Variations in Detachment Levels, Ramp Angles and Wedge Geometries Along the Alberta Thrust Front

Variable ramp angles between nine separate detachment horizons of varying lengths yield a spectrum of wedge geometries along the Alberta thrust front. In addition to the regionally significant Mississippian, Devonian and Jurassic detachments recognized try previous workers, six extensive Upper Cretaceous detachment horizons are identified here on the basis of detailed mapping, interpretation of high-resolution seismic data and regional correlation. In the Grande Cache area substantial shortening occurs at the Paleozoic level and between two detachments in the Blackstone Formation. In southern Alberta (between Grease Creek and Pincher Creek), shortening occurs between much higher level detachments in the Brazeau Group as well as above significant flats at the Mississippian, Devonian, Jurassic, Blackstone and Wapiabi levels. The majority of the 984 ramp angles measured fall in the 10 degrees-30 degrees range. Mean ramp angles are steepest near Grande Cache (30.4 degrees+/-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 Cretaceous clastics). There is a general trend of decreasing ramp angles from north to south, which is Likely due to the thick (2500 m) sequence of competent rocks involved in the deformation at Grande Cache and the presence of multiple detachments and many thinner (100-500 m) thrust sheets in southern Alberta.