Timber Mountain, Nevada Test Site, is a structural dome produced by resurgence of magma beneath the center of the Timber Mountain caldera. The dome is about 9 by 7 miles, elongate in a northwest direction. It is composed almost entirely of the tuff of Cat Canyon, a thick sequence of ash-flow tuffs that accumulated only within the collapsed caldera. Major uplift of the dome began after emplacement of the Cat Canyon. Dips on the dome are mostly gently outward but locally near the edge steepen to as much as 65 degrees. Structural uplift of higher parts of the dome is at least 4000 ft.
Dominant structural elements on Timber Mountain consist of a marginal arcuate fault system and a later longitudinal and radial graben system. The arcuate system may reflect the reactivated inner edge of the pre-existing caldera ring-fracture zone. The arcuate system is exposed only in the southeast quadrant of the dome where it is intruded by granite porphyry, basalt, rhyolite and silicic tuff dikes. Timber Mountain has a prominent longitudinal graben system and thus resembles resurgent domes of the Valles and Creede calderas. Additional grabens are radial to the center of the Timber Mountain dome. A semicircular graben near the center of the dome developed concurrently with the other grabens. Several small rhyolite intrusives emplaced along the longitudinal faults are analogous to the “middle rhyolite” of the Valles caldera.