17 Structural Inheritance in the Rocky Mountain Region

The Rocky Mountain region preserves a complex tectonic history shaped by multiple contractional and extensional events from the Proterozoic to present. This session examines the role of structural inheritance in guiding deformation across superposed orogens.

Monday, 18 May 2026
4:10 PM - 5:30 PM
Alvarado F

Ancestral Rocky Mountain and Laramide Deformation in the Sangre de Cristo Range, Colorado: The Evolution of a Multi-Stage Thin- and Thick-Skinned System

Paper number: 17-2
Program: Technical Sessions
Time: 4:35 PM – 4:55 PM

Authors

Abstract

The Sangre de Cristo Range of south-central Colorado records contractional deformation associated with the Pennsylvanian–Permian Ancestral Rocky Mountain (ARM) and Late Cretaceous–Early Paleogene Laramide orogenies. Differentiation of structures associated with either event is difficult due to similar geometries, and limited involvement of Mesozoic strata. We present a structural model of the Phanerozoic contractional history based on new detailed geologic mapping and thermochronology. Removal of the Pennsylvanian Minturn Formation and deposition of the Pennsylvanian–Permian Crestone Conglomerate on Proterozoic basement in the hanging wall of the moderately to steeply southwest-dipping, oblique reverse-sinistral Sand Creek fault (SCF) requires slip during ARM deformation; however, involvement of the Minturn Formation in the Deadman Creek thrust ~8km southwest suggests the presence of Pennsylvanian strata across the region prior to faulting. The SCF cuts across southwest-dipping strata in the footwall, suggesting out-of-sequence slip following folding and NE-directed slip on the structurally-lower Marble Mountain thrust. The steeply NE-dipping, oblique reverse-dextral Little Sand Creek fault (LSCF) system involves growth strata in the Crestone Conglomerate including an intraformational angular unconformity that truncates portions of the LSCF system, suggesting an ARM origin of the LSCF system and a proximal sediment source to the southwest during faulting. Involvement of Mesozoic strata in ENE-vergent thrusts on the eastern range front—and documented Laramide cooling in the hanging wall of the moderately NE-dipping Alvarado fault—suggests development of a Laramide thrust system that potentially truncates misaligned ARM structures. Modeling of multiple thermochronometers (biotite and K-feldspar Ar/Ar; zircon and apatite (U-Th)/He) from a 1.4 Ga granitoid in the hanging wall of the SCF permits up to ~100°C of cooling during ARM deformation. More robust time-temperature constraints during the Laramide orogeny suggests burial/re-heating to 160–180°C and little to no exhumation until the onset of Miocene extensional exhumation at ~17–16 Ma. We propose the structural evolution of the ARM/Laramide contractional system involves: 1) Northeastward propagation of the ARM deformational front and development of thin-skinned faults (e.g., the Marble Mountain thrust) in the Central Colorado trough. 2) Accommodation of deeper crustal shortening by high-angle, basement-involved oblique-reverse faults and erosion of Pennsylvanian–Permian strata to the southwest. Establishment of a thick-skinned transpressional system involving the SCF and LSCF system. 3) Development of a Laramide thrust system and burial of ARM structures to 6–8 km.

Geological Society of America Abstracts with Programs. Vol. 58, No. 4

doi: 10.1130/abs/2026RM-410651

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