Large‐volume rhyolite ash flow eruptions and submarine caldera collapse in the Lower Mesozoic Sierra Nevada, California

Abstract

The Mineral King roof pendant in the southern Sierra Nevada, California, provides a view of a Triassic and early Jurassic submarine caldera complex which was tilted to vertical and intruded by middle Cretaceous granitoids. An estimated 90 km3 of rhyolitic and 10 km3 of andesitic volcanic rocks are interstratified with marine sandstones, siltstones, limestones, and conglomerates. Periods of volcanotectonic activity alternated with periods of marine sedimentation, Volcanic eruptions at Mineral King resulted in (1) deposition of widespread, very thick sheets of rhyolite ash flow tuff and (2) construction of small (1–8 km3) andesitic stratocones. Growth faults, fault talus aprons, slide blocks, slumps, and debris flows found in association with these volcanic deposits indicate that the eruptive phases represent periods of great seismic activity. Interuptive phases were periods of volcanic and tectonic quiescence during which sequences of shallow to deep marine sediments were deposited. Four different times at Mineral King, eruption of large volumes of rhyolite ash flow tuff resulted in catastrophic caldera collapse and ponding of single ash flow tuffs to thicknesses greater than 0.5 km. Only minimum volumes can be estimated for three of these intracaldera tuffs because they are truncated by younger granites. Volumes are estimated to be at least 25 km3 each, placing these tuffs in the lower size range of epicontinental ring structures. The fourth caldera, the Vandever Mountain caldera, is entirely preserved in the roof pendant. The vertically dipping strata of the Vandever Mountain caldera provide an unusual opportunity to examine a submarine caldera in cross section. Evidence for caldera subsidence during the eruption of the Vandever Mountain rhyolite ash flow tuff includes the following: (1) the great thickness of the ash flow tuff and the rapid accumulation of the tuff during progressive draw‐down of a compositionally zoned magma chamber, (2) high‐angle faults which pond the ash flow tuff and offset underlying but not overlying stratigraphy, and (3) megabreccia and slide blocks up to 0.5 km long that were derived from the caldera wall and floor and incorporated into the ash flow tuff as it filled the caldera. Disruption of the caldera floor was intense near the caldera margin but negligible along most of its length, owing to pistonlike subsidence of an intact cylinder of crust. Small‐volume ash flow eruptions occurred along an incipient, “leaky” ring fracture zone prior to the caldera‐forming eruption of the Vandever Mountain ash flow tuff. The Vandever Mountain caldera was not affected by resurgent doming or late stage rhyolite magmatism in the cross‐sectional view provided by the Mineral King roof pendant.