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Age, origin, and significance of brittle faulting and pseudotachylyte along the Coast shear zone, Prince Rupert, British Columbia

¹ Department of Geology, Beloit College, Beloit, Wisconsin 53511, USA
² Department of Geology, College of William & Mary, Williamsburg, Virginia 23188, USA
³ Department of Geology, Carleton College, Northfield, Minnesota 55057, USA
⁴ Department of Geology, Pomona College, Claremont, California 91711, USA
⁵ Department of Geology and Geophysics, University of Wisconsin, Madison, Wisconsin 53706, USA

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.

Key Words: British Columbia • Coast Mountains • pseudotachylite • brittle deformation • dating

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