STRUCTURAL AND VOLCANIC EVOLUTION OF SOUTHERN CURLEW VALLEY, NORTHWEST UTAH

The Raft River detachment fault, Yellowstone hotspot, and Sevier rift influenced the Neogene and Quaternary development of geologic units, structures, and landforms in southern Curlew Valley, northwest Utah. Along the east flank of the Raft River Mountains and the south side of the Wildcat Hills, tuffaceous sedimentary rocks of the Miocene Salt Lake Formation occur depositionally on Paleozoic rocks. Exposures include massive gray air-fall tuffs that were erupted from volcanic centers along the track of the Yellowstone hotspot from ~10-6.5 Ma. The rocks were deposited in synextensional basins that probably developed in the hanging wall of the east-dipping Raft River detachment fault. The Miocene strata are complexly folded and tilted, and underlying Paleozoic rocks are brecciated, possibly the result of deformation within the hanging wall that occurred during detachment faulting. Concealed high-angle faults identified by geophysical and well log data likely represent two phases in the extensional history Curlew Valley. The faults apparently accommodate 1) extension in the hanging wall of the Raft River detachment fault, which may have been active until ~7.5 Ma, and 2) regional widespread mantle upwelling and development of the Sevier rift. Extension attributed to development of the Sevier rift was accompanied by eruptions of basalt along the southwest side of Curlew Valley during the late Miocene and early Pliocene (~8.6-5.0 Ma). Two apparent episodes of bimodal volcanism occurred near the center of Curlew Valley, in the vicinity of the Wildcat Hills: 1) rhyodacite and basalt erupted during the early Pliocene (~4.9-3.6 Ma), and 2) rhyolite and basalt erupted during the early Pleistocene (~2.3-2.2 Ma). Younger Pleistocene (~1.16-0.44 Ma) basalt flows that represent the youngest rift-related volcanism in northern Utah form three shield volcanoes west of and parallel to the fault-controlled east edge of Curlew Valley. The east-west limits of volcanism appear to define the approximate boundaries of the Sevier rift, and vent alignment parallel to regional structural grain is indicative of a tectono-magmatic system. Many of the mafic lavas of Curlew Valley have chemical similarities to Snake River Plain lavas, suggesting connectivity between the Sevier rift and Snake River Plain volcanic provinces.