ERUPTIVE HISTORY, GEOCHRONOLOGY, AND POST-ERUPTION STRUCTURAL EVOLUTION OF THE LATE EOCENE HALL CREEK CALDERA, TOIYABE RANGE, NEVADA

Previous geologic mapping in the northern Toiyabe Range of central Nevada suggested the presence of a caldera that sourced the late Eocene (34.0 Ma) tuff of Hall Creek, and the area was interpreted as the locus of large-magnitude Eocene to Miocene extension. We carried out geologic mapping, ⁴⁰Ar/³⁹Ar dating, and geochemical analyses to document the formation of the Hall Creek caldera, understand regional topography at the time it formed, and clarify the timing and kinematics of post-caldera extension. In the late Eocene, the northern Toiyabe Range was characterized by significant topography centered on an east-west-trending ridge in the area of present-day Mount Callaghan, probably localized along a Mesozoic anticline. Andesite lava flows that erupted ca. 34-35 Ma ponded in the erosional low areas north and east of this ridge, reaching thicknesses of 600 m in the Simpson Park Mountains. The Hall Creek caldera formed ca. 34.0 Ma during eruption of the ~400 km³ tuff of Hall Creek, a moderately crystal-rich rhyolite (71-77% SiO₂) ash-flow tuff. Caldera collapse was piston-like with an intact floor block, and the caldera filled with thick (~2600 m) intracaldera tuff and breccia shed from the caldera walls; the most extensive megabreccia deposits are concentrated on or near the caldera floor. Silicic and intermediate post-caldera lavas were locally erupted within 400 k.y. of the main eruption, and for the next ~10 m.y. sedimentary rocks and outflow tuffs from sources farther west ponded in the caldera basin surrounding low areas. Patterns of tuff deposition indicate that the area was characterized by east-west trending paleovalleys and ridges in the late Eocene and Oligocene, which permitted tuffs to disperse east-west but limited their north-south extent. Although a low-angle fault contact of limited extent separates Cambrian and Ordovician strata in the southwestern part of the study area, there is no evidence that this fault cuts overlying Tertiary rocks. Total extensional strain across the caldera is on the order of 15%, and there is no evidence for progressive tilting of 34-25 Ma rocks that would indicate protracted Eocene – Oligocene extension. The caldera appears to have been tilted as an intact block after 25 Ma, probably during the middle Miocene extensional faulting well documented to the north and south of the study area.