Triassic caldera at Tioga Pass, Yosemite National Park, California: Structural relationships and significance

A Middle or Late Triassic volcanic vent structure, named the Tioga Pass cal dera, is exposed near the eastern boundary of Yosemite National Park, Calif ornia The caldera and related volcanic and plutonic rocks-part of an early Mesozoic continental-margin magmatic are in east-central California-formed prior to or during an episode of contractional deformation in the are. Fiel d relationships show that a widespread 222 Ma rhyolitic ash-flow tuff was e rupted as an extensive outflow sheet during the formation of the caldera. T he Late Triassic Lee Vining Canyon pluton may represent the subvolcanic mag ma chamber that was partially evacuated during the eruption of the ashflow tuff. The caldera wall is now exposed as a highly irregular boundary betwee n prevolcanic basement and intracaldera rocks that formed by a combination of initial caldera collapse and subsequent intracaldera intrusive and extru sive events. Intracaldera racks include a thick section of Triassic metased imentary and metavolcanic rocks on Gaylor Peak, together with the Dana sequ ence on Mount Dana. All of the Triassic rocks of the Saddlebag Lake pendant later underwent str ong deformation and metamorphism involving folding and thrusting during Mid dle Jurassic time. The caldera fill is now exposed in the lower plate of an east-vergent Jurassic thrust, which emplaced loner Paleozoic through Juras sic(?) metasedimentary and metavolcanic rocks structurally above the calder a fill. The results of this study indicate that caldera formation may occur in a co ntractional are setting. Structural and stratigraphic relationships describ ed here may also provide clues to recognition of other caldera and vent com plexes in highly deformed metavolcanic sequences in the western United Stat es and elsewhere.