The Okitsu mélange in the Shimanto accretionary complex, the onshore extension of the modern Nankai accretionary prism, consists of a kilometer-size duplex of oceanic basalt and trench-fill sedimentary rocks, and is thought to represent rocks underplated to the prism along the subduction plate-boundary at seismogenic depth. An internal, horsebounding thrust of the duplex, referred to as the Suzume fault, juxtaposes basalt in the hanging wall and sedimentary rocks in the footwall. Structure and fabric of the fault was characterized at the mesoscale to investigate the processes and structural evolution along a plate-boundary décollement. The fault zone in the hanging wall consists of decimeterthick ultracataclasite bounded by a several m thick zone of fractured basalt, and likely records 2+ km displacement along the thrust. The footwall consists of decimeter-thick ultracataclasite bounded by a 20-m-thick zone of ductile shear in flattened sedimentary host rock, and likely records 30+ km of displacement. The asymmetric structure across the Suzume fault, as well as inferred displacement fields and timing relations, are consistent with a tectonic model in which the footwall records early ductile, compactive deformation of poorly consolidated sediments during underthrusting at the prism toe region, followed by extremely localized cataclasis at the underplating depth. In contrast, the hanging wall is deformed by intense cataclasis, and only during underplating. Deformation style and strain state in the footwall of the Suzume fault is qualitatively similar to the modern Costa Rica underthrust section at the toe region. Similarity in the structure and fabric of the hanging wall between the Suzume fault and modern décollement zones sampled through scientific drilling suggests that intense cataclasis under horizontal contraction likely is a common feature for the hanging wall of the décollement zone throughout the toe to underplating regions. Structures in the Suzume fault that are not in common with the modern décollements imply progressive consolidation during underthrusting from the toe to underplating depths may be responsible for the localization of shear in the footwall. At several kilometers depth, displacement along the plate boundary is likely accommodated within an extremely narrow zone as recorded in the ultracataclasite of the Suzume fault.

abstract: The study of fault zones is a critical component to understanding earthquake mechanics and seismic hazard evaluations. Models or simulations of potential earthquakes, based on fault zone properties, are a first step in mitigating the hazard. Theoretical models of earthquake ruptures along a bi-material interface result in asymmetrical damage and preferred rupture propagation direction. Results include greater damage intensity within stiffer material and preferred slip in the direction of the more compliant side of the fault. Data from a dense seismic array along the Clark strand of the SJFZ at Sage Brush Flat (SGB) near Anza, CA, allows for analysis and characterization of shallow (<1km depth) seismic structure and fault zone properties. Results indicate potential asymmetric rock damage at SGB, similar to findings elsewhere along the SJFZ suggesting an NW preferred rupture propagation. In this study, analysis of high resolution topography suggests asymmetric morphology of the SGB basin slopes are partially attributed to structural growth and fault zone damage. Spatial distributions of rock damage, from site mapping and fault perpendicular transects within SGB and Alkali Wash, are seemingly asymmetric with pulverization dominantly between fault strands or in the NE fault block. Remapping of the SJFZ through Alkali Wash indicates the fault is not isolated to a single strand along the main geologic boundary as previously mapped. Displacement measurements within SGB are analogous to those from the most recent large earthquake on the Clark fault. Geologic models from both a 3D shear wave velocity model (a product from the dense seismic array analysis) and lithologic and structural mapping from this study indicate surface observations and shallow seismic data compare well. A synthetic three-dimensional fault zone model illustrates the complexity of the structure at SGB for comparison with dense array seismic wave products. Results of this study generally agree with findings from seismic wave interpretations suggesting damage asymmetry is controlled by a NW preferred rupture propagation. / Dissertation/Thesis / Geologic Map of Sage Brush Flat / 3D fault zone model of the SJFZ at Sage Brush Flat / Masters Thesis Geological Sciences 2018