2. GEOLOGIC SETTING
The Colorado River extensional corridor in southern Nevada, California, and Arizona, is a ca. 70 x 250 km region consisting of highly extended crust (Fig. 1) (Faulds et al. , 2001, Faulds et al. , 1995, Faulds et al. , 1990, Howard et al. , 1996, Howard & John, 1987, Howard et al. , 1994). The corridor experienced crustal thinning from ca. 16 to 11 Ma and a flare-up of magmatism from ca. 18 to 8 Ma (Faulds et al. , 1995, Howard et al. , 1996). Several Miocene age plutons of felsic composition are found in the region, as well as extensive ash-flow tuffs, volcanic breccias, and lava flows (Fig. 1). Crustal sections were exposed in the region by tilting of fault blocks and subsequent erosion, the largest of which include upper to mid-crustal portions of the magmatic plumbing systems associated with region (Bachl et al. , 2001, Walker Jr et al. , 2007). There are several intrusive units in the corridor, the largest of which is Spirit Mountain batholith (Fig. 1).