The Nature and Significance of Contrasts in the Cordilleran Foreland Thrust and Fold Belt Between Southern Canada and the Northern U.S.A.

Raymond A. Price
Queen's University, Kingston, ON

The Cordilleran foreland thrust and fold belt of southern Canada and northern U.S.A. is “thin-skinned”. It comprises supracrustal rocks that were scraped off the under-riding western margin of the Laurentian craton and accreted to the over-riding Intermontane terrane during convergence between the Laurentia and the subduction zones west of Intermontane terrane, in which a strip of oceanic lithosphere >13,000 wide has descended into the mantle under Laurentia since the Late Jurassic. The southern Canadian Rockies and the northern Montana Rockies form disparate arcuate structural salients along the foreland belt. The abrupt changes in style and orientation of thrust and fold structures between the two salients occur across the Crowsnest Pass cross-strike discontinuity (CPCSD), along which there was an initial 230 km dextral offset of the boundary between the Laurentian craton and the Cordilleran miogeoclinal passive-margin basin. The CPCSD is a reactivated Paleoproterozoic crustal suture that can be traced on aeromagnetic anomaly maps into the western part of the thrust and fold belt from the craton southeast of Calgary. Neoproterozoic-Early Paleozoic displacement along the CPCSD truncated the northwestern part of the Mesoproterozoic Belt-Purcell basin. Both basins were “tectonically inverted” as the supracrustal rocks were detached from their basement and displaced over basin-margin ramps on to the flat surface of the craton. The configurations of the two structural salients have been inherited from the configurations of the rifts that defined the northeast- to north- and northwest-trending margin of the Cordilleran miogeocline and the northwest-trending margin of the Belt-Purcell basin. The contrasts in structural style reflect the contrasts in mechanical properties between the Mesoproterozoic and the Early Paleozoic strata.

AAPG Search and Discovery Article #90039©2005 AAPG Calgary, Alberta, June 16-19, 2005