TECTONIC EVOLUTION OF THE NORTHERN SANGRE DE CRISTO MOUNTAINS, COLORADO, USA

The Sangre de Cristo Mountains of southern Colorado and northern New Mexico are uniquely situated to record the Phanerozoic intraplate tectonism of North America. Geologic structures and orogenic sediments within the mountains provide evidence linking the rise of the Ancestral Rocky Mountains, the Laramide orogeny, and Rio Grande rifting.

Thrusts arising from a hinterland beneath the area now occupied by the San Luis Valley carried sheets of Proterozoic basement rocks >10 km northeast over a foreland of Paleozoic and Mesozoic rocks. Evidence for basement thrust sheets includes 1) low-angle faults carrying Proterozoic rocks over Paleozoic rocks, 2) Paleozoic sedimentary formations of contrasting facies in hanging walls and footwalls of thrusts, 3) windows of Paleozoic rocks beneath thrust sheets of Proterozoic rocks, and 4) repetition of thin (tens of meters) slices of strata beneath thrusts in windows. Thrusts extending into the foreland northeast of the basement sheets involve sedimentary rocks as young as 49 Ma.

A simple fault-bend thrust model is proposed to account for emplacement of the basement thrust sheets and the deformation of the foreland. Ancestral Rockies faults may have provided zones of inferred weakness for initiation and localization of Laramide thrusts, leading to development of hinterland ramps in basement rocks, floor thrusts in lower Paleozoic sedimentary rocks, and ramps cutting up to roof thrusts in Mesozoic shale. We propose that Laramide hinterland ramps emerged from the brittle-plastic regime of the Earth’s crust, perhaps >10 kilometers below the surface beneath the present San Luis Valley. On the west side of the range, bedrock shear zones with low-angle contractional microstructures show crystal-plastic deformation of quartz overprinted by extensional microstructures that may record the transition from Laramide contraction to Rio Grande extension of the crust about 30 Ma ago. The basement thrust sheets left no clear record of surface uplift in the Laramide foreland basins east and south of the northern Sangre de Cristo Mountains. Major rock uplift and exhumation of the range probably did not begin until Rio Grande rifting.