MOVEMENT ON EARLY EOCENE (~49.5 MA) SHOSHONE/SUNLIGHT/ABIATHAR DETACHMENT FAULT (SSADF) WAS RAPID, PRECIPITATED BY PLATE TECTONIC-INDUCED MEGATHRUST (MW 9.0+) EARTHQUAKES

Allochthons of the SSADF (formerly the proximal part of the so-called “Heart Mountain Detachment Fault”) are restricted primarily to displaced Paleozoic rocks distributed between Abiathar Peak, near Silver Gate, MT, and the Natural Corral area between Dead Indian Hill and Pat O’Hara Mtn., as well as those in the Logan and Sheep Mtn. areas, west of Buffalo Bill Reservoir, northwest WY. Breccia structure demonstrates that movement was catastrophic and rapid, and the dispersion of in situ and displaced remnants of the Crandall Conglomerate show that maximum movement of any single allochthon was about 25 km. There is no convincing evidence for inclusion of Wapiti Formation rocks in the displacement, and field evidence supports E.H. Stevens’ & W.G. Pierce’s concepts of the break-up of a massive allochthon and individual block movement, and development of a surface of tectonic denudation, respectively. Faulting was probably triggered by a catastrophic plate tectonic subduction megathrust earthquake. The associated volcanism was caused by arc magmatism from rollback of the Farallon Plate or development of a slab window between the Farallon and Resurrection plates. Continuous movement down the autochthonous slope of about 2^o was assisted by sustained seismicity and several massive allochthonous blocks rotated around a vertical axis as they were propelled downslope at speeds of 150+ km/hr. Detachment faulting was immediately succeeded by deposition of up to 2,000 m of Wapiti Formation. (Warner, A.J., and Bown, T.M., eds. 2024. The Heart Mountain Detachment Fault: A Critical Reappraisal. Bloomington, IN; Archway Publications:1-124.)