It shows grading and flow banding, and extends into the upper plate in a network of clastic dikes, some containing carbonized wood ( Figure 2a). ) is characterized by mixed lithology centimeter-scale and smaller rounded grains in a matrix of micron-scale fine-grained carbonate. The basal layer carbonate (called a microbreccia by Hauge and an ultracataclasite by Craddock et al. The basal layer of the HML contains clues for the emplacement mechanism. Still, the major difficulty with the catastrophic emplacement model is the enigma regarding the mechanism that allowed tens of kilometers of runout on an extremely shallow 2° slope. These supporting pieces of evidence relay on calcite twinning strain markers, anisotropy of magnetic susceptibility, and sedimentological, petrological, and geochemical observations. Recently, a growing body of evidence supporting catastrophic emplacement has accumulated. Pierce argue for catastrophic emplacement during a brief time interval with fragmentation of the upper plate during its motion, while Hauge suggested a gradual emplacement of the upper plate as a continuous allochthon. The mode of emplacement of the HML has long been a subject of investigation, with two different proposed models. Modified from Aharonov and Anders and based primarily on the work of W. Ī map and a cross section of the Heart Mountain landslide located in northwestern Wyoming and southeastern Montana. For this reason probably, despite early discovery of the upper plate block structures by Dake, their identification as erosional remnants of a much larger gravity-driven slide structure was made only later by Pierce. The very long runout of the HML over an extremely shallow dipping slope is a physical enigma that seemingly violates simple mechanistic considerations of a block sliding down a tilted slope. The most remarkable feature of this gigantic structure is that it slid on a 2° slope. The shear zone separating the upper from the lower plate cuts along the Ordovician Bighorn Dolomite formation, a few meters above a shale horizon. During the movement phase of the landslide, the upper plate consisted mostly of layered Paleozoic marine carbonates and shales underlying Eocene Andesitic volcanic rocks. Postmovement erosion has removed most of the toe area leaving several small kilometer square upper plate blocks like Heart Mountain, the feature after which the landslide is named ( Figure 1). This Eocene age landslide covers roughly 3400 km 2 with a toe that thrusts out over 50 km. The Heart Mountain landslide (HML) in northwestern Wyoming and southeastern Montana is the largest known subaerial landslide. 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. 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. This prediction is supported by ample field evidence for carbonate decomposition during the emplacement. 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. Temperatures at the shear zone quickly reach the decomposition temperature of carbonates. 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. Such a feedback arises when a porous, fluid-filled shear zone heats up because of frictional sliding.
The mechanism is a feedback between shear heating, thermal pressurization, and thermal decomposition of carbonates at the slide shear zone. We suggest here a mechanism for the catastrophic emplacement of the Heart Mountain landslide that is independent of slide triggering. This Eocene age slide slid ∼50 km along a shallow 2° slope, posing a long-standing enigma regarding its emplacement mechanism. The Heart Mountain landslide of northwestern Wyoming is the largest subaerial landslide known.
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