HIGH-VELOCITY FRICTION AND SYNKINEMATIC FLOW AT THE BASE OF GIANT GRAVITY SLIDES: CARBONATES VS SILICATES

Giant gravity slides often occur in submarine settings where they may trigger tsunamis. Their aerial equivalents tend to be smaller although the Heart Mountain Slide (HMS) in Wyoming and the Markagunt Gravity Slide (MGS) in Utah both represent >1,000 km³ of mobilized material. We compare the physical characteristics of these two giant slides formed respectively ~49 and 23 Ma ago. Because the HMS and MGS formed in distinct materials (carbonates and andesitic volcanics), the physical processes involved in the drastic reduction of basal friction differ profoundly and deserve further attention. These were investigated using geochemical, rock magnetic and magnetic fabric methods.

The HMS most likely resulted from a mid-Eocene eruption in the Absaroka volcanic province that triggered rupture and detachment of a ~1 km-thick Paleozoic sedimentary cover. The rapid initial sliding was enhanced by basal fluidization due to thermo-mechanical decomposition of carbonate rocks and coeval massive release of gaseous CO₂. The intense deformation at the base of the slide produced a ~3 m-thick ultracataclasite layer that surprisingly bears a consistent magnetic fabric. This fabric carried by synkinematic growth of magnetite grains, formed through breakdown of sulfides, and records the slide transport direction.

Similarly, the MGS formed due to failure of a volcanic series deposited on a mechanically weak foundation, possibly as a result of caldera inflation at the Oligo-Miocene boundary. The fast motion of the upper plate resulted in localized brecciation and frictional melting along secondary shear surfaces. Frictional heating formed a 2.5 cm-thick layer of silicate melt, preserved as pseudotachylyte. The magnetic fabric of this material also records the slide transport direction through the preferred alignment of synkinematic magnetite grains.

While similarities between these two giants abound (size, volcanic trigger), profound differences exist in the mechanisms responsible for lowering friction. In the case of HMS, decarbonation at temperatures as low as 400ºC played a major role in increasing pressure along the slip surface and reducing normal stress. In the case of MGS, the presence of a thin layer of superheated silicate melt reaching temperatures as high as 1,400ºC locally reduced friction which further promoted movement. However, while the gaseous lubricant at HMS seems broadly distributed across a large area, the silicate melt at MGS is more restricted to specific areas of the slip plane that had favorable melting conditions.