FRICTION-DRIVEN THERMAL DECOMPOSITION AND SYNKINEMATIC FABRICS OF CARBONATES IN THE HEART MOUNTAIN SLIDE, WYOMING

Cataclastic deformation has long been considered to produce unfoliated rocks, i.e., materials devoid of a tectonic fabric. However, the 3 m-thick carbonate ultracataclasite formed by the Heart Mountain Slide, in Wyoming, has a consistent subhorizontal magnetic fabric across its thickness. This ultracataclasite layer developed in the Eocene during one of the largest known continental landslide on Earth (~3,500 km²). A mid-Eocene eruption in the Absaroka volcanic province most likely triggered rupture and subsequent detachment of a ~1 km-thick Paleozoic sedimentary cover. The rapid sliding was enhanced by basal fluidization due by thermo-mechanical decomposition of carbonate rocks and coeval massive production of gaseous CO₂. The ultracataclasite layer has a spatially-consistent, strong magnetic fabric, whereas host carbonates preserve their weak sedimentary fabric. The anisotropy of magnetic susceptibility (AMS) arises from synkinematically deformed, pseudo-single domain magnetite grains formed by thermal decomposition of primary iron sulfides. The ultracataclasite does not generally show a visible macroscopic fabric. Yet, the strong AMS in these deformed rocks records the transport direction of the landslide. These results indicate that friction-driven decomposition of carbonates may generate consistent synkinematic fabrics and therefore that some cataclasites/ultracataclasites may indeed bear a foliation.