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Pennsylvanian sinistral faults along the southwest boundary of the Uncompahgre uplift, Ancestral Rocky Mountains, Colorado

¹ Department of Geological Sciences, University of Kentucky, Lexington, Kentucky 40506-0053, USA

Resolution of the large-scale kinematics of the Pennsylvanian-Permian Ancestral Rocky Mountains requires definition of sense of slip and time of movement on specific faults within the system of fault-bounded basins and uplifts (e.g., Paradox basin, Uncompahgre uplift). Along an east-west–striking segment of the boundary between the Paradox basin and Uncompahgre uplift, a system of steep faults defines a horst and graben system, which includes the Grenadier fault block. Positive flower structures (anticlines) along both the bounding (Coal Bank Pass and Molas Creek faults) and internal (Snowdon fault) faults of the Grenadier fault block indicate strike-slip displacement, and oblique subsidiary folds show sinistral slip. On both sides of the Snowdon fault, stratigraphic thinning of parts of the Pennsylvanian Hermosa Group toward the fault documents synsedimentary growth of the positive flower structure, suggesting Pennsylvanian sinistral slip along this part of the Ancestral Rocky Mountains. Sinistral strike-slip along one east-west–striking segment of the fault system between the Paradox basin and Uncompahgre uplift is compatible with reverse slip on northwest-striking segments, demonstrating that a regional assembly of data to define sense of slip and time of movement on specific faults will better constrain regional-scale models for kinematics and mechanics of Ancestral Rocky Mountains deformation.

Keywords: Ancestral Rocky Mountains • sinistral fault • synsedimentary fault • Pennsylvanian • Uncompahgre uplift