Abstract

Northwest-striking brittle faults containing cataclasite, fault gouge, and, in one location, pseudotachylyte are common in the vicinity of the Coast shear zone near Prince Rupert, British Columbia. The pseudotachylyte locality, found in a dextral strike-slip fault zone along the Coast shear zone, contains spherulites, alkali feldspar microlites, and amygdules, suggesting that the pseudotachylyte crystallized rapidly from a melt phase within 5 km of the surface. 40Ar/39Ar incremental heating of the pseudotachylyte matrix yielded a weighted mean plateau of 29.8 ± 0.6 Ma and an inverse isochron of 29.8 ± 1.5 Ma with an 40Ar/36Ar intercept of 296.0 ± 15.2. These results show that pseudotachylyte associated with brittle faulting can be dated precisely, and imply that some dextral coast-parallel displacement occurred across the Coast shear zone in the Oligocene and that the majority of exhumation in the Coast Mountains at the latitude of Prince Rupert (∼54°N) was accomplished by 30 Ma.

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