STRUCTURAL INHERITANCE AND THE ROLE OF BASEMENT ANISOTROPIES IN THE LARAMIDE STRUCTURAL AND TECTONIC EVOLUTION OF THE NORTH AMERICAN CORDILLERAN FORELAND, WYOMING: TOWARDS A UNIFIED HYPOTHESIS

The Laramide belt of the North American Cordillera is a thick-skinned orogen that continues to garner attention due to many unresolved ambiguities, particularly in the subsurface. Recent seismic studies provide a better understanding of Laramide tectonism at deep crustal levels. However, mechanisms for deformation accommodation in the upper crust remain unclear.

A structural/tectonic analysis of Precambrian fabrics and structural grain of basement-cored Laramide arches and uplifts in Wyoming using only previously collected data, along with a hypothesis on the potential role of these features in Laramide orogenesis, is presented. This work provides evidence for the presence of Neoarchean convergence zones dominantly directed from SW–NE towards the Wyoming province forming NNW anisotropies. In addition, regional compressional forces from convergence formed WNW- and NE-striking conjugate shears. Precambrian basement fabrics characterize all three directions of major anisotropy and they likely have a complex history of deformation since the Precambrian, most recently, during the Laramide orogeny. This Precambrian deformation system was likely a fundamental tectonic control in Laramide arch/uplift formation in Wyoming.

During the Laramide orogeny, reactivation of anisotropies occurred throughout Laramide contraction, forming somewhat symmetrical, but discrete zones of transpression, displaced along a SW–NE-directed Laramide deformational front. Reverse-left oblique-slip faults developed from reactivation of WNW fabrics and, where connected, acted as relay zones facilitating major arch development along NNW-striking faults. Internal controls for Laramide orogenesis in the upper crust are likely related to these basement anisotropies, which may link the evolution of foreland arches at deeper crustal levels to surface structures.