Two of the World’s largest Terrestrial Landslides

October 15th (Tuesday), 6 p.m., Teton Co. Library Auditorium – Open to Public. Presentation: “Two of the World’s largest Terrestrial Landslides, Presented by Bob Biek, Utah Geological Survey.

The earth’s surface has often been “sculpted” by incredibly violent events – think volcanoes, giant earthquakes. Such a catastrophic event not seen in recorded human history is giant landslides, of a size that literally staggers the imagination. One of such events is recorded in the rock record not terribly far from Jackson, in SW Utah, and Bob Biek of the Utah Geological Survey will take us on a tour that unveils this event.

Gravitational collapse of the south flank of the Oligocene to Miocene Marysvale volcanic field produced three gigantic catastrophic gravity slides that form an overlapping contiguous complex covering an area >8000 km2, with the largest gravity slide (Markagunt gravity slide) being >3500 km2. Each gravity slide exhibits the full range of structural features commonly seen in modern landslides, but on a huge scale—they are among Earth’s largest terrestrial landslides. Gravity-slide masses consist of andesitic lava flows, volcaniclastic rocks, and regional ash-flow tuffs that moved as a mostly intact sheet along four distinct structural segments: (1) a high-angle breakaway segment, (2) a bedding-plane segment ~55–65 km long, (3) a ramp segment ~1–2 km long where the gravity slide cut up-section and the basal fault “daylighted,” and (4) a former land-surface segment where the upper plate moved at least 35 km over the landscape. Features such as basal breccias and shears, clastic dikes (injectites), jigsaw-puzzle fracturing, and stunning pseudotachylytes (frictionites) indicate high-velocity movement aided by overpressured fluids. The principal zone of failure was in mechanically weak, clay-rich sedimentary strata at the base of the volcanic section. The gravity slide masses become younger westward, and the breakaway area of each is overprinted by one the largest volcanic features in the field, namely calderas and Utah’s largest exposed batholith. Failure was preceded by slow gravitational spreading accommodated by the Paunsaugunt thrust fault system, which is rooted in Middle Jurassic evaporite-bearing strata at a depth of ~2 km.

These gravity slides remained undiscovered for so long precisely because of their gigantic size and initially confusing mix of extensional, translational, and compressional structures overprinted by post-gravity-slide basin-range tectonism. They provide examples of lithified landslide structures so large that they may be mistaken for tectonic features produced over millions of years. However, these gravity slides post-date and lie at right angles to regional compressional tectonic structures and thus offer compelling case studies for how to differentiate features resulting from surficial verses tectonic processes.