Evaluation of thrusting and folding of the Deadman Creek thrust fault, Sangre de Cristo Range, Saguache County, Colorado

Abstract

The Deadman Creek Thrust Fault was mapped in a structural window on the west side of the Sangre de Cristo Range. The study area, located in southern Colorado, is a two square mile area halfway between the town of Crestone and the Great Sand Dunes National Park. The Deadman Creek Thrust Fault is the center of this study because it delineates the fold structure in the structural window. The fault is a northeast-directed low-angle thrust folded by subsequent additional compression. This study was directed at understanding the motion of the Deadman Creek Thrust Fault as affected by subsequent folding, and the driving mechanism behind the folding of the Pole Creek Anticline as part of a broader study of Laramide thrust faulting in the range. This study aids in the interpretation of the geologic structure of the San Luis Valley, which is being studied by staff of the United States Geological Survey (USGS), to understand Rio Grande Rift basin evolution by focusing on rift and pre-rift tectonic activity. It also provides a geologic interpretation for the Saguache County Forest Service, Great Sand Dunes National Park, and its visitors. The Sangre de Cristo Mountain Range has undergone tectonic events in the Proterozoic, Pennsylvanian (Ancestral Rocky Mountains), Cretaceous-Tertiary (Laramide Orogeny) and mid-Tertiary (Rio Grande Rift). During the Laramide Orogeny the Deadman Creek Thrust Fault emplaced Proterozoic gneiss over Paleozoic sedimentary rocks and Proterozoic granodiorite in the area. Continued deformation resulted in folding of the fault to form the Pole Creek Anticline. The direction of motion of both the fault and fold is northeastward. A self-consistent net of cross-sections and stereonet plots generated from existing and new field data show that the anticline is an overturned isoclinal fold in Pole Creek Canyon, which shows an increasing inter-limb angle and a more vertical axial surface northwestward toward Deadman Creek Canyon. Southwest-directed apparent normal fault motion reflects out-of-syncline thrust faulting primarily on the forelimb of the anticline, which has subsequently been overturned by further tightening of the anticline. The driving force of the anticline is inferred to be a propagating reverse fault breaking toward the surface and causing the Deadman Creek Thrust Fault to fold, forming the Pole Creek Anticline. This fault appears to have a complex geometry that causes the fold axis to change orientation in two locations within the study area. Furthermore, diverse fault motions indicated in stereonet plots suggest a complex deformation system in these massive rock units. A syncline (Alpine Gulch Syncline) to the southwest of the Pole Creek Anticline becomes more open to the southeast. The driving force for the Alpine Gulch Syncline is not understood, but may also have affected the Pole Creek Anticline. Additional complexities include two minor faults north of the Pole Creek Canyon mouth, an inferred fault in Pole Creek Canyon, and a second inferred fault in Deadman Creek Canyon. These complexities make structural interpretation challenging.