Abstract

A Middle or Late Triassic volcanic vent structure, named the Tioga Pass caldera, is exposed near the eastern boundary of Yosemite National Park, California. The caldera and related volcanic and plutonic rocks—part of an early Mesozoic continental-margin magmatic arc in east-central California—formed prior to or during an episode of contractional deformation in the arc. Field relationships show that a widespread 222 Ma rhyolitic ash-flow tuff was erupted as an extensive outflow sheet during the formation of the caldera. The Late Triassic Lee Vining Canyon pluton may represent the subvolcanic magma chamber that was partially evacuated during the eruption of the ash-flow tuff. The caldera wall is now exposed as a highly irregular boundary between prevolcanic basement and intracaldera rocks that formed by a combination of initial caldera collapse and subsequent intracaldera intrusive and extrusive events. Intracaldera rocks include a thick section of Triassic metasedimentary and metavolcanic rocks on Gaylor Peak, together with the Dana sequence on Mount Dana.

All of the Triassic rocks of the Saddlebag Lake pendant later underwent strong deformation and metamorphism involving folding and thrusting during Middle Jurassic time. The caldera fill is now exposed in the lower plate of an east-vergent Jurassic thrust, which emplaced lower Paleozoic through Jurassic(?) metasedimentary and metavolcanic rocks structurally above the caldera fill.

The results of this study indicate that caldera formation may occur in a contractional arc setting. Structural and stratigraphic relationships described here may also provide clues to recognition of other caldera and vent complexes in highly deformed metavolcanic sequences in the western United States and elsewhere.

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