Two large east-west—elongate, fault-bounded troughs were sites of intense rhyolitic to andesitic volcanism and coeval faulting in the central part of the Great Basin during Oligocene and early Miocene time. The orientation and age of these volcano-tectonic features fit within a regional pattern of south-southwest migration of calc-alkalic volcanism that lasted from 42 to 18 m.y. ago. The troughs and many smaller calderas predate crustal extension from Basin and Range faulting, which began subsequent to the migration of calc-alkalic activity in middle Miocene time.

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