USING REGIONAL KINEMATIC DATASETS TO BETTER UNDERSTAND TECTONIC SYSTEMS: AN EXAMPLE FROM THE NORTH AMERICAN LARAMIDE FORELAND

Understanding the nature of progressive deformation recorded in geologic structures has been a hallmark of Steven Wojtal’s career. His work shows the importance of asking critical questions on how displacement fields, paleostress, and rheology are interrelated, and ultimately on what can be deduced about broader tectonic processes from our necessarily limited observations. Herein we explore these questions, using regional datasets of minor fault kinematics, anisotropy of magnetic susceptibility (AMS) fabrics, and paleomagnetic declinations, and their relations to large-scale fold-fault systems across the Laramide foreland belt, a broad region of thick-skinned, contractional deformation within the North American Cordilleran. The belt developed during the Late Cretaceous to Paleogene, marked by changing subduction dynamics, and within Precambrian basement having complex architecture. The Laramide is characterized by an anastomosing network of curved, basement-cored arches that vary from N-S to E-W trending. Arches are bound by major reverse faults that propagated upward into folded sedimentary cover rocks, and merged downward into diffuse lower crustal shortening and/or a deep detachment. Additional en echelon folds and faults developed along arch flanks and in associated basins. Widespread layer parallel shortening (LPS) preceded large-scale faulting and folding, with development of systematic minor fault sets and subtle AMS fabrics. Maximum shortening and paleostress directions estimated from minor fault data are consistent and oriented regionally WSW-ENE, similar to the direction of relative motion between the North America and Farallon plates, but with directions refracted along curved to oblique trending arches, partly related to reactivation of basement weaknesses. Paleomagnetic data indicate limited block rotations, except near some oblique faults, which allow restoration of the early strain field. LPS magnitudes are regionally small but increase in areas of future folds, interpreted to reflect increased stress related to slip on underlying basement faults that propagated upward and linked laterally. Ultimately, development of the Laramide belt may reflect an imposed velocity field related to flat-slab subduction and deep lithospheric interactions far from the plate margin.