Structure and subduction processes along the Oregon-Washington margin |
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| Authors: | Guy R Cochrane Brian T R Lewis Kevin J McClain |
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| Institution: | (1) School of Oceanography WB-10, University of Washington, 98195 Seattle, WA, USA;(2) Present address: U.S. Geological Survey, 345 Middlefield Rd. MS-999, 94025 Menlo Park, CA, USA;(3) Present address: 8056 S. Niagara Way, 80112 Anglewood, CO, USA |
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| Abstract: | Seismic reflection and refraction data off Washington and Oregon are used to determine the style of sediment deformation and to infer the physical properties of accreted sediments on the lower slope. Onshore-offshore seismic refraction data off Washington are used to determine the location of the  trench , or where the plate bending starts.We find that off Washington the subduction zone is characterized by a trench whose physiographic expression is buried under several kilometers of sediments and is tens of kilometers landward of the lower slope, which is accreting seaward as the result of the offscraping of sediments.Seismic reflection data support previous observations that offscraping occurs along seaward and landward dipping thrust faults. Refraction data indicate that a sediment package thrust up along a seaward dipping fault (off Washington) was not measurably changed in velocity with respect to a Cascadia basin section. However a package uplifted by thrusting along a landward dipping fault (off Oregon) did have increased velocity. It is suggested that the increased velocities off Oregon could be the result of erosion and exposure of more deeply buried and compacted sediments, rather than the result of dewatering due to tectonic stress. Off Washington the sensitivity of velocity to porosity and resolution of the seismic method does not preclude dewatering due to tectonic stress, but it does limit the degree of dewatering.In the deeper parts of the lower slope section off Washington and Oregon velocities as high as 3 to 4 km/sec are found. Heat flow data indicate that the temperatures in this high velocity regime are greater than 100°C. It is hypothesized that lithification related to clay diagenesis may be partly responsible for the high velocities, rather than simply compaction. It also appears that the high velocity sediments are subducted while the unlithified low velocity sediments are offscraped. |
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| Keywords: | Accretion subduction seismic-refraction seismic-reflection Oregon margin OBS sediment deformation overpressuring |
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