Listric normal faulting on the Cascadia continental margin

Abstract

Analysis of multichannel seismic reflection profiles reveals that listric normal faulting is widespread on the northern Oregon and Washington continental shelf and upper slope, suggesting E‐W extension in this region. Fault activity began in the late Miocene and, in some cases, has continued into the Holocene. Most listric faults sole out into a subhorizontal décollement coincident with the upper contact of an Eocene to middle Miocene mélange and broken formation (MBF), known as the Hoh rock assemblage onshore, whereas other faults penetrate and offset the top of the MBF. The areal distribution of extensional faulting on the shelf and upper slope is similar to the subsurface distribution of the MBF. Evidence onshore and on the continental shelf suggests that the MBF is overpressured and mobile. For listric faults which become subhorizontal at depth, these elevated pore pressures may be sufficient to reduce effective stress and to allow downslope movement of the overlying stratigraphic section along a low‐angle (0.1°–2.5°) detachment coincident with the upper MBF contact. Mobilization, extension, and unconstrained westward movement of the MBF may also contribute to brittle extension of the overlying sediments. No Pliocene or Quaternary extensional faults have been identified off the central Oregon or northernmost Washington coast, where the shelf is underlain by the rigid basaltic basement of the Siletzia terrane. Quaternary extension of the shelf and upper slope is contemporaneous with active accretion and thrust faulting on the lower slope, suggesting that the shelf and upper slope are decoupled from subduction‐related compression.