STRUCTURAL GEOMETRY OF THE EASTERN FISH LAKE PLATEAU, UTAH

The Fish Lake Plateau lies in the transition zone between the structurally distinct Colorado Plateau and Basin and Range provinces. Multiple normal faults cut the Cenozoic volcanic and underlying sedimentary units of Fish Lake Plateau (FLP). Our research is focused on documenting the structural geometry and deformation history of eastern FLP. A broad gentle SE slope, NW trending topographic lineaments with drainages to the SE, and several NW trending topographic lineaments characterize the landscape of the eastern FLP. The structural geometry of the eastern FLP is dominated by three en-echelon NNW trending grabens: the Fremont River graben, UM Creek graben, and West Tidwell Canyon graben. Graben-bounding faults strike 340°±10°, with lengths between 7-15 km, and graben widths of 1-2.5 km. Total displacement on individual faults is ~100-250 m. A set of graben perpendicular faults strike 070°±10°. Cross-section restoration reveals modest extension (~3-7%) in a ENE-WSW direction. Two temporally distinct deformation events affected the FLP. An offset and tilted 4.9 Ma trachybasalt flow and ~1 Ma terrace deposits in the Fremont River graben, which formed post-faulting, indicate that the NNW trending grabens formed between 1-5 Ma. Decreasing width/height ratios ([graben wall to wall]/[valley floor + 50 m]), indicate a progression from a graben with low amounts of incision and scarp retreat to a graben with high amounts of incision and scarp retreat. Assuming these differences are time-dependent, the decreasing width/height ratios from east to west of the West Tidwell Canyon graben (22±5), UM Creek graben (15±7), and the Fremont River graben (9±4) suggest that the West Tidwell Canyon graben is the youngest and the Fremont River graben is the oldest. The faults perpendicular to the graben walls formed contemporaneously with the NNW grabens most likely due to stresses produced during deformation. A second deformation event forming the NE striking faults occurred after 1 Ma. The grabens in the eastern FLP likely formed independently of Basin and Range deformation, and may be the result of block-segmentation associated with late Cenozoic plateau uplift.