SEISMIC CHARACTERIZATION OF A COMPOUND TECTONIC WEDGE BENEATH THE ROCKY-MOUNTAIN FORELAND BASIN, ALBERTA

Interpretation of reflection seismic data from the Sundre area of sout hern Alberta has revealed the detailed internal geometry of a ''compou nd tectonic wedge'' beneath a gently eastward-dipping homocline. The w edge extends for a distance of 8 km into the foreland basin east of th e exposed apex of the triangle zone and is separated from the overlyin g homocline by a smooth, listric upper detachment occurring within the Upper Cretaceous Brazeau Group. Maximum dip of the upper detachment i s 20 degrees. The lower detachment of the wedge rides within Lower Cre taceous rocks, varying locally in position between shales of the Alber ta Group and coals of the Blairmore Group. It is approximately 1.5 km deeper in the section than the upper detachment at the foreland limit of the wedge. Thickening of the wedge occurred by displacement along a series of frontal ramps which branch from the lower detachment and ge nerally merge with the upper detachment toward the foreland. A backthr ust also developed within the wedge, resulting in the formation of a p op-up with a vertical uplift of approximately 300 m. The backthrust to the pop-up branches from the lower detachment and cuts upsection towa rd the hinterland, steepening to a dip of nearly 60 degrees at its tip point. The pop-up terminates to the south into a fault propagation fo ld which has only a small displacement.