STRUCTURAL AND FLUVIAL HISTORY OF THE FISH LAKE BASIN, HIGH PLATEAUS, CENTRAL UTAH

The Fish Lake basin, in the High Plateaus of central Utah is located in the structural transition zone between the Basin & Range and Colorado Plateau provinces. Fish Lake (~2700 m) occupies a northeast-southwest trending topographic depression, bounded by the Fish Lake Hightop Plateau and the Mytoge Mountains. Fish Lake drains northeast into the Fremont River. Previous workers, including G.K. Gilbert and C.E. Dutton, interpret the basin to be a structural graben and suggest neotectonic faulting effected a recent drainage reversal at the southwestern end of Fish Lake.

We mapped 150 km² in the southern Fish Lake Plateau to better understand the structural geometry of the basin. The area is underlain by a 600 m-thick sequence of densely-welded ash-flow tuffs including (from bottom to top): 1) phenocryst-rich andesite, 2) highly vesicular to non-vesicular eutaxitic porphyritic andesite, 3) glass-rich phenocryst-poor andesite, and 4) spherulite-rich porphyritic andesite. The sequence is capped by the Osiris Tuff, a densely-welded porphyritic dacite dated by previous workers at 21 Ma. Tuffs experienced a protracted pre-eruptive history evidenced by magma mixing textures as well as post-eruptive deuteric alteration. Fish Lake basin is an asymmetric graben formed by a suite of northeast-southwest striking normal faults. Displacement of ~300 m is accomplished along a major northwest-dipping fault flanked by related synthetic and antithetic faults. Faults are steeply dipping with a listric geometry and total extension across the Fish Lake basin is <10%.

Late Pleistocene moraines cover faults indicating no recent tectonism in the Fish Lake basin. Faulting in the Fish Lake basin probably occurred 5-10 Ma ago, contemporaneous with deformation in nearby regions of the High Plateaus. Geomorphic features are inconsistent with a drainage reversal at the southwestern end of Fish Lake. We suggest that Fish Lake was originally an internally drained basin that has been captured by the Fremont River. Paired terraces on the Fremont River ~120 m above the present river elevation may represent the Fremont’s pre-capture profile. We are attempting to date this surface using cosmogenic radionuclide techniques.