FLUID-FLOW ALONG A STRIKE-SLIP-FAULT AT THE TOE OF THE OREGON ACCRETIONARY PRISM - IMPLICATIONS FOR THE GEOMETRY OF FRONTAL ACCRETION

Sediment offscraping and accretion takes place predominantly by landwa rd-vergent thrusting along most of the Oregon accretionary prism north of latitude 44-degrees-50'N. This indicates very low basal shear stre ss, probably due to near-lithostatic fluid pressure on the decollement horizon. Recent multichannel seismic and sidescan sonar studies have demonstrated the existence of an oblique, basement-involved, left-late ral strike-slip fault, termed the Wecoma fault. It cuts abyssal plain sediments and oceanic basement of the subducting plate. Where the Weco ma fault intersects the wedge thrust front at 45-degrees-09'N, accreti on to the wedge occurs through an anomalous seaward-vergent thrust. Li near, fault-parallel erosional gullies, which trend obliquely across t he slope on the seaward flank of the frontal anticline in the accretio nary wedge, are interpreted as marking the surface traces of splays of the Wecoma fault. Data collected during six Alvin dives on and around these gullies demonstrate that fluid expulsion from the deforming sed iments is preferentially concentrated along the fault traces. Evidence includes the presence of chemosynthetic biological communities, perva sive veining and fracturing of the rock, and extensive carbonate cemen tation, all forming preferentially within the gullies. This strike-sli p fault apparently acts as a high-angle fluid escape conduit, channeli ng fluids from deep within the wedge to the surface. We propose that t his fluid conduit causes the local inversion of thrusting to seaward v ergence by allowing the escape of highly overpressured fluids from the incipient decollement horizon. The consequent increase in basal effec tive normal stress favors the development of a seaward-vergent thrust. The discovery of this strike-slip fault acting as an efficient fluid conduit to the surface of the wedge emphasizes the generally unappreci ated importance of strike-slip (and normal) faults in dewatering accre tionary prisms. Such dewatering features may significantly influence t he development of large-scale structures, and they have broad applicab ility to other tectonic settings, including collision zones and contin ental fold-and-thrust belts.