Tilted Middle Tertiary Ash-Flow Calderas & Subjacent Granitic Plutons, Southern Stillwater Range, Nevada: Cross Section of an Oligocene Igneous Center: Stillwater Caldera Complex, Southern Stillwater Range & Wonder Mining District

This trip examines features of calderas and cogenetic plutonic rocks that comprise the Stillwatercaldera complex in the southern Stillwater Range, west-central Nevada (fig. 1, p. 49). This unusual complex provides new insights into the nature of ash-flow calderas, their structural and magmatic histories, and the hydrothermal systems that may be associated with these calderas. The late Oligocene (approx. 28-24 Ma) Stillwater caldera complex is composed of three partly overlapping calderas (Job Canyon, Poco Canyon, and Elevenmile Canyon calderas) and subjacent cogenetic granitic plutons (IXL and Freeman Creek plutons) that were steeply (≥60°) tilted by latest Oligocene extensional faulting (fig. 2; John, 1993a, 1995a; Hudson and others, 2000). Because of the steep tilting shortly after caldera formation, the calderas are exposed in structural cross-sections that represent a 10-km range in paleodepth from near the paleosurface to mid-crustal depths, and they provide a rare opportunity to view the transition from volcanic to hypabyssal to plutonic environments within a single magma system. Late Miocene to HoloceneBasin and Range faulting subsequently uplifted the Stillwater Range several kilometers relative to the surrounding basins, thereby exposing the steeply tilted caldera complex. Major features of calderas in the Stillwater caldera complex are listed in table 1.