Graduate Thesis Or Dissertation
 

Diffuse deformation patterns along the North American plate boundary zone, offshore western United States

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  • Plate boundaries are commonly regions of complex, diffuse deformation with the motion across the boundary accommodated by numerous structural systems, rather than being narrow, discrete zones of deformation. One such boundary occurs where the North American plate makes contact with Juan de Fuca, Gorda, and Pacific plates along the west coast of the United States, forming a wide zone of deformation which crosses the ocean-continent transition. Two offshore regions associated with this boundary zone, the Gorda plate and California Continental Borderland, have undergone significant deformation in order to accommodate the changing relative tectonic motion across the boundary. The Gorda plate, seaward of the Cascadia Subduction Zone is deforming as a modified, vertically-hinged flexural-slip buckle that utilizes, via-reactivation, the relict spreading-fabric faults and newly-formed leftlateral strike-slip faults that cut the original fabric. In this way, it appears that the Gorda plate, as the youngest and weakest plate in the system, absorbs the motion of the surrounding plates, essentially buffering the strain accumulation in other regions. In southern California, the plate boundary is dominated by the San Andreas transform system, but with deformation distributed over structures within a zone whose width west of the main San Andreas fault exceeds 300 km. While much of this deforming zone is onshore, a significant component is offshore within the California Borderland and Western Transverse Ranges provinces. Deformation related to the growth of the San Andreas fault system within the Borderland has been ongoing since the late Oligocene, resulting in a poly-deformed terrane of distributed deformation, reflecting the shift from subduction to a highly evolved transform system. Two areas within the Borderland illuminate aspects of the regions complex history: the intersection of the Santa Cruz-Catalina Ridge and northern Channel Islands platform which reflect the current transpressional tectonic regime, and Dall Bank within the Outer Borderland which contains evidence of earlier (Oligocene-Miocene) deformational phases.
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