The newly recognized Markagunt gravity slide (MGS) represents gravitationally induced, catastrophic collapse, ca. 21–22 Ma, of the southwest sector of the Oligocene to Miocene Marysvale volcanic field in southwest Utah (USA). The ∼90-km-long slide mass consists of ∼1700–2000 km3 of andesitic volcanic mudflow breccias and subordinate lava flows, sandstones, and intertongued regional ash-flow tuffs. The MGS extends over an area of >3400 km2 and advanced >30 km to the south over the Miocene land surface. The low-angle basal slip surface is in Eocene–Oligocene Brian Head Formation, a weak, clay-rich, tuffaceous sedimentary unit at the base of the volcanic section. The presence of a basal zone of cataclastic and sheared breccia with associated clastic dikes, and pseudotachylyte along secondary shear planes, provides strong evidence of catastrophic emplacement. The uniformity of directional indicators, stratigraphy, and overall geometry of the MGS suggests that it represents a single-emplacement event coeval with late-stage magmatic doming of the volcanic field. The MGS thus represents one of the largest subaerial volcanic landslides on Earth, and along with the comparable Heart Mountain gravity slide in northwest Wyoming, constitutes a class of catastrophic collapse hazard not widely recognized within modern volcanic fields.

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