POTENTIAL ACTIVE FAULTING IN SOUTHWESTERN COLORADO REVEALED BY LIDAR AND RECONNAISSANCE-LEVEL MAPPING

The San Juan Mountains region in southwest Colorado has few mapped active faults and is tectonically situated between the Rio Grande Rift to the east, and a zone of northwest-directed oblique-dextral shear to the south in northern New Mexico. New lidar datasets encompassing the San Juan Mountains provide an opportunity to assess fault activity for seismic hazard assessment in southwest Colorado. Here, we investigate the recency of slip on the Cannibal and Stinking Springs faults through reconnaissance geomorphic and lineament mapping.

The Cannibal fault is a NS-oriented ~50-km-long fault that parallels the eastern margin of the Clear Creek Graben. In places, the Cannibal fault is expressed as topographic lineaments across late Pleistocene glacial and alluvial deposits. A Pinedale-age moraine complex near Spring Creek Pass is crossed by a lineament that may represent west-side-down vertical separation across the Cannibal fault.

The Stinking Springs fault is a ~45-km-long en-echelon stepping fault system that is expressed as NW-oriented escarpments within Cretaceous sandstone. The fault crosses three Quaternary terraces near Dutton Creek that are vertically separated across the fault, which may be explained by Quaternary fault displacement, but may also be explained by a miscorrelation of terraces across the fault. The surfaces preserve an apparent increase in vertical separation with age demonstrating possible progressive deformation. The youngest terrace is vertically separated by 28 m. Soil descriptions and correlation of the terraces are planned to assess the geomorphic relationships and possibility of Quaternary fault activity.

Our mapping reveals multiple sites where Quaternary surfaces may be faulted, providing preliminary evidence for possible Quaternary activity on these faults. Future work, including more detailed mapping and cosmogenic dating of offset landforms, will provide constraints on the fault slip rate and recency of activity. Only the Cannibal fault is included in previous active fault compilations. The updated fault characterization of the Cannibal and Stinking Springs faults may redistribute and more accurately represent the seismic hazard in southwest Colorado.