Abstract

Sediment offscraping and accretion takes place predominantly by landward-vergent thrusting along most of the Oregon accretionary prism north of latitude 44°50'N. This indicates very low basal shear stress, probably due to near-lithostatic fluid pressure on the décollement horizon. Recent multichannel seismic and sidescan sonar studies have demonstrated the existence of an oblique, basement-involved, left-lateral strike-slip fault, termed the Wecoma fault. It cuts abyssal plain sediments and oceanic basement of the subducting plate. Where the Wecoma fault intersects the wedge thrust front at 45°09'N, accretion to the wedge occurs through an anomalous seaward-vergent thrust. Linear, fault-parallel erosional gullies, which trend obliquely across the slope on the seaward flank of the frontal anticline in the accretionary 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 sediments is preferentially concentrated along the fault traces. Evidence includes the presence of chemosynthetic biological communities, pervasive veining and fracturing of the rock, and extensive carbonate cementation, all forming preferentially within the gullies. This strike-slip fault apparently acts as a high-angle fluid escape conduit, channeling fluids from deep within the wedge to the surface. We propose that this fluid conduit causes the local inversion of thrusting to seaward vergence by allowing the escape of highly overpressured fluids from the incipient décollement horizon. The consequent increase in basal effective 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 unappreciated importance of strike-slip (and normal) faults in dewatering accretionary prisms. Such dewatering features may significantly influence the development of large-scale structures, and they have broad applicability to other tectonic settings, including collision zones and continental fold-and-thrust belts.

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