In order to understand the movement of large rock masses or allochthons on low-angle surfaces, we have studied the 3400-km2 Heart Mountain slide block of northwestern Wyoming and southwestern Montana. The Heart Mountain slide block was initiated on a 2° gradient, with its toe thrust a minimum of 45 km across an early Eocene landscape. The slide block moved on a basal layer that ranges in thickness from a few tens of centimeters to several meters. This basal layer commonly has a concrete-like appearance of rounded, mixed-lithology grains in a fine-grained carbonate matrix, and in some locations it has features similar to sedimentary deposits, including both normal and inverse grading, flow banding, turbidite-like structures, and clastic dikes containing pieces of carbonized wood. Nowhere did we observe crosscutting relationships in the basal layer or overlying clastic dikes, as would be expected from incremental or noncatastrophic emplacement. Results from cathodoluminescence and δ18O, δ13C, and 87Sr/ 86Sr isotopic compositions from the basal layer support a single movement event followed by hydrothermal and meteoric fluids percolating through a permeable basal layer. These observations suggest that a catastrophic movement on the detachment resulted in frictional heating at the base of the slide. When the generated heat was at least 800°C, calcining of carbonates occurred, yielding calcium and magnesium oxide powders and carbon dioxide gas. The calcium oxide powder became mechanically fluidized by the pressurized carbon dioxide gas, leading to a reduced coefficient of friction at the base of the slide, which in turn permitted the long runout on such a low-angle surface. This mechanism might be applied to explain a wide range of catastrophic sliding events where carbonate rocks are involved.
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