TRANSFORM PUSH, OBLIQUE SUBDUCTION RESISTANCE, AND INTRAPLATE STRESS OF THE JUAN-DE-FUCA PLATE

The Juan de Fuca plate is a small oceanic plate between the Pacific an d North America plates. In the southernmost region, referred to as the Gorda deformation zone, the maximum compressive stress sigma(1) const rained by earthquake focal mechanisms is N-S. Off Oregon, and possibly off Washington, NW trending left-lateral faults cutting the Juan de F uca plate indicate a sigma(1) in a NE-SW to E-W direction. The magnitu de of differential stress increases from north to south; this is infer red from the plastic yielding and distribution of earthquakes througho ut the Gorda deformation zone. To understand how tectonic forces deter mine the stress held of the Juan de Fuca plate, we have modeled the in traplate stress using both elastic and elastic-perfectly plastic plane -stress finite element models, We conclude that the right-lateral shea r motion of the Pacific and North America plates is primarily responsi ble for the stress pattern of the Juan de Fuca plate. The most importa nt roles are played by a compressional force normal to the Mendocino t ransform fault, a result of the northward push by the Pacific plate an d a horizontal resistance operating against the northward, or margin-p arallel, component of oblique subduction. Margin-parallel subduction r esistance results in large N-S compression in the Gorda deformation zo ne because the force is integrated over the full length of the Cascadi a subduction zone. The Mendocino transform fault serves as a strong bu ttress that is very weak in shear but capable of transmitting large st rike-normal compressive stresses. Internal failure of the Gorda deform ation zone potentially places limits on the magnitude of the fault-nor mal stresses being transmitted and correspondingly on the magnitude of strike-parallel subduction resistance. Transform faults and oblique s ubduction zones in other parts of the world can be expected to transmi t and create stresses in the same manner.