Incoming Oceanic Plate Research Articles

Multiple Types of Porosity – P‐Wave Velocity Relationships for the Nankai Trough

The empirical relationship between the porosity and P-wave velocity is a useful tool for probing large-scale underground physical properties and stress states based on P-wave velocity structures acquired by seismic surveys. In this study, the porosity-velocity curves were examined using local core sample and logging datasets acquired along the Kumano transect in the Nankai Trough, Southwest Japan. We tested Hashimoto et al.’s (2010) hypothesis that slope apron and accreted sediments have different relationships. Our advantage is using a large amount of logging and core data obtained at multiple sites in the Nankai Trough under various geological conditions, from the incoming oceanic plate to the inner wedge. We identified multiple types of porosity – P-wave velocity relationships: 1) The first type agrees with the low-velocity model in the global empirical relation of Erickson and Jarrard 1998), which is observed at lithostatic stress state condition (incoming upper Shikoku basin and slope sediments), 2) The second type consistent with high-velocity models of the global empirical relation observed at compressive stress state condition (accreted sediments), which have a higher velocity and larger dependence on the porosity than the first type with the same porosity, and 3) The third type for incoming sediments with high smectite content has lower velocity than the first type. Based on our results, the transition between the first and second porosity – velocity relationships occurs in the prism toe, implying that compressive stress due to subduction controls acoustic properties and the lithification process of these sediments.

U-Pb geochronology and isotopic geochemistry of adakites and related magmas in the Ediacaran arc section of the SW Iberian Massif: The role of subduction erosion cycles in peri-Gondwanan arcs

In the peri-Gondwanan Ediacaran arc section outcropping in the SW of the Iberian Massif (Mérida region), changes in the subduction angle and subduction rate, variable participation of highly modified mantle sources together with the role of old crustal material and juvenile mafic rocks driven by the incoming slab, occur with a recognizable cyclicality and can be traced according to the geochronology, geochemistry and isotopic sources of the magmatic bodies. Our results support the initial existence of a mantle wedge highly modified due to percolation of significant volumes of variably old crustal materials, whose isotopic sources are almost identical to those found in the oldest known metasedimentary sequence described in SW Iberian Massif, the Serie Negra Group. Old crustal sources, analogous to those of this series were introduced to high depths by common subduction an also probably by significant subduction erosion. During the oldest period recorded in the study arc section, between c. 602 and c. 550 Ma, subduction episodes that involve high rates of crustal material along with an eventual incorporation of mafic and ultramafic materials from the incoming oceanic plate, favoured by low subduction angles, lead to the generation of more silicic magmas with adakitic geochemical affinity and isotopic signature (Nd-Sr) with crustal tendency. However, an increase in the subduction angle and change to a roll-back stage dated at c. 540 Ma, caused significant decrease in the access of cortical material to the subduction channel, favouring generation of typical calc-alkaline magmas derived from the modified mantle wedge, which is also consistent with more juvenile Nd-Sr ratios. The correlation found between the magmatic events and the involved tectonic processes suggests that subduction erosion mechanisms have been very likely underestimated as active dynamic processes along the peri-Gondwana margin during at least Neoproterozoic to Early Cambrian times.

Spatial variations of incoming sediments at the northeastern Japan arc and their implications for megathrust earthquakes

Abstract The nature of incoming sediments is a key controlling factor for the occurrence of megathrust earthquakes in subduction zones. In the 2011 Mw 9 Tohoku earthquake (offshore Japan), smectite-rich clay minerals transported by the subducting oceanic plate played a critical role in the development of giant interplate coseismic slip near the trench. Recently, we conducted intensive controlled-source seismic surveys at the northwestern part of the Pacific plate to investigate the nature of the incoming oceanic plate. Our seismic reflection data reveal that the thickness of the sediment layer between the seafloor and the acoustic basement is a few hundred meters in most areas, but there are a few areas where the sediments appear to be extremely thin. Our wide-angle seismic data suggest that the acoustic basement in these thin-sediment areas is not the top of the oceanic crust, but instead a magmatic intrusion within the sediments associated with recent volcanic activity. This means that the lower part of the sediments, including the smectite-rich pelagic red-brown clay layer, has been heavily disturbed and thermally metamorphosed in these places. The giant coseismic slip of the 2011 Tohoku earthquake stopped in the vicinity of a thin-sediment area that is just beginning to subduct. Based on these observations, we propose that post-spreading volcanic activity on the oceanic plate prior to subduction is a factor that can shape the size and distribution of interplate earthquakes after subduction through its disturbance and thermal metamorphism of the local sediment layer.

Seismic Structure of the Oceanic Crust Around Petit‐Spot Volcanoes in the Outer‐Rise Region of the Japan Trench

AbstractThe structure of an incoming oceanic plate in a subduction zone systematically changes toward the trench because of the flexure of the plate. Changes in seismic velocity from the outer rise toward the trench are widely considered to be caused by the development of bending‐related faults. However, few studies have focused on the effects of igneous activities preceding the subduction, which lead to structural changes of the incoming plate. Here we present active‐source seismic data acquired around petit‐spot volcanoes situated in the outer‐rise region of the Japan Trench. The resultant velocity model showed low seismic velocities (Vp < 4.0 km/s) at the top of the oceanic crust under the petit‐spot volcanoes. Moreover, the velocity reduction was significantly larger than that caused only by bending‐related faults. Our results suggest that petit‐spot volcanism is a contributing factor affecting structural variation and modification of the incoming oceanic plate in this subduction zone.

Strike-slip 23 January 2018 MW 7.9 Gulf of Alaska rare intraplate earthquake: Complex rupture of a fracture zone system

Large intraplate earthquakes in oceanic lithosphere are rare and usually related to regions of diffuse deformation within the oceanic plate. The 23 January 2018 MW 7.9 strike-slip Gulf of Alaska earthquake ruptured an oceanic fracture zone system offshore Kodiak Island. Bathymetric compilations show a muted topographic expression of the fracture zone due to the thick sediment that covers oceanic basement but the fracture zone system can be identified by offset N-S magnetic anomalies and E-W linear zones in the vertical gravity gradient. Back-projection from global seismic stations reveals that the initial rupture at first propagated from the epicenter to the north, likely rupturing along a weak zone parallel to the ocean crustal fabric. The rupture then changed direction to eastward directed with most energy emitted on Aka fracture zone resulting in an unusual multi-fault earthquake. Similarly, the aftershocks show complex behavior and are related to two different tectonic structures: (1) events along N-S trending oceanic fabric, which ruptured mainly strike-slip and additionally, in normal and oblique slip mechanisms and (2) strike-slip events along E-W oriented fracture zones. To explain the complex faulting behavior we adopt the classical stress and strain partitioning concept and propose a generalized model for large intra-oceanic strike-slip earthquakes of trench-oblique oriented fracture zones/ocean plate fabric near subduction zones. Taking the Kodiak asperity position of 1964 maximum afterslip and outer-rise Coulomb stress distribution into account, we propose that the unusual 2018 Gulf of Alaska moment release was stress transferred to the incoming oceanic plate from co- and post-processes of the nearby great 1964 MW 9.2 megathrust earthquake.

Three-dimensional variations of the slab geometry correlate with earthquake distributions at the Cascadia subduction system

Significant along-strike variations of seismicity are observed at subduction zones, which are strongly influenced by physical properties of the plate interface and rheology of the crust and mantle lithosphere. However, the role of the oceanic side of the plate boundary on seismicity is poorly understood due to the lack of offshore instrumentations. Here tomographic results of the Cascadia subduction system, resolved with full-wave ambient noise simulation and inversion by integrating dense offshore and onshore seismic datasets, show significant variations of the oceanic lithosphere along strike and down dip from spreading centers to subduction. In central Cascadia, where seismicity is sparse, the slab is imaged as a large-scale low-velocity feature near the trench, which is attributed to a highly hydrated and strained oceanic lithosphere underlain by a layer of melts or fluids. The strong correlation suggests that the properties of the incoming oceanic plate play a significant role on seismicity.

Three-dimensional imaging of impact of a large igneous province with a subduction zone

How the thickened crust of a large igneous province on an incoming oceanic plate is accommodated at a subduction zone remains an open question. New Zealand is one of the few places to study this, as at ca. 105 Ma the ca. 35 km-thick Hikurangi Plateau impacted the Gondwana subduction zone in what is now the South Island. Here we report on results from a forty-station portable seismograph array in the southern South Island, designed to delineate the leading edge of the subducted plateau. Three-dimensional images of Vp and Vp/Vs reveal the southwestern part of the plateau was a relatively narrow salient, and the first part to be subducted. The plateau then rotated clockwise about this salient until the southern edge of the plateau was parallel to subduction strike and subduction ceased at ca. 100 Ma. Our results suggest that the global-scale plate reorganization event at 105–100 Ma was due to a cessation of subduction caused by the Hikurangi Plateau choking the Gondwana subduction zone, rather than the subduction of mid ocean ridges as previously proposed. The choking of Gondwana subduction by the plateau also led to a concentration of slab pull in the adjacent subducted oceanic crust, explaining the episode of basin opening and intraplate magmatism there that occurred at the same time. Our study underlines the havoc caused by impact of a large igneous province with a subduction zone.

Spatiotemporal evolution of slow slip events in a nonplanar fault model for northern Cascadia subduction zone

AbstractSlow slip events (SSEs) are identified as the quasi‐stable fault deformation in the deep transition zone from locked to continuous sliding in many subduction zones. In the well‐instrumented Cascadia margin, a class of Mw6.0 slow slip events arise beneath Port Angeles every ∼14 months, as inferred from two decades of continuous geodetic monitoring. The along‐strike bending of the incoming oceanic plate beneath north Washington is a unique geometric feature whose influence on slow slip processes is still unknown. Here we incorporate a realistic fault geometry of northern Cascadia in the framework of rate‐ and state‐dependent friction law, to simulate the spatiotemporal evolution of slow slip events on a nonplanar subduction fault. The modeled SSEs capture the major characteristics revealed by GPS observations. The central 150 km long fault segment beneath Port Angeles acts as a repetitive slip patch, where SSEs appear every ∼1.5 years with a maximum slip of ∼2.5 cm. Two minor slip patches with smaller areas and cumulative slips straddle this central slip patch. The along‐strike segmentation of slow slip is inversely related to the local fault dip and strike angles of the slow slip zone, suggesting strong geometrical control on the slow slip process. This correlation holds even after removing the effect of W/h∗, ratio between velocity‐weakening SSE fault width and characteristic nucleation size. Besides the GPS‐detectable fast‐spreading phase, we find that each SSE cycle consists of deep pre‐SSE preparation and post‐SSE relaxation phases, which may be the driving mechanism for the deep tremor activity between major SSE episodes discovered in Cascadia.

Fluid flow and water–rock interaction across the active Nankai Trough subduction zone forearc revealed by boron isotope geochemistry

Compositional changes, dehydration reactions and fluid flow in subducted sediments influence seismogenesis and arc magmatism in subduction zones. To identify fluid flow and water–rock interaction processes in the western Nankai Trough subduction zone (SW Japan) we analyzed boron concentration and boron isotope composition (δ11B) of pore fluids sampled across the subduction zone forearc from depths of up to ∼922m below seafloor during four Integrated Ocean Drilling Program (IODP) Expeditions. The major structural regimes that were sampled by coring include: (1) sedimentary inputs, (2) the frontal thrust zone, (3) the megasplay fault zone, and (4) the forearc basin. From mass balance consideration we find that consumption of boron (B) by ash alteration and desorption of B from the solid phase, mediated by organic matter degradation, produces a net decrease in B concentrations with depth down to ∼120μM and variable δ11B values in the range of ∼+20‰ and +49‰. Interstitial water in sediments on the incoming oceanic plate are influenced by more efficient mobilization of exchangeable B from the solid phase due to higher temperatures and alteration of the oceanic crust that acts as a sink for 10B. At the tip of the megasplay fault zone, elevated B concentration and B isotopic composition suggest that underthrust coarse-grained slope sediments provide a pathway for fluids out of the upper (<2km) accretionary prism. Silt and sand layers in the underthrust section of the downgoing plate favor fluid escape in seaward direction from depths equivalent to the temperature range of 60–150°C. At both locations the δ11B signature evolves during updip migration through re-adsorption. Mass balance considerations suggest a shallower fluid source depth compared to pore fluids sampled previously near the décollement zone along the central portion of the Nankai margin.

Along‐trench variations in the seismic structure of the incoming Pacific plate at the outer rise of the northern Japan Trench

AbstractTo investigate along‐trench variations in the seismic structure of the incoming oceanic plate and their effect on water transportation by the oceanic plate, we conducted a wide‐angle seismic survey of a trench‐parallel transect 270 km long on the outer rise of the northern Japan Trench. The resulting seismic structure models show that the central part of the transect is characterized by rough topography, thick oceanic crust, low seismic velocities, and high Vp/Vs ratios, suggesting pervasive fracturing and high water content (hydration) there. These observations are consistent with the presence of an ancient fracture zone associated with ridge propagation. The trenchward extension of this fracture zone corresponds to an area of low interplate seismicity, low seismic velocities, and high Vp/Vs ratio around the depth of the subduction interface. Our results suggest that this ancient scar on the oceanic plate influences along‐trench variations in interplate seismic coupling through its effect on water transportation.

First measurement of the displacement rate of the Pacific Plate near the Japan Trench after the 2011 Tohoku-Oki earthquake using GPS/acoustic technique

The subduction rate of an oceanic plate may accelerate after large earthquakes rupture the interplate coupling between the oceanic and overriding continental plates. To better understand postseismic deformation processes in an incoming oceanic plate, we directly measured the displacement rate of the Pacific Plate near the Japan Trench after the 2011 Tohoku-Oki earthquake using a GPS/acoustic technique over a period of 2 years (September 2012 to September 2014). The displacement rate was measured to be 18.0 ± 4.5 cm yr−1 (N302.0°E) relative to the North American Plate, which is almost twice as fast as the predicted interseismic plate motion. Because the sum of steady plate motion and viscoelastic response to the Tohoku-Oki earthquake roughly accounts for the observed displacement rate, we conclude that viscoelastic relaxation is the primary mechanism responsible for postseismic deformation of the Pacific Plate and that significant subduction acceleration did not occur at least not during the observation period.

Background seismicity rate at subduction zones linked to slab‐bending‐related hydration

AbstractTectonic properties strongly control variations in seismicity among subduction zones. In particular, fluid distribution in subduction zones influences earthquake occurrence, and it varies among subduction zones due to variations in fluid sources such as hydrated oceanic plates. However, the relationship between variations in fluid distribution and variations in seismicity among subduction zones is unclear. Here we divide Earth's subduction zones into 111 regions and estimate background seismicity rates using the epidemic type aftershock sequence model. We demonstrate that background seismicity rate correlates to the amount of bending of the incoming oceanic plate, which in turn is related to the hydration of oceanic plates via slab‐bending‐related faults. Regions with large bending may have high‐seismicity rates because a strongly hydrated oceanic plate causes high pore fluid pressure and reduces the strength of the plate interface. We suggest that variations in fluid distribution can also cause variations in seismicity in subduction zones.

Bending-related faulting of the incoming oceanic plate and its effect on lithospheric hydration and seismicity: A passive and active seismological study offshore Maule, Chile

We have studied the dependency between incoming plate structure, bending-related faulting, lithospheric hydration, and outer rise seismic activity offshore Maule, Chile. We derived a 2D Poisson's ratio distribution from P- and S-wave seismic wide angle data collected in the trench-outer rise. High values of Poisson's ratio in the uppermost mantle suggest that the oceanic lithosphere is highly hydrated due to the water infiltration through bending-related normal faults outcropping at the seafloor. This process is presumably facilitated by the presence of a seamount in the area. We conclude that water infiltrates deep into the lithosphere, when it approaches the Chile trench, producing a reduction of crustal and upper mantle velocities, supporting serpentinization of the upper mantle. Further, we observed a mantle Vp anisotropy of 8%, with the fast velocity axis running normal to the abyssal hill fabric and hence in spreading direction, indicating that outer rise processes have yet not affected anisotropy.The first weeks following the megatrust Mw=8.8 Maule earthquake in 2010 were characterized by a sudden increase of the outer rise seismic activity, located between 34°S and 35°30′S. We concluded that this phenomenon is a result of an intensification of the water infiltration process in the outer rise, presumably triggered by the main shock, whose epicenter was located some 100km to the south east of the cluster.

The thermal structure of the subduction thrust within accretionary and erosive margins

We investigate differences in the thermal structure of the subduction thrust between accretionary and erosive margins using a finite element model. Global averages of plate margin geometries, sediment thickness, plate age, and convergence rate are used to construct generic models of accretionary and erosive margins. Of these parameters, our analysis shows that the largest uncertainty in these models is the geotherm for the incoming oceanic plate. Despite these uncertainties, the subduction thrust of similarly aged accretionary margins is slightly warmer than erosive margins primarily due to the effect of sediment insulating the subduction thrust at accretionary margins. If the updip limit of seismicity is thermally controlled, warmer accretionary margins suggest shallower seismogenic updip limits, counter to the observation that erosional margins are more likely to generate tsunamigenic earthquakes. This discrepancy can be reconciled if frictional heat generation at erosive margins is larger than at accretionary margins.

Effect of time-evolving age and convergence rate of the subducting plate on the Cenozoic adakites and boninites

Partial melting of subducting oceanic crust expressed as high-Mg volcanic rocks such as adakites and boninites has been actively studied for decades, and Lee and King (2010) reported that time-evolving subduction parameters such as the age and the subduction rate of the converging oceanic plate play important roles in transient partial melting of the subducting oceanic crust (e.g., Aleutians). However, few subduction model experiments have considered time-evolving subduction parameters, posing problems for studies of transient partial melting of subducting oceanic crust in many subduction zones. Therefore, we constructed two-dimensional kinematic–dynamic subduction models for the Izu–Bonin, Mariana, Northeast Japan, Kuril, Tonga, Java–Sunda, and Aleutian subduction zones that account for the last 50Myr of their evolution. The models include the time-evolving age and convergence rate of the incoming oceanic plate, so the effect of time-evolving subduction parameters on transient partial melting of oceanic crust can be evaluated. Our model calculations revealed that adakites and boninites in the Izu–Bonin and Aleutian subduction zones resulted from transient partial melting of oceanic crust. However, the steady-state subduction model using current subduction parameters did not produce any partial melting of oceanic crust in the aforementioned subduction zones, indicating that time-evolving subduction parameters are crucial for modeling transient eruption of adakites and boninites. Our model calculations confirm that other geological processes such as forearc extension, back-arc opening, mantle plumes and ridge subduction are required for partial melting of the oceanic crust in the Mariana, Northeast Japan, Tonga, and southeastern Java–Sunda subduction zones.

Hydrothermal heat mining in an incoming oceanic plate due to aquifer thickening: Explaining the high heat flow anomaly observed around the Japan Trench

To explain the origin of a high heat flow anomaly observed within 150 km seaward of the Japan Trench, we construct a thermal model for an oceanic plate prior to subduction that includes the effect of hydrothermal circulation within a high-permeability aquifer in its uppermost part. The model includes the effects of aquifer thickening, which is expected to occur near subduction zones where plate bending prior to subduction causes fracturing and faulting within the oceanic plate. Using typical parameter values for the Japan Trench, we find that hydrothermal circulation in the thickening aquifer mines heat from the underlying basement and can account for the observed high heat flow anomaly. The ratio of heat supply below the aquifer as a result of aquifer thickening to the inverse of the thermal resistance of the sediment layer is a control parameter for the system. As long as the aquifer permeability is higher than ∼10−13 m2, a typical value for the uppermost part of the oceanic plate, variations in other details of the hydrothermal circulation such as the exact value of the aquifer permeability and the size of the convection cells do not significantly change model results. Despite its strong influence on seafloor heat flow seaward of the trench, this hydrothermal heat mining does not affect significantly the thermal structure of the subducted oceanic plate. This finding indicates that surface heat flow anomaly around the trench may not correspond to temperature anomaly within the subducted oceanic plate and the megathrust seismogenic zone.

Marine cold seeps and their manifestations: geological control, biogeochemical criteria and environmental conditions

Characteristics of cold seeps at different geologic settings are the subject of this review primarily based on results of the Research Consortium SFB 574. Criteria are drawn from examples on the erosive convergent margin off Costa Rica, the accretionary margin off Chile supplemented by examples from the transform margin of the Golf of Cadiz and the convergent Hikurangi margin off New Zealand. Others are from well-studied passive margins of the Black Sea, the Golf of Mexico, the eastern Mediterranean Sea and the South China Sea. Seeps at all settings transport water and dissolved compounds to the ocean through the seafloor by different forcing mechanism and from different depths of the submerged geosphere (10s of meters to 10s of km). The compounds sustain oasis-type ecosystems by providing bioactive reductants sulfide, methane and hydrogen. Hereby, the interaction between fluid composition, flux rates and biota results in a diagnostic hydrocarbon–metazoan–microbe–carbonate association; currently, well over 100 active sites are known. The single most important reaction is microbially mediated anaerobic oxidation of methane with secondary reactions involving S-biogeochemistry and carbonate mineral precipitation. Seep fluids and their seafloor manifestations provide clues as to source depth, fluid–sediment/rock interaction during ascent, lifetime and cyclicity of seepage events but less so on the magnitude of return flow. At erosive margins, Cl-depleted and B-enriched fluids from clay dehydration provide criteria for source depth and temperature. The upward material flow generates mud volcanoes at the seafloor above the projected location of dehydration at depth. At accretionary margins, fluids are derived from more shallow depths by compaction of sediments as they ride on the incoming oceanic plate; they are emitted through thrust faults. At highly sedimented margins, organic-rich and evaporite-containing strata (when present) determine the final fluid composition, by emitting characteristically gas hydrate-derived methane, brine-associated non-methane hydrocarbons or leached elements and their isotopes (Li, δ7Li, B, Ba) from host sediments. Smectite–illite transformation and associated Cl-depletion from release of interlayer water is a pervasive process at these margins. Rare earth element pattern in conjunction with redox-sensitive metals retained in seep carbonates indicate whether or not they precipitated in contact with oxic bottom water or suboxic fluids; clear environmental characterization, though, currently remains inconclusive. More deeply sourced fluids as in transform margins may be characterized by their 87Sr/86Sr ratios from interaction with oceanic crustal rocks below. Quantification of flow and reliable estimates of total volatile output from fore-arcs remain a challenge to seep research, as does understanding the role of geologically derived methane in the global methane cycle.

Systematic changes in the incoming plate structure at the Kuril trench

Recent seismic structural studies in trench‐outer rise regions have shown that Vp within the incoming oceanic plate systematically decreases toward the trench, probably owing to bending and fracturing of the plate. To understand the mechanisms acting to reduce Vp, Vs is critical because the Vp/Vs ratio is a sensitive indicator of lithology, porosity, and the presence of fluid. In the outer rise region of the Kuril trench, we conducted an extensive seismic refraction and reflection survey that revealed systematic changes in Vp, Vs, and Vp/Vs. Our results suggest that water content within the incoming oceanic plate increases toward the trench accompanied by the development of bending‐related fractures at the top of the oceanic crust, consistent with the seawater percolation. Our results support the idea that plate bending and fracturing during the bending in the outer rise of the trench play an important role in the water cycle of subduction zones.

Effects of heterogeneous hydration in the incoming plate, slab rehydration, and mantle wedge hydration on slab-derived H2O flux in subduction zones

We quantify the effects of heterogeneous hydration in the incoming oceanic plate, rehydration in the subducting slab, and hydration in the lowermost mantle wedge on the pattern of H2O release from the slab in the forearc and arc regions of subduction zones. The release and uptake of H2O by dehydration and hydration reactions, respectively, are calculated using a subduction zone thermal model and thermodynamic calculations for the distribution of mineralogically bound H2O for idealized lithologies. We consider two end-member models for the initial hydration state of the incoming plate: (1) uniform hydration, in which H2O is stored homogeneously within each lithologic unit, and (2) localized hydration, in which H2O is stored at its saturation level in discrete zones. The former is commonly assumed in thermo-petrologic models, while the latter approximates the effect of localized hydration along faults in the oceanic plate inferred from geophysical observations. Our modeling results show that for the same bulk H2O content, localized hydration results in shallower H2O release compared to uniform hydration, and that the H2O flux off the subducting slab beneath the forearc and arc regions can be almost twice as large from a locally hydrated slab as from a uniformly hydrated slab. Rehydration can occur in the subducting slab and delays the liberation of H2O. The effect of pervasive rehydration in the slab on the pattern of fluid release in the forearc and arc regions is relatively small if the slab is uniformly hydrated, but it is large if the slab is locally hydrated. Hydration of the overlying mantle also delays the liberation of H2O, but the volume of H2O absorbed in the mantle is small compared to that released from the subducting slab, thereby implying that mantle wedge hydration has only a modest effect on the pattern of fluid release.

Electromagnetic detection of plate hydration due to bending faults at the Middle America Trench

Water plays an important role in the processes occurring at subduction zones since the release of water from the downgoing slab impacts seismicity and enhances arc volcanism. Geochemical indicators suggest that the Nicaraguan slab is anomalously wet, yet the mechanism of slab hydration remains poorly constrained. Extensional bending faults on the incoming oceanic plate of the Middle America Trench offshore Nicaragua have been observed to penetrate to mantle depths, suggesting a permeable pathway for hydration of the crust and serpentinization of the upper mantle. Low seismic velocities observed in the uppermost mantle of the incoming plate have been explained as serpentinization due to deep fluid penetration but could also be explained by intrinsic anisotropy and fractures in the absence of fluid circulation. Here we use controlled-source electromagnetic imaging to map the electrical resistivity of the crust and uppermost mantle along a 220km profile crossing the trench offshore Nicaragua. Along the incoming plate our data reveal that crustal resistivity decreases by up to a factor of five directly with the onset of the bending faults. Furthermore, a strong azimuthal anisotropy compatible with conductive vertical fault planes is observed only on the faulted trench seafloor. The observed resistivity decrease and anisotropy can be explained by a porosity increase along vertical fault planes, which we interpret as evidence that the lithospheric bending faults provide the necessary permeable fluid pathways required for serpentinization of the uppermost mantle. This implies that most serpentinisation happens at the trench, with the width of the faulting region and the density of fractures controlling the extent of upper mantle alteration. This observation explains why the heavily faulted trench offshore Nicaragua is associated with an anomalously wet slab, whereas other sections of the Middle America Trench containing fewer bending faults have less fluid flux from the subducting slab.