Explanation for 1:12 000 Scale Geologic Map of the South Virgin Mountains, Southeastern Nevada and Northwestern Arizona: Supplement 6 from "The Geology of the Gold Butte Breakaway Zone and the Mechanical Evolution of Normal Fault Systems" (Thesis)

The Gold Butte breakaway zone is the easternmost and oldest of the major Tertiary normal fault systems in the central Basin and Range province of the southwestern U.S. The normal faults of the breakaway zone crop out across the South Virgin Mountains (SVM), and define a narrow boundary zone between the Colorado Plateau and the highly extended central Basin and Range Province. Geochronologic data, including 40Ar/39Ar muscovite ages, (U-Th)/He apatite ages, and (U-Th)/Pb monazite ages, suggest that extension within the breakaway zone occurred rapidly at ~15 Ma, consistent with earlier work (Fitzgerald, 1991). Approximately 400 km2 of the SVM was mapped at a scale of 1:12 000. This mapping shows that extension initiated on a set of steeply west dipping normal faults. Later faults soled into the earlier faults rather than cutting them, requiring motion to continue on both fault sets, with the earlier faults remaining active to dips of less than 30°. Total extension across the SVM is at least ~21 km. The latest deformation to affect the region was isostatic uplift of the footwall to the Lakeside Mine Fault Zone, with resultant formation of a basement dome and associated folding and late stage faulting adjacent to the dome. Seismic reflection data suggest that the crustal thickness of the region is 30 to 35 kilometers. When combined with the high average elevation of the denuded basement block, this suggests that extension of the upper crust has been compensated by emplacement of fluid mid to lower crust. The lower crust and Moho are seismically transparent, so the lower crust is probably not made up of basaltic intrusions; rather, it has probably flowed in from surrounding areas. A mechanical model has been developed which considers the behavior of an elastic upper crust isostatically compensated by flow in the lower crust. This model shows that gradual isostatic upwarping of the thinning region should generate compressional flexural stresses near the base of the elastic upper crust. These stresses may shut down the active faults and force new normal faults to root outside of the extended region