The Heart Mountain Detachment, Wyoming: Processes involved in a Mega-block Slide

The processes of the displacement of the Heart Mountain block in northwestern Wyoming have long been debated. A slab of rock, 1300 km2 in area by 1 – 2 kilometers thick, slid a distance of at least 45 kilometers along a nearly-horizontal surface. Usually faults with this much movement would be thick (e.g., tens of meters), but the Heart Mountain detachment fault is merely a couple millimeters wide in most places. For movement to take place, a fault zone this thin would require that friction be very low, much less than is normally the case. How did this fault reduce to a low enough coefficient of friction to slide a block of this area? Some possible processes involve breakdown of carbonate fault rocks by frictional heating (calcining), which produces nanoparticle sized grains that could have essentially acted as ball bearings. Calcining also produced carbon dioxide gas, which could have acted as a high-pressure “lubricant” in the fault. Samples were collected along the Heart Mountain detachment and are being studied through use of scanning electron microscopy, geochemical analysis, and petrographic analysis. Through these analyses, we can also evaluate whether the block moved in a single catastrophic event or in multiple steps. Evidence from these methods of analysis shows that the detachment event occurred in at least two stages and that nanoparticles were involved in the faulting event. These findings have helped to better characterize the mechanics of movement on the Heart Mountain Detachment