THE BRITTLE DEFORMATION SEQUENCE AT DEAD INDIAN HILL AND THE HEART MOUNTAIN DETACHMENT

The Heart Mountain Detachment in northwestern Wyoming is one of the largest terrestrial mega-scale gravity slides mapped on Earth. Emplacement closely followed the termination of the Laramide Orogeny and was contemporaneous with the widespread Eocene magmatic activity of the Absaroka Volcanic Province. Our study focuses on the relationship between the detached hanging wall and footwall of an allochthonous section at Dead Indian Hill. Fracture data was collected (+1,700 measurements in total) from selected outcrops of Cambrian to Eocene strata from the vicinity of White Mountain, Dead Indian Hill and Heart Mountain to provide a stratigraphic framework needed to decipher the regional sequence of brittle factures. Three sets of fractures (joints and meso-faults) exist in the footwall of the detachment in Cambrian to Cretaceous strata, but are not present in Eocene strata; hence, they are interpreted as Laramide fractures. Laramide age joints and subsequent thrust faults progressively rotate in a clockwise direction from North to Northeast. For fracture sets in the allochthonous hanging wall; after corrections for dip, slope and a clockwise rotation ~30^o around a vertical axis, the angular and abutting relationships in the hanging wall are the same as the Laramide fracture sets within the footwall wall on the west slope of Dead Indian Hill and accordingly are Laramide age fractures. At least two fracture sets exist within the Eocene Willwood Formation below Heart Mountain in the Bighorn Basin. In conjunction with the fractures in the Eocene sediment, similar oriented fractures are recognized in truncated Absaroka intrusions, as well as, post-Laramide fractures and early Eocene reactivations (primarily as strike-slip faults) in the hanging wall of the detachment.