ANATOMY AND CHARACTERISTICS OF MEGA-SCALE VOLCANIC LANDSLIDES OF THE MARYSVALE VOLCANIC FIELD, SOUTHWESTERN UTAH

Continued geologic mapping of the polygenetic Oligocene to Miocene Marysvale volcanic field, which straddles the Basin and Range and High Plateaus in southwest Utah, has revealed at least two gigantic catastrophic landslide events resulting from the southward collapse of the south flank of the volcanic field. The previously identified Markagunt gravity slide (MGS, >5000 km²) was emplaced at 22 to 21 Ma, while the newly discovered Sevier gravity slide (SGS, >2000 km²) was emplaced at 25 to 23 Ma. The MGS and SGS each consist of three distinct structural areas: (1) a slide zone containing both a breakaway segment and a bedding-plane segment (~65 km long for the MGS and ~55 km long for the SGS), the latter typically within the Eocene-Oligocene Brian Head Formation, (2) a ramp zone ~1-2 km wide where the slide masses cut up section from the slide zone, and (3) a former land surface zone with runouts > 35 km over the Miocene landscape. These volcanic mass movements remain relatively coherent in the slide zone where largely undeformed blocks retain most of their original stratigraphy with some degree of internal deformation by minor faults and folds. Deformation is much greater in the ramp and former land surface zones, where chaotic tilted blocks of volcanic rocks commonly occur in a brecciated and sheared matrix; this deformation represents a debris-avalanche style of movement. Each mega-scale volcanic landslide is a continuous catastrophic emplacement event, with the resulting mass being a gradational combination of slide and debris-avalanche movements. Characteristics of these astonishingly large gravity slides suggest that these types of volcanic fields are preconditioned for collapse by virtue of their: (1) large volume of available slide material that rapidly accumulated in a thick volcanic wedge, (2) a substrate of weak subhorizontal strata, (3) underlying intrusive complexes whose growth tilts the volcanic pile, and (4) development of pre-collapse summit fractures and/or normal faults by intrusive doming or during gradual lateral spreading on deep-seated thrust faults that weaken the structural integrity of the volcanic field.