The dehydration of subducting oceanic crust and upper mantle has been inferred both to promote the partial melting leading to arc magmatism and to induce intraslab intermediate-depth earthquakes, at depths of 50-300 km. Yet there is still no consensus about how slab hydration occurs or where and how much chemically bound water is stored within the crust and mantle of the incoming plate. Here we document that bending-related faulting of the incoming plate at the Middle America trench creates a pervasive tectonic fabric that cuts across the crust, penetrating deep into the mantle. Faulting is active across the entire ocean trench slope, promoting hydration of the cold crust and upper mantle surrounding these deep active faults. The along-strike length and depth of penetration of these faults are also similar to the dimensions of the rupture area of intermediate-depth earthquakes.

[1] Fluid distribution in convergent margins is by most accounts closely related to tectonics. This association has been widely studied at accretionary prisms, but at half of the Earth's convergent margins, tectonic erosion grinds down overriding plates, and here fluid distribution and its relation to tectonics remain speculative. Here we present a new conceptual model for the hydrological system of erosional convergent margins. The model is based largely on new data and recently published observations from along the Middle America Trench offshore Nicaragua and Costa Rica, and it is consistent with observations from other erosional margins. The observations indicate that erosional margins possess previously unrecognized distinct hydrogeological systems: Most fluid contained in the sediment pores and liberated by early dehydration reactions drains from the plate boundary through a fractured upper plate to seep at the seafloor across the slope, rather than migrating along the décollement toward the deformation front as described for accretionary prisms. The observations indicate that the relative fluid abundance across the plate-boundary fault zone and fluid migration influence long-term tectonics and the transition from aseismic to seismogenic behavior. The segment of the plate boundary where fluid appears to be more abundant corresponds to the locus of long-term tectonic erosion, where tectonic thinning of the overriding plate causes subsidence and the formation of the continental slope. This correspondence between observations indicates that tectonic erosion is possibly linked to the migration of overpressured fluids into the overriding plate. The presence of overpressured fluids at the plate boundary is compatible with the highest flow rates estimated at slope seeps. The change from aseismic to seismogenic behavior along the plate boundary of the erosional margin begins where the amount of fluid at the fault declines with depth, indicating a control on interplate earthquakes. A previously described similar observation along accreting plate boundaries strongly indicates that fluid abundance exerts a first-order control on interplate seismogenesis at all types of subduction zones. We hypothesize that fluid depletion with depth increases grain-to-grain contact, increasing effective stress on the fault, and modifies fault zone architecture from a thick fault zone to a narrower zone of localized slip.Components: 9574 words, 6 figures.

[1] A seismic wide-angle and refraction experiment was conducted offshore of Nicaragua in the Middle American Trench to investigate the impact of bending-related normal faulting on the seismic properties of the oceanic lithosphere prior to subduction. On the basis of the reflectivity pattern of multichannel seismic reflection (MCS) data it has been suggested that bending-related faulting facilitates hydration and serpentinization of the incoming oceanic lithosphere. Seismic wide-angle and refraction data were collected along a transect which extends from the outer rise region not yet affected by subduction into the trench northwest of the Nicoya Peninsula, where multibeam bathymetric data show prominent normal faults on the seaward trench slope. A tomographic joint inversion of seismic refraction and wide-angle reflection data yield anomalously low seismic P wave velocities in the crust and uppermost mantle seaward of the trench axis. Crustal velocities are reduced by 0.2-0.5 km s À1 compared to normal mature oceanic crust. Seismic velocities of the uppermost mantle are 7.6-7.8 km s À1 and hence 5-7% lower than the typical velocity of mantle peridotite. These systematic changes in P wave velocity from the outer rise toward the trench axis indicate an evolutionary process in the subducting slab consistent with percolation of seawater through the faulted and fractured lithosphere and serpentinization of mantle peridotites. If hydration is indeed affecting the seismic properties of the mantle, serpentinization might be reaching 12-17% in the uppermost 3-4 km of the mantle, depending on the unknown degree of fracturing and its impact on the elastic properties of the subducting lithosphere.Citation: Ivandic, M., I. Grevemeyer, A. Berhorst, E. R. Flueh, and K. McIntosh (2008), Impact of bending related faulting on the seismic properties of the incoming oceanic plate offshore of Nicaragua,

We present a new travel time tomography velocity model and seismic reflection images that delineate the rift architecture and magmatic features of the rifted margin in the northeastern South China Sea. These data reveal moderately stretched crust~25 km thick along the continental shelf and thin but laterally variable crustal thickness in the distal margin. Along the continental slope, crust rapidly thins tõ 4 km in a basin characterized by tilted fault blocks that sole into a low-angle detachment. Strain was localized to a degree within the highly stretched basin but failed to progress to breakup and seafloor spreading. Crust in the distal margin is~12-15 km thick. Few extensional structures are apparent in the distal margin, but seismic velocities are suggestive of highly thinned and magmatically intruded continental crust. The magmatic features we interpret include volcanic zones at the top of the basement that deform or disrupt overlying postrift strata, sills intruded into the postrift sedimentary section, and a high-velocity (~6.9-7.5 km/s) lower crustal layer that we take to be magmatic underplating or pervasive lower crustal intrusions. These features primarily occur in the distal margin and may have been emplaced during postrift seafloor spreading. The postrift magmatism may have been induced by convective removal of continental lithosphere following breakup and the onset of seafloor spreading in the South China Sea.

Conventional two‐dimensional seismic reflection investigations have been generally relied upon to provide images of large to medium scale structural features in accretionary prisms. We undertook a three‐dimensional seismic reflection survey of a small part of a prism arcward of the Middle America Trench off Costa Rica to more correctly image structure and to use the improved structural information to examine the processes of accretion. This survey reveals small features, with dimensions of hundreds of meters, while also defining features thousands of meters in lateral extent, both of which were underappreciated in conventional two‐dimensional data from the same area. We have imaged active off scraping at the trench and both duplexing and out‐of‐sequence faulting a few kilometers arcward of the trench. Fault spacing and reflector geometry vary dramatically over a space of several hundred meters. Some of these variations are related to visible changes in morphology of the underlying oceanic basement, but others are not so easily documented. Fault surface reflections define an architecture which may control gross fluid motion through the prism. This architecture is apparently formed by duplexing and out‐of‐sequence faulting and has been maintained by periodic motion on some of the out‐of‐sequence faults. The slope sediment apron records multiple phases of deformation. Abundant small offset reverse faults break the seafloor and indicate recent shortening of a broad region of the underlying prism. A primary result of this survey is appreciation of the structural diversity across a small width of an accretionary prism.