Trench Subduction Research Articles

Impact of bending-related faulting and oceanic-plate topography on slab hydration and intermediate-depth seismicity

Abstract It is commonly assumed that intermediate-depth seismicity is in some way linked to dehydration reactions inside subducting oceanic plates. Although there is growing evidence that the hydration state of an oceanic plate is controlled by its structure and degree of faulting, we do not have a quantitative understanding of this relationship. Double seismic zones offer the possibility of investigating changes in oceanic-plate hydration not only along strike but also with depth beneath the slab surface. To quantify the impact of oceanic-plate structure and faulting on slab hydration and intermediate-depth seismicity, with a focus on the genesis of double seismic zones, we correlate high-resolution earthquake catalogs and seafloor maps of ship-based bathymetry for the northern Chilean and Japan Trench subduction zones. The correlations show only a weak influence of oceanic-plate structure and faulting on seismicity on the upper plane of the double seismic zone, which may imply that hydration is limited by slow reaction kinetics at low temperatures 5–7 km below the seafloor and by the finite amount of exposed wall rock in the outer-rise region. These factors seem to limit hydration even if abundant water is available. Seismicity in the lower plane is, in contrast, substantially enhanced where deformation of the oceanic plate is high and distributed across intersecting faults. This likely leads to an increase in the volume of damaged wall rock around the faults, thereby promoting the circulation of water to mantle depths where serpentinization is faster due to elevated temperatures. Increased lower-plane seismicity around subducting oceanic features such as seamounts or fracture zones may also be caused by enhanced faulting around these features. Our results provide a possible explanation for the globally observed presence of rather homogeneous upper-plane seismicity in double seismic zones as well as for the commonly patchy and inhomogeneous distribution of lower-plane seismicity.

Pleistocene-Holocene Monogenetic Volcanism at the Malko-Petropavlovsk Zone of Transverse Dislocations on Kamchatka: Geochemical Features and Genesis

The Malko-Petropavlovsk zone of transverse dislocations (MPZ) was formed on the extension of the deep Avachinsky transform fault in perpendicular relation to the subduction trench. It is a natural boundary between variously aged slabs in Kamchatka (103–105 Ma under Southern Kamchatka and 87–92 Ma under the Eastern volcanic belt). Monogenetic cinder cones in the MPZ are randomly distributed along these long-lived rupture zones. Here we present new geochemical and isotopic results of monogenetic volcanism in the MPZ. Based on whole rock and trace element geochemistry, Pb-Sr-Nd isotopic ratios of monogenetic cinder cone magmas were shown to tap the enriched mantle source (low 143Nd/144Nd isotopic ratios (0.512959–0.512999), and changed as 87Sr/86Sr (0.703356–0.703451) and 206Pb/204Pb (18.30–18.45), 208Pb/207Pb (38.00–38.12) isotopic ratios). High Nb/Yb and La/Yb ratios, without significant inputs of the slab’s components (the lowest Ba, Th content), indicate decompression melting predominately. Calculations of the pressure (9–11 kbar) and temperature (1160–1240 °C) conditions using a glass thermobarometer suggest that magma of monogenetic cinder cones resided near the Moho boundary prior to eruption. This correlates with the crustal discontinuity under the MPZ according to geophysical observations (converted-wave seismic exploration and magnetotelluric sounding). The majority of well-preserved monogenetic cinder cones were formed in Holocene, after the last glaciation, but eruptions were not observed historically. This, however, suggests that similar eruptions in the MPZ may occur in the future. Given that the MPZ hosts major population centres of Kamchatka (Petropavlovsk, Elizovo, Vilyuchinks, and Paratunka: ~ 250,000 people or ~ 80% of the whole Kamchatkan population live in the major cities on the coastline of the MPZ), we highlight the urgent need to install a continuous monitoring system around the MPZ cones, geophysical investigation, and more serious attention from the local government and scientists. In particular, a detailed study of the MPZ regarding age, volume, and volcanic hazard assessment (pyroclastic vs extrusive) will help reduce potential risks of eruptions from monogenetic volcanoes for humans and infrastructures.

Fluid mechanics of free subduction on a sphere. Part 1. The axisymmetric case

To understand how a spherical geometry influences the dynamics of gravity-driven subduction of the oceanic lithosphere on Earth, we study a simple model of a thin and dense axisymmetric shell of thickness $h$ and viscosity $\eta _1$ sinking in a spherical body of fluid with radius $R_0$ and a lower viscosity $\eta _0$ . Using scaling analysis based on thin viscous shell theory, we identify a fundamental length scale, the ‘bending length’ $l_b$ , and two key dimensionless parameters that control the dynamics: the ‘flexural stiffness’ $St = (\eta _1/\eta _0)(h/l_b)^3$ and the ‘sphericity number’ $\varSigma = (l_b/R_0)\cot \theta _t$ , where $\theta _t$ is the angular radius of the subduction trench. To validate the scaling analysis, we obtain a suite of instantaneous numerical solutions using a boundary-element method based on new analytical point-force Green functions that satisfy free-slip boundary conditions on the sphere's surface. To isolate the effect of sphericity, we calculate the radial sinking speed $V$ and the hoop stress resultant $T_2$ at the leading end of the subducted part of the shell, both normalised by their ‘flat-Earth’ values (i.e. for $\varSigma = 0$ ). For reasonable terrestrial values of $\eta _1/\eta _0$ ( $\approx$ several hundred), sphericity has a modest effect on $V$ , which is reduced by $< 7\,\%$ for large plates such as the Pacific plate and by up to 34 % for smaller plates such as the Cocos and Philippine Sea plates. However, sphericity has a much greater effect on $T_2$ , increasing it by up to 64 % for large plates and 240 % for small plates. This result has important implications for the growth of longitudinal buckling instabilities in subducting spherical shells.

Hadal fauna of the South Sandwich Trench, Southern Ocean: Baited camera survey from the Five Deeps Expedition.

The South Sandwich Trench is a 965 km long arcuate subduction trench spanning the South Atlantic and Southern oceans, isolated from most other hadal (areas where water depths exceed 6000 m) environments by thousands of kilometres. This feature represents the coldest known hadal ecosystem with ambient sub-zero temperatures recorded from bathyal depths to over 7000 m water depth. Very little research has been carried out into the hadal fauna of this trench since the Russian expeditions of the 1960s and 70s. Here, we report the first visual assessment of hadal fauna from the South Sandwich Trench from a series of seven baited camera and trap deployments between 6044 m and the deepest point at 8265 m water depth, a location called Meteor Deep. Three species of hadal fish (Liparidae), with very low population densities were observed at depths shallower than expected, likely due to the piezo-thermal effect decreasing their depth range. Four species of scavenging amphipods were recovered, extending the known distribution of Eurythenes andhakarae, Hirondellea dubia, and Bathycallisoma schellenbergi to the South Sandwich Trench. Large densities of brittle stars (Ophiuroidea) were seen in the shallower hadal depths, while dense aggregations of holothurians (Elpididae) were observed within trench-fill basins located along the deep trench axis coincident with a visible surficial layer of surface derived detritus. In addition, gastropods, sponges, and stalked crinoids were observed. Importantly, this study highlights the endemic community present at hadal depth within this somewhat unique high pressure-low temperature environment.

A global assessment of fishes at lower abyssal and upper hadal depths (5000 to 8000 m)

The diversity of bait-attending fishes at lower abyssal and upper hadal depths was assessed using baited cameras in 14 subduction trenches, three fracture zones and two holes, spanning all five oceans. A total of 184 lander deployments from depths >5000 m (plus some additional observation from a submersible) were analysed to resolve the bathymetric extent of fishes and variations between locations. The most common families were the Macrouridae (grenadiers), Ophidiidae (cusk eels) and the Liparidae (snailfishes). Other less common families such as the Zoarcidae (eel pouts), Synphobranchidae (cut-throat eels) are reported, and the Stephanoberycidae (pricklefishes), and Ateleopodidae (jellynoses) were observed for the first time at hadal depths. While acknowledging many cryptic species and difficulty in identifying some groups from video, this study more than doubles the number of species and records of fishes at hadal depths and serves as an updated and illustrated collation of deepest fish records. We also discuss putative influences of temperature and oxygen on the depth range within the liparidae family, of which we found nine new species at hadal depths.

Depositional age and provenance of high-grade paragneisses from the Mérida Andes, Venezuela: Implications for the Ediacaran–Cambrian tectonic setting of northwestern Gondwana

Isotopic, geochemical, and geochronological data are provided to constrain sedimentary sources and depositional ages of high-grade paragneisses of the Iglesias Complex in the Mérida Andes (Venezuela). U-Pb geochronology of detrital zircons suggests a maximum depositional age of 540–530 Ma, whereas age spectra reveal sources from Pan-African-Brasiliano belts, the Amazon Craton, and Oaxaquia-Putumayo basement. Reworking of such Gondwanan sources is also reflected in whole-rock Nd and Hf crustal residence ages of ~2.0–1.3 Ga. Post-depositional disturbance of the U-Pb system by metamorphism during the early Paleozoic and Permo–Triassic is assessed through in-situ analysis of zircon rims and discordant analyses. Field evidence, geochemical and isotopic signatures are consistent with pelitic-psammitic and volcanic-volcaniclastic compositions for sedimentary protoliths. The former show provenances from continental intermediate sources, whereas the latter are probably sourced in an immature volcanic arc, suggesting the initiation of subduction of the Iapetus oceanic crust beneath northwestern Gondwana around 530 Ma. Sedimentary precursors were likely deposited during the latest Ediacaran–earliest Cambrian in an extensive continental shelf, fed by detritus draining from the topographic highs of the Pan-African-Brasiliano belts, across the Amazon craton and into the Iapetus Ocean. Thus, the main direction of sediment flow was opposite to the present-day transport from the Andes Cordillera by the Pacific Ocean to the Amazon fan in the Atlantic Ocean. Chemical trends in gneisses suggest fluvial sedimentary protoliths and cold-climate weathering at high latitudes. Metasedimentary rocks in the Mérida Andes show similarities with equivalent units in Peri-Gondwanan crustal fragments such as the Acatlán Complex and the Santander Massif, but no correlation with Ediacaran metasedimentary rocks of the Maya Block. Ediacaran–Cambrian fossiliferous sedimentary rocks in the Eastern Venezuela Basin and the Colombian foreland basin are possible equivalents but these units remained unmetamorphosed, as its paleoposition was far from the subduction trench during the Early Ordovician climax of the Famatinian orogeny.

Sedimentary Structure Derived From Multi‐Mode Ambient Noise Tomography With Dense OBS Network at the Japan Trench

AbstractWe derive the 3‐D S‐wave velocity structures of sediments and upper crust in the region off Ibaraki at Japan Trench subduction zone by applying ambient noise tomography to a dense array of short‐period ocean bottom seismometers (OBSs). The cross‐spectra were calculated using 27 or 142‐day continuous seismic data, and the phase velocities of the fundamental and the first‐higher Rayleigh wave modes are obtained in the frequency ranges of 0.1–0.25 Hz and 0.17–0.3 Hz, respectively. Our 1‐D S‐wave velocity inversion based on the trans‐dimensional Markov chain Monte Carlo method revealed multiple sedimentary layers above the acoustic basement and the upper crustal structure. The 1‐D structure was then used as a reference model to conduct ambient noise tomography and non‐linear inversion of the 3‐D S‐wave velocity structure by collecting data of the local 1‐D S‐wave velocity structure. Our 3‐D S‐wave velocity structure revealed three main points: (a) The acoustic basement is situated at a depth of ∼4 km depth; (b) the crustal structure is more complex than that of the sedimentary layers; and (c) the southern region has a complex crustal structure in which subducting seamounts were identified by previous P‐wave velocity tomographies.

Eurythenes atacamensis sp. nov. (Crustacea: Amphipoda) exhibits ontogenetic vertical stratification across abyssal and hadal depths in the Atacama Trench, eastern South Pacific Ocean

Eurythenes S.I. Smith in Scudder, 1882 (Crustacea: Amphipoda) are prevalent scavengers of the benthopelagic community from bathyal to hadal depths. While a well-studied genus, molecular systematic studies have uncovered cryptic speciation and multiple undescribed lineages. Here, we apply an integrative taxonomic approach and describe the tenth species, Eurythenes atacamensis sp. nov., based on specimens from the 2018 Atacamex and RV Sonne SO261 Expeditions to the southern sector of the Peru-Chile Trench, the Atacama Trench (24–⁠21°S). Eurythenes atacamensis sp. nov. is a large species, max. observed length 83.2 mm, possesses diagnostic features, including a short gnathopod 1 palm and a chelate gnathopod 2 palm, and a distinct genetic lineage based on a 16S rRNA and COI phylogeny. This species is a dominant bait-attending fauna with an extensive bathymetric range, spanning from 4974 to 8081 m. The RV Sonne SO261 specimens were recovered along a 10-station transect from abyssal to hadal depths and further examined for demographic and bathymetric-related patterns. Ontogenetic vertical stratification was evident across the trench axis, with only juveniles present at abyssal depths (4974–6025 m). Total length-depth analysis revealed that the size of females was unrelated to depth, whereas juveniles followed a sigmoidal relationship with a step-up in size at depths >7200 m. Thus, these bathymetric trends suggest that juveniles and females employ differing ecological strategies in subduction trench environments. This study highlights that even dominant and ecologically important species are still being discovered within the abyssal and hadal environments. Continued systematic expeditions will lead to an improved understanding of the eco-evolutionary drivers of speciation in the world’s largest ecosystem.

Effect of tsunami load induced by earthquake at Manila Trench on transmission tower

The Manila Trench subduction zone is an active convergent plate margin between the South China Sea and northern Philippines. This strongly indicates a high probability for larger earthquakes that could potentially trigger a tsunami in the future and anticipated to affect neighboring countries such as Malaysia which has power plants near the coastal areas. The potential disastrous impact of tsunami waves on a power plant causes loss of lives, damage to assets, and loss of electricity for the coastal communities similar to year 2011 Tohoku tsunami in Japan. Therefore, application of shallow water equation (SWE) in tsunami modelling namely generation and propagation phase and nonlinear shallow water equation (NSWE) for run-up and inundation phase of a tsunami event are pertinent in simulating various scenarios of tsunami. Results from tsunami simulation is used as tsunami lateral load. Linear Static Analysis is then utilized to analyze stability of 275-kV transmission tower.

Full-Waveform Inversion for Imaging Faulted Structures: A Case Study from the Japan Trench Forearc Slope

Full-waveform inversion (FWI) of limited-offset marine seismic data is a challenging task due to the lack of refracted energy and diving waves from the shallow sediments, which are fundamentally required to update the long-wavelength background velocity model in a tomographic fashion. When these events are absent, a reliable initial velocity model is necessary to ensure that the observed and simulated waveforms kinematically fit within an error of less than half a wavelength to protect the FWI iterative local optimization scheme from cycle skipping. We use a migration-based velocity analysis (MVA) method, including a combination of the layer-stripping approach and iterations of Kirchhoff prestack depth migration (KPSDM), to build an accurate initial velocity model for the FWI application on 2D seismic data with a maximum offset of 5.8 km. The data are acquired in the Japan Trench subduction zone, and we focus on the area where the shallow sediments overlying a highly reflective basement on top of the Cretaceous erosional unconformity are severely faulted and deformed. Despite the limited offsets available in the seismic data, our carefully designed workflow for data preconditioning, initial model building, and waveform inversion provides a velocity model that could improve the depth images down to almost 3.5 km. We present several quality control measures to assess the reliability of the resulting FWI model, including ray path illuminations, sensitivity kernels, reverse time migration (RTM) images, and KPSDM common image gathers. A direct comparison between the FWI and MVA velocity profiles reveals a sharp boundary at the Cretaceous basement interface, a feature that could not be observed in the MVA velocity model. The normal faults caused by the basal erosion of the upper plate in the study area reach the seafloor with evident subsidence of the shallow strata, implying that the faults are active.

Controls on the origin and evolution of deep-ocean trench-axial channels

The type and volume of sediment entering subduction zones affects the style of plate-boundary deformation and thus sedimentary and tectonic cycles. Because submarine channels significantly increase the transport efficiency of turbidity currents, their presence or absence in subduction trenches is a primary control on trench fill. To date, comprehensive architectural characterization of trench-axial channels has not been possible, undermining efforts to identify the factors controlling their initiation and evolution. Here, we describe the evolution of the Hikurangi Channel, which traverses the Hikurangi Trench, offshore New Zealand. Analysis of two- and three-dimensional seismic data reveals that the channel was present only during the last ~3.5 m.y. of the ~27 m.y. of the trench’s existence; its inception and propagation resulted from increased sediment supply to the trench following amplified hinterland exhumation. To test if the controls on the evolution of the Hikurangi Channel are universal, multivariate statistical analysis of the geomorphology of subduction trenches globally is used to investigate the formative conditions of axial channels in modern trenches. Terrigenous sediment supply and thickness of sediment cover in a trench are the dominant controls; subsidiary factors such as trench length and rugosity also contribute to the conditions necessary for trench-axial channel development. Axial channels regulate sediment distribution in trenches, and this varies temporally and spatially as a channel propagates along a trench. The presence of a trench-axial channel affects plate-boundary mechanics and has implications for the style of subduction-margin deformation.

Worldwide distribution and depth limits of decapod crustaceans (Penaeoidea, Oplophoroidea) across the abyssal-hadal transition zone of eleven subduction trenches and five additional deep-sea features

Abstract Decapod crustaceans are conspicuous members of marine benthic communities to at least 7,700 m deep. To assess the bathymetric extent of this taxonomic group, baited landers were deployed to across the abyssal-hadal transition zone of 11 subduction trenches spanning the Pacific, Atlantic, Southern, and Indian oceans and additional sites. Decapods were dominated by penaeid shrimps (superfamily Penaeoidea), in particular Benthesicymus Spence Bate, 1881 and Cerataspis Gray, 1828, with the former being found deeper. Benthesicymus cf. crenatus Spence Bate, 1881 was observed in the Kermadec, Mariana, New Hebrides, Puerto Rico, Peru-Chile, Tonga, San Cristobal, and Santa Cruz trenches, plus the South Fiji Basin and the Wallaby-Zenith Fracture Zone. They were not recorded in the Abaco Canyon, Agulhas Fracture Zone, Java Trench, or any of the polar locations. Cerataspis cf. monstrosus Gray, 1828 was present in the Kermadec, Mariana, New Hebrides, Puerto Rico, and Java trenches, the Abaco Canyon, Agulhas Fracture Zone, Wallaby-Zenith Fracture Zone and the South Fiji Basin, but absent from the Tonga, San Cristobal and Santa Cruz trenches. Hymenopenaeus nereus (Faxon, 1893) was only recorded in the Peru-Chile Trench. Unidentified species belonging to superfamily Oplophoroidea were observed to a maximum depth of 6,931 m. Decapods are thus are primarily represented at hadal depths by penaeoid shrimps, consistently present at tropical and temperate latitudes to ~7,700 m, while absent from equivalent depths in polar regions. Their maximum depth may be limited due to hydrostatic pressure, while potentially affected by temperature and oxygen in some instances. Muscle samples of three specimens from 6,000 m (Mariana and Kermadec trenches) were found to have high levels of trimethylamine N-oxide (TMAO; 260 mmol kg–1), the major piezolyte, a protectant against hydrostatic pressure, in other deep-sea organisms. We speculate that physiological limits to TMAO concentration may prevent them from inhabiting the greatest hadal depths.

Synthetic analysis of the efficacy of the S-net system in tsunami forecasting

The Seafloor Observation Network for Earthquakes and Tsunamis along the Japan Trench (S-net) is presently the world’s largest network of ocean bottom pressure sensors for real-time tsunami monitoring. This paper analyzes the efficacy of such a vast system in tsunami forecasting through exhaustive synthetic experiments. We consider 1500 hypothetical tsunami scenarios from megathrust earthquakes with magnitudes ranging from Mw 7.7–9.1. We employ a stochastic slip model to emulate heterogeneous slip patterns on specified 240 subfaults over the plate interface of the Japan Trench subduction zone and its vicinity. Subsequently, the associated tsunamis in terms of maximum coastal tsunami heights are evaluated along the 50-m isobath by means of a Green’s function summation. To produce tsunami forecasts, we utilize a tsunami inversion from virtually observed waveforms at the S-net stations. Remarkably, forecasts accuracy of approximately 99% can be achieved using tsunami data within an interval of 3 to 5 min after the earthquake (2-min length), owing to the exceedingly dense observation points. Additionally, we apply an optimization technique to determine the optimal combination of stations with respect to earthquake magnitudes. The results show that the minimum requisite number of stations to maintain the accuracy attained by the existing network configuration decreases from 130 to 90 when the earthquake size increases from Mw 7.7 to 9.1.

The great tectonic discontinuity of Bahía de Banderas, Mexico

Detachment of the Baja California peninsula from the North American plate altered the crustal region between them, including an extension of the crust between the peninsula and Mexico's mainland. In the SE portion of this region Bahía de Banderas, a 60-km indentation in the littoral of Mexico, is a boundary region across which significant changes in topography, seismicity, seismic attenuation, trench subduction angle, continental platform width, geomagnetic field, and crustal thickness have been documented. Here we identify two additional, major discontinuities: one in gravity and the second in the regional electrical resistivity to 50-km depth, both associated with the region in which the crust changes from extended to un-extended. Under the former, we ratify and extend the presence of the oceanic slab obtained elsewhere with seismological determinations. Plate rollback is also reported in the boundary region. Gravity and electrical resistivity models locate the NW and SE limits of the rolled-back portion of the oceanic slab; within this region, we identify upwelling asthenospheric material. Bahía de Banderas region represents a major tectonic discontinuity, implying the existence of a continental sliver prolonging ~160 km into the San Blas basin north of the bay, the Tres Marías sliver, independent from the Jalisco block, in which crustal fragmentation processes appear to be still in progress. We argue that this sliver is tectonically different from the Jalisco block since at least the late Miocene, when extensional tectonics stretched and thinned its corresponding crust and lithosphere.

A tuffaceous volcaniclastic turbidite bed of Early Miocene age in the Temburong Formation of Labuan, North‐West Borneo and its implications for the Proto‐South China Sea subduction in the Burdigalian

AbstractA 2 m thick bed of smectite‐rich clay within a sequence of distal deep water turbidites and marine mudstones of the Temburong Formation on the island of Labuan, Sabah, North‐West Borneo, is an altered, re‐sedimented pyroclastic tuff which is geochemically of rhyolite‐dacite composition. The tuff bed was deposited in deep water, although probably <1,000 m, by traction currents and retains relic siliceous shards, feldspar phenocrysts and biotite crystals but was dominated by ash‐grade material. The tuff is interpreted as a turbidite deposit and most probably records long distance sub‐marine transport down the depositional slope from an active volcanic system in the hinterland. It has a zircon U–Pb weighted mean age of 19.6 ± 0.1 Ma whilst combined forminiferal and nannofossil ages from the enclosing marine mudstones range from 21.5 to 19.4 Ma; together these indicate an early Miocene age for the Temburong Formation. The tuff is the result of a very short‐lived volcanic arc magmatic episode and was either related to similar aged volcanics of the Sintang Suite in central Borneo or a product of the Proto‐South China Sea subduction beneath the Cagayan arc north‐east of Borneo. The age indicates that deep water sedimentation in North‐West Borneo continued into the Burdigalian suggesting the subduction trench of the Proto‐South China Sea was active into the early Miocene, beneath the depocentre of the Temburong turbidite fan. By contrast, sandstones representative of the typical marine Temburong Formation yield abundant zircons which are dominately Cretaceous in age indicating an important switch in source provenance during the Early Miocene from the Crocker Formation that is dominated by Permian‐Triassic zircons. This difference in zircon populations likely reflects an important tectonic event in the early Miocene that resulted in uplift of central Borneo to supply sediment to the Sabah Trough whilst the input of material derived from the western terrains diminished. The Temburong Formation was sourced by reworking of uplifted Rajang Group, Sapulut or Trusmadi formations.

The sediments of Lake Singkarak and Lake Maninjau in West Sumatra reveal their earthquake, volcanic and rainfall history

Natural hazards such as earthquakes, volcanic activity, and heavy rainfall causing floods and debris avalanches are common phenomena in many tropical settings, including the island of Sumatra, located in the Indonesian archipelago. To enhance our understanding of the recurrence of these often destructive events, we studied the sedimentary infill of two lakes in the Padang highlands, West Sumatra. This includes Lake Singkarak, a tectonically-formed lake located on a step-over of a major strike-slip fault system (the Sumatran Fault), and Lake Maninjau, a caldera lake. Both lakes are located ~300 km from the Sunda subduction trench and surrounded by steep slopes and a chain of active volcanoes. Hence, considering their unique tectonic setting, these lakes may potentially record a wide range of natural hazards that affect the region. A combination of seismic-reflection profiles and short sediment cores revealed that both lakes indeed record various types of natural hazards, each with their own sedimentary response to a specific type of event. Lake Singkarak can be used to study past floods and major debris avalanches in addition to high-magnitude megathrust earthquakes, while traces of past intraplate earthquakes have been identified in both lakes. Furthermore, we argue that Lake Singkarak is an ideal recorder of volcanic activity in the region, while Lake Maninjau itself can pose a volcanic hazard to its surroundings as demonstrated by potential activity of the volcano below the lake.

The Nickel isotope composition of the authigenic sink and the diagenetic flux in modern oceans

We investigated Ni isotopes in a stratigraphic sequence of pelagic clays collected during ODP core leg 185 on site 1149 in Western Pacific Ocean near the Izu-Bonin subduction trench in order to determine the Ni isotope composition of the authigenic Mn-oxides sink and evaluate the composition of the diagenetic Ni input flux. This predominant oxic sink likely controls the Ni isotope budget in modern oceans. The sequence presented here is a 170 m-thick sequence of pelagic sediments deposited on some of the oldest oceanic crust of the seafloor, and the base was dated at 104 Ma. Nickel isotope values (δ60/58Ni relative to NIST SRM 986) vary in the range of 0.04 ± 0.04 to 1.03 ± 0.03‰. There is a trend between depth in the stratigraphic sequence and increasing Mn/Fe, Mn/Al, higher Ni concentration and heavier Ni isotope values. This trend is accounted for by authigenic Mn-oxyhdroxides precipitation in the sediment and scavenging of dissolved metals in the sediment porewaters. This indicates that authigenic oxide minerals in deep-sea pelagic clays are a relevant sink for Ni in modern oceans. Results enable us to determine the authigenic oxic output flux of 3.58 × 108 mol/yr of Ni with a Ni isotope composition of 1.2‰, this oxic output is ~0.4‰ lighter than the oxic output flux in FeMn crusts and nodules. This study shows that Ni isotope variations along the stratigraphic sequence could be the result of mixing between a pure pelagic clays end-member at ~0.1‰ and a pure Mn-oxyhydroxides end-member at ~1.2‰. We suggest that to keep the system at steady-state conditions the oxic output flux is compensated for by a diagenetic input flux of 3.7 × 108 mol/yr which is characterized by a highly fractionated Ni isotope composition of ~2.9‰.

Scavenging amphipods from the Wallaby-Zenith Fracture Zone: Extending the hadal paradigm beyond subduction trenches

Our understanding of the ecology of the hadal zone (> 6000 m depth) is based solely on subduction trenches, leaving other geomorphological features, such as fracture zones, troughs, and basins, understudied. To address this knowledge gap, the Wallaby-Zenith Fracture Zone, Indian Ocean (WZFZ; ~ 22°S, 102°E; maximum depth 6625 m measured during Expedition SO258) was studied using free-fall baited landers. We assessed the amphipod distribution and community assemblage of this non-subduction hadal feature and compared it to subduction hadal features. Eleven species were identified across the abyssal-hadal transition zone using a paired morphological and DNA barcoding approach. The community composition was found to change gradually from abyssal to hadal depths, which contrasts with the ecotone shift characteristic of subduction trenches. A large population of Bathycallisoma schellenbergi (Birstein & Vinogradov, 1958), a quintessential hadal amphipod, was present at the flat bottom of the WZFZ. Further, an mtDNA phylogeny resolved a degree of phylogeographic structure between the B. schellenbergi WZFZ population and four previously sampled Pacific Ocean subduction trench populations, indicating these features are not interconnected through ongoing gene flow. Combined, these data indicate that some amphipods have far broader distributions than previously understood, with some species present in both hadal subduction trenches and non-subduction fracture zones and basins interspersed across the abyssal plains. This initial exploration highlights that whilst non-subduction features are an overlooked minor fraction of the total hadal area, they are essential to our understanding of the ecological and evolutionary dynamics across the hadal zone.

Hadal zones of the Northwest Pacific Ocean

Understanding the extent of the hadal ecosystem (habitats exceeding 6000 m water depth) is convoluted due to the complexity of seafloor geomorphology that accounts for 45% of the total ocean depth range. Furthermore, at such great depths, features such as fracture zones and basins, although numerous, are less prominent and therefore have drawn less focus compared to the conspicuous subduction trenches that are typically associated with hadal science. Here we focus on the Northwest Pacific Ocean, where the majority of hadal features are located, to evaluate the true extent of the deepest marine ecosystem. This analysis has highlighted that the Mariana Trench, in terms of continuous hadal habitat, is in fact five isolated areas, with the most northern being what Russian scientists used to call the Volcano Trench. Conversely, we identified that there are no physical partitions either north or south of the Japan Trench to isolate it from the neighbouring Kuril-Kamchatka or Izu-Bonin trenches respectively, thus it forms one continuous hadal habitat. By evaluating the frequency and distribution of smaller features, such as basins and fracture zones, we conclude that in the northwest Pacific, the total area occupied by depths > 6000 m is 2,793,011 km2, which is considerably larger than the 686,114 km2 accounted for by subduction trenches alone. These results demonstrate not only that the hadal ecosystem may be far larger than previously anticipated but that the geomorphology is crucial in understanding the distribution and genetic connectivity of endemic hadal species that inhabit these great depths.

Stagnant forearc mantle wedge inferred from mapping of shear-wave anisotropy using S-net seafloor seismometers

Shear-wave anisotropy in Earth’s mantle helps constrain the lattice-preferred orientation of anisotropic minerals due to viscous flow. Previous studies at the Japan Trench subduction zone using land-based seismic networks identified strong anisotropy in the mantle wedge, reflecting viscous flow induced by the subducting slab. Here we map anisotropy in the previously uninvestigated offshore region by analyzing shear waves from interplate earthquakes that are recorded by a new seafloor network (the S-net). The newly detected anisotropy is not in the mantle wedge but only in the overlying crust (∼0.1 s time delay and trench-parallel fast direction). The distinct lack of anisotropy indicates that the forearc mantle wedge offshore is decoupled from the slab and does not participate in the viscous flow, in sharp contrast with the rest of the mantle wedge. A stagnant forearc mantle wedge provides a stable and cold tectonic environment that is important for the petrological evolution and earthquake processes of subduction zones.