DISCOVERY AND DISTINGUISHING CHARACTERISTICS OF THE MARKAGUNT GRAVITY SLIDE, SOUTHWEST UTAH

The early Miocene Markagunt gravity slide (MGS) in southwest Utah, USA, is so large that it took decades of geologic mapping and research to recognize that it represents one of the earth’s largest terrestrial landslides. Previous geologists mapped parts of what we now call the MGS, including (1) in 1993, a report on the “Markagunt Megabreccia,” a chaos of displaced volcanic rocks that covered >500 km² of the Markagunt Plateau; (2) thrust faults described in 1986-1999 on the southeast flank of the Oligocene to Miocene Marysvale volcanic field and interpreted to represent gravitational spreading; and (3) a zone of inferred detachment at the base of the volcanic rocks on the field’s southwest flank. We built on this foundation by mapping the Panguitch and Beaver 30' x 60' quadrangles and found that the field's entire southwest flank had detached along weak, underlying volcaniclastic strata. We now understand the MGS to be a large, contiguous volcanic sheet of allochthonous andesitic lava flows, volcaniclastic rocks, and intertonguing regional ignimbrites that represents southward catastrophic failure, preceded by the gravitational spreading. 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 its initially confusing mix of extensional, translational, and compressional structures overprinted by post-MGS basin-range tectonism.

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, examination of the slide’s base allows us to distinguish between rootless (gravity slide) structures and rooted (tectonic) structures. The former exhibit single-generation basal and lateral cataclastic breccias and associated clastic dikes—compelling evidence of overpressured fluids on its basal failure plane. Pseudotachylytes on low-angle shear planes and in associated dikes as much as 10 cm wide demonstrate high temperatures on slip surfaces and an inferred high rate of emplacement. These and other structures, coupled with the uniformity of kinematic indicators and the overall geometry of the MGS, show that it represents a single catastrophic emplacement event.