Wharton Basin Research Articles

Geochemical and temporal evolution of Indian MORB mantle revealed by the Investigator Ridge in the NE Indian Ocean

The Investigator Ridge, located along the prominent Investigator Fracture Zone in the Wharton Basin, northeast Indian Ocean, was chosen to investigate the temporal and geochemical evolution of the Indian upper mantle. We present new 40Ar/39Ar age, as well as major and trace element and Sr-Nd-Pb-Hf isotope data, from a large part of this so far scarcely sampled ridge. The 40Ar/39Ar ages range from 100.0 Ma to 64.1 Ma, and except for the two youngest samples (65.5 and 64.1 Ma) of likely Christmas Island Seamount Province (CHRISP) origin, display decreasing ages along the ridge from south to north. This pattern is consistent with the ages derived from paleomagnetic anomalies on the local oceanic crust that also decrease in age northwards and suggests an origin of the studied rocks at or near the now extinct Wharton spreading center. These ages also provide information about the displacements along the multiple Investigator Fracture Zones indicating large left-lateral as well as right-lateral offsets of the paleo spreading axis. The fracture zone also shows signs of recent tectonic reactivation. The Investigator Ridge samples are derived from a DUPAL-like mantle source similar to present-day Indian MORB. This source contains a depleted mantle type component, an enriched, moderately HIMU-like, component such as common “C” or “FOZO”, and an additional enriched component of recycled subcontinental lithospheric mantle material. In both the Investigator Ridge and CHRISP samples, the trace element and isotope compositions systematically change with age implying a temporal geochemical evolution of the Indian Mantle Domain in this region. It contained higher proportions of subcontinental lithospheric material delaminated during breakup of Gondwana and mixed with depleted upper mantle material in its early phase. With time, the source evolved in the direction of “C” or “FOZO”, which then became the dominant enriched component and is observed in the youngest samples.

Hadal zones of the Southwest Pacific and east Indian oceans

The hadal zone (water depths > 6000 m) are unlike the overlying shallower marine regions (bathyal and abyssal) as it does not follow a continuum from the continental shelves to abyssal plains, but rather exhibits a globally disjunct series of discrete deep-sea habitats confined within geomorphological features. From an ecological perspective, hadal communities are often endemic to individual or adjacent features and are partitioned and isolated by geomorphological structures. To examine the size, shape, depth and degree of isolation of features where hadal fauna inhabit, this study explores the broad seafloor geomorphology, and distinctly partitioned hadal areas, across the Southwest Pacific and East Indian oceans using global bathymetric datasets. This research revealed the area occupied by hadal depths to be 716,915 km2 of which 58% are accounted for by trenches, 37% in basins and troughs, and 5% fracture zones. The largest feature in terms of area > 6000 m depth is the Wharton Basin with 218,030 km2 spanning 376 discrete areas. The largest continuous hadal habitats were the Kermadec and Tonga trenches at 145,103 and 111,951 km2 respectively, whereas features such as the Java Trench comprise two hadal components partitioned by a bathymetric high. Conversely, no physical barrier exists between the New Britain and Bougainville trenches thus any literature pertaining to hadal species or habitats from these trenches can be merged. This study highlights that the hadal zone mainly comprises two main geomorphological features (trenches and basins) that differ in size, depth and complexity. Hadal basins cover vast, generally shallower areas, comparable to abyssal plains, whereas trenches, despite a lesser footprint, represent greater depth ranges and complexity. As such, sampling designs and interpretation of ecological data must differ and hadal basins likely play an increasingly important role in understanding ecological shifts from abyssal to hadal ecosystems.

Analysis of TEC variations and prediction of TEC by RNN during Indonesian earthquakes occurred from 2004 to 2024 and comparison with IRI-2020 model

The Ionosphere, a crucial region of Earth’s atmosphere extending from approximately 50 to 1000 km above the Earth’s surface, plays a pivotal role in global communication, navigation, and satellite-based technologies. Among its various parameters, Total Electron Content (TEC) stands out as a key metric, representing the integrated electron density along a specific path through the ionosphere. This research focuses on analyzing TEC variations during Indonesian earthquakes between 2004 and 2024 using a Recurrent Neural Network (RNN) model. The study investigates the ionospheric response to seismic activity, particularly in Indonesia, a region prone to earthquakes due to its location within the Pacific Ring of Fire and complex tectonic setting involving multiple lithospheric plates. The RNN model, designed to predict TEC variations during earthquakes, utilizes data on solar and geomagnetic activity, including the Kp index, solar wind, and geomagnetic storm activity, obtained from the OMNIWEB data centre, along with TEC data from the IONOLAB data servers for the BAKO station in Cibinong, Indonesia. The earthquakes considered for observation include significant events such as the 2004 Indian Ocean earthquake and tsunami, the 2012 Wharton Basin earthquake, and subsequent seismic events up to 2024. Statistical metrics such as Mean Bias Deviation (MBD), Relative Error (REL_E), Absolute Error (ABS_E), and Root Mean Square Error (RMSE) are used to evaluate the predictive capability of the RNN model and compare it with the International Reference Ionosphere (IRI) 2020 model, a widely accepted empirical model for describing ionospheric parameters. The comparison reveals the performance of the RNN model in capturing TEC variations during seismic events, providing valuable insights for earthquake monitoring and early warning systems.

Provenance and sedimentary environment of REY-rich sediments from the Wharton Basin, Indian Ocean

Rare earth elements and yttrium (REY) in deep-sea sediments have emerged as potential strategic resources. In order to investigate the provenance, sedimentary environment, and mechanism of REY enrichment of pelagic sediments in Indian Ocean, we analyzed the grain size, bulk major and trace element concentrations and mineralogy of clay fractions (<2 μm) of sediments in the Wharton Basin, Indian Ocean. Based on the characteristics of biostratigraphic, we speculated that the sediments were deposited during the period from 11 Ma to 2 Ma. Sediments in the upper part (0–100 cm) of the sequence were mainly derived from aeolian material in Australia, whereas sediments in the lower part (100–258 cm) were mainly sourced from volcanic and hydrothermal materials which associated with submarine volcanic activity. The oxygen-rich environment below the calcite compensation depth (CCD) is crucial for the formation of REY-rich sediments. Additionally, there is a strong correlation between ΣREY, clay-grained sediments, Al2O3, and Fe2O3, suggesting that clay fractions contribute significantly to the enrichment of REY in the deep-sea sediments.

Hydrogenetic and Diagenetic Controls on the Specific Surface Area of Polymetallic Nodules in Deep Ocean Basins

Polymetallic nodules (nodules) are a predominant deep-sea mineral resource due to theirenrichment with critical metals, such as Co, Ni, and Cu, and rare earth elements (REEs). The loose and porous nature of nodules contributes to their adsorption and enrichment in trace metals from seawater and pore water. Consequently, the specific surface area (SSA) of nodules is a key factor requiring further study. However, controls on the SSA of nodules with various genetic types remain poorly characterized. This study aimed to investigate controls on nodule SSA by analyzing the transition metals, REEs, mineralogy, and SSA of nodules recovered from basins in the Atlantic, Indian, and Pacific oceans, including the Northwest Pacific Basin (NPB), Bauer Basin (BB), Tiki Basin (TB), Wharton Basin (WB), Central Indian Basin (CIB), and Angola Basin (AB). Nodule SSAs were compared among the various basins by calculating the BET SSA (based on the equation proposed by Brunauer, Emmett, and Teller, 1938). The results suggest thatnodules from the PNB, WB, CIB, and AB are mainly hydrogenetic, and those nodules have a relatively high SSA, high Co, low Ni and Cu, positive Ce anomalies, and low X-ray diffraction (XRD) intensities at ~10 Å. The nodules from the BB and TB are mainly diagenetic in origin, characterized by a relatively low SSA, low Co, high Ni and Cu, negative Ce anomalies, and high XRD intensities at ~10 Å. The SSAs of nodules were significantly positively correlated with Co, δCe, and light REEs (LREEs), and negatively correlated with the XRD intensity at ~10 Å, Ni, and Cu. The SSAs of nodules from the NPB ranged from 329.440 m2/g to 418.711 m2/g, comparable to the SSAs of Co-rich crusts on seamounts. This study proposes that nodule SAA is regulated by nodule genesis and that hydrogenetic nodules have a higher SSA.

Spatial distribution, source identification and flux estimation of polycyclic aromatic hydrocarbons and organochlorine pesticides in basins of the Eastern Indian Ocean

Although polycyclic aromatic hydrocarbons (PAHs) and organochlorine pesticides (OCPs) have been recorded worldwide, information on their presence in the Eastern Indian Ocean (EIO), especially south of 10°S, remains limited. We investigated the distribution and depositional fluxes of PAHs and OCPs, and the major sources and ecological risks of PAHs in EIO surface sediments from the Central Indian Ocean (CIOB) and Wharton Basin (WB). The concentration of Σ18 PAHs and ∑10 OCPs had an average value (± SD) of 138.4 ± 52.34 and 0.8 ± 0.20 ng g−1, respectively. PAHs may mainly affected by traffic emission and biomass and wood combustion. Persistent organic pollutant accumulation rate (PAR) and depositional flux (DF) values showed that abundant PAHs might lost during top-down transport. The low trans- chordane (CHL)/cis-CHL ratio and PAR of OCPs may indicated few OCPs were inputted into the EIO recently. The results of binary isotope mixing modeling indicate the predominance of marine organic matter (MOM) in total organic carbon (TOC) of sediments. Fluoranthene (Flour) and pyrene (Py) might have potential biological effects in the EIO. The study provided background values for PAHs and OCPs in the Indian Ocean, and preliminarily revealed the fate of POPs in the open oceans.

The transformation and accumulation mechanism of rare earth elements in deep-sea sediments from the Wharton Basin, Indian Ocean

Recently, rare-earth elements and yttrium (REYs, including La-Lu and Y) have been found to be remarkably enriched in Pacific and Indian Ocean deep-sea sediments, as the extreme enrichment of critical metals have received unprecedented attention. However, the origins, transformation and accumulation mechanism of REYs in deep-sea sediments remain controversial. In this study, the mineralogical and geochemical characteristics of REY-rich sediments from the Wharton Basin, Indian Ocean (core GC34) were systematically analyzed. Our results indicate that bioapatites are the most significant host phase of REYs, ambient solutions (seawater and pore water) are the main sources of REYs in deep sea sediments. Besides being the other important host phase of REYs, Fe-Mn micronodules also act as the links between sources and sinks of REYs in deep-sea sediments. Based on the variations in the characteristic profiles of ΣREY, δCe, and MREE/MREE* with depth, combined with the fractionation characteristics of LREE, MREE and HREY in bulk sediments, bioapatites, and Fe-Mn micronodules, we present a model of REYs transformation and accumulation processes in the deep-sea sediments, which mainly occurs in two intervals. The first and dominant accumulation process is in the sediment–water interface (SWI) and shallow sediments, where Fe-Mn micronodules and bioapatites adsorb and/or substitute REYs directly from ambient solutions, resulting in the initial accumulation of REYs in bioapatites and micronodules. The secondary accumulation process in the deep subsurface sediments, due to desorption and resorption processes in bioapatites and release processes in micronodules. The significant connection between P/Th and ΣREY suggests that the physical enrichment of REY-rich materials (bioapatites and micronodules) contributed to the REYs enrichment in deep-sea sediments.

Large Active Faults and the Wharton Basin Intraplate Earthquakes in the Eastern Indian Ocean

Large Active Faults and the Wharton Basin Intraplate Earthquakes in the Eastern Indian Ocean

Evidence of Strong Upper Oceanic Crustal Hydration Outboard the Alaskan and Sumatran Subduction Zones

AbstractThe hydration state of subducting oceanic crust has been proposed to influence subduction zone processes like seismic coupling at the megathrust interface and arc magmatism downdip. Plate bending in the outer rise region is thought to help rehydrate the incoming oceanic lithosphere before subduction. Although numerous seismic refraction studies provide constraints on the amount of water stored in the lower crust and uppermost mantle, little information exists about how much free water is present in the upper 1–2 km of oceanic crust. Here, we present results from the application of advanced techniques to long‐offset multi‐channel seismic data acquired outboard the Alaskan and South Sumatran subduction zones. Our results show that the incoming upper crustal seismic layer, layer 2A, is significantly hydrated in both areas. Favorable conditions offshore the Alaska Peninsula promote the creation of a dense system of bending‐related faults, facilitating the infiltration of fluids that increases average water estimates to ∼3.9 wt.% H2O in the outer rise. As the crust subducts and temperature increases, some free water may react with the host rocks to form mineral‐bound water that is carried to greater depths, in agreement with elevated water contents found in arc lavas of Shumagin Gap volcanoes. Offshore Sumatra, we propose that similar layer 2A water estimates (∼3.2 wt.% H2O average) and heterogeneous hydration within 2B are associated with the ongoing, slow, complex deformation occurring in the Wharton Basin and thus potentially contribute to the presence of a long‐lived slow slip event recently inferred there at seismogenic depths.

Source and Distribution of Emerging and Legacy Persistent Organic Pollutants in the Basins of the Eastern Indian Ocean.

Persistent organic pollutants (POPs) have received significant and ongoing attention. To establish favorable regulatory policies, it is vital to investigate the occurrence, source, and budgets of POPs worldwide. POPs including phthalic acid esters (PAEs), organophosphate esters (OPEs), brominated flame retardants (BFRs), and highly chlorinated flame retardants (HFRs) have not yet been examined in the Eastern Indian Ocean (EIO). In this study, the distribution of POPs has been investigated from surface sediments with the depth of 4369-5742 m in the Central Indian Ocean Basin (CIOB) and Wharton Basin (WB) of EIO. The average (±SD) concentrations of ∑11PAEs, ∑11OPEs, ∑4 BFRs, and ∑5HFRs were 1202.0 ± 274.36 ng g-1 dw, 15.3 ± 7.23 ng g-1 dw, 327.6 ± 211.74 pg g-1 dw, and 7.9 ± 7.45 pg g-1 dw, respectively. The high abundance of low-molecular-weight (LMW) PAEs, chlorinated OPEs, LMW BDEs, and anti-Dechlorane Plus indicated the pollution characteristics in the EIO. Correlation analysis demonstrated that LMW compounds may be derived from the high-molecular-weight compounds. The monsoon circulation, currents, and Antarctic Bottom Water may be the main drivers. POP accumulation rate, depositional flux, and mass inventory in the Indian Ocean were also estimated.

GNSS TEC‐Based Detection and Analysis of Acoustic‐Gravity Waves From the 2012 Sumatra Double Earthquake Sequence

AbstractThe Wharton Basin earthquake sequence on April 11, 2012, offshore Sumatra, represents the two largest (Mw &gt; 8.0) strike‐slip earthquakes ever recorded. Ground fault displacements generated a spectrum of acoustic‐gravity waves due to solid Earth–atmosphere coupling. Wave‐like perturbations in Total Electron Content (TEC) were therefore observed in ground‐based Global Positioning System data. The waves arrive about 10 min after each earthquake and their spectral analysis reveals the presence of acoustic resonance frequencies of 3.8 and 4.4 mHz. The acoustic wave speeds of 0.9–1.2 km/s suggest coseismic ground movement as the primary wave generating mechanism instead of seismic Rayleigh waves. Gravity waves with frequencies below 2 mHz traveling with lower speeds of 0.21 km/s are also detected. Ray tracing using a simple numerical model traced the source of observed ionospheric perturbations to within 150 km distance of the epicenters. Large amplitude ionospheric disturbances were found to travel mostly in a north‐south direction, an observation explained by the orientation of Earth’s geomagnetic field.

Strike-slip seismicity at the Andaman-Sumatra Subduction Zone: Role of the fracture zones and age of the subducting lithosphere

A better comprehension of the shallow strike-slip seismicity along the Sunda Trench is achieved by superimposing the Wharton basin fracture zones, magnetic isochrones, and focal mechanisms to the geometry of the subducting plate. The shallow strike-slip seismicity is generally associated with the north-south trending fracture zones of the basins, which segment the subducting lithosphere into eight main compartments named A-H. Strike-slip earthquakes with magnitudes higher than 4.5 Mw in the time period of 2004–2019 are examined with respect to the thickness and density of the subducting lithosphere, estimated from the lithospheric age. Bulk density ranges from ~3258 to ~3272 and lithospheric thickness from ~74 to ~112 km between the younger fossil spreading axis and the older Compartment H, respectively. ‘Subductability’, a parameter calculated from lithospheric thickness and bulk density, is coined to represent the easiness of subduction and varies for each spreading compartment depending on the lithospheric age. We propose the correlation between the shallow strike-slip seismicity and the contrasts of lithospheric thickness, density and subductability. Considering the available seismic records from the past four decades, a prominent cluster of strike-slip earthquakes is observed along Fracture Zone B-C, which displays a strong subductability contrast, whereas other fracture zones displaying only minor seismicity correspond to low subductability contrasts. Despite a high subductability contrast, slab tearing on the subducting Investigator Fracture Zone between Compartments G and H prevents the generation of strike-slip earthquakes. This result emphasizes the importance of physical and mechanical properties of the subducting lithosphere in localizing the seismicity at subduction zones.

Active Tectonics And Active Faults: Why These Terms Still Lack Consensus On Definitions

The terms: Active tectonics and active faults have emerged as some of the most frequently used terms in geological literature, and traditionally, the main purpose of these definitions has historically remained devoted to either geological or engineering uses. However, most of the existing literature on the definitions has been gathered since &gt;230 years that were spent on the understanding of the science of earthquakes, but a clear-cut consensus lacks on how to define active tectonics and active faults, for various reasons that are discussed at length here. Therefore, this paper presents a brief overview of the terms with a motivation to rekindle the discussion on what is considered active in tectonics. It also explores whether the traditional definitions are valid or not, and should we look for other alternatives. We present a brief historical background knowledge and understanding on the active faults, and particularly in some of the tectonically stable and presumably inactive portions of the Earth’s crust. The two major strike-slip faulting events (Mw 8.6 and Mw = 8.2) that have occurred in the Wharton Basin, Indian Ocean in 2012 are discussed in detail. The events are specially quoted to make a case for reactivation of old fracture systems as these earthquakes ruptured the ~30-90 Ma old Indian oceanic crust. This clearly demonstrates that much older geological structures could also be re-activated, thereby questioning the traditional definition of the typical time span that is used to define active tectonics and active faults.

Transfer of stress from the 2004 Mw9.2 Sumatra subduction earthquake promoted widespread seismicity and large strike‐slip events in the Wharton Basin

AbstractThe 2004 Mw9.2 Sumatra and 2012 Mw8.6 Wharton Basin (WB) earthquakes provide the unprecedented opportunity to investigate stress transfer from a megathrust earthquake to the subducting plate. Comprehensive analyses of this study revealed that the 2004 earthquake excited widespread seismicity in the WB, especially in regions of calculated stress increase greater than 0.3 bars. The 2004 earthquake stressed all three rupture planes of the 2012 Mw8.6 strike‐slip mainshock and the largest Mw8.2 aftershock with mean values of Coulomb stress between 0.3 and 2.1 bars. For the 77 Mw ≥ 4 regional events since 2012, at least one nodal plane for 95% of the events, and both nodal planes for 72% of the events experienced stress increase due to the 2004 earthquake. Results of the analyses also revealed that the regional stress directions in the WB may have controlled the sub‐fault orientations of the 2012 Mw8.6 strike‐slip earthquake.

A note on stress rotations due to the 2004 Mw 9.2 Sumatra–Andaman megathrust earthquake

Here we study the evolution of stress and its rotations in the Sumatra–Andaman Subduction Zone (SASZ) after the 2004 Mw 9.2 megathrust earthquake, using moment tensor stress inversions. Models indicate returning of North Andaman stress regime to the inter-seismic oblique-compression, after a short stint of extension. Likewise, Little Andamans experience extension indicating a trench-ward guidance of extensional stresses. Similarly, Nicobar regime is presently normal-oblique, from early post-seismic reverse-oblique, and North Sumatra experiences pure-reverse regime. The influence of the Wharton Basin stress field and 2012 earthquakes on the SASZ fore-arc deformation is also probed. A near-complete co-seismic stress-relief is observed at Nicobar, followed by North Sumatra and Little Andaman. Co-seismic and post-seismic model comparisons indicate strong correlation with zones of co-seismic stress relief and regions of rapid post-seismic reloading. The North Andaman shows a northward compressive shear than the margin-normal component of subduction, as evident from prominent oblique stress regime with lower co-seismic stress relief and lack of post-seismic back rotations. At Andaman Spreading Ridge (ASR), transition from oblique to extensional stress indicates lateral shear to back-arc spreading enhancement.

A complete structural model and kinematic history for distributed deformation in the Wharton Basin

The equatorial eastern Indian Ocean hosts a diffuse plate boundary, where widespread deformation accommodates the relative motion between the Indian, Australian and Capricorn sub-plates. We integrate IODP Expedition 362 borehole data, which for the first time provides an accurate, ground-truthed chronostratigraphy of the sedimentary sequence east of the Ninety East Ridge (NER), with 2D seismic reflection profiles and multibeam bathymetry to assess the styles of faulting between the NER and the Sunda subduction zone, timing of activity and comparison with physical and rheological properties. We identify four distinct fault sets east of the NER in the northern Wharton Basin. N-S (350-010°) orientated faults, associated with the N-S fracture zones formed at the now extinct Wharton spreading centre, are still active and have been continuously active since at least 10 Ma. NNE- and WNW-trending fault fabrics develop between the fracture zones. The orientations and likely sense of displacement on these three sets of faults defines a Riedel shear system responding to ∼NNE-SSW left-lateral strike-slip activity at depth, demonstrated by the recent 2012 great intraplate earthquakes. We also find evidence of ∼NE-SW reverse faults, similar in style to E-W reverse faults observed west of the NER, where reverse faulting is more dominant. We show that the activity of this strike-slip system increased ca. 7-9 Ma, contemporaneous with reverse faulting and intraplate deformation west of the NER.

Joint inversion of GPS and high-resolution GRACE gravity data for the 2012 Wharton basin earthquakes

The Wharton basin is situated in the north-eastern part of the Indian Ocean. In 2012 it hosted the largest intraplate strike-slip earthquakes ever recorded by geophysical networks. The Mw 8.6 earthquake of April 11, 2012, was preceded by a major foreshock (Mw 7.2) on January 10 and was followed two hours afterward by a Mw 8.2 event. These three large events occurred at the diffuse boundary between the Indian and Australian plates and were almost pure strike-slips on sub-vertical rupture surfaces. Using GRACE data, we first extracted the coseismic and postseismic gravity signals caused by these earthquakes. Then we fitted both GPS and the highest available spatial resolution of GRACE data using the geometry of the fault system suggested by Hill et al. (2015). We propose a regularization, which allows to solve for a linear problem in order to invert GPS and GRACE data under constraints on the rake angle. Our inversion yields a uniform displacement field on all elements of a given fault plane. Our solution shows that even the main displacement occurred on WNW trending faults, comparable displacement also occurred on a rupture striking NNE. Hence, we show that the deformation in this diffuse plate boundary region in 2012 was accommodated by displacements along both fault-systems.A viscoelastic relaxation of the asthenosphere with a Maxwell viscosity 1019 Pa∙s successfully explains the postseismic displacements at GPS sites and postseismic gravity signals. The limited postseismic aftershock activity suggests small postseismic slip in the area of the 2012 Wharton earthquakes contrary to what is often observed after large subduction event. Because a part of the observed signal could be related to afterslip, our obtained Maxwell viscosity value should be considered as a lower limit of the asthenospheric viscosity below the Wharton basin.

Timing and causes of the mid-Cretaceous global plate reorganization event

Global plate reorganization events (GPREs) can have profound impact on plate velocities, climate and the biotic cycle but the timing and triggers for GPREs remain debated. The timing and causes of the mid-Cretaceous GPRE, one of only two GPREs since the breakup of Pangea, remains poorly constrained because it occurred during the Cretaceous Normal Superchron. Here, we provide a new plagioclase 40Ar/39Ar age of 103.82±0.54 Ma (2σ) at DSDP site 256 along the world's clearest curved fracture zones in the Wharton Basin, off NW Australia. Using Monte Carlo simulations, we compute that the mid-Cretaceous GPRE commenced at 111.2−1.2+1.3 Ma (95% confidence). The mid-Cretaceous GPRE was associated with a significant plate deceleration (minimum ∼26%), similar to the Eocene GPRE, implying that this process is fundamental to plate reconfigurations. We propose that the mid-Cretaceous GPRE was caused by slab-breakoff and cessation of the south-dipping subduction zone in the Mesotethys Ocean at ca. 111 Ma. Ultimately, we posit that subduction zone initiations and cessations are the primary triggers for both post-Pangean GPREs and that top-down processes may be the fundamental drivers for GPREs.

Amphipods from the Wallaby-Zenith Fracture Zone, Indian Ocean: new genus and two new species identified by integrative taxonomy

The Wallaby-Zenith Fracture Zone, within the Wharton Basin of the East Indian Ocean, is a geomorphologically complex structure with depths from abyssal to hadal (∼6600 m) with limited published taxonomic data. To fill this knowledge gap, this study describes two new species of scavenging amphipods, Stephonyx sigmacrus sp. nov. (Uristidae) and Civifractura serendipia gen. et sp. nov. (Alicellidae), collected using baited traps from a depth of 4932 m. As identification of deep-sea amphipods is challenged by phenotypic plasticity and convergent evolution, we combined a morphological with a molecular phylogenetic analysis of two mitochondrial (16S rDNA and COI) and two nuclear (Histone 3 and 28S rRNA) regions, where possible (no genetic material was obtainable from S. sigmacrus sp. nov.). The genetic data uncovered cryptic taxonomy and elevated diversity within the non-monophyletic Alicellidae family and provided evidence for establishing a new genus. Additionally, we identified morphological variations from the Tectovalopsis wegeneri description, genetically resolved two previously identified specimens of Diatectonia sp. as T. wegeneri, and identified two new species of Tectovalopsis for future description. These investigations highlight the application of integrative taxonomic approaches and represent a potential model for resolving incongruent morphological and molecular phylogenies for deep-sea amphipods and other specimen-limited taxa.Paper – zoobank.org:pub:79BC3A33-4FB2-404B-B7CB-EC401B392817Civifractura – zoobank.org:act:E513D9AB-C258-4079-B192-BB453D7F0CBACivifractura serendipia – zoobank.org:act:B7110CA9-365C-4D0A-899D-5BFFDAD4A05EStephonyx sigmacrus – zoobank.org:act:DDAB1B8D-8359-49FE-B1C7-42E10CE7C9F8

Constraining the Oceanic Lithosphere Seismogenic Zone Using Teleseismic Relocations of the 2012 Wharton Basin Great Earthquake Sequence

AbstractThe great 2012 Mw 8.6 strike‐slip earthquake beneath the Wharton Basin generated a complex aftershock sequence that maps onto a system of conjugate faults. Analysis of high‐precision aftershock locations with improved depth constraint is used here to characterize the seismogenic limits of the oceanic lithosphere. The study presents teleseismic double‐difference earthquake relocation results for 695 events in and around the 2012 Wharton Basin intraplate earthquake sequence. We highlight seven major clusters of seismicity and show that the 2012 earthquake sequence ruptured in the oceanic crust and upper mantle. The refined aftershock locations projected onto available mainshock finite‐fault models show that aftershocks occur outside the largest coseismic slip region and tend to cluster in low slip areas, a pattern commonly seen for large continental and megathrust sequences. For events with depth phases, the relocated focal depths generally correspond to predicted depths from pP‐P time observations. Reported pP‐P observations for intraplate events correspond to depths ranging from ~5 to 35–40 km, such that the deepest events occur within the expected limit of brittle seismic failure at 600 °C, here defined by a half‐space cooling model of the region. The 74 low magnitude events in our catalog that locate below the 600 °C isotherm do not have consistent depth phase observations and cannot be interpreted as strong evidence of rupture into the ductile regime. The refined double‐difference catalog supports that, along with deep coseismic rupture, moderate‐sized earthquakes ruptured across the full extent of the elastic oceanic lithosphere in the Wharton Basin.