Texture development of clay-rich sediments across the Costa Rica subduction zone

Abstract

During sedimentation, burial, and deformation at active continental margins, clay-rich sediments develop crystallographic preferred orientations (textures) due to the progressive alignment of phyllosilicates. Such textures help to interpret sedimentation and compaction conditions as well as tectonic processes at convergent margins. At the Costa Rica Trench, subduction and plate boundary deformation between the downgoing oceanic Cocos Plate and the overriding Caribbean Plate was investigated during Integrated Ocean Drilling Program Expeditions 334 and 344 within the Costa Rica Seismogenesis Project. Samples of varying depths from the Cocos Plate, the frontal prism, and the slope of the Caribbean Plate were analyzed regarding their composition and texture. Composition is quite similar for all sample locations of the hemipelagic section across the trench as determined by X-ray powder analysis. Texture analysis reveals that phyllosilicates in samples from the incoming plate show in general weaker textures than those from upper and middle slope of the overriding plate. Samples from the frontal accretionary prism, however, mostly correspond to the incoming plate fabric according to their oceanic origin. Texture intensity depends on the internal parameters grain size and shape, porosity, and composition as well as compaction and tectonics. In samples from the continental wedge and the frontal accretionary prism, we are able to distinguish tectonically undisturbed compacted sediments from core sections that suffered faulting and folding due to subduction-related deformation. This helps to constrain a more detailed image of sedimentary compaction and localized as well as distributed deformation across the active continental margin offshore Costa Rica.