Abstract

The Chewack-Pasayten fault is a major steeply dipping tectonic boundary that trends N30°W across north-central Washington and adjacent British Columbia for at least 250 km. It is the eastern boundary of the Cenozoic Methow trough and is here interpreted to have been the eastern boundary of the early Late Cretaceous Tyaughton trough. It separates Paleozoic and Mesozoic plutonic rocks on the northeast from Mesozoic and Cenozoic volcanic and sedimentary rocks on the southwest.

A 1.5-km-wide zone of cataclastic fluxion structure (S) containing a mineral lineation (L) is present in the granitic rocks adjacent to the fault. S is steeply dipping and has a N15°W trace that is en echelon to the fault trace. L is subhorizontal in S. Detailed interpretation of fabric elements in this zone are used herein to interpret one phase of the tectonic history of the fault. Plagioclase shape and crystallographic fabrics suggest rigid body rotation of plucked augen. Quartz-shape fabrics suggest ductile flattening and recrystallization. Quartz c-axis fabrics suggest crystal glide in response to compression perpendicular to S similar to recent experimental results. Microfractures in quartz tentatively are interpreted as stress relief phenomena. Mica shape and crystallographic fabrics suggest crystal glide, rotation, and fracture. All of these results indicate that the strain in the cataclastic zone may be described by an ellipse oriented with the axis of maximum finite shortening perpendicular to S and the axis of maximum finite elongation parallel to L. This is interpreted to reflect left-lateral simple shear motion during the early Late Cretaceous development of the cataclastic zone. The exposed portion of the cataclastic zone originally developed at a depth of about 2 km and at a temperature of 300 to 400 °C.

Slickensided shear joints in volcanic rocks record compression nearly at right angles to the fault that is probably related to middle Late Cretaceous folding. Associated fault motion, if any, would have been high-angle reverse. Later, at higher structural levels, normal motion on the fault disrupted the cataclastic zone and created the Methow trough in early Cenozoic time.

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