JOINTS RELATIONSHIPS: ASSESSMENT OF ROTATION WHILE SLIDING OF THE HEART MOUNTAIN DETACHMENT AND DEVELOPMENT OF RELIEF AT DEAD INDIAN HILL, WYOMING

The Heart Mountain Detachment in NW Wyoming is one of the largest terrestrial gravity-slides. Emplacement of the slide followed the Laramide Orogeny and was contemporaneous with widespread Eocene activity in the Absaroka Volcanic Field. Analysis of fracture-sets (joints and faults), determined from abutting, cross-cutting and angular relationships identify seven regionally consistent fracture-sets, from select outcrops in the continuous Paleozoic-Mesozoic stratigraphy below the detachment at Dead Indian Hill, above the detachment in an important kilppe on the west slope, below Heart Mountain in the Eocene Willwood Formation and at the classic White Mountain locality. The structural relationships, elucidated from ~1,800 measurements, depict two episodes of principle stress rotation – Pre-Eocene and Eocene. The Pre-Eocene sequence, identifiable in Cambrian to Cretaceous stratigraphy pinned to the basement core of the Dead Indian Hill fold, contains three fracture-sets where the principle horizontal stress had progressively rotated 48^o E from the North to Northeast. Being pinned to basement, these relate to stress-fields associated with Pre-Laramide and Laramide tectonic events prior to the detachment. The start of the Eocene fracture sequence is marked by a distinct realignment of the principle horizontal stress to the Northwest and absence of Pre-Eocene fractures in chronostratigraphic units like, local Absaroka dikes and the Willwood Formation sandstones below Heart Mountain. Four Eocene fracture-sets, including the Heart Mountain Detachment, define a 60^o W rotation of the horizontal stress-field, from the Northwest to West. In the Paleozoic carbonate units of the Dead Indian Hill klippe, Pre-Eocene and eldest Eocene joint-sets maintain the same angular and temporal pattern, after corrections for bedding dip. However, as a whole, the fracture network is offset ~30^o counterclockwise around a vertical axis from their pinned counterparts, i.e, the Heart Mountain Detachment rotated ~30^o W while sliding. Reactivations of high-angle fractures (joints) as strike-slip faults and normal faults indicate the detachment had been transported to the Southeast as a volcanic edifice thermally collapsed against the ridged Laramide platform and created the western topographic relief of Dead Indian Hill.