7: Seismic evidence for the structure and Cenozoic tectonics of the Pacific Coast States

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Abstract
Processes involving interactions at the local boundaries between lithosphere plates have played a dominant role in shaping the structure and Cenozoic tectonics of the Pacific Coast States. The distribution of earthquake hypocenters indicates that contemporary deformation between the Pacific and North American plates includes a zone several hundred kilometres wide about the San Andreas fault zone between the Salton Trough and Cape Mendocino. Brittle deformation of the lithosphere in this zone, however, is confined to the upper 15 to 20 km of the crust as indicated by maximum earthquake focal depths. Low seismic activity north of Cape Mendocino suggests that subduction of the Kula plate beneath the Cascade andecitic volcanoes has either ceased or is proceeding at a very low rate. Moderate earthquake activity with focal depths as great as 40 to 80 km beneath the Cape Mendocino and Puget Sound regions at either end of the Cascade Range may, however, be related to contemporary local subduction processes. The gross crustal and upper-mantle structure based on seismic body wave data for the major tectonic provinces within the Pacific Coast States is characterized by diversity. Total crustal thickness ranges from 60 km beneath the Sierra Nevada to 25 km beneath the Columbia Plateau and 20 km beneath the Salton Trough. In general, however, crustal thickness correlates poorly with average topographic elevation, and the depth to and thickness of the intermediate layer (the lower portion of the crust with P-wave velocities between about 6.5 to 7.0 km/s) varies widely from one province to another. Yet, the gross crustal structure determined for most of the provinces satisfies an approximate condition of mass balance (isostacy) consistent with average gravity anomalies measured over the respective tectonic provinces. The exceptions, which include structures determined for the western Snake River Plain, Idaho, and the Salinian block of the central California Coast Ranges, point to the possibility of tectonically significant anomalies in simple velocity-density relations or complications in the velocity structure not resolved by available seismic data.

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