Fluid inclusion evidence for minimum 11 km vertical offset on the Wasatch fault, Utah

Abstract

The footwall of the Wasatch fault in the Corner Creek area near Salt Lake City is hydrothermally altered and deformed quartz monzonite of the Oligocene Little Cottonwood stock. Secondary fluid inclusions are associated with hydrothermal alteration minerals and structural deformation. Thermometric measurements of fluid inclusion characteristics indicate entrapment of a CO2-H2O-NaCl fluid at minimum temperatures of 223–353 °C and minimum fluid pressures of 900–2800 bar. The 2800-bar fluid pressure is near lithostatic pressure at a depth of 11 km, the minimum displacement of the fault that is required for exhumation of the observed alteration and fluid inclusions and more than three times greater than most previous estimates. The large displacement estimate is supported by the occurrence of pyrophyllite in shales in the footwall, by the petrology of metamorphosed shale surrounding an unroofed pluton in the footwall, and by geologic reconstruction of the eroded footwall.