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The age and origin of pseudoachylyte in the Coast Mountains, British Columbia
Davis, Kenneth J.
Davis, Kenneth J.
Abstract
Fault-related pseudotachylyte was discovered in the Coast Steep Zone (CSZ), a crustal-scale lineament that extends 1000 km along strike of the Coast Mountains. The pseudotachylyte was found near a NW-dipping near-vertical dextral fault zone in the Prince Rupert area, British Columbia, and contains typical frictional melt textures such as amygdales, spherulites, microlites, flow structures and injection veins. Collectively, the microstructures indicate rapid quenching from a melt, close association with cataclasis, and formation in the upper ~2 km of the crust. Chemical data indicate that the pseudotachylyte matrix is more mafic that the host diorite, and is enriched in biotite and amphibole. Differences in chemical composition between pseudotachylyte and host rock are interpreted to arise from preferential cataclasis and melting of biotite and hornblende due to lower fracture toughness. *’Ar/*’Ar dating of the pseudotachylyte matrix gives a date of 30.51 + 0.7 Ma that is interpreted to be the age of pseudotachylyte generation by seismogenic faulting. The fault that contains pseudotachylyte is related to a set of dextral faults in the CSZ that may have been active at ~30 Ma as right-lateral stress was transferred from the offshore Queen Charlotte transform fault.
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2001-01-01
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Geology