Utah’s ancient mega-landslides.

During the late Oligocene to early Miocene, the Marysvale volcanic field of southwestern Utah experienced three consecutive, catastrophic, mega-scale collapse events: the Sevier, Markagunt, and Black Mountains gravity slides, which we refer to collectively as the Marysvale gravity slide complex (MGSC). Each collapse ranks among Earth’s largest terrestrial landslides; collectively they span more than 8000 km^2^ with runout distances that exceed 35 km over the less than 3° dipping former land surface. The allochthonous stratigraphic sequences, kinematic indicators, and basal structures including clastic dikes, pervasive fragmentation, and pseudotachylyte collectively suggest emplacement of each gravity slide occurred at high velocity during a single event. Because they are newly discovered, we wonder if other volcanic fields elsewhere in the world host unrecognized mega landslides. Might other areas be susceptible to such large-scale collapse events in the future?  Each MGSC gravity slide exhibits the full range of structural features commonly seen in modern landslides, but on a gigantic scale. Each consists of four distinct structural segments: (1) a high-angle breakaway segment, (2) a bedding-plane segment, (3) a ramp segment, and (4) a land-surface segment where the upper plate moved over the former landscape. The gravity slide complex remained undiscovered for so long precisely because of its gigantic size and an initially confusing mix of extensional, translational, and compressional structures overprinted by post-MGSC basin-range tectonism. Each slide is a large contiguous sheet of andesitic lava flows, mudflow breccias, source intrusions, and intertonguing ash-flow tuffs that record southward, gravitationally induced catastrophic failure of the southern flank of the Marysvale volcanic field. Failure was preceded by slow gravitational spreading on the Paunsaugunt thrust fault system. Triggering mechanisms are poorly understood, but the principal zone of failure was in mechanically weak, clay-rich volcaniclastic strata at the base of the volcanic section. In September 2017 we convened a 6-day GSA-sponsored Thompson Field Forum on the characteristics and implications of the MGSC. New collaborative research now underway, funded by the National Science Foundation, includes geologic mapping, further age control, characterization and implications of pseudotachylytes and injectites, and modelling of mobility mechanisms that allowed extraordinary long runouts. Biek (2022) Utah’s ancient mega-landslides. USGS Landslide Hazards Seminar, 13 July 2022. Video: https://www.usgs.gov/media/videos/uta...