Welded tuffs deformed into megarheomorphic folds during collapse of the McDermitt Caldera, Nevada‐Oregon

Abstract

The Montana Mountains contain part of a resurgent dome complex within the southwestern sector of the McDermitt caldera. Exposed intracaldera units consist of welded tuffs, lavas, domes and epiclastic deposits. The majority of volcanic units have a comenditic composition, but a few units are nonperalkaline and biotite bearing. A deformed series of comenditic welded tuffs occur in a strongly folded rheomorphic zone more than 500 m. thick. The geometry of vitrophyres, flow bands, foliations, and lineations causes the rheomorphic zone to superficially resemble a series of intrusive domes. However, the contacts between units are conformable, the vitrophyres are continuous from one fold to the next, and the units display pumice, shards, and other features common in welded tuffs. The deformed tuff sequence lies on top of biotite tuffs and epiclastic deposits that are tilted and faulted (but not folded) and beneath relatively undeformed tuffs and lake deposits. This relationship suggests that their large‐scale folds in the deformed sequence developed shortly after caldera collapse