THE AGE, ORIGIN, AND SIGNIFICANCE OF PSEUDOTACHYLYTE IN THE COAST MOUNTAINS, BRITISH COLUMBIA

The Coast Steep Zone is a major crustal-scale structure that extends for 1000 km along the Coast Mountains in southeastern Alaska and British Columbia. Most previous work along the Coast Steep Zone has focused on the high-temperature plastic deformational history of this zone, but its significance as a brittle structure is poorly understood. Fault-related pseudotachylyte occurs along the trace of the Coast Steep Zone near Prince Rupert, British Columbia. Pseudotachylyte occurs in a 1-2 meter wide, near-vertical brecciated fault zone that strikes to the NW and cuts diorite of the 91 Ma Ecstall pluton. Slickenfiber lineations are sub-horizontal and consistent with dextral slip. The presence of spherulites, alkali feldspar microlites, and amygdules in the opaque glassy matrix suggest that the pseudotachylye crystallized rapidly from a melt phase within 2 km of the surface. 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 the pseudotachylyte and host rock are interpreted to arise from preferential cataclasis and melting of mafic phases. ⁴⁰Ar/³⁹Ar incremental-heating of the pseudotachylyte matrix yields a weighted mean plateau age of 29.81 ± 0.60 Ma and an inverse isochron age of 29.81 ± 1.52 Ma interpreted to be the age of pseudotachylyte generation by seismogenic faulting. These results imply that some dextral coastwise displacement occurred across the Coast Steep Zone in the Oligocene and this movement was contemporaneous with the opening of the Queen Charlotte basin to the west. Furthermore, the majority of exhumation in the Coast Mountains in the Prince Rupert region was accomplished by 30 Ma.