NORTHERN EXTENT OF THE TINTIC VALLEY THRUST FAULT AND RELATED SYNOROGENIC DEPOSITS IN THE WESTERN PART OF THE PROVO SALIENT, SEVIER OROGENIC BELT, UTAH

New 1:24,000-scale geologic mapping is ongoing under the UGS/USGS STATEMAP program in three 7.5' quadrangles centered on the Vernon Hills in eastern Tooele County, Utah. This mapping provides constraints on the complex western geology of the Provo (or Charleston-Nebo) salient, a major eastward-convex salient in the Sevier fold and thrust belt of central Utah. The eastern half and southern margin of the salient, its major thrust sheets, and corresponding synorogenic deposits have been well described by previous workers, and are considered type localities for arcuate fold and thrust belt geometry. The western part of the salient is more complex and is strongly overprinted by post-thrusting extension and basin formation. Major named thrust faults of the western portion of the salient include (west to east): Sheeprock thrust, Tintic Valley thrust, East Tintic-Stockton thrust, and Midas thrust. The true extent, offset, and timing of movement along these faults is poorly known. Bedrock exposed in the Vernon Hills comprises a portion of the western salient and may include important exposures of the Tintic Valley thrust and related synorogenic deposits. Exposures consist of overturned and faulted Lower Ordovician through Upper Mississippian rocks in the south, and complexly folded and faulted Upper Mississippian through Permian rocks of the Oquirrh basin in the north. The juxtasposed stratigraphy and structural relations between these two zones is similar to exposures along the Tintic Valley thrust in the Gilson Mountains to the south, and likely represents the thrust's northern extent. Near and overlying this structure is a section of steeply dipping, faulted and folded, locally-sourced red-bed conglomerate, sandstone, mudstone, and limestone. These deposits likely comprise a section of synorogenic sediments deposited during movement along the Tintic Valley thrust and/or internal shortening and folding of the upper thrust plates within the Provo salient. Results from new 40Ar/39Ar dating of reworked volcanic ash, interbedded within the conglomerate, may constrain early internal development of the Provo salient. Other volcanic and younger lacustrine rocks in the quadrangles record subsequent extensional collapse and basin formation within the western part of the Provo salient.