24 Tectonics, Volcanism, and Sediment Dispersal Within and Proximal to the Rio Grande Rift and Southern Rocky Mountains (Posters)

This session integrates research on coupled tectonics, volcanism, and sediment dispersal in the Rio Grande Rift and southern Rockies. We examine how extensional faulting controls volcanic processes and reshapes sediment routing from Colorado through Texas.

Tuesday, 19 May 2026
8:00 AM - 5:30 PM
Alvarado D/E

Pennsylvanian-Permian Folding and Thrust Faulting in the Sangre de Cristo Mountains, Southern Colorado: New Insights into Ancestral Rocky Mountain Deformation along the Northeastern Margin of the Uncompahgre Uplift

Paper number: 24-6
Program: Technical Sessions
Time: 8:00 AM – 8:00 AM

Authors

Abstract

The northeastern margin of the Uncompahgre uplift of the Ancestral Rocky Mountains (ARM) preserves evidence for syndepositional deformation within the Pennsylvanian-Permian deposits of the Central Colorado trough. The Gibson Peak syncline in the Sangre de Cristo Mountains near Crestone, Colorado is a major NE-vergent, NW-trending fold developed within the Crestone Conglomerate Member of the Pennsylvanian-Permian Sangre de Cristo Formation. This syncline is located ~1–4 km northeast of the SW-dipping Crestone thrust, which juxtaposes Proterozoic crystalline basement and Mississippian, Devonian, and Ordovician strata in the hanging wall against Pennsylvanian-Permian strata in the footwall. Previous studies interpret the Crestone thrust and Gibson Peak syncline as predominantly ARM structures, whereas other studies suggest they are primarily Laramide in age. We investigate these relationships with detailed geologic mapping and field-based structural analysis of the Crestone thrust and Gibson Peak syncline coupled with multi-method thermochronology from a sample in the hanging wall of the Crestone thrust.

 

Detailed geologic mapping within the Pennsylvanian–Permian section documents NE-vergent growth folding and two intraformational unconformities within the Crestone Conglomerate in the footwall of the Crestone thrust. Along the southwest limb of the Gibson Peak syncline, vertical to overturned SW-dipping beds are unconformably overlain by upright NE-dipping beds with up to 30° of angular discordance across the unconformity. The Gibson Peak syncline in the uppermost exposed part of the section, ~1.5 km above the base of the Sangre de Cristo Formation, has an interlimb angle that is ~20–30° less than interlimb angles in beds ~0.5 km above the base, consistent with progressive growth during deposition. Minor thrust faults within 250 m of the Crestone thrust record a mean subhorizontal shortening direction of 041°, oriented 88° from the mean fold axis trend of the Gibson Peak syncline, indicating kinematic compatibility between NE-directed thrusting and folding. Thermal history models of a 1.7 Ga granite sample from the hanging wall of the Crestone thrust based on multiple thermochronometers (apatite U-Pb and fission track, K-feldspar Ar/Ar, zircon fission track and (U-Th)/He) indicate minimal cooling (<50°C) during the Laramide orogeny, suggesting minor to no Laramide thrust-related exhumation. We interpret the Gibson Peak syncline to be an ARM structure associated with the NE-directed thrusting of the Crestone thrust or a similar, structurally higher thrust that has been eroded away. 

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

doi: 10.1130/abs/2026RM-410559

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