CRUST AND UPPER MANTLE STRUCTURES SURROUNDING THE ROCKY MOUNTAIN FORELAND BASIN IN SOUTHWESTERN CANADA

The Alberta foreland basin marks a boundary zone between the Precambrian North American craton and the Phanerozoic Cordillera. The crystalline basement, currently buried beneath a thick (up to 5 km) sedimentary cover, is a network of intricate lithospheric domains assembled during episodes of Precambrian tectonic events. In this study, we utilize teleseismic body waves from regional networks to determine the structures of the crust and upper mantle beneath the western margin of the Alberta basin, a previously under-sampled region.

Based on our results of receiver functions, the Moho depth increases from ~35 km under the Cordillera to ~45 km under the foothills of the Rockies in southern Alberta. Since both regions exhibit similar (~0.43 g/cm³) density contrasts across the Moho, which are consistent with predictions from global 1D reference models, the reduced Moho depth in the former region is possibly isostatically compensated by the thermal buoyancy of the hot upper mantle. This interpretation is supported by a pronounced low (<-2%) velocity zone, which extends down to ~300 km beneath the southern Canadian Cordillera, based on finite-frequency tomography. The largest mantle velocity gradient between the slow Cordillera and the high (>2%) velocity craton is observed in the vicinity of the Rocky Mountain Trench, potentially representing an upper mantle suture of the orogen. Our results further suggest significant variations in lithospheric structures along the strike of the mountain belt. The craton boundary dips westward in southern Alberta, which contradicts the presumed (eastward) orientation in the back arc of the Canadian Cordillera. Overall, the combination of crustal and lithospheric constraints from regional seismic body waves provides compelling evidence for secular modification of the craton margin in southwestern Canada.