GRABENS OF THE FISH LAKE PLATEAU, HIGH PLATEAUS, UTAH

The Fish Lake Plateau of south-central Utah is located in the structural transition zone between the Basin & Range and Colorado Plateau provinces. The structural geometry of the Fish Lake Plateau is characterized by a suite of grabens of varying size, orientation, and age. The plateau is underlain by Tertiary volcanic rocks that include three distinct units, a basal andesite, a trachyte, and the Osiris Tuff. Volcanic rocks unconformably overlie the early Tertiary Flagstaff Formation. The oldest graben on the Fish Lake Plateau is the upper Fremont River graben a 12 x 3 km, NW-SE trending basin. The graben is filled with a syn-rift sequence of poorly cemented, sandstone and conglomerate. The rift fill deposits are eroded and capped by boulder armored paleo-surfaces ~150 m above the modern Fremont River. We determined ³He exposure ages from pyroxene phenocrysts in large andesite boulders exposed on the paleo-surfaces. Corrected ³He exposure ages range from 700 to 900 ka. These are minimum ages and as such constrain a maximum incision rate of 0.2 mm/yr for the Fremont River. The main Fish Lake basin is a 4 km-wide asymmetric graben formed by a suite of NE-SW striking faults. Volcanic rocks are displaced by ~300 m along a major NW-dipping fault and flanked by related synthetic and antithetic faults. The Crater Lakes occupy an internally drained graben in the Mytoge Mountains 2-5 km NE of Fish Lake. Displacement along these faults ranges from 60 to 140 m. Transverse faults bound the north and south ends of the Crater Lakes graben and a horst occurs at the north end. Cedarless Flat is a 3 x 1 km graben located 15 km SE of Fish Lake. The Cedarless Flat graben is bounded by two NE-SW striking faults that accommodate ~110 m of displacement. Gravity data suggests up to 60 m of valley fill sediment. The faults truncate a previously existing drainage network that has since incised into a ~700 ka Fremont River terrace. We postulate that two episodes of graben development occurred on the Fish Lake Plateau. The first episode (pre-900 ka) formed NW-SE trending grabens while the second episode (post-900 ka) formed NE-SW trending grabens.