Trench Subduction Research Articles

Impact of the Offshore Seismograph Network and 3‐D Seismic Velocity Structure Model on Centroid Moment Tensor Analysis for Offshore Earthquakes: Application to the Japan Trench Subduction Zone

Impact of the Offshore Seismograph Network and 3‐D Seismic Velocity Structure Model on Centroid Moment Tensor Analysis for Offshore Earthquakes: Application to the Japan Trench Subduction Zone

Cretaceous to early Paleogene sediment provenance transition from continental to magmatic arc systems in the Northwestern Pacific Region

Unraveling the Paleo-Kuril Arc's origins is key to understanding northwest Pacific tectonics. The Paleo-Kuril Arc is viewed as an intraoceanic arc from trench subduction between the Izanagi and Pacific Plates. Alternatively, several studies suggest the Paleo-Kuril Arc as a continental magmatic arc, hypothesizing the existence of a mid-oceanic ridge and Paleogene subduction, placing the Paleo-Kuril Arc near the Okhotsk Block's southern edge. This study clarifies these hypotheses, previously clouded by limited geochronological data on deposits in the Paleo-Kuril Arc. We conducted U–Pb dating to examine the origins of detrital zircons from the Cretaceous–Paleogene Tokoro and Nemuro Belts of the Paleo-Kuril Arc. Cluster analysis, merging new and existing data, identified two unique detrital zircon age clusters. The abundance of Precambrian zircons in Cretaceous–Paleocene Paleo-Kuril Arc sandstones (Type 1 Cluster) suggests a continental magmatic origin, supporting the ridge subduction model. However, an early Eocene shift to a consistent local volcanic source (Type 2 Cluster) highlights a significant provenance change. This geochronological evidence, indicating a separation from continental sources, calls for further research to decode the simultaneous development of sediment sources in different geological belts, potentially tied to the ridge subduction event.

Seamount Subduction Dynamics and Long‐Term Evolution of the Franciscan Active Margin

AbstractThe Snow Mountain Volcanic Complex (SMVC; northern California, USA) is a well‐preserved example of a coherently‐exhumed subducted seamount. This study reappraises the genesis and evolution of this complex and surrounding units through detailed field, petro‐structural and geochronological analyses. This work demonstrates that the SMVC (a) erupted at ∼166 Ma as a hotspot volcano on the Farallon Plate, (b) entered the Franciscan subduction trench at ∼118 Ma, and (c) was subsequently subducted to a depth of ∼20 km (within the seismogenic zone), as shown by local blueschist‐facies assemblages formed at 0.6 GPa, 240°C. Transient subduction interfaces are preserved above, within, and below the SMVC, making it an exceptional target to study seamount subduction dynamics. Like other seamounts, the subduction‐related deformation was mainly accommodated along kilometer‐scale internal thrust zones lubricated by serpentinite/metasediments, and within centimeter‐thick crack‐seal veins recording pulsed fluid flow near peak metamorphism. No unequivocal proof of seismic activity was found. The integration of other seamounts (some potentially belonging to a former seamount chain) in the Franciscan Complex suggests that exhumed seamounts are more abundant than previously thought. Moreover, pressure‐temperature‐time estimates of subduction metamorphism for the surrounding units, combined with previous work constrain the thermal maturation of the subduction zone through time and the in‐sequence emplacement of the SMVC. Rapid changes in age of the subducted oceanic plate when subducted additionally hint to the subduction of large‐offset transform faults on the former Farallon plate. Such a process might have been linked to changes in accretion dynamics and magmatic flare‐ups in the arc.

Variability of quaternary marine terraces and uplift rates in the Andean coastal zone at 23°S

During the Quaternary most of the coastal areas along the Andes were uplifted due to subduction related processes. At 23°S, along the Mejillones Peninsula and the shoreline of the Coastal Cordillera, there is an abundance of Quaternary marine terraces, coastal cliffs, eroded dikes, alluvial fans, and fault scarps which preserve evidence of this uplift. The hyper-arid conditions have minimised subaerial erosion to the extent that the Quaternary marine terraces (QMT) can be carefully studied to understand the amount of permanent deformation in the forearc and the variability of uplift rates along the coast. A morpho-metric analysis using remotely sensed imagery from ALOS-PALSAR, drone survey and local differential GPS data allowed us to define the best-preserved QMT sequences and the elevation at which they were found with respect to the modern mean sea level. We propose a correlation between the individual paleo-shorelines of each marine terrace with a Quaternary Marine Isotope Stage (MIS). We determined two representative areas with well-preserved marine terraces: Morro Mejillones, in the western zone of the Mejillones Peninsula (∼12 × 10 km2) and the combined Hornitos – Punta Chacaya zone (∼16 × 7 km2), in the northern part of the Mejillones Bay. Within these areas we identified 12 QMT's in Morro Mejillones with estimated uplift rates between 0.31 and 0.76 m/ka over the last 600 ka, 3 QMT's In Hornitos with uplift rates between 0.28 and 0.4 m/ka over the last 220 ka, and 4 QMT's in Punta Chacaya with uplift rates between 0.25 and 0.39 m/ka over the last 330 ka. We link differences in uplift rates as related primarily to the distance from the coastal zone to the subduction trench and, locally, to their variable location with respect to faults active during the Quaternary.

A Seismogenic Slab Source Model for Aotearoa New Zealand

Abstract Intraslab seismicity within the Hikurangi and Puysegur subduction zones constitutes >50% of recorded (Mw≥4.0 events) earthquakes in Aotearoa New Zealand. Here, we develop a source model for intraslab seismicity using recently augmented datasets including models of subduction interface geometries, an earthquake catalog, and a regional moment tensor catalog. For the areal zones of uniform seismicity, we consider the whole of each slab, as well as demarcations between shallower (depth ≤40 km) and deeper regions. Thereafter, we evaluate the magnitude–frequency distributions in each zone. To compute smoothed seismicity distributions, we apply a novel quasi-3D approach that involve: (1) delineation of midslab surfaces (defined by regions of maximum earthquake density), (2) orthogonal projections of hypocenters onto the midslab profiles, (3) uniform gridding of 0.1° down-dip on the midslab, and (4) application of smoothing kernel on the projected hypocenters. We also develop a model to characterize the focal mechanisms of the intraslab earthquakes using the regional moment tensor catalog. This model has median strike angles subparallel to subduction trenches and median dip angles ≥60° in both the subduction zones. The distribution of rake angles suggests that the Hikurangi slab has an extensional regime in the shallower parts but a compressional regime in the deeper parts, indicative of slab flexure. In contrast, the Puysegur slab predominantly exhibits a compressional regime.

Observation of a Synchronicity between Shallow and Deep Seismic Activities during the Foreshock Crisis Preceding the Iquique Megathrust Earthquake

We analyze at a broad spatial scale the slab seismicity during one of the longest and best recorded foreshock sequence of a subduction earthquake to date: the M8.1 2014 Iquique earthquake in Chile. We observe the synchronisation of this sequence with seismic events occurring in the deep slab (depth ~100km). This synchronisation supports the existence of long-range seismic bursts already observed in the Japan Trench subduction. It suggests that, like for the 2011 Tohoku earthquake, the deep slab was involved in the nucleation process of the Iquique earthquake. We interpret these observations by the presence of pressure pulses propagating in transient fluid channels linking the deep slab where dehydration occurs to the shallow seismogenic zone before the earthquake. These observations may seem surprising but they are in line with the short-lived pulse-like channelized water escape from the dehydration zone predicted by recent studies in slab mineralogy and geochemistry.

Incoming plate structure at the Japan Trench subduction zone revealed in densely spaced reflection seismic profiles

The structure of the incoming plate is an important element that is often considered to be related to the occurrence of great earthquakes in subduction zones. In the Japan Trench, where the 2011 Tohoku earthquake occurred, we collected seismic profiles along survey lines separated by 2–8 km to examine the structural characteristics of the incoming Pacific plate in detail. The average thickness of the incoming sediments was < 500 m along most of the Japan Trench, and it was < 300 m at ~ 38° N, where the large shallow megathrust slip occurred during the 2011 Tohoku earthquake. We mapped bending-related normal faults, including their dip direction and amount of throw. The numbers of eastward (oceanward) and westward (trenchward) dipping normal faults were generally comparable in the Japan Trench. Eastward dipping normal faults were dominant in the northern and southern parts of the Japan Trench, whereas westward dipping normal faults were more numerous in the central part. Graben-fill sediments deposited at the landward edge of the graben were bounded by eastward dipping normal faults. Trench-fill sediments were also observed along the trench axis. The sediment fills locally increased the thickness of the input sediments where they were deposited. The along-axis variation in input sediment and sediment fill distribution, and the variations in normal fault dip direction between the central and other parts of the Japan Trench may correspond to different slip styles along the plate boundary.

Plate Age and Uppermost Mantle Structure Across the Juan de Fuca and Gorda Plates

AbstractTo test the relationship between plate age and uppermost mantle structure for the Juan de Fuca and Gorda plates, as well as the mode of lithospheric cooling, we produce a new 3D oceanic shear wave speed (VS) model. This model is constructed using a new high‐quality Rayleigh wave phase speed data set across the plates and is based on a novel thermo‐seismic hybrid parameterization. In the lithosphere, we find that the VS structure is primarily (68% of study area) governed by conductive cooling. Exceptions are observed, suggesting advective heat influx along ridges and hydrothermal circulation through cracks near the subduction trench and transform faults. In the asthenosphere of the Juan de Fuca plate, no simple monotonic relationship between seismic structure and plate age exists, but instead there is a non‐monotonic slow‐fast‐slow VS pattern from west to east. The two slow asthenospheric VS anomalies indicate the presence of partial melt beneath both the mid‐ocean ridge and the subduction trench. The partial melt beneath the trench is enigmatic.

[formula omitted] wave teleseismic tomography of the subducted Chile rise

We investigate the crustal and upper mantle structure of the Aysén region of Chile from 44° to 49° S, a place where the diverging oceanic Nazca and Antarctic plates subduct beneath the South American continent. The Seismic Experiment in the Aysén Region of Chile (SEARCH) project operated a network of up to 60 land-based seismometers in this region between 2004 and 2006, centered over a 6 Myr old subducted spreading center. The data are used to examine the P wave velocity structure beneath the region using relative-arrival teleseismic travel time tomography, using 2534 P wave residuals from 173 teleseismic earthquakes. It is possible to image the velocity structure beneath the seismic network from ∼30 down to ∼300 km depth. The model can resolve structures at a spatial scale between ∼60 and ∼200 km and shows a large difference between the northern and southern parts of the region. To the north, a ∼100 km thick fast anomaly exists which dips away from the subduction trench; likely to be related to the subducting Nazca plate. Going to the south, as the age of this plate at the subduction trench decreases and arc volcanism shuts off, the fast anomaly migrates further from the trench, suggesting that the Nazca plate subducts at a reduced angle over a larger distance before the subduction angle steepens. The distinct sections of the fast anomaly suggest that slab tears exist across the fracture zones between subducted plate segments. Where the 6 Myr old subducted ridge segment is predicted to lie, there is a region of low velocity between ∼100 and ∼300 km depth, and no fast region associated with a subducting slab is present, indicating the presence of an asthenospheric window between the subducted Nazca and Antarctic plates.

Thermomechanical modelling of lithospheric slab tearing and its topographic response

Lithospheric slab tearing, the process by which a subducted lithospheric plate is torn apart and sinks into the Earth’s mantle, has been proposed as a cause for surface vertical motions in excess of 100 s of meters. However, little is known about the mechanisms that help initiate and control the propagation of slab tearing and the associated uplift. This study aims to explore these processes by means of 3D thermo-mechanical geodynamic modelling of a slab retreat oblique to a continental margin, using the Gibraltar Arc region (Betic Cordillera) as a scenario for inspiration. Our results suggest that the obliquity of the continental passive margin relative to the subduction trench leads to an asymmetric distribution of subduction forces and strength, facilitating the initiation of slab tearing. The model results predict a lateral migration of the tearing point at a velocity ranging between 38 and 68 cm/yr for a sublithospheric-mantle viscosity of up to 1e+22 Pa s. This fast slab tearing propagation yields uplift rates of 0.23–2.16 mm/yr above the areas where the subducted slab is torn apart, depending on mantle viscosity. Although a more detailed parametric exploration is needed, this range of uplift rates is compatible with the uplift rates required to overcome seaway erosion along the Atlantic-Mediterranean marine corridors during the Late Miocene, as proposed for the onset of the Messinian Salinity Crisis.

Free water input into the New Britain subduction zone in Solomon Sea estimated from reflected seismic data

AbstractIt is of great significance to examine the distribution of the fluid within the subduction zone to estimate the water flux into the subduction zone and to study the motion state of the subducting plate. The interaction of the Western Pacific Plate and the Indo‐Australian Plate controls the subduction and expansion of the Solomon Basin where the mechanisms governing plate activity remain unclear but are typically associated with high pore fluid pressures and stabilizing frictional conditions. Combining the long streamer seismic data (200) and nine well logs inside the work area, we used constrained sparse spike inversion to obtain the P‐wave impedance profile. Then we constructed reservoir rock physics models to predict the fluids distribution in New Britain Trench subduction system in Solomon Sea. The genetic algorithm was used to guide the inversion of the pore fluid distribution. The water content profile revealed the shallow structure of the New Britain Trench subduction system in Solomon Sea, including the water content within the basalt layers. We determined that in certain cases, faults identified from the reflection data were associated with low‐impedance anomalies, and this may indicate that they are zones of high porosity that act as conduits for fluid flow. The continuity of the distribution of water content under the New Britain Trench suggests that there is a large amount of free water that subducts deeply and may be important in controlling the motion state of the subducting plate. We assume that the water saturation is 1. Therefore, the free water flux entering the deep circle with the subducting plate is estimated to be 158 Tg/Ma/m totally, including 53.724 Tg/Ma/m in the sediment and 105.06 Tg/Ma/m in the basalt. This study provides insight into the structure of free water in the New Britain subduction zone and reveals the potential for the direct estimation of the 2D structural distribution of water content using geophysical survey data.

Shear wave velocity structure at the Fukushima forearc region based on H/V analysis of ambient noise recordings by ocean bottom seismometers

SUMMARYThis study presents the shear wave velocity (VS) structures of sedimentary sequences and a section of the upper crustal layer in the Fukushima forearc region of the Japan Trench subduction zone, which were obtained by analysing the horizontal-to-vertical (H/V) spectral ratios of ambient vibration records. The H/V curves were derived using 31 d of continuous seismic data from 3 broad-band and 16 short-period ocean bottom seismometer (OBS) stations. Using the broad-band data, H/V ratios from 0.01 to 10 Hz were derived, but the ratios below 0.1 Hz frequencies were unusually large and temporally unstable. Characterization of seismic noise energy from ∼1 yr of seismic data of three broad-band OBSs revealed variable and elevated energy conditions below 0.1 Hz due to typical long-period oceanic noise; we link these observations with the unstable H/V ratios below this frequency. Therefore, H/V analysis was performed in the frequency range of 0.1–10 Hz for both broad-band and short-period OBSs to obtain subsurface VS profiles. For the forward calculation of the H/V ratios in the inversion process, we used the recently developed ‘hvgeneralized’ method, which is based on the diffuse field assumption, and accounts for the water layer on top of stratified media. Moreover, available prior geological and geophysical information was utilized during the inversion of the H/V curves. We found that subsurface VS ranged from approximately 30 m s−1 at the seabed to approximately 4900 m s−1 at 7000 m below the sea floor (mbsf). Starting with the best model candidate at each OBS location, the effect of the water layer on the H/V curve in the deep ocean was investigated by comparing synthetic H/V curves with and without the water layer. The synthetic H/V analysis revealed that the water layer had a significant effect on H/V amplitudes at higher frequencies (&amp;gt;1 Hz), whereas comparatively little effect was observed at lower frequencies (&amp;lt;1 Hz). This study provides an empirical basis for H/V analysis using OBS data to determine VS down to several kilometres of sedimentary sequences to the upper crust with high-resolution.

Tsunami scenario triggered by the activity of the Mentawai Fault Zone offshore western Sumatra Island

Subduction zones worldwide pose tsunami risks, mainly linked to megathrust activity near subduction trenches. However, tsunamis can originate from various sources, including marine volcanic eruptions, submarine landslides, and strike-slip earthquakes. In the Sumatra subduction zone, a seismic gap in the Mentawai region heightens the tsunami risk. This region’s tectonics are complex due to the oblique subduction of the India-Australia oceanic plate beneath the Eurasian continental plate, leading to the sliver faults system of Mentawai Fault Zone (MFZ) and Sumatra Fault Zone (SFZ). The SFZ on Sumatra Island has limited tsunami potential, except at its northern and southern offshore extensions. In contrast, the MFZ, situated in the marine Northern Bengkulu basin, holds a higher tsunami-generation potential. This study aims to assess the MFZ activity in generating tsunamis, estimate the maximum wave height, and analyze propagation, and arrival times at several crucial sites in Western Sumatra Province. We utilized the COMCOT tsunami model to simulate scenarios with two fault mechanisms, i.e., strike-slip and back thrust, and two magnitudes (Mw 7.6, and 8.2). The results indicate that the most hazardous tsunami, generated by a strike-slip fault with Mw 8.2, produces a 2-meter tsunami on the east coast of Siberut Island and the west coast of Padang City, West Sumatra. The scenarios reveal that Mentawai Island’s eastern part lacks evacuation time, with an almost instantaneous tsunami arrival. In contrast, western Sumatra, including Pariaman and Padang City, has 13 to 20 minutes for evacuation planning. Thus, disaster risk reduction strategies in these locations should consider these findings.

Tectono-sedimentary evolution of the NE dinarides margin: vestiges of the Adria-Europe convergence in cretaceous sedimentary formations of central Serbia

The Cretaceous sedimentation along the NE Dinarides margin occurred in basins above the Europe-dipping Neotethyan Sava subduction zone positioned between Adria- and Europe-derived continental units. The Cretaceous sedimentation on the upper plate of the Sava subduction system took place in a fore-arc basin, developed in frontal parts of the active European continental margin. The Cretaceous sedimentation in the lower Adria plate domain of the Sava subduction system includes sediments deposited in the basin developed over the passive continental margin of the Internal Dinarides and the sediments deposited in the Sava subduction trench. While the Cretaceous sedimentation on the entire Adriatic continental margin was associated with an overall contraction, which led to the progressive subsidence towards the end of the Cretaceous, the fore-arc basin on the European continental margin displays three depositional cycles during the Early Cretaceous-Cenomanian, Turonian-Santonian, and Campanian-early Paleogene, reflecting three stages of deformation, contraction, extension, and ultimately contraction again during the Adria-Europe collision.

Subduction-related mantle accretion and makeover revealed by mantle xenoliths at the Pacific margin of NE Eurasia

Subduction is thought to be a major factor in the origin and makeover of continental lithospheric mantle, but essential aspects of these processes continue to be debated. Here, we report textural, chemical and Re Os isotope data on mantle xenoliths from far eastern Russia located at different distances from the Asian Pacific coast to examine how the proximity to subduction zones affects lithospheric mantle compositions. The xenoliths come from the Khanka block close to the border with China and several sites in the Sikhote-Alin orogenic belt (SAOB) near the Japan Sea coast. The inboard Khanka xenoliths are spinel lherzolites with chemical compositions, suggesting that they are residues of low- to moderate-degrees of melt extraction from fertile mantle and essentially unaffected by later events for 1.9 Ga. Such rocks are also common among peridotites in the SAOB, located closer to the subduction margins, but the SAOB suite also includes: a) heterogeneous rocks with veins and patches of pyroxenites or hydrous minerals, and b) rocks enriched in iron (8.8–13.5 wt%), Al 2 O 3 and incompatible minor elements. The positive correlation between elevated 187 Os/ 188 Os ratios (>0.135) and FeO contents in some Fe-rich SAOB xenoliths is interpreted to indicate that the metasomatic melts were derived from the asthenosphere hybridized with recycled materials from the Pacific slab. The lack of correlation between Re -depletion Os model ages (T RD ) and depletion indices, as well as the similarity of Os isotopic compositions between the SAOB peridotites and abyssal peridotites are interpreted to show that the SAOB mantle possibly formed and accreted to Eurasia from the asthenosphere during eastward migration of the Paleo-Pacific subduction trench. Based these results, we infer that the SAOB lithospheric mantle experienced multi-stage metasomatism after its formation, the most significant of which is localized infiltration of subduction-related basaltic melts with recycled characteristics induced by the extension of Japan Sea and release of volatiles from the slab. By contrast, the inboard Khanka lithospheric mantle shows little, if any, evidence for metasomatism. Such contrasting metasomatic modification implies different degrees and styles of makeover processes at different distances relative to the subduction margins. • The Khanka mantle was little affected by metasomatism since it was formed at ~1.9 Ga. • The Sikhote-Alin lithospheric mantle was formed by subduction and accretion. • Mantle metasomatism beneath Sikhote-Alin was caused by melts with recycled signature. • The nature and degrees of metasomatism are related to the distance from subduction front.

Independent radiation of snailfishes into the hadal zone confirmed by Paraliparis selti sp. nov. (Perciformes: Liparidae) from the Atacama Trench, SE Pacific

Snailfishes are among the most rapidly radiating families of marine fishes, resulting in a global distribution from the coastal intertidal to deep subduction trenches. The true diversity and distribution of deep-water snailfishes, particularly at hadal depths (>6000 m) and in the Southern Hemisphere, remain uncertain due to the rarity of samples. Here, we present the snailfish diversity at near-hadal and hadal depths in the Atacama Trench, which runs along the southwest coast of South America. Using free-fall baited cameras and traps, we documented at least three species of hadal snailfishes between 5920 and 7608 m based on distinct morphologies. One snailfish specimen was recovered from 6714 m, which we describe herein as Paraliparis selti sp. nov., based on a combined morphological and molecular taxonomic approach (16S, COI, and Cyt-b). Paraliparis selti sp. nov. is morphologically distinct from described snailfishes due to a combination of high number of; vertebrae (65) particularly the abdominal vertebrae (12), dorsal fin rays (60), anal fin rays (52), and caudal fin rays (8); comparatively low number of pectoral fin rays (18) which forms a deep notch with two widely spaced non-rudimentary rays. Micro-CT was used to minimise dissection of the specimen and to provide a digital holotype. Paraliparis selti sp. nov. highlights the importance of the Liparidae at hadal depths and provides evidence for at least two independent radiations of snailfishes into the hadal zone.

Seismic Full‐Waveform Inversion of the Crust‐Mantle Structure Beneath China and Adjacent Regions

AbstractWe present the first‐generation full‐waveform tomographic model (SinoScope 1.0) for the crust‐mantle structure beneath China and adjacent regions. The three‐component seismograms from 410 earthquakes recorded at 2,427 stations are employed in iterative gradient‐based inversions for three successively broadened period bands of 70–120 s, 50–120 s, and 30–120 s. Synthetic seismograms were computed using GPU‐accelerated spectral‐element simulations of seismic wave propagation in 3‐D anelastic models, and Fréchet derivatives were calculated based on an adjoint‐state method facilitated by a checkpointing algorithm. The inversion involved 352 iterations, which required 18,600 wavefield simulations. SinoScope 1.0 is described in terms of isotropic P‐wave (VP), horizontally and vertically polarized S‐wave velocities (VSH and VSV), and mass density (ρ), which are independently constrained with the same data set coupled with a stochastic L‐BFGS quasi‐Newton optimization scheme. It systematically reduced differences between observed and synthetic full‐length seismograms. We performed a detailed resolution analysis by repairing input random parametric perturbations, indicating that resolution lengths can approach the half propagated wavelength within the well‐covered areas. SinoScope 1.0 reveals strong lateral heterogeneities in the lithosphere, and features correlate well with geological observations, such as sedimentary basins, Holocene volcanoes, Tibetan Plateau, Philippine Sea Plate, and various tectonic units. The asthenosphere lies below the lithosphere beneath East and Southeast Asia, bounded by subduction trenches and cratonic blocks. Furthermore, we observe an enhanced image of well‐known slabs along strongly curved subduction zones, which do not exist in the initial model.

Observation of rapid long-range seismic bursts in the Japan Trench subduction leading to the nucleation of the Tohoku earthquake

Earthquake precursory phenomena like foreshocks are common but the physical processes which produce them and lead to rupture are poorly understood. Looking for clues about these processes, we explore here the long-term evolution of seismicity in a broad section of the Japan Trench slab around the 2011 Tohoku earthquake location and report the existence of long-range seismic bursts moving rapidly (∼1-2 days or less) between the deep and the shallow slab. Their presence is evidenced by synchronizations of deep and shallow seismic activities in the slab. Probabilities that these synchronizations could be due to chance are infinitesimal (<10−5). We show that a significant part of the activity in this subduction occurs during these bursts and that they are responsible for the foreshock sequence which preceded the earthquake. These observations support the existence of fluid channels connecting the deep slab, where water is released during dehydration, to the shallow seismogenic zone.

Formation and Evolution of Supradetachment Basins During Continental Extension: Insights From the Fuxin Basin in NE China

Supradetachment basins can record the stratigraphy and development of metamorphic core complexes (MCCs). The Fuxin supradetachment basin, which lies immediately to the west of the Yiwulvshan MCC in NE China, provides an excellent opportunity to establish the relationship between supradetachment basins and MCCs. In this study, we conducted field investigations, sedimentary facies analysis, and seismic profile interpretation to decipher the sedimentary processes and structural evolution of the basin and link them to the development of the Yiwulvshan MCC. The Fuxin Basin is filled predominantly by syn-rift volcanic sedimentary and post-rift clastic rocks, which developed in four stages, namely, proto-rift, fault subsidence, transition, and compression. The Fuxin Basin developed simultaneously with the two stages (earlier faulting-dominated and later exhumation stages) of development of the MCC. Erosion of the core of the Yiwulvshan MCC provided clastic material to the Fuxin Basin. Based on multi-stage reconstruction of the formation and evolution of the Fuxin Basin and Yiwulvshan MCC, we propose that rollback of the Paleo-Pacific Plate and retreat of the subduction trench provided the geodynamic setting for the crustal extension that formed the basin and MCC.

Hadal Biodiversity, Habitats and Potential Chemosynthesis in the Java Trench, Eastern Indian Ocean

The Java Trench is the only subduction trench in the Indian Ocean that extends to the hadal zone (&amp;gt; 6,000 m water depth), and except for sevenbenthic trawls acquired around the 1950s, there has been little to no sampling at hadal depths undertaken since. In 2019, we undertook a 5-day expedition comprising a scientific dive using a full ocean depth-rated submersible, the DSV Limiting Factor, seven hadal-lander deployments, and high-resolution bathymetric survey. The submersible performed a video transect from the deepest point of the trench, up a 150 m high near-vertical escarpment located on the forearc, and then across a plateau at a depth of ∼7,050 m to make in situ observations of the habitat heterogeneity and biodiversity inhabiting these hadal depths. We found the Java Trench hadal community to be diverse and represented by 10 phyla, 21 classes, 34 orders and 55 families, with many new records and extensions in either depth or geographic range, including a rare encounter of a hadal ascidian. The submersible transect revealed six habitats spanning the terrain. The deepest trench axis comprised fine-grained sediments dominated by holothurians, whereas evidence of active rock slope failure and associated talus deposits were prevalent in near-vertical and vertical sections of the escarpment. Sediment pockets and sediment pouring down the steep wall in “chutes” were commonly observed. The slope terrain was dominated by two species in the order Actiniaria and an asteroid, as well as 36 instances of orange, yellow, and white bacterial mats, likely exploiting discontinuities in the exposed bedrock, that may indicate a prevalence of chemosynthetic input into this hadal ecosystem. Near the top of the escarpment was an overhang populated by &amp;gt; 100 hexactinellid (glass) sponges. The substrate of the plateau returned to fine-grained sediment, but with a decreased density and diversity of epifauna relative to the trench floor. By providing the first visual insights of the hadal habitats and fauna of the Java Trench, this study highlights how the habitat heterogeneity influences patchy species distributions, and the great benefit of using a hadal-rated exploratory vehicle to comprehensively assess the biodiversity of hadal ecosystems.