BASIN INVERSION AND TECTONIC HEREDITY CONTROL FIRST-ORDER STRUCTURE IN A FORELAND THRUST-FOLD BELT: SOUTHEASTERN CANADIAN CORDILLERA

A new palinspastic map of an 850-km segment of the foreland thrust-fold belt between northern Montana (~48⁰ N) and NE B.C. (~54⁰ N) that is based on restorations of 6 published balanced regional structure sections shows the hanging-wall surface traces of the main thrust faults in both their present and restored locations. It provides a framework for reconstructing the 3-D shapes, locations, and mutual relationships among: the 1500 - 1400 Ma Belt-Purcell intracontinental rift basin, the 850 - 600 Ma Windermere intracontinental rift basin, and the Cordilleran miogeocline (a 525 – 385 Ma passive-margin sedimentary basin); and also for reconstructing the tectonic relationships of these basins to the Paleoproterozoic basement as outlined under Alberta by magnetic and gravity anomaly maps, radiometric dates from deep boreholes, and by petroleum industry and Lithoprobe deep geophysical imaging. Isopach maps of the estimated total thickness of the sedimentary fill in the three restored basins illustrate the truncation and overlap relationships among them; and also show that: 1.) The locations and orientations of first-order structural culminations in the thrust-fold belt were tectonically inherited. As basin-fill strata were detached along basin-margin fault zone ramps and displaced over the planar surface of the craton they formed 3 – 10 km thick hanging-wall thrust ramps that controlled the orientations of a foreland wedge of critical-taper thrusting and related folding. 2.) The major negative Bouguer gravity anomaly of the SE Canadian Cordillera coincides with the area of thrust overlap of the displaced miogeocline and underlying Windermere Supergroup relative to the matching autochthonous basin-margin ramp that lies beneath the Cordillera. 3.) South of 50⁰ N, the NE margins of the miogeocline and the Windermere basin are deflected ~220 km SW across the Belt Supergroup along the Crowsnest Pass cross-strike discontinuity (CPCD), a transverse NE trending fault zone created by reactivation of a tectonic suture in the Paleoproterozoic basement that extends under the Cordillera from the Vulcan structure SE of Calgary. 4.) Near the CPCD the edge of the miogeocline overlaps the Windermere basin and lies on the Purcell Supergroup; SE of the CPCD the Belt-Purcell basin was the part of the Paleozoic cratonic platform that has been called “Montania”.