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Science for Decisionmaking: Coastal and Marine Geology at the U.S. Geological Survey
types of geologic processes that have operated, and are still operating, in the nearshore and offshore along the continental margins. Through time, this diversity also results in variations in the distributions of common coastal features, rivers, aquifers, marine and coastal habitat, and marine resources.
This variability is perhaps most easily seen by comparing the geologic structure of the areas making up the continental margins of the United States, which can be categorized into eight major provinces:
Province 1—The Pacific Northwest
A volcanically and tectonically active province that includes a spreading center and a subductive compressive margin. The province is characterized by:
volcanic and earthquake processes and massive active margin faulting;
extensive hydrothermal activity along the spreading center, resulting in the formation of metal-rich sulfide deposits and chemosynthesis-based biological communities;
areas of simultaneous rapid uplift and subsidence;
a glacially shaped margin with a major river depositing large quantities of sediment that builds the edge of continental margin; and
strong littoral currents, high persistent wave energy, and periodic tsunamis.
Province 1 is one of the most complex, dynamic, and least understood of the U.S. continental margin. A systems-science view of the area starts with understanding the active oceanic rift generating new oceanic crust. This young, thin crust is being subducted under the continent and the subsequent melt zone forms a line of live volcanoes from northern California to British Columbia. Thus, a tectonic system is operating from the spreading center in the west to the compressive folds under the shelf and beach and to the active volcanoes.
The present coastline runs at right angles across this tectonic grain and therefore the beaches and shelves have a complex history of uplift and subsidence. The active compressional history has resulted in a complex ocean-bottom bathymetry that is host to benthic life of the deep marine and the marvelous tidal pools of the Oregon coast. The landward extent of this complex tectonic system is represented by faults, earthquakes, mudslides, and volcanoes.
Province 2—Central and Southern California
A shearing margin characterized by:
areas of extremely rapid uplift and subsidence;
broad continental borderland with active real-time strike-slip faults associated with massive earthquakes;