A LEFT-LATERAL STRIKE-SLIP-FAULT SEAWARD OF THE OREGON CONVERGENT MARGIN

We have mapped a recently active left-lateral strike-slip fault (the Wecoma fault) on the floor of Cascadia Basin west of the Oregon convergent margin, using SeaMARC I sidescan sonar, Seabeam bathymetry and multichannel seismic and magnetic data. The fault intersects the base of the continental slope at 45-degrees 1O'N and extends northwest (293-degrees) for at least 18.5 km. The fault's western terminus was not identified, and the eastern end of the fault splays apart and disrupts the lower continental slope. The fault extends to the base of the 3.5-km-thick sedimentary section and overlies a basement discontinuity that may be related to movement along the Wecoma fault. Prominent seafloor features crosscut by the fault individually display between 120 and 2500 m of left-lateral separation, allowing the general history of fault motion to be evaluated. The fault's average slip rate since 10-24 ka is inferred to be 5-12 mm/yr, based on the age of an offset submarine channel. Surficial structural relationships, in conjunction with the maximum inferred slip rate, indicate that fault movement initiated at least 210 ka and that the fault has been active during the Holocene.