High‐Velocity Slip and Thermal Decomposition of Carbonates: Example From the Heart Mountain Slide Ultracataclasites, Wyoming

Abstract

AbstractThe Heart Mountain Slide in Wyoming is one of the largest known terrestrial gravity slides (3,500 km2) formed ∼49 Ma ago by the nearly horizontal detachment of Paleozoic‐Eocene cover sliding on top of autochthonous formations. At the White Mountain locality, exposures offer an exceptional opportunity to investigate high strain rate/high velocity processes in carbonates. Here we use the anisotropy of magnetic susceptibility (AMS) of 274 samples to shed light on ultracataclastic deformation along this detachment. Contrary to predictions, the carbonate ultracataclasite displays a consistent AMS fabric, particularly in the upper ultracataclasite. The AMS in this unit is controlled primarily by magnetite formed through the breakdown of iron sulfides caused by frictional heating. Additional thermomagnetic experiments reveal that the new magnetic fabric began forming ∼250ºC and continued up to ∼400ºC when calcination of carbonate minerals caused a major drop in friction. The main cataclastic slip direction inferred from AMS is ∼N033°, at odds with the previously accepted NNW‐SSE direction. We validate these AMS fabrics through 3D shape preferred orientation analysis and micro X‐ray scanning of the same specimens. These results, however, may only represent cataclastic flow directions at the local scale as a result of synkinematic rotation of the White Mountain block. Alternatively, these results may call for a re‐evaluation of the large scale movement of the slide. Finally, this study demonstrates the usefulness of a magnetic approach in deciphering deformation processes in carbonates, particularly in high strain rate cases such as seismic faults.