Structural and Hydrologic Evolution of the Central Oregon Accretionary Prism

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Accretionary prisms form along the landward side of deep sea trenches in response to offscraping of oceanic sediments during lithospheric subduction. Several interrelated factors influence the evolution of these thick sediment prisms including sediment composition, physical properties, rate of underthrusting, and fluid de-watering and migration within the offscraped sediment. Fluid migration and expulsion play an important role in the mechanical behavior, fluid pressure and lithification of sediments in the deformation zone. The present renewal award will complete study of the structure of the Oregon accretionary margin using multichannel seismic techniques. Under previous funding, 2000 km of data was acquired along this margin. Progress in analyzing the data has been excellent. Both the sources and migration pathways of fluids can be imaged in the data. Continued analysis and interpretation will focus on: 1) the role of basement topography in controlling structural evolution of the margin; 2) the nature of the fault zone along which underthrusting occurs; 3) oblique faults which were discovered during initial analysis of the data; and 4) the porosity of the sediment sequence. These data are the basis for a major ODP drilling cruise to the area in late 1992.