Abstract

Rocks within the 1-km-thick Western Old Woman Mountains shear zone (WSZ) contain ductilely deformed quartz and ductilely and brittlely deformed feldspars, indicating greenschist to lower amphibolite facies mylonitization. Foliation measured in 73 Ma mid-crustal granitoids within the zone generally dips west-southwest, and sense-of-shear indicators demonstrate top-to-the-west sense of movement parallel to gently southwest plunging lineation. The top of the shear zone is not exposed, but 10 km to the west unmetamorphosed Late Cretaceous-age upper crust is present. The WSZ is thus most simply interpreted as a normal-sense low-angle shear zone. Timing of deformation is constrained to the interval 73 to ca. 65 Ma by the age of deformed granitoids and 40Ar/39Ar chronology. This interval coincides with a period of rapid cooling of rocks now exposed in the Old Woman Mountains that probably resulted from 4 to 8 km of unroofing. We suggest that movement along the WSZ is responsible for at least some of this unroofing The proposed history involves tectonic denudation along a low-angle ductile shear zone and is similar to that demonstrated for Tertiary Cordilleran metamorphic core complexes.

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