The long runout of the Heart Mountain landslide: Heating, pressurization, and carbonate decomposition

The Heart Mountain landslide of northwestern Wyoming is the largest subaerial landslide known. This Eocene age slide slid ∼50 km along a shallow 2° slope, posing a long‐standing enigma regarding its emplacement mechanism. We suggest here a mechanism for the catastrophic emplacement of the Heart Mountain landslide that is independent of slide triggering. The mechanism is a feedback between shear heating, thermal pressurization, and thermal decomposition of carbonates at the slide shear zone. Such a feedback arises when a porous, fluid‐filled shear zone heats up because of frictional sliding. If the shear zone is confined, the generated heat leads to pore pressure rise, which in turn reduces frictional resistance to sliding, leading to acceleration. Temperatures at the shear zone quickly reach the decomposition temperature of carbonates. Since the shear zone of the Heart Mountain slide is located within a dolomite layer, it is expected that thermal decomposition of dolomite occurred within the Heart Mountain shear zone. This prediction is supported by ample field evidence for carbonate decomposition during the emplacement. Simulation of the sliding dynamics of the Heart Mountain block, accounting for feedback between shear heating, thermal pressurization, and thermal decomposition of carbonates, successfully reproduce the travel distance of the Heart Mountain block. The simulation results also predict that the maximum sliding velocity ranged between tens of meters per second to more than 100 m s−1 (depending on model assumptions) and that the duration of sliding was of the order of a few tens of minutes, in agreement with previous assessments.