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

The Heart Mountain Slide in Wyoming is one of the largest known terrestrial gravity slides (3,500 km(2)) formed similar to 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 similar to 250 degrees C and continued up to similar to 400 degrees C when calcination of carbonate minerals caused a major drop in friction. The main cataclastic slip direction inferred from AMS is similar to N033 degrees, 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. Plain Language Summary The Heart Mountain Slide in northwest Wyoming is one of the largest landslides on continents. It occurred about 49 millions years ago when rock masses primarily made of limestones started sliding on top of other limestones. The movement of the upper body caused tremendous friction at the base of the slide and resulted in frictional heating. As the temperature kept rising the slide reached high speed possibly up to 112 m/s. We studied the rocks crushed by this extraordinary event and, for the first time, we can demonstrate that the temperature probably did not exceed 400 degrees C. Also, we show that these rocks preserve a fabric in their magnetic minerals that indicates a SSW to NNE sliding direction instead of NNW to SSE as previously thought. The magnetic analyses hence give valuable information about these crushed rocks that no other methods have provided before. Our results and methods demonstrate that this approach could be applied to the analysis of seismic faults in limestones.

Automated summary

This article focuses on the Heart Mountain Slide in Wyoming and uses the anisotropy of magnetic susceptibility (AMS) to study the ultracataclastic deformation of carbonates. The results reveal a different sliding direction than previously thought and demonstrate the changes in magnetic susceptibility due to the breakdown of iron sulfides caused by frictional heating. This study is significant for understanding deformation processes in carbonates, particularly in seismic faults.