Introduction: The 36-18 Ma southern Great Basin, USA, ignimbrite province and flareup: Swarms of subduction-related supervolcanoes

Abstract

During the middle Cenozoic, from 36 to 18 Ma, one of the greatest global expressions of long-lived, explosive silicic volcanism affected a large segment of southwestern North America, including central Nevada and southwestern Utah in the southern Great Basin. The southern Great Basin ignimbrite province, resulting from this flareup, harbors several tens of thousands of cubic kilometers of ash-flow deposits. They were created by more than two hundred explosive eruptions, at least thirty of which were super-eruptions of more than 1000 km3. Forty-two exposed calderas are as much as 60 km in diameter. As in other parts of southwestern North America affected by the ignimbrite flareup, rhyolite ash-flow tuffs are widespread throughout the southern Great Basin ignimbrite province. However, the province differs in two significant respects. First, extrusions of contemporaneous andesitic lavas were minimal. Their volume is only about 10% of the ignimbrite volume. Unlike other contemporaneous volcanic fields in southwestern North America, only a few major composite (strato-) volcanoes predated and developed during the flareup. Second, the central sector and especially the eastern sector of the province experienced super-eruptions of relatively uniform, crystal-rich dacite magmas; resulting deposits of these monotonous intermediates measure on the order of 16,000 km3. Following this 4 m.y. event, very large volumes of unusually hot and dry trachydacitic magmas were erupted. These two types of magmas and their erupted volumes are apparently without parallel in the middle Cenozoic of southwestern North America.