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/or Laramide Deformation in the Arkansas Hills, central Colorado; Who’s Fault is it?

Paper number: 17-1
Program: Technical Sessions
Time: 4:15 PM – 4:35 PM

Authors

Abstract

The Central Colorado trough of the Ancestral Rocky Mountains (ARM) system preserves evidence of late Paleozoic deformation adjacent to major range-bounding structures. In the southern Arkansas Hills, near the western margin of the Apishapa uplift, angular unconformities and growth folds involving Pennsylvanian-Permian strata record ARM deformation adjacent to the Pleasant Valley (PV) fault. The PV fault is a major north-striking structure that juxtaposes Proterozoic basement against the Pennsylvanian-Permian Sangre de Cristo Formation and is locally cut by the Late Cretaceous Whitehorn Granodiorite (Kwg). Previous studies have interpreted the PV fault to be active during Pennsylvanian-Permian deposition, while others interpret the fault as a W-dipping normal fault associated with the Rio Grande Rift. We investigate these relationships with detailed geologic mapping and field-based structural analysis of the PV fault, Kwg, and adjacent growth folds and angular unconformities.

 

Map patterns of the PV fault indicate a moderate east dip, and minor fault data collected within and adjacent to the PV fault core record overall ENE-WSW shortening with thrust-sense kinematics. The uppermost Sangre de Cristo Formation in the footwall is folded into a west-vergent overturned syncline, consistent with west-directed thrusting. The PV fault does not appear to offset the intrusive contact between Kwg and Sangre de Cristo Formation, indicating little to no slip along the PV fault following the intrusion of Kwg at ~67 Ma. However, argillic alteration is pervasive within Kwg in a ~20-200 m-wide, NW-trending zone that is aligned along strike with the PV fault, suggesting structurally controlled fluid flow. Minor reverse faults in Kwg record overall ESE-WNW directed shortening during the Laramide orogeny. Detailed mapping within the Pennsylvanian-Permian section has confirmed an intraformational angular unconformity within the Sangre de Cristo Formation with up to 74° of angular discordance. Based on fold limb thickness along anticlines, up to ~0.5 km of section is missing beneath the unconformity. Detailed bedding orientation data indicates that map-scale SE-plunging folds predate the development of the unconformity, whereas one E-plunging syncline developed during deposition of strata below and above the unconformity. We interpret the PV fault and associated folding within the Pennsylvanian-Permian section to dominantly record overall ENE-WSW ARM shortening with limited Laramide overprint.

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

doi: 10.1130/abs/2026RM-410558

© Copyright 2026 The Geological Society of America (GSA), all rights reserved. Permission is hereby granted to the author(s) of this abstract to reproduce and distribute it freely, for noncommercial purposes. Permission is hereby granted to any individual scientist to download a single copy of this electronic file and reproduce up to 20 paper copies for noncommercial purposes advancing science and education, including classroom use, providing all reproductions include the complete content shown here, including the author information. All other forms of reproduction and/or transmittal are prohibited without written permission from GSA Copyright Permissions.