The Timber Mountain–Oasis Valley caldera complex lies within a volcanic field in southern Nevada that once covered 11,000 km2. The caldera complex, active from 16 to 9.5 m.y. ago, was the source of nine voluminous rhyolitic ash-flow sheets and numerous smaller rhyolitic tuffs and lava flows. Several centers of basaltic and related volcanism were active before the complex formed, continued around its periphery during caldera activity, and have since overlapped the caldera complex. Extensional normal faulting and perhaps deep-seated right-lateral deformation preceded, accompanied, and followed evolution of the caldera complex and its surrounding volcanic field.
The youngest major structure of the complex is the Timber Mountain resurgent caldera, 25 by 30 km across. Several stages of its development can be documented: (1) magmatic insurgence accompanied by gentle tumescence, formation of a ring-fracture zone, and minor rhyolitic volcanism; (2) eruption about 11 m.y. ago of a voluminous ash-flow sheet, caldera collapse during the eruption, and postcollapse caldera infilling by sediments and rhyolite flows; (3) renewed ash-flow eruptions and further caldera collapse; (4) resurgent doming of the cauldron block; and (5) postcollapse rhyolitic volcanism and filling of the caldera by sediments.
Only parts of the older calderas are preserved, but they can be interpreted in terms of evolutionary cycles similar to that of the Timber Mountain caldera. Major tectonic intersections appear to have controlled the locations and certain structural features of each major volcanic source area.
Differentiation at high crustal levels of the silicic magmas related to the caldera complex produced compositionally zoned ash-flow sheets. High-level differentiation also is represented at most of the basaltic centers of the field. Each caldera cycle probably represents a separate batch of rhyolitic magma that rose high into the crust, differentiated in place, and partly erupted to the surface. Each of these magmas probably rose independently through the crust, but all of them were related ultimately to a single magmagenetic system, as were the basaltic magmas of the field. The silicic magma bodies consolidated to form large shallow granitic plutons, and the caldera complex now overlies a small composite granitic batholith.