Particel size distribution and microstructures of sediments from ODP Leg 205 sites (Table 1)

Ocean Drilling Program Legs 170 and 205 offshore Costa Rica provide structural observations which support a new model for the geometry and deformation response to the seismic cycle of the frontal sedimentary prism and decollement. The model is based on drillcore, thin section, and electron microscope observations. The decollement damage zone is a few tens of meters in width, it develops mainly within the frontal prism. A clear cm-thick fault core is observed 1.6 km from the trench. The lower boundary of the fault core is coincident with the lithological boundary between the frontal prism and the hemipelagic and pelagic sediment of the Cocos plate. Breccia clast distributions in the upper portion of the decollement damage zone were studied through fractal analysis. This analysis shows that the fractal dimension changes with brecciated fragment size, implying that deformation was not accommodated by self-similar fracturing. A higher fractal dimensionality correlates with smaller particle size, which indicates that different or additional grain-size reduction processes operated during shearing. The co-existence of two distinct fracturing processes is also confirmed by microscopic analysis in which extension fracturing in the upper part of the damage zone farthest from the fault core is frequent, while both extension and shear fracturing occur approaching the fault core. The coexistence of extensional and shear fracturing seems to be best explained by fluid pressure variations in response to variations of the compressional regime during the seismic cycle. During the co-seismic event, sub-horizontal compression and fluid pressure increase, triggering shear fracturing and fluid expulsion. Fractures migrate upward with fluids, contributing to the asymmetric shape of the decollement, while slip propagates. In the inter-seismic interval the frontal prismrelaxes and fluid pressure drops. The frontal prismgoes into diffuse extension during the intervalwhen plate convergence is accommodated by creep along the ductile fault core. The fault core is typically a barrier to deformation, which is explained by its weak, but impermeable, nature. The localized development of a damage zone beneath the fault core is characterized by shear fracturing that appears as the result of local strengthening of the detachment.

Phospholipid fatty acid and quinone composition of sediments from ODP Leg 205 sites

Assessing the habitability of deep-sea sediments undergoing compaction, compression, and subduction at convergent margins adds to our understanding of the limits of the terrestrial biosphere. In this work, we report exploratory biomarker data on sediments obtained at Ocean Drilling Program (ODP) Sites 1253, 1254, and 1255 during drilling at the Costa Rica subduction trench and forearc sedimentary wedge. The samples selected for postcruise biomarker analyses were located within intervals of potentially enhanced fluid flow within the décollement and sedimentary wedge fault zones (Sites 1254 and 1255) and within basal carbonates at the reference site (Site 1253). The passage of fluids that are geochemically distinct from ambient interstitial water provides a disequilibrium setting that may enhance habitability. Biomarker data show low levels of microbial biomass in subseafloor sediments sampled at the Costa Rica convergent margin as deep as ~370 meters below seafloor.