CENOZOIC EXTENSIONAL REACTIVATION OF THE CRETACEOUS MUDDY MOUNTAIN THRUST FAULT, SEVIER OROGENIC BELT, SOUTHERN NEVADA

The Muddy Mountain thrust (MMT), of the Muddy Mountains (MM) of southern Nevada, is a frontal thrust of the Sevier orogenic belt. In much of the MM, the thrust as defined by Bohannon (1983, GSA Memoir 157) places Cambrian Bonanza King Formation over Jurassic Aztec Sandstone or, locally, Cretaceous foreland basin deposits. The thrust strikes ENE to NE and dips gently ~W except in the northern MM where a section called the Arrowhead fault strikes E-W and dips steeply.

Two lines of evidence from new detailed mapping suggest that the MMT was reactivated as a low-angle normal fault during Cenozoic extension. (1) High-angle normal faults cut the upper plate of and end at the MMT. The geometry and kinematics of these faults suggest that as they moved, slip was transferred from them onto the MMT reactivating it. (2) The units directly above the MMT differ between blocks bounded by the upper-plate normal faults. In several areas, Mississippian or Devonian units lie directly above the MMT both between and down dip of locations where Cambrian rocks immediately overlie the MMT. These relations are contrary to a standard thrust where a single unit lies directly above it and / or it cuts up section in the transport direction. Extensional reactivation of the MMT allows the upper-plate normal faults to drop higher parts of the thrust plate down onto the MMT as it moved in extension.

Reactivation has implications for slip on the Arrowhead fault. The Arrowhead fault crops out where the MMT footwall changes from autochthonous Aztec Sandstone to folded rocks of the Summit - Willow Tank thrust plate (defined by Bohannon, 1983). The Arrowhead fault is interpreted here as a lateral ramp in the MMT that was reactivated as an extensional transfer fault.

The age of reactivation is unclear, but is probably Miocene. Regionally, major extension occurred at that time. Miocene extension occurred on the Mormon Peak detachment (~50 km N) and on the Saddle Island detachment (~ 25 km S). The Miocene Horse Spring Formation does not provide a timing bracket because this unit was deposited in a basin partly defined by the MMT plate. Younger normal faults cut the Horse Spring Formation and the MMT. These post-reactivation faults strike NE and NW, and appear to be a conjugate set. Offset of the MMT by these faults suggests that reactivation of the MMT occurred relatively early in the extensional deformation.