seismic profiles, this investigation recognized three distinct stages in the evolution of channel-levee complexes (CLCs): the Middle Miocene erosional deep-water channel-levee complex (CLC-1), the Pliocene erosional-depositional deep-water channel-levee complex (CLC-2), and the Pleistocene depositional deep-water channel-levee complex (CLC-3). Comparative analysis revealed that the CLC-3 exhibits unique characteristics, including shallower channel depth (D), wider channel width (WC), extensive U-shaped cross-sectional area (S), broader natural levee width (WL) and height (H), and the highest tortuosity coefficient (LC/LS) when contrasted with the other two CLCs. Furthermore, this study substantiated how tectonic processes, primarily the collision between the Indo-Eurasian plates, resulting in the Himalayan and Indo-Burma Range orogenies, have shaped the region's geological and climatic framework, thereby influencing monsoon circulation and precipitation patterns, which in turn augmented detrital material influx into the basin. Moreover, this tectonic collision has led to gradual changes in the slope gradient of the Rakhine Basin and its deep-water areas, impacting internal flow velocities and channel curvature, thus governing deep-water channel systems' development and spatial distribution. Also, episodic sea level fluctuations were identified as significant contributors to sediment transport from shelves to deep-water regions and sediment deposit reworking. Overall, this study underscores the role of tectonic changes after the Eocene in driving the evolutionary dynamics of the deep-water channel system within the Rakhine Basin, Myanmar.