Structural Geology of the Central Utah Thrust Belt

Abstract The central Utah thrust belt is located between the Uinta-Cottonwood arch to the north and the Tushar and Monroe Mountain calderas to the south. Structural analysis distinguishes three structural-tectonic zones along the thrust belt, including, from north to south, the Charleston-Nebo salient, the Gunnison sector, and the Salina sector. The Charleston-Nebo salient is underlain by the Charleston-Nebo thrust sheet and includes, from east to west: (1) a frontal imbricate fan in the footwall of the Charleston-Nebo thrust and restricted to the Mesozoic section of the adjacent Uinta Basin, (2) a frontal structural uplift involving the Pennsylvanian-Permian section in the hanging wall of the leading edge to the Charleston-Nebo thrust, (3) a central structural depression where the Triassic and Jurassic sections of the thrust sheet have been preserved, and (4) the Cascade Peak culmination along which a locally overturned hanging wall anticline to the Charleston-Nebo thrust is exposed. The Cascade Peak culmination is believed to be underlain by several Paleozoic-Mesozoic thrust sheets that define an antiformal stack. The Gunnison sector of the thrust belt is characterized by a frontal triangle zone that has developed above a detachment surface within the Jurassic Arapien Shale, a central zone of horizontally stacked thrust sheets that involve Paleozoic through Middle Jurassic units, and an internal zone of vertically stacked thrust sheets that define a duplex within Precambrian and Paleozoic strata. The Salina sector is characterized by a frontal triangle zone within and above the Arapien Shale that is underlain by a Paleozoic-cored fault-bend fold, the Salina anticline, located in the hanging wall of the Salina thrust. The recently discovered Covenant oil field is located along the Salina anticline where a back thrust has resulted in a perched thrust sheet on the crest of the larger-scale fault-bend fold. To the west of the Salina anticline, the Salina sector of the thrust belt includes a series of vertically stacked thrust sheets composed of Paleozoic through Jurassic strata. These stacked thrust sheets appear to have developed as an emergent imbricate fan rather than as a duplex. The deformation fronts of the Gunnison and Salina sectors are located in the vicinity of the Jurassic Ephraim fault system, across which the mudstone-, shale-, and evaporite-rich Arapien Shale thickens to the west. The Arapien Shale defined a significant detachment horizon during both Cretaceous shortening and Tertiary extension. The Ephraim fault system is therefore believed to have played a significant role, both directly and indirectly, in the structural evolution of the central Utah thrust belt.