1. EVALUATION OF THRUSTING AND FOLDING OF THE DEADMAN CREEK THRUST
FAULT, SANGRE DE CRISTO RANGE,
SAGUACHE COUNTY,
COLORADO
By: Jacob Weigel
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

2. 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.