Abstract

The Misty Range is a homoclinal, southwest-dipping panel of Paleozoic carbonate rocks in the Canadian Rocky Mountains southwest of Calgary that has been juxtaposed against underlying Mesozoic siliciclastic rocks along the northeast-verging Misty thrust fault. At the south end of the Misty Range, a steep, oblique ramp marks an abrupt southward change along the hanging wall of the Misty thrust, from a detachment zone in the Upper Devonian Palliser Formation to another detachment zone in the Jurassic Fernie Group. This 2-km-high ramp is expressed, in the hanging wall rocks, as a large, oblique, southeast-plunging fold.

A west-verging back-thrust that rises from a detachment zone in the Mississippian Mount Head Formation extends across the hanging wall ramp fold and is truncated by the Misty thrust. The Misty thrust evidently propagated southward as an oblique tectonic wedge into this detachment zone, forming a “triangle zone” structure with the back-thrust as the roof thrust. As displacement on the Misty thrust increased, and the hanging wall ramp of the Misty thrust moved into the triangle zone, the back-thrust was progressively back-folded and became convex. Subsequently, the lower part of the back-thrust was partly overridden by the Misty thrust and became overturned, before becoming detached and incorporated in the Misty thrust sheet.

The configuration and evolution of the tectonic wedge and the oblique hanging wall ramp provide a natural prototype for the interpretation of ramps that mark the lateral terminations of oil and gas reservoir structures in the Foothills belt, but which are concealed by overlying deformed Mesozoic rocks.

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