Normal Faulting and Graben Development as Catalysts for
Late Cenozoic Landscape Change, Fish Lake Plateau, Utah

Bailey Christopher, College of William & Mary
Erika Wenrich, College of William & Mary
Hanna Bartram, College of William & Mary
David Marchetti, Western Colorado State University
Scott Harris, College of Charleston

Normal fault systems create accommodation space for sediment infill and, in continental settings, segment and modify existing drainage networks. The interactions between extensional structures, sedimentation, and erosion play an important role in regional landscape evolution.

In south-central Utah the Fish Lake Plateau (FLP) forms a distinct high-elevation structural block in the transition zone between the Colorado Plateau and Basin & Range provinces. The FLP is underlain by a thick sequence of Tertiary volcanic rocks that unconformably overlie Cretaceous to Paleogene strata. Multiple generations of steeply-dipping normal faults cut bedrock units and strongly influence the topographic character of the FLP. Early-formed drainage networks developed on a gently southeast-sloping landscape that resulted from progressive rotation in the hanging wall of the Thousand Lake fault system, a major structure with ~1 km of throw that juxtaposes mid-Tertiary volcanic rocks against Jurassic strata of the Colorado Plateau.

Multiple graben complexes segment the FLP. In the eastern FLP, a suite of en-echelon NNW-trending grabens becomes progressively younger towards the east. Graben-bounding faults offset Pliocene trachybasalt and basin-fill deposits are overlain by 0.5 to 0.9 Ma boulder diamicts on Fremont River strath terraces. Fish Lake, a ~10 km2 lake at 2,700 m in elevation, occupies a wide graben produced by ~600 m of displacement on a suite of NE-striking faults. Gravity data indicates the Fish Lake basin is underlain by 100 to 225 m of sediment, including at least two sequences of Pleistocene glaciogenic sediment. The Fish Lake graben complex cuts older NNW-trending grabens and truncates the early-formed SE-trending drainage network.

Normal faulting on the FLP accommodated only modest crustal extension (~5%), yet severed older drainage networks producing internally drained basins and local sediment depocenters. The variable orientation of FLP grabens records orthogonal extension of the region during the last 5 My. Normal faults in the FLP region likely formed due to a combination of 1) far-field stresses associated with Basin & Range extension, 2) a uniaxial stress field generated by segmentation associated with uplift and topographic isolation of the FLP, and 3) fluvial erosion along the flanks of the FLP which produces a free surface that initiates gravity-driven extension in the mechanically weak Paleogene strata and dilational normal faulting in overlying volcanic units.