Abstract

The Wild Horse River map-area is situated near the Crowsnest Pass cross-strike discontinuity, which marks the southern termination of the Western Main Ranges subprovince of the Southern Canadian Rocky Mountains. The Crowsnest Pass cross-strike discontinuity is a transverse northeast-trending tectonic feature marked by profound changes in structural style and in Paleozoic stratigraphy between the Laurentia craton and the Cordilleran miogeocline. Cambrian and Ordovician strata in the Wild Horse River map-area accumulated on the northwest side of the discontinuity in two contrasting tectonostratigraphic domains. One domain consists of an incomplete stratigraphic succession that was deposited on the Windermere high, a high-standing fault block within the Paleozoic miogeocline that was intermittently tilted eastward. The other domain contains a thicker more complete shaly succession that accumulated adjacent to the margin of the Laurentia craton. This latter succession contains local intercalations of diatreme-related volcanic rocks, the emplacement of which may have been controlled by basement faults that defined the margin of the miogeocline. The east verging Lussier River thrust fault separates the two domains. The dominant structures east of it are the Porcupine Creek anticlinorial fan structure and the east-verging Ruault Lake thrust fault. The Ruault Lake fault has been synformally backfolded in the west flank limb of the Porcupine Creek fan structure; both it and the fan structure are truncated by the out-of-sequence Lussier River fault. All convergence-related structures are cut and thermally overprinted by approximately 111 Ma granitic stocks that date all the thrusting and folding as pre-Albian. The transverse Boulder Creek fault, which branches from the Lussier River fault, may be a dextral tear fault that was reactivated as a down-to-the-north normal fault during Tertiary displacement along the Southern Rocky Mountain Trench fault.