The Heart Mountain Detachment Fault: A Critical Reappraisal

The Heart Mountain Detachment Fault (HMDF) is an immense geologic structure in northwest Wyoming that, as defined prior to this study, extends from just inside the northeast entrance of Yellowstone National Park to areas southwest and northeast of Cody, WY. This enigmatic structure has puzzled geologists for more than 100 years. The consensus is that a massive slab of limestones and dolomites 320-485 million years old broke away from an outcrop near Silver Gate, MT about 49 million years ago and slid nearly 75 miles to the southeast at speeds of nearly 100 mph to more than 700 mph and broke up into several giant blocks up to five miles in diameter. Heart Mountain, north of Cody, WY, the namesake for this displacement, is where some of its geologic relationships were first recognized over 125 years ago. Our research demonstrates that all of the geologic features attributed to the single event HMDF are instead the result of two geologic events separated in time by more than 47 million years. The first we term the Shoshone/Sunlight/Abiathar Detachment Fault (SSADF) that occurred 49.5 million years ago, and the second we call the Heart Mountain/McCullough Peaks Sturzstrom (HMMPS) that occurred 2.08 million years ago. A sturzstrom is a huge landslide that travels a great horizontal distance. Both massive displacements were instigated by violent tectonic processes. The SSADF was triggered by a powerful megathrust earthquake initiated by plate tectonic movements and followed by extensive volcanism, whereas the HMMPS was caused by a gigantic super eruption of a volcano in what is present-day Yellowstone National Park.