Abstract

The northwestern Cascades structural province can be interpreted as an accretionary complex comprising fault-bounded blocks of pre-Tertiary metamorphic rocks of diverse age and lithologic type. This paper documents the deformation in a portion of the Chilliwack Group, a unit in this complex. The Chilliwack Group is a thick sequence of volcaniclastic sedimentary rocks, calc-alkaline volcanic rocks, and limestone that is metamorphosed to low-grade blueschist facies. The rocks underwent ductile deformation during a Late Cretaceous orogenic event, producing a subhorizontal foliation and, in appropriate lithologies, subhorizontal stretching lineations that trend N20°W. Finite strain sustained by coarse clastic rocks produced RXZ values averaging 3.5. The deformation at least partially postdates the high pressure metamorphic event, based on the presence of bent and broken high-pressure mineral grains. Although early studies postulated west-vergent thrust imbrication of units in the northwest Cascades, the N20°W direction of apparent elongation in the Chilliwack Group, consistent with the direction of motion along segments of the Shuksan fault elucidated in other more recent studies, may reflect significant, highly oblique components of convergence during formation of the western North Cascades collisional orogen.

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