KINEMATIC ANALYSIS OF BRITTLE FAULTING IN THE PRINCE RUPERT AREA, COAST MOUNTAINS, BRITISH COLUMBIA

Brittle faults with slickenside surfaces, cataclasite, and pseudotachylyte occur along the trace of the Coast shear zone near Prince Rupert, British Columbia. The Coast shear zone (CSZ) is a crustal-scale boundary that separates rocks of the Western Metamorphic Belt and Central Gneiss Complex. The Western Metamorphic Belt contains upper amphibolite facies rocks that were deformed, metamorphosed, and intruded by mid-Cretaceous quartz diorite plutons that cooled through 500°C by about 75 Ma. The Central Gneiss Complex contains upper amphibolite to granulite facies rocks intruded by Late Cretaceous to Eocene quartz diorite and tonalite plutons that cooled through 500°C by 50 Ma. High-temperature, melt-present and solid state ductile fabrics along the CSZ indicate an early history of east-side-up and dextral strike-slip motion followed by west-side-up dip-slip motion. Fault slip data collected from small-scale brittle faults that overprint the earlier high-temperature fabrics suggest that at least two episodes of brittle faulting occurred along the trace of the Coast shear zone. P-T axes indicate an overall E-W extension during both normal and dextral faulting. The relative timing of normal and dextral faulting could not be determined; however, a precise 40Ar/39Ar date from pseudotachylyte associated with the dextral faulting shows that seismogenic brittle faulting occurred at 29.8 Ma. This date coincides with the opening of the Queen Charlotte basin to the west and suggests that dextral faulting, perhaps associated with this extensional event, occurred along the pre-existing Coast shear zone.