The geometry of a relict triangle zone at the boundary of the Foothills and Front Ranges in the southern Canadian Rocky Mountains is constrained by detailed surface mapping over 700 m of relief and by seismic reflection data. The geometry and progressive development of the triangle zone along a strike length of 15-20 km, in the displacement transfer zone between the Coleman and Misty thrusts, is illustrated using closely spaced balanced cross sections, palinspastic restorations, and a sequentially restored cross section.

Structural geometries show that a northeast- (foreland-) verging, mainly carbonate wedge of Mississippian to Triassic rock was inserted along a major upper detachment zone in shale, near the base of the Jurassic-Cretaceous clastic package. This was accompanied by southwest- (hinterland-) verging displacements along the upper detachment zone, tectonic thickening of the clastic package exceeding 200%, and backthrusting. Later northeast-verging deformation slightly modified the triangle zone by steepening structures, tightening folds, and minor thrusting. Recognition of relict triangle zones within the fold and thrust belt may document important changes in the rate of thrust front advancement, and aid in the delineation of potential hydrocarbon traps, similar to those discovered along th present-day thrust-belt margin in the southern Canadian Rocky Mountains.