VARIATIONS IN DETACHMENT LEVELS, RAMP ANGLES AND WEDGE GEOMETRIES ALONG THE ALBERTA THRUST FRONT

Variable ramp angles between nine separate detachment horizons of vary ing lengths yield a spectrum of wedge geometries along the Alberta thr ust front. In addition to the regionally significant Mississippian, De vonian and Jurassic detachments recognized try previous workers, six e xtensive Upper Cretaceous detachment horizons are identified here on t he basis of detailed mapping, interpretation of high-resolution seismi c data and regional correlation. In the Grande Cache area substantial shortening occurs at the Paleozoic level and between two detachments i n the Blackstone Formation. In southern Alberta (between Grease Creek and Pincher Creek), shortening occurs between much higher level detach ments in the Brazeau Group as well as above significant flats at the M ississippian, Devonian, Jurassic, Blackstone and Wapiabi levels. The m ajority of the 984 ramp angles measured fall in the 10 degrees-30 degr ees range. Mean ramp angles are steepest near Grande Cache (30.4 degre es+/-10.0 degrees in Paleozoic carbonates; 21.0 degrees+/-8.9 degrees in Cretaceous clastics) and shallowest at Pincher Creek (11.3 degrees/-3.1 degrees in Paleozoic carbonates; 9.2 degrees-12.8 degrees in Cre taceous clastics). There is a general trend of decreasing ramp angles from north to south, which is Likely due to the thick (2500 m) sequenc e of competent rocks involved in the deformation at Grande Cache and t he presence of multiple detachments and many thinner (100-500 m) thrus t sheets in southern Alberta.