PALINSPASTIC RESTORATION OF THE SOUTHERN CANADIAN ROCKIES: IMPLICATIONS FOR LARGE-SCALE VARIATIONS IN CRUSTAL STRUCTURE OF THE SOUTH-CENTRAL CANADIAN CORDILLERA

Autochthonous Paleoproterozoic continental crust that extends under the Rocky Mountain thrust and fold belt from beneath the Western Canada Sedimentary Basin is uniformly ~40 km thick, even under the western part of the foreland thrust and fold belt where is has been buried tectonically to depths of >15 km. However, under the south-central Canadian Cordillera the thickness decreases abruptly and is highly variable, partly because of the inhomogeneous crustal stretching that occurred during the Mesoproterozoic intra-continental rifting that produced the Belt-Purcell basin and during the Neoproterozoic-Early Cambrian continental rifting and continental break-up that established the Cordilleran miogeocline, partly because of superposed crustal boudinage during Eocene regional transtension, and perhaps partly because of superposed shear and tectonic erosion during Jurassic to Paleocene thrusting and folding, when allochthonous northeast-verging thrust sheets were scraped off the under-riding Laurentian “basement” and accreted to the over-riding Intermontane terrane as it converged obliquely with Laurentia.

The hanging wall of each thrust sheet provides a template for reconstructing the footwall rocks that were left behind when it was detached. The thrust sheet also carries a stratigraphic record of the character and tectonic history of the “basement” in the area from whence it came. These concepts provide the basis for reconstructing a footwall map of the basal detachment of the thrust and fold belt as it existed while the overlying thrust sheets were being scraped off (i.e. before the Eocene transtension and crustal boudinage, and before long-term deep tectonic burial and thermal mobilization of the basement rocks). After step-wise 3-D palinspastic restoration of Eocene crustal stretching, and then of Paleocene to Jurassic thrusting, large basin-margin hanging-wall thrust ramps within the allochthonous rocks of the thrust and fold belt can be correlated to relict Mesoproterozoic and Neoproterozoic-Early Paleozoic basin-margin fault ramps marked by zones of abrupt crustal thinning in the parautochthonous continental crust beneath the accreted terranes of the south-central Canadian Cordillera. This provides new insights on the nature and tectonic significance of the structure of the crust in the interior of the Cordillera.