STRUCTURE AND TECTONIC DEVELOPMENT OF THE SOUTHERN ROCKY-MOUNTAIN TRENCH

The Rocky Mountain trench is one of the youngest, most prominent, and most enigmatic structures of the Canadian Cordillera. Approximately 65 0 km of seismic-reflection data, providing regional three-dimensional coverage over an area of 10,000 km(2), include six crossings of the Ro cky Mountain trench between 49 degrees N and 50 degrees 15'N. Prominen t reflections from mid-Proterozoic Moyie sills outline thrust-and-fold structures of a Late Jurassic to Early Cretaceous fault system that w as cut by the Rocky Mountain trench fault in the Tertiary. The near-ba sement reflections outline a 10 km high west facing basement ramp, the hinge line of which spatially coincides with the Rocky Mountain trenc h in this area. This ramp is part of a mid-Proterozoic margin upon whi ch the Belt-Purcell supergroup was deposited and is preserved beneath the trench. During Mesozoic contraction, the basal detachment of the F oreland belt closely followed the craton-cover contact across the base ment ramp. Thrusting ceased and extension was initiated when a culmina tion of thick basinal strata was juxtaposed with the basement ramp. In the Eocene-Miocene, the basement ramp and the culmination above it fo cused stress, reactivating the basal detachment and causing extensiona l faulting in the southern Rocky Mountain trench. The Rocky Mountain t rench fault may be linked via the basal detachment to the Flathead fau lt on the east and the Eocene extensional faults that flank the Ominec a belt on the west.