The Rocky Mountain trench, a linear system of valleys, extends for approximately 1600 km from northern Montana to the B.C.-Yukon border. In the southern Canadian Cordillera, where line SBC 1A crosses the trench, it is bounded on its east side by a west-dipping normal fault. Miocene lake deposits have been found in this part of the trench, indicating that the southern Rocky Mountain trench fault is significantly younger than the Mesozoic compressional structures of the Rocky Mountains to the east and the Purcell mountains to the west.
The southern Rocky Mountain trench fault appears to flatten above the basement at the top of a prominent, 10 km high west-facing basement ramp. This basement ramp is part of a mid-Proterozoic margin upon which the Belt-Purcell supergroup was deposited. During Mesozoic contraction, the Rocky Mountain basal detachment (RMBD) closely followed the craton-cover contact across this ramp, forming a major culmination above it. When thrusting ceased, the RMBD was reactivated as an extensional fault and focused stress towards the surface at the basement ramp, causing extensional faulting in the trench.
To the north, the southern Rocky Mountain trench is aligned with the northern Rocky Mountain trench, which contains a strike-slip fault system that has at least 450 km of dextral displacement since the mid-Cretaceous.
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