Subduction Front Research Articles

Lithospheric structure beneath Tibet based on gravity data from World Gravity Map 2012

An appreciation of the geodynamics of the Cenozoic great collision between the India and Eurasia plates necessitates an understanding of the lithospheric structure beneath Tibet. Here, we utilise the World Gravity Map 2012 (WGM2012) gravity data to derive the lithospheric density variations and gravity anomaly beneath Tibet, employing the multi-scale inversion and regularised downward continuation methods. The findings indicate that the E-W striking structures persist to a depth of 90 km. The deep lithospheric mantle displays block-like density anomalies that exhibit a distinct N-S trend with increasing depth. The results clearly indicate a decoupling between the lithospheric crust and mantle. It is possible that changes may occur along the Indian plate’s subduction front, which subducts to the south of 32°N along the western portion of the subduction zone, as indicated by the apparent density of the lithospheric mantle. In contrast, in the central and eastern regions, the Indian lithosphere appears to subduct primarily north of the Indus-Tsangbo suture zone (30°N), where it may undergo tearing and delamination beneath the Lhasa block. This phenomenon promotes the convection of soft materials and the upwelling of mantle material, which causes the Tibetan lithosphere to thin and stretch in an E-W direction. At a depth of 150 km beneath central Tibet, alternating lower-density belts trending north–south may be connected at greater depth. These structures likely reflect lithospheric deformation, which plays a crucial role in shaping the geodynamics of the region.

Vertical Carbon Export During a Phytoplankton Bloom in the Chukchi Sea: Physical Setting and Frontal Subduction

AbstractIn order to quantify pelagic‐benthic coupling on high‐latitude shelves, it is imperative to identify the different physical mechanisms by which phytoplankton are exported to the sediments. In June–July 2023, a field program documented the evolution of an under‐ice phytoplankton bloom on the northeast Chukchi shelf. Here, we use in situ data from the cruise, a simple numerical model, historical water column data, and ocean reanalysis fields to characterize the physical setting and describe the dynamically driven vertical export of chlorophyll associated with the bloom. A water mass front separating cold, high‐nutrient winter water in the north and warmer summer waters to the south—roughly coincident with the ice edge—supported a baroclinic jet which is part of the Central Channel flow branch that veers eastward toward Barrow Canyon. A plume of high chlorophyll fluorescence extending from the near‐surface bloom in the winter water downwards along the front was measured throughout the cruise. Using a passive tracer to represent phytoplankton in the model, it was demonstrated that the plume is the result of subduction due to baroclinic instability of the frontal jet. This process, in concert with the gravitational sinking, pumps the chlorophyll downwards an order of magnitude faster than gravitational sinking alone. Particle tracking using the ocean reanalysis fields reveals that a substantial portion of the chlorophyll away from the front is advected off of the northeast Chukchi shelf before reaching the bottom. This highlights the importance of the frontal subduction process for delivering carbon to the sea floor.

Deep‐Focus Earthquake Mechanisms at the Subducting Nazca Plate (Peru‐Brazil Border): Cold Slab Behavior in a Warm Plate

AbstractWe calculate focal mechanisms and centroid depths for deep‐focus earthquakes (DFEs) along the Peru‐Brazil border. We obtained a total of 28 focal solutions for events with magnitudes between 4.2 and 7.5 Mw and occurring between 2014 and 2022. Focal mechanisms indicate predominance of normal faulting, demonstrating a rather uniform down‐dip compression (DDC) regime within the plate. The orientations of the nodal planes suggest that earthquakes tend to occur along faults parallel to the local slab strike, although other fault types are documented. Stress orientations derived from the focal mechanisms agree with patterns expected if faulting were initiated by transformational faulting on a metastable olivine wedge (MOW) under DDC. Centroid depths range between 557 and 659 km, defining a narrow seismic zone within the lower portion of the subducting plate and an aseismic upper portion. We suggest that DFEs nucleate through transformational faulting within a narrow MOW preserved at a colder slab segment right above the lower mantle and juxtaposed to a shallower, warmer segment at around 500 km depth. This thermal complexity was possibly produced through flat subduction initiated by the subduction of the Nazca Ridge. We speculate that subduction of other aseismic ridges is possibly controlling the thermal state of the Nazca slab as a whole and, consequently, the depth distribution of DFEs along the South America subduction front.

Seasonal Tracer Subduction in the Subpolar North Atlantic Driven by Submesoscale Fronts

AbstractSubmesoscale flows (0.1–10 km) are often associated with large vertical velocities, which can have a significant impact on the transport of surface tracers, such as carbon. However, global models do not adequately account for these small‐scale effects, which still require a proper parameterization. In this study, we introduced a passive tracer into the surface mixed layer (ML) of a northern Atlantic Ocean simulation based on the primitive‐equation model CROCO, with a horizontal resolution of Δx = 800 m, aiming to investigate the seasonal submesoscale effects on vertical transport. Using surface vorticity and strain rate criteria, we identified regions with submesoscale fronts and quantified the associated subduction, that is the export of tracer below the ML depth. The results suggest that the tracer vertical distribution and the contribution of frontal subduction can be estimated from surface strain and vorticity. Notably, we observed significant seasonal variations. In winter, the submesoscale fronts contribute up to 40% of the total vertical advective transport of tracer below the ML, while representing only 5% of the domain. Conversely, in summer, fronts account for less than 1% of the domain and do not contribute significantly to the transport below the ML. The findings of this study contribute to a better understanding of the seasonal water subduction due to fronts in the region.

Fluid-related manganese deposits at the incoming plate of the Southern Mariana subduction zone

Manganese is one of the most abundant transition metals in marine systems, controlling the behaviors of a variety of important elements in oceans worldwide. Deposited manganese minerals in marine systems mainly belong to poorly crystalline manganates or sometime tecto-manganates in the form of crusts and nodules origin from ambient bottom seawater, sediment porewater, or ascending hydrothermal fluids. Here, we report the discovery of two kinds of fluid-related manganese deposits on the incoming plate of the Southern Mariana subduction zone. Hausmannite that rich in copper and zinc with a strong negative Ce anomaly is identified in one sample. Notable features of this unusual manganese mineral indicate that reduced fluid derived from the nearby fluid discharge site reacted with original manganese oxides and then precipitated as hausmannite in the sub-oxic subsurface. However, another fluid derived sample located approximately 60 m away, shows normal hydrothermal mineral and elemental features. The atypical and diverse fluid-related manganese deposits more likely indicate multiple episodes of fluid activities at the subduction front. As bend-faults are proven to induce the fluid discharge at the outer-rise region of the incoming plate, manganese deposits in the present study suggest that an appreciable amount of manganese in the upper lithosphere of the incoming plate may be discharged into seawater through fluid activities before subduction. As such, the mass of manganese subducted into the subduction zone may be much lower than previously expected.

Evidence for long-lived (>100 Myr) continental intraplate volcanism: Mongolia since the last ocean closure

Since the closure of the Mongol-Okhotsk Ocean in the Mesozoic, Mongolia has been in an intraplate tectonic setting; the nearest plate boundary being ∼3,000 km to the east, at the active Pacific subduction front. Throughout this time, Mongolia has experienced magmatism in the form of distinctive, small-volume volcanic fields dispersed along the central and eastern parts of the country. On the basis of geochemical, isotopic, palaeomagnetic and zircon data, the magmatism can be discriminated from preceding post-collisional magmatism. Gradual change from a lithospheric to an asthenospheric mantle source suggests lithospheric delamination occurred beneath Mongolia, starting at ∼140 Ma and terminating at ∼107 Ma. Accordingly, the onset of intraplate magmatism is set at 107 Ma. Regardless of the spatial and temporal occurrence and evolution of the intraplate magmatism in Mongolia, the geochemistry of the resultant volcanic rocks throughout time remains remarkably similar, although the cause of magmatism has been much debated.Through evaluation of available K-Ar and Ar-Ar data from the literature, along with newly-obtained data from three different volcanic fields, we have identified that the intraplate volcanism in Mongolia has been near-continuous since its onset, with hiatuses of only <6 Myr. Since 35 Ma, hiatuses have reduced to <1 My. In light of these findings, we re-evaluate the various models that have been proposed for the origins of this long-lived volcanism and suggest the cause of magmatism results from asthenospheric upwellings initiated by a delamination event in the Mesozoic, but have been prolonged by enhanced mantle flow relating to northward progression of India, the closure of Neo-Tethys, and ultimately the Indo-Asian collision. This example of intraplate magmatism is one of the longest-lived volcanic intraplate regimes on Earth that does not appear to relate to a geophysically-recognisable high heat-flux mantle plume.

STRUCTURAL CHARACTERISTICS OF KIRAZLI DISTRICT, ÇANAKKALE, TÜRKİYE

The Biga Peninsula, which geologically evolved in relation with the the closure of the Tethyan oceans during pre-Cenozoic and subsequent syn-, post-collisional extensional tectonic regime throughout the Cenozoic located in the northwestern Türkiye. The Kirazlı district and and surrounding regional study area covers the central part of Biga Peninsula where Permo-Triassic Rhodope and Sakarya Zones sutured during closure of Paleo-Tethyan marginal seas along Intra-Pontide Suture (IPS). Neo-Tethyan northward subduction and closure, limit the Biga Peninsula from south along İzmir-Ankara-Erzincan Suture (IAES). Following syn- and post-collisional, slab break-off, slab roll-back events accompanied by crustal extension, metamorphism, core-complex formation, southward retreat of subduction front and resulted in generally southward younging magmatism. &#x0D; Field observations showed that Kirazlı district was exampled both Rhodope and Sakarya basement rocks along its eastern sector those were covered by Oligocene volcanic sequence at the rest of district. Resultant Cenozoic structural architecture of post-collisional tectonic regime includes three and four predominant fault orientations respectively in regional and district scale study areas. Investigating the tectonic characteristics of these faults at both regional and district scales will be instrumental in unveiling the temporal and spatial relationships among Cenozoic structures. To achieve this goal, the study examines and discusses the tectonic properties associated with the identified predominant fault orientations.

Oceanic Plateau and Spreading Ridge Subduction Accompanying Arc Reversal in the Solomon Islands

AbstractWe study the collision of the Ontong Java Plateau, reversal of the Solomon Islands arc, and subduction of the Woodlark Basin spreading ridge in the southwest Pacific. Double‐sided subduction and intra‐arc deformation, not just polarity reversal, characterize the rapid Australia‐Pacific plate convergence. The rifted margins of the 0–6.2‐Ma Woodlark Basin separate its unflexed young lithosphere without a trench from the older flexed lithosphere with &gt;8‐km‐deep trenches subducting to either side. A gap evident in the seismicity and tomography between the NE‐subducted slabs of the Solomon Sea and Australia plates indicates that the Woodlark lithosphere likely melts and/or is quickly (&lt;1 m.y.) resorbed into the mantle. A section of the Pacific Plate remains attached within the gap and provides the slab pull to form the Malaita accretionary prism. Quaternary arc‐composition volcanoes that occur on both sides of the Woodlark Basin subduction front are not sourced from a mantle wedge overlying a Wadati‐Benioff zone, but come from and through breaks in the young incoming and subducting lithosphere and the overturning mantle beneath. Though the basin is too weak to sustain plate flexure, arc volcanoes on the subducting crust result in locally strong coupling to the forearc, manifest by tsunamigenic earthquakes. At 150–400‐km depth, the subducted slabs under the Solomon Islands are near vertical. They are near‐horizontal between the 410 and 660‐km mantle discontinuities beneath the Woodlark Basin and further south, whereas beneath the Solomon Sea Basin they dip southwest and penetrate the 660‐km discontinuity, having been detached by the north‐dipping Solomon Sea slab.

Subduction of Submarine Arc Volcanoes Beneath the Solomon Islands Arc

In the Solomon Islands, arc magmas are erupting on the subducting Australia Plate. These island (Simbo) and submarine arc volcanoes (Kana Keoki, Coleman and Pavuvu) are about to be recycled by rapid subduction. We identify eight of their former equivalents beneath the forearc by the morphologies and deformation structures that are characteristic of seamount subduction. Tsunamigenic earthquakes recently nucleated just ahead of two of the subducting seamounts. A third (Pavuvu), that has indented the subduction front and uplifted the lower forearc, is associated with a historic earthquake gap. It is positioned such that a rupture there has the potential for tsunami waves to impact the capital, Honiara.

Gravity Modeling to Understand the Subsurface Geology of the Central Part of West Bandung Regency (Citatah Karst Area, Cipatat-Padalarang)

Understanding the subsurface system of the central part of West Bandung Regency (including Karst Area), Indonesia, is an important undertaking carried out by geological and geophysical investigations. This research aims to identify geological subsurface features, especially faults,that have an impact on geohazards in the area. To achieve this goal, deep analysis of geology, geomorphology, and geophysical data is conducted. The central part of West Bandung regency is crossed by the Cimandiri Fault, which is located east of the transitional zone between the frontal subduction of Java and the oblique subduction of Sumatra. The preliminary result shows that the active fault has an impact on geohazards (earthquakes and landslides) in the area. Gravity reduction, regional and residual separation, as well as forward modeling with the Pluoff Equation, are all parts of the examination of the gravity data. A second-order polynomial trend surface was subtracted from the entire Bouguer gravity grid to create the residual gravity anomaly map. The amplitudes of the anomalies range from −12 to 8 mGal. The density distribution from the inverse calculation using 4 horizontal line sections (AB, CD, EF, and GH) was set up in the range of −0.3 to +0.3 g/cc. Geological surface markers for the model can be correlated with the information from the Cianjur Sheet geological map. Regional structure is also interpreted in the model to show the main structure of the reverse fault in the study area. HIGHLIGHTS The gravity method using density properties can provide good information about the subsurface karst system Modeling data gravity contributes to an understanding of geological subsurface interpretation Identifying geological subsurface features, especially faults that cause earthquakes and trigger landslides, is part of geohazard mitigation GRAPHICAL ABSTRACT

Impact of the Iquique Ridge on structure and deformation of the north Chilean subduction zone

The subduction of seamounts and basement ridges affects the structure, morphology, and physical state of a convergent margin. To evaluate their impact on the seismo-tectonic setting of the subduction zone and the tectonic development of the lower subducting and upper overriding plate, it is essential to know the precise location of subducted topographic features under the marine forearc. Offshore Northern Chile, the Iquique Ridge represents a broad zone of complex and heterogeneous structure of variable width on the oceanic Nazca Plate, which complicates attempts to project it beneath the forearc of the Chilean subduction zone. Here we use a state-of-the-art seismic reflection data processing approach to map structures related to ridge subduction under the marine forearc with unprecedented accuracy and resolution and evaluate their impact on the deformation of both the plate boundary and the upper plate. We show that significant ridge-related topography is currently subducting south of 20.5 °S and that the combined effect of horst and graben subduction with subduction of Iquique ridge-related thickened and elevated crust causes an upward bulging of the entire upper plate from the plate interface up to the seafloor as well as the presence of kilometer-scale anticlinal structures observed in multibeam bathymetric data that are approximately aligned with horsts seaward of the trench. In the area affected by the subducting ridge, a frontal prism is absent, which may relate to frontal subduction erosion caused by the excess lower plate topography. In contrast farther towards the north, where only isolated seamounts subduct, a small frontal prism and a slope/apron sediment cover down to 3000 m water depth are found.

The Gibraltar slab dynamics and its influence on past and present-day Alboran domain deformation: Insights from thermo-mechanical numerical modelling

The origin and tectonic evolution of the Gibraltar Arc system is the result of a complex geodynamic evolution involving the convergence of the Eurasian and African plates and the dynamic impact of the Gibraltar slab. Although geologic and geophysical data collected in the last few years have increased our knowledge of the Gibraltar Arc region, it is still unclear which are the mechanical links between the Gibraltar slab and the past deformation of the overriding Alboran lithosphere, as well as to which degree this subduction system is presently active. In this study, we use 2D numerical modelling to investigate the impact of the Gibraltar slab dynamics on the deformation of the overriding Alboran lithosphere. Our model simulates a WE generic vertical section at an approximate latitude of 36°N and considers an initial setup at about Burdigalian times (∼20 Ma), when the subduction front position is relatively well constrained by recent tectonic reconstructions. Our modelling shows a switch in the overriding plate (OP) stress state from extensional stresses during the slab rollback to compressional stresses near the trench when the rollback velocity decreases, caused by the change in slab-induced mantle flow. We also find that much of the crustal and lithospheric deformation occur during fast slab rollback and OP extension in the first 10 Myr of evolution, while after that only moderate deformation associated with subduction is predicted. Finally, we find that despite the subduction rollback ceases, the ongoing motion of the deeper portion of the slab induces a mantle flow that causes some amount of west-directed basal drag of the Alboran lithosphere. This basal drag generates interplate compresional stresses compatible with the distribution of intermediate-depth earthquakes in western Alboran.

Phytoplankton transport out of the euphotic zone by frontal subduction and gravitational sinking in the Santa Barbara Channel, CA, USA

Production of particulate organic carbon (POC) in nutrient-rich coastal waters over continental shelves, its export to depth, and its transport to deeper ocean waters is a poorly quantified component of the global carbon cycle. A critical step in quantifying this vertical transport is identifying shelf processes that export phytoplankton out of the euphotic zone. During cruises of the Santa Barbara Coastal Long Term Ecological Research project, we discovered substantial chlorophyll a (chl a) below the euphotic zone in the Santa Barbara Channel, a part of the southern California Current System. Observations from towed, undulating vehicles revealed deep chlorophyll layers near fronts where upwelled waters from central California converged with lower-density waters from the Southern California Bight. The mean fraction ± 1 standard deviation (SD) of chlorophyll biomass below the euphotic zone spanning the entire Santa Barbara Channel was ~7 ± 9% during 13 cruises averaged across all seasons. In one spring cruise, the fraction was ~30%, and in other cruises the layers were absent. Phytoplankton export out of the euphotic zone by subduction was indicated by spatial coherence between chl a and sloping density surfaces. Vertical plumes of chl a crossing density surfaces indicated enhanced gravitational export within cyclonic eddies. Chl a in water samples below the euphotic zone, away from fronts and cyclonic flows, suggested additional phytoplankton export. Our results emphasize the importance of subduction in the export of phytoplankton and POC out of the euphotic zone in coastal upwelling systems.

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.

Thermocline Salinity Minima Due To Wind‐Driven Differential Advection

AbstractObservations from the global ocean have long confirmed the ubiquity of thermohaline inversions in the upper ocean, often accompanied by a clear signal in biogeochemical properties. Their emergence has been linked to different processes such as double diffusion, mesoscale stirring, frontal subduction, and the recently discussed submesoscale features. This study uses the central Baltic Sea as a natural laboratory to explore the formation of salinity inversions in the thermocline region during summer. We use realistic high‐resolution simulations complemented by field observations to identify the dominant generation mechanism and potential hotspots of their emergence. We propose that the strongly stratified thermocline can host distinct salinity minima during summer conditions resulting primarily from the interaction between lateral surface salinity gradients and wind‐induced differential advection. Since this is a generic mechanism, such salinity inversions can likely constitute a typical feature of the upper ocean in regions with distinct thermoclines and shallow mixed layers.

The India‐Eurasia convergence system: Late Oligocene to early Miocene passive roof thrusting driven by deep‐rooted duplex stacking

• The India-Eurasia convergence zone hosts a set of post-Eocene tectonic events. • Enhanced duplex stacking from continuous indian underthrusting. • Passive roof thrust system promoted the tectonic exchange. The Late Oligocene-Early Miocene epoch represents a critical stage in the collisional history of the India-Eurasia dominant collision zone. Within this timeframe, a set of tectonic events occurred simultaneously or progressively. However, the key responsible for triggering these nearly coeval events remains unclear. This study used an integrated analysis of both geological and geophysical data to document the tectonic interactions throughout the Himalayan Orogenic Belt from west to east. Deep seismic reflection profiles outline crustal geometry of a decoupled Indian subduction front, and the overlying sheets over the Main Himalayan Thrust (MHT) is evidenced with a sequence of detachment-associated ramp-anticlines. Meanwhile, regional magnetotelluric (MT) profiles document rheologic connections in the form of a high-conductivity anomaly running top-to-the south between the southernmost Tibet and the areas beyond the Yarlung-Zangbo Suture Zone to the south. The overall architecture provides a complete picture of the complex deformation pattern beneath the tectonic convergence system. Together with previous studies in surface geological investigations, we propose that the enhanced duplex stacking of the underthrusting Indian crust increased crustal shortening of the convergence system. The consequent sudden exposure of the northern Himalayan domes released the accumulated stress to trigger the onset of a south-dipping passive roof thrusting through the convergence system to the southernmost Tibetan Plateau. Recognition of this exchange pattern from crustal duplex stacking to passive roof thrusting replenished an understanding of the tectonic interactions of the ongoing India-Eurasia collision.

Back‐Arc Dynamics Controlled by Slab Rollback and Tearing: A Reappraisal of Seafloor Spreading and Kinematic Evolution of the Eastern Algero‐Balearic Basin (Western Mediterranean) in the Middle‐Late Miocene

AbstractIn spite of clear fan‐shaped magnetic anomalies in the Eastern Algero‐Balearic Basin (EABB), the way how and the time when seafloor spreading occurred are still debated. In this work, a new seismo‐stratigraphic interpretation based on deep‐penetration reflection seismic data correlated to reduced‐to‐the‐pole magnetic anomalies and on‐offshore litho‐stratigraphic correlation of Pre‐Messinian units brings new constraints on its age and mode of opening. Our results reveal that the seafloor spreading of the EABB occurred at an intermediate half‐spreading rate of 3.7 ± 0.5 cm/yr for 2.45 ± 0.18 Myr during the Langhian‐Serravallian times, that is, after the Corsica‐Sardinia block rotation and the collision of Lesser Kabylia with Africa. We revise the kinematics of the Algero‐Balearic domain into three stages: (a) birth of a highly stretched continental basin accommodating the southern drift of the Kabylies driven by slab rollback between ∼23 and ∼15 Ma, (b) fast opening of a new EABB between 15.2 and 12.7 Ma by clockwise rotation of a Greater Alboran Block (GALB) promoted by slab tearing, and (c) late East‐West opening of the western basin in response to the westward retreat of the Gibraltar slab and westward slab tearing propagation and the concomitant migration of the GALB. The last stages match both the late formation of Subduction‐Transform Edge Propagator faults at the toes of the Algero‐Balearic margins and the post‐collisional volcanic migration along the Algerian margin interpreted as related to slab tearing propagation. This new scenario favors a significant stretching and splitting of the GALB into several continental fragments resulting from the westward propagation of the arcuate subduction front by lateral tearing of a narrow slab.

Metallogenic fingerprint of a metasomatized lithospheric mantle feeding gold endowment in the western Mediterranean basin

AbstractSpinel peridotite xenoliths (one plagioclase-bearing) hosted in alkaline basalts from Tallante (southeast Spain) record the mineralogical and geochemical fingerprint of the subcontinental lithospheric mantle (SCLM) evolution beneath the southern Iberian margin. Mantle metasomatism in fertile lherzolites caused the crystallization of clinopyroxene + orthopyroxene + spinel clusters through the percolation of Miocene subalkaline melts during the westward migration of the subduction front in the western Mediterranean. In the Pliocene, heat and volatiles provided by alkaline host-magmas triggered very low melting degrees of metasomatic pyroxene-spinel assemblages, producing melt quenched to silicate glass and reactive spongy coronae around clinopyroxene and spinel. Refertilization of the Tallante peridotites induced the precipitation of base-metal sulfides (BMS) included in metasomatic clino- and orthopyroxene. These sulfides consist of pentlandite ± chalcopyrite ± bornite aggregates with homogeneous composition in terms of major elements (Ni, Fe, Cu) and semi-metals (Se, As, Te, Sb, Bi), but with wide variability of platinum-group elements (PGE) fractionation (0.14 &amp;lt; PdN/IrN &amp;lt; 30.74). Heterogeneous PGE signatures, as well as the presence of euhedral Pt-Pd-Sn-rich platinum-group minerals (PGM) and/or Au-particles within BMS, cannot be explained by conventional models of chalcophile partitioning from sulfide melt. Alternatively, we suggest that they reflect the incorporation of distinct populations of BMS, PGM, and metal nanoparticles (especially of Pt, Pd, and Au) during mantle melting and/or melt percolation. Therefore, we conclude that Miocene subalkaline melts released by asthenosphere upwelling upon slab tearing of the Iberian continental margin effectively stored metals in metasomatized domains of this sector of the SCLM. Remarkably high Au concentrations in Tallante BMS (median 1.78 ppm) support that these metasomatized domains provided a fertile source of metals, especially gold, for the ore-productive Miocene magmatism of the westernmost Mediterranean.

Seismic evidence on the complex rupture system of the 23 January 2018 MW 7.9 offshore Kodiak event

In this study, we probe the formation mechanisms of the 23 January 2018 Mw7.9 Kodiak earthquake in the Gulf of Alaska region from detailed structural analysis, combined with the regional crustal stress status evaluation via seismology methods measured by the b‐value imaging. The large earthquake not only aroused heated debate due to its intraplate deformation and complex rupture system approaching to the Aleutian subduction zone, but also the complex rupture system. Aftershocks of this event propagated along the ENE‐striking and NNW‐striking fractures in planar view, whereas it shows double‐layer seismicity zones at depth. The b‐value image indicates the highly fractured crustal stress status above 12 km and above 25 km, respectively, while the faulting was locked at a depth between 12 and 20 km. We suggest that mantle peridotite serpentinization induced the aseismic zone gap. Besides, the bathymetric map, the multichannel profiles, and the sub‐bottom seismic evidence across the study area have shown the sinistral seismogenic strike‐slip faults may have been reactivated from the deep‐seated oceanic fracture zones involving the subduction front. In addition, all these faults conjunction has formed the topographic uplift or fault blocks and contribute to the complex earthquake ruptures.

Observing and quantifying ocean flow properties using drifters with drogues at different depths

AbstractThis paper presents analyses of drifters with drogues at different depths – 1, 10, 30, 50 m – that were deployed in the Mediterranean Sea to investigate frontal subduction and upwelling. Drifter trajectories were used to estimate divergence, vorticity, vertical velocity, and finite-size Lyapunov exponents (FTLEs), and to investigate the balance of terms in the vorticity equation. The divergence and vorticity are O(f) and change sign along trajectories. Vertical velocity is O(1 mm/s), increases with depth, indicates predominant upwelling with isolated downwelling events, and sometimes changes sign between 1 and 50 m. Vortex stretching is one of, but not the only, significant term in the vorticity balance. 2D FTLEs are 2 × 10−51/s after 1 day, twice larger than in a 400-m-resolution numerical model. 3D FTLEs are 50% larger than 2D FTLEs and are dominated by the vertical shear of horizontal velocity. Bootstrapping suggests uncertainty levels of ~10% of the time-mean absolute values for divergence and vorticity. Analysis of simulated drifters in a model suggests that drifter-based estimates of divergence and vorticity are close to the Eulerian model estimates, except when drifters get aligned into long filaments. Drifter-based vertical velocity is close to the Eulerian model estimates at 1 m but differs at deeper depths. The errors in the vertical velocity are largely due to the lateral separation between drifters at different depths, and partially due to only measuring at 4 depths. Overall, this paper demonstrates how drifters, heretofore restricted to 2D near-surface observations, can be used to learn about 3D flow properties throughout the upper layer of the water column.