THE MARKAGUNT GRAVITY SLIDE, SOUTHWEST UTAH: A LANDSLIDE BIGGER THAN RHODE ISLAND (Invited Presentation)

Despite being larger than the state of Rhode Island, the newly discovered Miocene (22-21 Ma) Markagunt gravity slide (MGS) in southwestern Utah, USA, exhibits the full range of structural features commonly seen in modern landslides, but on a gigantic scale. The MGS, a large contiguous volcanic sheet of allochthonous andesitic lava flows, volcaniclastic rocks, and intertonguing regional ignimbrites, represents southward catastrophic failure, preceded by gravitational spreading, of the southwestern sector of the Oligocene to Miocene Marysvale volcanic field. The MGS remained undiscovered for so long precisely because of its gigantic size (>5000 km², >95 km long, >35 km runout, estimated volume 3000 km³; dimensions revised from Geology, v. 42, no. 11, p. 943-946) and an initially confusing mix of extensional, translational, and compressional structures overprinted by post-MGS basin-range tectonism. Only the Eocene Heart Mountain gravity slide in Wyoming is a terrestrial slide of comparable size; it was considered unique until discovery of the MGS. The MGS is better exposed than the Heart Mountain slide and the basal slide plane of the MGS is easier to understand because it is largely in weak volcaniclastic beds.

The MGS is significant because it provides a stunning example of gravity-slide structures so large that they may be mistaken for tectonic features. However, the presence of basal and lateral cataclastic breccias, clastic dikes, jigsaw puzzle fracturing, internal shears, pseudotachylytes, the uniformity of kinematic indicators, breakaway and antithetic faults, and the overall geometry of the MGS show that it represents a single catastrophic emplacement event. That features as large as the MGS can remain undetected despite decades of geologic mapping and research in the area suggests to us that other volcanic fields around the world may hold evidence of as-yet-undiscovered, exceptionally large gravity slides. Furthermore, some modern volcanic fields may possess the conditions capable of generating similarly large slides. Although smaller­-scale flank failures have been recognized on hundreds of individual volcanoes worldwide, the MGS catastrophic sector collapse represents a new class of low-frequency but high-impact hazard that may exist in other large volcanic fields.