130-9 Superposition of Laramide Shortening and Ancestral Rocky Mountain Transpression in the Sangre De Cristo Mountains, Southern Colorado

Session: Evolution of Orogenic Belts Through Time: Insights from Sedimentation, Deformation, Magmatism, and Metamorphism, Part II

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Authors:

Abstract:

The Sangre de Cristo Mountains in southern Colorado record polyphase contraction that spans the Paleoproterozoic through the Early Cenozoic. In particular, it has been suggested that the range has experienced deformation associated with both the Pennsylvanian–Permian Ancestral Rocky Mountain (ARM) and Late Cretaceous–Early Paleogene Laramide orogenies. We investigate the geometries and kinematics of contractional structures and evaluate possible syndepositional deformation within Pennsylvanian–Permian strata through 1:10,000 scale geologic mapping and detailed structural analysis around the Music Pass/Sand Creek area. Mapping has revealed a possible intraformational unconformity within the Pennsylvanian–Permian Crestone Conglomerate of the Sand Creek syncline, as well as possible syn-depositional faults within the Crestone Conglomerate associated with the Little Sand Creek fault (LSCF) system. Fault kinematic data within the LSCF system indicates oblique dextral/northeast-side-up slip along high angle, northeast-dipping faults compatible with northeast-southwest directed shortening. The LSCF system is kinematically compatible with the Sand Creek fault (SCF) which juxtaposes Mesoproterozoic basement against Pennsylvanian–Permian strata and records oblique-sinistral/southwest-side-up slip along a moderately-to-steeply southwest-dipping zone, also consistent with northeast-southwest directed shortening. The northwest-southeast trend of the upright Sand Creek syncline is also compatible with this shortening direction and may have formed as a fault propagation fold associated with the LSCF system. High angle, oblique-reverse slip associated with both the LSCF system and the SCF, as well as the splayed nature of the LSCF system, is consistent with a transpression dominated contractional regime. Oriented fault rock samples from both structures reveal minor quartz plasticity in the form of undulose extinction, deformation lamellae, and localized bulging recrystallization in cataclastic zones. This minor quartz plasticity, as well as the abundance of epidote coated faults within the Crestone Conglomerate, suggests deformation temperatures on the upper end of the brittle regime during faulting. Although field relations suggest an ARM origin for these structures, Laramide thrust faults involving Mesozoic strata are present along the eastern flank of the range, and the extent of overprinting due to Laramide contraction is still not clear. Detailed thermochronology of the Mesoproterozoic Music Pass quartz monzonite, located in the upthrown block of both the LSCF and the SCF, should provide more definitive timing for the exhumation history of this fault complex. 

Geological Society of America Abstracts with Program. Vol. 57, No. 6, 2025

doi: 10.1130/abs/2025AM-8464

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