MECHANISMS FOR PRECAMBRIAN BASEMENT CONTROL OF LARAMIDE AND ANCESTRAL ROCKY MOUNTAIN STRUCTURES

Basement-involved foreland structures are unusually variable in both orientation and structural vergence. A substantial part of this variability can be ascribed to the effect of basement anistropies such as Precambrian faults, pervasive basement fabrics and lateral lithologic changes. In both the Laramide and Ancestral Rockies orogens of the western United States, reactivation of Precambrian faults can be either dramatic (e.g., uplift of the anomalously east-west-striking Uinta arch on inverted Proterozoic normal faults) or minimal (e.g., Cheyenne Belt). Laramide reactivation of Proterozoic NE, E-W, and NW striking faults contributed to the complex fabric of basement uplifts.

Structural modelling of Laramide arches as either regional or localized detachment folds above a mid-crustal zone of decoupling also suggests that basement influenced Laramide structures from below. We postulate that decoupling occurred at a compositionally and thermally controlled brittle-ductile detachment zone and that lateral changes in basement lithologies influenced the depth to this detachment. For example, balanced cross-sections across the Cheyenne Belt, a Proterozoic suture in southern Wyoming, show a systematic change in the calculated depth to detachment from middle crustal levels in Colorado to lower crustal levels in central Wyoming. ENE-directed slip across this north-dipping zone of detachment explains the northward plunge of the Medicine Bow and Sierra Madre arches into the Hanna Basin. This change in depth to detachment may be due to changes in crustal lithology, with the Archean rocks to the north more resistant to ductile deformation than the Proterozoic rocks to the south. Alternatively, variations in geothermal gradients could cause this change in detachment zone depth, with higher geothermal gradients south of the Cheyenne Belt resulting from Laramide magmatism associated with the Colorado Mineral Belt. Similar changes in detachment zone depth below both Laramide and Ancestral Rocky Mountain structures may explain many of their structural complexities.