Trench-slope Basin Research Articles

Beyond the Akiyoshi Orogeny: unravelling overturned structures and tectonic processes in the Permian accretionary complex of the Mine-Akiyoshidai area, Yamaguchi, western Japan

The concept of the Akiyoshi Orogeny emerged from geological studies of the overturned Akiyoshi Limestone. However, our understanding of the stratigraphy and geological structure of the siliceous–clastic rocks surrounding the Akiyoshi Limestone remains incomplete. We therefore conducted a geological survey within the Beppu unit, which is distributed north of the Akiyoshi Limestone. We found substantial overturning of the strata, even within the siliceous–clastic rocks that form the upper section of the ocean plate stratigraphy within the accretionary complex. The Tsunemori Formation, representing sediments of the forearc or trench slope basin, has also undergone overturning. Extensive overturned structures therefore exist throughout the entire accretionary complex of the Mine-Akiyoshidai area. To understand the mechanism behind these overturned structures within the accretionary complex, we focused on the collision and deformation resulting from the interaction between the accretionary wedge and a massive seamount, along with the back-stop. The overturning of the forearc or slope basin sediments suggests that post-accretion deformation along a large lateral fault may have occurred during the collision between the South and North China blocks. These tectonic events during and after accretion may represent the true nature of the Akiyoshi Orogeny. 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

Early stages of trench-slope basin development: Insights from mass-transport deposits and their interactions with turbidite systems (southern Hikurangi margin, New Zealand)

Geological complexity, progressive tectonic overprint and prevailing submarine location of subduction margins generally hinder investigating the early stages of development of their oldest trench-slope basins, witnesses of subduction initiation and early history. Along the eastern North Island of New Zealand, the inner portion of the Hikurangi subduction wedge is emerged, thereby offering a unique opportunity to examine, at outcrop-scale, the tectonostratigraphic evolution of a trench-slope basin (Castlepoint trench-slope basin) that was linked to the onset of subduction. We present new occurrences of lowermost Miocene gravity-driven systems, suggesting a more intricate depositional framework to the ones previously inferred during this key period. Results show that the early stages of development of a trench-slope basin coeval with the birth of a subduction margin may deviate from traditional models either comprising (1) sustainable sediment sources connected to the basin very early in the history of the margin and (2) a sedimentation-deformation feedback mechanism promoting the long-term development of an aggradational turbidite system downslope. Analysis of the mass-transport deposits (MTDs) also revealed that the outboard migration of deformation was discontinuous and uneven along the margin during the earliest Miocene. Two major tectonic events, separated by a period of reduced tectonic activity, were recorded at the inboard border of the Castlepoint trench-slope basin, each resulting in seaward motion, oversteepening and frontal denudation of a thrust sheet. Each thrust sheet provided contrasting failed material and morphometric characteristics to the associated deposits, thereby allowing us to discriminate the nature of the nappe and related controls responsible for shedding each MTD as well as refine the timing of nappe emplacement along this part of the margin. Overall, this study draws new insights on the early structural evolution and stratigraphic infill during the birth of subduction zones, insights which may, in turn, help improve understandings of active margin settings.

Lithogeochemical features and formation environment of the Late Jurassic–Early Creaceous tuff-terrigenous rocks of Ust-Belsky Mountains (North-west of Koryak highlands)

The paper presents the results of studying the lithochemical features of the Late Jurassic-Early Cretaceous tuff-terrigenous rocks of the Ust-Belsky and Algan terranes. Tuff-terrigenous rocks represented by medium-fine-pelitic-grained quartz-feldspar and feldspar greywackes. According to the results, they were formed due to the erosion of intermediate and basic composition volcanic formations, in the marine environment, near the coast. The main source was associated with the island arc in the northwest of the region. Tuff-terrigenous rocks of Ust-Belsky terrane formed in different parts of the forearc basin. Tuff-terrigenous rocks of Algan terrane formed in the trenchslope basin.

Early Stages of Trench-Slope Basin Development: Insights from Mass-Transport Deposits and Their Interactions with Turbidite Systems (Southern Hikurangi Margin, New Zealand)

Early Stages of Trench-Slope Basin Development: Insights from Mass-Transport Deposits and Their Interactions with Turbidite Systems (Southern Hikurangi Margin, New Zealand)

Lateral, longitudinal, and temporal variation in trench-slope basin fill: examples from the Neogene Akitio sub-basin, Hikurangi Margin, New Zealand

ABSTRACT The fill of trench-slope basins is complex, varying temporally, laterally, and longitudinally. New data from the Neogene stratigraphy of the Akitio Sub-basin, Wairarapa, are presented to investigate such fill variation. The preserved basin fill spans an area 70 km long by 10 km wide, representing deposits from a trench-slope basin. Integration of sedimentological, micropalaeoentological, and geological mapping data charts basin fill evolution. Over 15 km of strata were logged, defining 17 lithofacies associations, which were mapped across the basin; these are interpreted to represent both shallow and deep-water environments. The deep-water strata show a temporal evolution from ponded turbidite deposition, to a period of basin spill via conduits connecting to downstream basins, development of aggradational channel-levees, and finally unconfined submarine fan deposition. Shallow marine deposits mostly developed on the up-dip basin margin occur contemporaneously with basinal mass-transport deposits, and in association with the growth of basin bounding structural ridges. Comparison with the evolution of the offshore, actively filling Akitio Trough highlights controls on trench-slope basin fill: a first-order influence of external controls, e.g. tectonism to create the basin; a second-order progression from under- to overfilled; and third-order lateral variation reflecting autogenic process and the effects of local structures on seafloor gradients. These factors combine to vary sedimentation in trench-slope-basins spatially and temporally.

Episodes of seabed rise and rapid drowning controlling the development of regressive and transgressive rhodolithic limestones in a tectonically-active subduction setting (Early Miocene, Wairarapa region, New Zealand)

ABSTRACT Early Miocene limestones from the Wairarapa region of New Zealand were deposited on thrust-bounded margins of trench-slope basins, and consist of two superposed stratal units, namely units A and B, interpreted as forced regressive and forced transgressive foralgal and rhodolith-bearing deposits, respectively. Unit A is floored by a regressive surface of marine erosion cut into shelfal to bathyal siliciclastic successions, and is overlain across a transgressive surface of erosion by Unit B. A drowning surface abruptly places the latter deposits in contact with deeper hemipelagites and turbidites. The succession is explainable within a tectono-eustatic based framework as follows: (1) Structurally controlled rapid relative sea-level fall, (i.e. sea-bed rise), from upper bathyal to shelfal depths promoted development of regressive (‘lowstand’) mixed carbonate–siliciclastic deposits. Continuation of seabed rise resulted in deposition of pure carbonate sediments, which are represented by channel-fill rhodolithic rudstones; (2) early stage of relative sea-level rise (i.e. seabed drowning) caused emplacement of transgressive glauconitic limestones, consisting of either foralgal deposits, or incised rhodolithic limestones; (3) continued relative sea-level rise terminated carbonate production. The present study documents carbonate production and termination in a tectonically active and confined intra-slope setting, reflected in the development and distribution of unusual forced regressive, and transgressive rhodolithic-heterozoan carbonates.

Overview of deep water acoustics

<p indent=0mm>The aim of research on ocean acoustics is to reveal the phenomenon and physical mechanism of sound propagation in complex marine environments, which is the basis of the high technology and application of underwater acoustics. From the view of the sound field in deep ocean and SONAR application, the progress of ocean acoustic theories and experiments by developed countries has been reviewed firstly. The history of basic research in China is introduced briefly. In 1997 and 2012, the progress of ocean acoustics in China has been summarized at the First and Third International Conference on Shallow Water Acoustics, respectively. In recent years, with the development of maritime strategy, research on ocean acoustics in China is moving from shallow water to deep water. Some progress on deep water acoustics has been made based on the funding from the Global Change and Air-sea Interaction Project and the National Natural Science Foundation of China. The scientific research achievements on deep water acoustics are reviewed from the aspects of acoustic experimental technology, sound propagation in complex ocean environments, sound field spatial correlation, characteristics of ambient noise and application on ocean tomography, ocean reverberation, geoacoustic inversion, and source localization. The distributed sound signal recorders are developed which can work at the maximal depth of <sc>12000 m.</sc> Experiment data from the South China Sea, the Pacific Ocean and the Indian Ocean surveys have been recorded with them. Three-dimensional propagation phenomena with the presence of seamount and trench-slope basin have been observed and explained by ray theory. Characteristics of spatial correlation of sound field for different ocean environments have been investigated. The sound field fluctuation and the probability distribution of the transmission losses (TLs) caused by internal waves, mesoscale eddy, surface waves and bottom roughness are systematically studied. Longitudinal and vertical correlations within the direct zone and convergence zone are relatively high for the main energy coming from the rays reflected by sound channel. Then long-time ambient noise measurements with typhoon passing by the submerged buoys have been done. The noise model for the South China Sea has been developed. A passive ocean tomography method from ambient noise is proposed to estimate sound speed profile. A geoacoustic inversion method for deep water environments is proposed by using the TLs in the shadow zone, which is very sensitive to bottom parameters. By using the relationship of arrival angle and source ranges, three source localization methods have been proposed, which can estimate the source depth and range at the same time with high precision. Finally, the key research directions of deep water acoustics are given, such as characteristics of sound field in typical ocean environments, effects of ocean environments on SONAR performance, weak signal enhancement methods based on the interference of multi-path arrivals, and environment measurements and target detection methods with unmanned platforms. It has reference meaning for future work of deep water acoustics.

Shelf-derived mass-transport deposits: origin and significance in the stratigraphic development of trench-slope basins

ABSTRACT Continental shelves generally supply large-scale mass-wasting events. Yet, the origin and significance of shelf-derived mass-transport deposits (MTDs) for the tectonostratigraphic evolution of subduction complexes and their trench-slope basins have not been extensively studied. Here, we present high-resolution, outcrop-scale insights on both the nature of the reworked sediments, and their mechanisms of development and emplacement along tectonically active margins, by examining the Middle Miocene shelf-derived MTDs outcropping in the exhumed southern portion of the Hikurangi subduction margin. Results show that periods of repeated tectonic activity (thrust propagation, uplift) in such compressional settings not only affect and control the development of shelfal environments but also drive the recurrent generation and destruction of oversteepened slopes, which in turn, favour the destabilisation and collapses of the shelves and their substratum. Here, these events produced both large-scale, shelf-derived sediment mass-failures and local debris flows, which eventually broke down into a series of coalescing, erosive, genetically linked surging flows downslope. The associated MTDs have a regional footprint, being deposited across several trench-slope basins. Recognition of tectonic activity as another causal mechanism for large-scale shelf failure (in addition to sea-level changes, high-sedimentation fluxes) has implications for both stratigraphic predictions and understanding the tectonostratigraphic evolution of deep-marine fold-and-thrust belts.

Deformation–sedimentation feedback and the development of anomalously thick aggradational turbidite lobes: Outcrop and subsurface examples from the Hikurangi Margin, New Zealand

ABSTRACT Concepts of the interaction between autogenic (e.g., flow process) and allogenic (e.g., tectonics) controls on sedimentation have advanced to a state that allows the controlling forces to be distinguished. Here we examine outcropping and subsurface Neogene deep-marine clastic systems that traversed the Hikurangi subduction margin via thrust-bounded trench-slope basins, providing an opportunity to examine the interplay of structural deformation and deep-marine sedimentation. Sedimentary logging and mapping of Miocene outcrops from the exhumed portion of the subduction wedge record heavily amalgamated, sand-rich lobe complexes, up to 200 m thick, which accumulated behind NE–SW-oriented growth structures. There was no significant deposition from low-density parts of the gravity flows in the basin center, although lateral fringes demonstrate fining and thinning indicative of deposits from low-density flows. Seismic data from the offshore portion of the margin show analogous lobate reflector geometries. These deposits accumulate into complexes up to 5 km wide, 8 km long, and 300 m thick, comparable in scale with the outcropping lobes on this margin. Mapping reveals lobe complexes that are vertically stacked behind thrusts. These results illustrate repeated trapping of the sandier parts of turbidity currents to form aggradational lobe complexes, with the finer-grained suspended load bypassing to areas downstream. However, the repeated development of lobes characterized by partial bypass implies that a feedback mechanism operates to perpetuate a partial confinement condition, via rejuvenation of accommodation. The mechanism proposed is a coupling of sediment loading and deformation rate, such that load-driven subsidence focuses stress on basin-bounding faults and perpetuates generation of accommodation in the basin, hence modulating tectonic forcing. Recognition of such a mechanism has implications for understanding the tectono-stratigraphic evolution of deep-marine fold and thrust belts and the distribution of resources within them.

Variable In Situ Stress Orientations Across the Northern Hikurangi Subduction Margin

AbstractWe constrain orientations of the horizontal stress field from borehole image data in a transect across the Hikurangi Subduction Margin. This region experiences NW‐SE convergence and is the site of recurrent slow slip events. The direction of the horizontal maximum stress is E‐W at an active splay thrust fault near the subduction margin trench. This trend changes to NNW‐SSE in a forearc trench slope basin on the offshore accretionary wedge, and to NE‐SW in the onshore forearc. Multiple, tectonic, and geological processes, either individually or in concert, may explain this variability. The observed offshore to onshore stress rotation may reflect a change from dominantly compressional tectonics at the deformation front, to a strike‐slip and/or extensional tectonic regime closer to the Taupo Volcanic Zone, further inland. In addition, the offshore stress may be affected by topography and/or stress rotation around subducting seamounts, and/or temporal stress changes during the slow slip cycle.

Sedimentology and stratigraphy of syn-subduction Miocene fine-grained turbidites deposited in first stages of trench-slope basin development: Whakataki Formation, North Island, New Zealand

This study provides a detailed sedimentological, stratigraphic and depositional model for the lower Whakataki Formation. The fine-grained nature of the turbidite successions and common Tbcde successions suggests deposition associated with medial to distal submarine fan deposits. However, the textural and compositional immaturity, combined with a high carbonaceous and carbonate content of the clastic sediments indicates a proximal detrital input. In the examined section of the lower Whakataki Formation the sedimentology and facies associations identified that the turbidite successions were deposited as dilute low-density turbidity currents that transported sediment down-slope, overflowing or bypassing upper- and mid-trench-slope basins into a structurally confined lower trench-slope basin. Flows entering the structurally confined basin were deflected to the north-northeast, producing palaeo-flow along the axis of the basin and represent deposition in the early stages of trench-slope basin development. Here, we provide a depositional model for a lower (immature) trench-slope basin and highlight that the application of the ‘classical’ submarine-fan model to all fine-grained turbidite deposits is not always appropriate.

Elemental and isotopic compositions of trench-slope black shales, Bohemian Massif, with implications for oceanic and atmospheric oxygenation in early Cambrian

This study examines a lower Cambrian pyrite-bearing black shale–graywacke succession in the Czech Republic interpreted as infill of a deep-marine, extensional trench-slope basin on top of an accretionary wedge that developed during subduction of an oceanic plate beneath the northern margin of Gondwana. The new U–Pb detrital zircon geochronology of the graywacke constrains the maximum age of deposition to 533 +5/−6 Ma, whereas the Re–Os dating of pyrite yielded a younger age of 507 ± 16 Ma. Distribution of major and trace elements and calculated enrichment factors (EF) indicate that this succession was deposited under changing redox conditions over a short time span and that the basin was presumably controlled by tectonic subsidence and varying supply of terrigenous arc-derived material. This depositional setting is reflected by largely variable EFs and δ98Mo and δ53Cr values. Euxinic conditions detected at the base of the sampled black shale interval are documented by the highest values of EFs of redox-sensitive metals (e.g., Mo, U, V, Ni, Co, As) and also the Corg/P (>1000) and DOPT (>0.7) values. Black shales show lower δ56Fe values due to the excess of authigenic pyrite-Fe with a mean δ56Fe value of −0.02‰ over detrital Fe in graywacke with a mean δ56Fe value of +0.25‰. However, the Fe isotopic signatures of the black shales are not consistent with iron shuttling, mixing of authigenic and detrital sources, or hydrothermal metal enrichment. Instead, they most likely resulted from partial oxidation of pyrite through the syndepositional oxidizing hydrothermal fluids (Si–Ba enrichment), which resulted in precipitation of isotopically heavy Fe-oxyhydroxides. We propose that our maximal recorded δ98Mo value (+ 0.98‰) might represent the best estimate for the ancient local seawater Mo composition at around 533 Ma and argues against deep-ocean oxygenation in the early Cambrian. On the other hand, the Se/Co ratios of synsedimentary pyrite indicate a mean atmosphere O2 value of ~27%.

Sealing potential of contourite drifts in deep-water fold and thrust belts: Examples from the Hikurangi Margin, New Zealand

Contourite drifts form an important component of many deep-water sedimentary systems. Although their role as reservoirs, particularly along passive margins, is recognised, their sealing potential has been relatively understudied, especially along convergent margins. A range of contourite drifts has recently been recognised along the Hikurangi subduction margin, off the east coast of the North Island of New Zealand. Those occurring in shallower depths comprise giant elongate mounded and plastered drifts; they are mud-rich and blanket the crests and flanks of thrust-cored ridges. These drifts may seal reservoirs that principally comprise the turbiditic component of deformed trench-slope basin fill that was deposited adjacent to the ridges. The locations of modern hydrocarbon seeps appear to confirm the sealing potential of these drifts, which could trap significant hydrocarbon accumulations. The faults that underpin the ridges appear to act as primary charge pathways. Because the drifts are documented to drape the crests and flanks of the structural highs against which the reservoirs pinch out, there is a specific spatial and structural correspondence between seal, reservoir and charge pathways in this location. Such an association could be of broader economic significance if developed in convergent settings elsewhere.

Mesozoic Subduction Accretion History in Central Tibet Constrained From Provenance Analysis of the Mugagangri Subduction Complex in the Bangong‐Nujiang Suture Zone

AbstractThe Mugagangri Complex in central Tibet provides a record of the subduction accretion history between the Lhasa and Qiangtang terranes and consists of coherent sedimentary sequences, strongly dismembered formations, and siliciclastic‐matrix block‐in‐matrix mélange. We identified three different groups of sandstone within the Mugagangri Complex. Group 1 is volcaniclastic lithic (Q41F19L40, Lm16Lv77Ls7) and exhibits youngest U‐Pb detrital zircon (YDZ) ages of 177–170 Ma. Group 2 is litho‐quartzose (Q67F6L27, Lm23Lv63Ls13) with YDZ ages of 237–213 Ma. Groups 1 and 2 show provenance affinity with the Qiangtang terrane to the north. Group 3 is similar to Group 2 in detrital mode (Q67F2L31) and U‐Pb detrital zircon age spectra but is distinguished by more sedimentary fragments (Lm17Lv24Ls59), a 1,200–1,100 Ma age peak that is possibly characteristic of the Lhasa terrane to the south and YDZ ages of 284–246 Ma. During (and possibly prior to) Late Triassic time, the recycled orogen‐derived sandstones, including Group 2 and possibly Group 3, were deposited on the ocean floor adjacent to the Qiangtang terrane. During the early Middle Jurassic, the arc‐derived sandstone (Group 1) was deposited and accreted through the northward subduction of Bangong‐Nujiang oceanic lithosphere. During the late Middle to Late Jurassic, Groups 2 and 3 may have been deposited in a trench or trench slope basin, with Group 3 receiving detritus from both the Lhasa and Qiangtang terranes. These three groups of sandstone and ocean plate stratigraphy were mixed in the mélange during the accretion between the Lhasa and Qiangtang terranes.

Tectonic Influences on Trench Slope Basin Development via Structural Restoration Along the Outer Nankai Accretionary Prism, Southwest Japan

AbstractThree‐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.

Boreal Molluscan Records Around the Jurassic–Cretaceous Boundary in East Asia Provide Clues for the Paleobiogeographical Reconstruction in the Mid-Latitudes of the Northwest Pacific

Studying marine paleobiogeographical conditions in the mid-latitudes of the Northwest Pacific around the Jurassic–Cretaceous boundary would be expected to contribute to a better understanding of the paleoclimatic and/or paleoenvironmental background of the evolution of the Late Mesozoic terrestrial ecosystem in East Asia. However, uncertainty about the paleogeography of the eastern margin of the Asian Continent has meant difficulties for paleobiogeographical discussion. In this paper, the strata containing Boreal faunal elements, Buchia, and cylindroteuthidid belemnites in East Asia (Far East Russia, Heilongjiang in northeastern China, and Japan) and their tectonic settings are reviewed. The Uda and Torom (northern Sikhote-Alin), Suibin (Heilongjiang), and Tetori (northern Central Japan) regions were located from north to south in the eastern margin of the already amalgamated Asian Continent around the Jurassic–Cretaceous boundary and can be considered the “fixed points for paleobiogeographical reconstruction.” On the other hand, Buchia-bearing strata in the Komsomolsk (northern Sikhote-Alin) and Dong'an (Heilongjiang) regions can be considered to have been deposited in the fore-arc basin or trench slope basin on the accretionary complex along the East Asian continental margin. The strata around the Jurassic–Cretaceous boundary in the Partizansk Basin (southern Sikhote-Alin) contain both Buchia and Tethyan ammonoids and were deposited on the Paleozoic continental basement or block (Sergeevka Belt). The paleo-position of these three regions around the Jurassic–Cretaceous boundary is highly debated.The Tethyan–Pacific ammonoids, Boreal belemnites, and Tetori bivalve fauna, showing some similarities with those in the Boreal Realm and Early Cretaceous strata in Heilongjiang, are present in the late Tithonian–Berriasian Mitarai Formation of the Tetori Group in the Tetori Region. This unit, deposited in the eastern margin of the North China Block, provides evidence that Boreal faunal elements reached the mid-latitudes of the Northwest Pacific. The position of the ecotone of the Boreal and Tethys realms in the Northwest Pacific can be discussed based on the comparison of the faunal elements among almost coeval strata in the Tetori Region (fixed point), the Sergeevka Belt, and the South Kitakami Belt (Pacific side of Northeast Japan), which is usually correlated with the Sergeevka Belt but contains only Tethyan faunal elements. Further studies of the records of Tethyan and Boreal taxa in the “fixed points” and other localities could provide clues to reveal the paleoclimatic and/or paleoenvironmental background of the evolution of the terrestrial and marine ecosystems in East Asia around the Jurassic–Cretaceous boundary.

Tectonic, diapiric and sedimentary chaotic rocks of the Rakhine coast, western Myanmar

The western margin of Myanmar is the northern extension the active Sunda (India-Eurasia) subduction zone. Coastal regions and offshore islands have remarkable exposures of chaotic rock terranes along wave-cut terraces that allow characteristics of tectonic, sedimentary and diapiric mélanges to be recognized. Tectonic shear zones (tectonic mélanges) contain fragments of Cretaceous ophiolites (chrome-spinel-bearing peridotites and radiolarian cherts) that are in contact with thrust packets of Eocene turbidite units (broken formations). The turbidites contain shale-rich beds that have been sheared during soft-sediment deformation (sedimentary broken formations) and are sandwiched between undeformed thick sandy beds. These are mass transport deposits (MTDs) that most likely formed during deposition of the initial detritus of the Himalayan orogenic zone, probably trench slope basins on the accretionary prism. The ophiolitic and turbiditic thrust slices have been exhumed and are currently being intruded by active mud volcanoes that bring fragments of units up from depth to the surface, forming diapiric mélanges. These diapiric mélange bodies contain only small fragments (<50 cm) that are randomly oriented and do not exhibit shear fabrics. Because the region lacks superimposed deformation characteristic of most orogenic belts, the origins of all three rock bodies can easily be distinguished.

High-precision radioisotopic ages for the lower Midian (upper Wordian) Stage of the Tethyan time scale, Shigeyasu Quarry, Yamaguchi Prefecture, Japan

Global correlation of strata to the Guadalupian Series (Permian Period) of the International Geologic Time Scale remains provisional due to a lack of the sufficient biostratigraphically constrained radioisotopic ages. Five new high-precision UPb radioisotopic ages were obtained from tuffs in the lower Midian (upper Wordian) volcano-siliciclastic succession of the Akiyoshi Plateau, southwestern Japan. Two undisturbed and continuous tuffs occur within fusulinid-bearing laminated limestone turbidites, with precise biostratigraphic control. The Colania douvillei and Lepidolina shiraiwensis fusulinid Zones in the Akiyoshi succession provide an early Midian age in the Tethyan provincial time scale. The range of ages (267.46 ± 0.04 Ma to 265.76 ± 0.03 Ma) over 25 m of section including the limestone turbiditic beds suggest that this lower Midian Stage of the Tethyan time scale is correlated with the upper Wordian Stage of the International Geologic Time Scale. The sedimentologic, bio- and geochronologic data affirm the effectively conformable nature of the siliciclastic‑carbonate succession of the Tsunemori Formation at the Shigeyasu Quarry, and the presence of acidic volcanic tuffs are consistent with a forearc basin or trench slope basin depocenter for the Tsunemori Formation.

Relicts of mud diapirism of the emerged wedge-top as an indicator of gas hydrates destabilization in the Manila accretionary prism in southern Taiwan (Hengchun Peninsula)

Due to oblique collision of the Luzon Volcanic arc and Eurasian passive margin, in Taiwan we observe southward progression of mountain building processes. Consequently, the southernmost tip of Taiwan represents the most recently emerged part of the orogen - the Hengchun Peninsula. The area shows evidence of a trench-slope basin that emerged in the Holocene with landforms that bear submarine characteristics. In the Late Pleistocene, lagoonal sediments blanket the basin, however the deeper deposits (Maanshan Formation) can be found on the surface in several areas. We document that the Maanshan Formation was brought to the surface through upwelling structures—mud diapirs—as a consequence of fast and overpressured sedimentation from the emerging orogen. Although fast erosional processes continuously reorganize the new landscape, we are able to capture relicts of fast eroding inactive mud volcanoes. These unique features are preserved on the surface due to shielding by coral terraces during the shallowing and the lagoonal period of the basin. Stable carbon δ13C and oxygen δ18O isotope analysis, performed on samples of carbonate blocks of cold vents collected on the outcrops of the Maanshan Formation, confirm classic seep carbonates origins. We also identify a mud-core anticline below the study area acting as the main source for the mud volcanoes and a possible cause of the uplift of the trench-slope basin. Two multichannel seismic profiles from marine surveys along with a synthesis of several previous surveys helped construct a model of the characteristics of mud diapirism in southeastern offshore Taiwan.

The subduction-accretion history of the Bangong-Nujiang Ocean: Constraints from provenance and geochronology of the Mesozoic strata near Gaize, central Tibet

The Mesozoic strata, within the Bangong-Nujiang suture zone in central Tibet, recorded critical information about the subduction-accretion processes of the Bangong-Nujiang Ocean prior to the Lhasa-Qiangtang collision. This paper reports detailed field observations, petrographic descriptions, sandstone detrital zircon U-Pb ages and Hf isotopic analyses from an accretionary complex (preserved as Mugagangri Group) and the unconformably overlying Shamuluo Formation near Gaize. The youngest detrital zircon ages, together with other age constraints from literature, suggest that the Mugagangri Group was deposited during late Triassic-early Jurassic, while the Shamuluo Formation was deposited during late Jurassic-early Cretaceous. Based on the differences in lithology, age and provenance, the Mugagangri Group is subdivided into the upper, middle and lower subunits. These units are younging structurally downward/southward, consistent with models of progressive off-scrapping and accretion in a southward-facing subduction complex. The upper subunit, comprising mainly quartz-sandstone and siliceous mud/shale, was deposited in abyssal plain environment close to the Qiangtang passive margin during late Triassic, with sediments derived from the southern Qiangtang block. The middle and lower subunits comprise mainly lithic-quartz-sandstone and mud/shale, containing abundant ultramafic/ophiolitic fragments. The middle subunit, of late Triassic-early Jurassic age, records a transition in tectono-depositional setting from abyssal plain to trench-wedge basin, with sudden influx of sediments sourced from the central Qiangtang metamorphic belt and northern Qiangtang magmatic belt. The appearance of ultramafic/ophiolitic fragments in the middle subunit reflects the subduction initiation. The lower subunit was deposited in a trench-wedge basin during early Jurassic, with influx of Jurassic-aged zircons originating from the newly active southern Qiangtang magmatic arc. The lower subunit records the onset of arc magmatism related to the northward subduction of the Bangong-Nujiang Ocean. The Shamuluo Formation, comprising mainly lithic-feldspar-sandstone with limestone interlayers, was deposited in a post-collisional residual-sea or pre-collisional trench-slope basin, with sediments derived entirely from the Qiangtang block.