LISTRIC NORMAL FAULTING ON THE CASCADIA CONTINENTAL-MARGIN

Analysis of multichannel seismic reflection profiles reveals that list ric normal faulting is widespread on the northern Oregon and Washingto n continental shelf and upper slope, suggesting E-W extension in this region. Fault activity began in the late Miocene and, in some cases, h as continued into the Holocene. Most listric faults sole out into a su bhorizontal decollement coincident with the upper contact of an Eocene to middle Miocene melange and broken formation (MBF), known as the Ho h rock assemblage onshore, whereas other faults penetrate and offset t he top of the MBF. The areal distribution of extensional faulting on t he shelf and upper slope is similar to the subsurface distribution of the MBF. Evidence onshore and on the continental shelf suggests that t he MBF is overpressured and mobile. For listric faults which become su bhorizontal at depth, these elevated pore pressures may be sufficient to reduce effective stress and to allow downslope movement of the over lying stratigraphic section along a low-angle (0.1 degrees-2.5 degrees ) detachment coincident with the upper MBF contact. Mobilization, exte nsion, and unconstrained westward movement of the MBF may also contrib ute to brittle extension of the overlying sediments. No Pliocene or Qu aternary extensional faults have been identified off the central Orego n or northernmost Washington coast, where the shelf is underlain by th e rigid basaltic basement of the Siletzia terrane. Quaternary extensio n of the shelf and upper slope is contemporaneous with active accretio n and thrust faulting on the lower slope, suggesting that the shelf an d upper slope are decoupled from subduction-related compression.