The Rocky Mountain trench is one of the youngest, most prominent, and most enigmatic structures of the Canadian Cordillera. Approximately 650 km of seismic-reflection data, providing regional three-dimensional coverage over an area of 10,000 km2, include six crossings of the Rocky Mountain trench between 49¿N and 50¿15'N. Prominent reflections from mid-Proterozoic Moyie sills outline thrust-and-fold structures of a Late Jurassic to Early Cretaceous fault system that was cut by the Rocky Mountain trench fault in the Tertiary. The near-basement reflections outline a 10 km high west facing basement ramp, the hinge line of which spatially coincides with the Rocky Mountain trench in this area. This ramp is part of a mid-Proterozoic margin upon which the Belt-Purcell supergroup was deposited and is preserved beneath the trench. During Mesozoic contraction, the basal detachment of the Foreland belt closely followed the craton-cover contact across the basement ramp. Thrusting ceased and extension was initiated when a culmination of thick basinal strata was juxtaposed with the basement ramp. In the Eocene-Miocene, the basement ramp and the culmination above it focused stress, reactivating the basal detachment and causing extensional faulting in the southern Rocky Mountain trench. The Rocky Mountain trench fault may be linked via the basal detachment to the Flathead fault on the east and the Eocene extensional faults that flank the Omineca belt on the west. ¿ American Geophysical Union 1996