ALBERT

Description: There are two general types of ocean-continent subduction zones forming either accretionary or erosive continental margins. In the accretionary case, the overriding continental plate grows by the accretion of material tectonically detached from the downgoing oceanic plate. In the erosive case, however, the overriding continental plate shrinks due to tectonic erosion by the downgoing oceanic plate. Two major endeavors of the International Ocean Discovery Program (IODP), the Nankai Trough Seismogenic Zone Experimen (NanTroSEIZE) and the Costa Rica Seismogenesis Project (CRISP), investigate the processes and controlling factors of deformation and seismogenesis at accretionary and erosive margins including the incidence of large magnitude earthquakes and related tsunamis. Focussing here on the Costa Rica erosive margin, we study material properties of marine sediments promoting either distributed and continuous deformation or localized and discontinuous deformation in the forearc wedge. Our results will also be compared to similar investigations on sediments from the Nankai accretionary prism. Forearc stability and inherent tectonic failure processes at active continental margins very much depend on the strength of the composing sediments. Forearc sediments can either be prone to fracturing and more localized deformation or, alternatively, to creep and distributed deformation. Strength and deformation behavior can vary significantly depending on small differences in composition and fabric of the sediments as has been shown in a similar study on samples from the Nankai trench and forearc (Stipp et al., 2013). Whole-round core samples recovered during IODP Expeditions 334 and 344 from a depth range of 7–125m below seafloor were experimentally deformed in a triaxial cell under consolidated and undrained conditions at confining pressures of 460–1000 kPa, room temperature, axial displacement rates of 0.01–0.1 mm/min, and up to axial compressive strains of ~ 45%. First results show significant differences in the consolidation state and the mechanical behavior of between upper plate and incoming plate sediments (Fig. 1). Similar to previous findings from the Nankai trench, two “rheological groups” can be distinguished: structurally weak and structurally strong samples. One sample from the incoming plate shows a previously unrecognized transition from structurally strong to structurally weak behavior at elevated confining pressure of 1000 kPa. All samples, deformed and undeformed, are designated to texture analysis via synchrotron x-ray diffraction and anisotropy of magnetic susceptibility (AMS). The observed differences in mechanical behavior may hold a key for understanding strain localization and brittle faulting in forarc regions.