Magnetic fabrics from the Costa Rica margin: sediment deformation during the initial dewatering and underplating process

Drilling off of the Costa Rica margin during Ocean Drilling Program Leg 170 penetrated the complete sediment section seaward of the Middle Americas Trench (MAT) at one location (Site 1039), and sections through the margin wedge, décollement, and subducting sediments at two locations (Sites 1040 and 1043) near the toe of the wedge. Analyses of these sediments and their structures indicated that the margin wedge is not constructed by offscraping of the present incoming sediments, but is an older (perhaps accreted) structure. Nearly the entire section of sediments entering the MAT is thrust beneath the margin wedge. Anisotropy of magnetic susceptibility (AMS) and paleomagnetic data provide insight into the initial stages of fabric development associated with formation of the active décollement fault. Within the décollement zone, fabric development is localized to the upper few meters of the décollement, as indicated by higher AMS P values and by fabric orientations. Below the décollement, the uppermost 180 m of the underthrust hemipelagic sediment section is also deformed, as indicated by comparison of fabrics and their orientations between the seaward reference site and the sites that penetrated the décollement. AMS foliations become progressively more anisotropic, and the orientation of these fabrics, following reorientation of the cores using paleomagnetism, dip landward within the underthrust sediment package. These data indicate that deformation involves thinning of the incoming sediment package by compaction produced by distributed shearing of the upper portion of the sediment package. The fabric data suggest that the uppermost 180 m of the underthrust sediments are undergoing incipient underplating beneath the margin wedge.