ACCRETED GRAINS AS THE "SMOKING GUN" OF THE HEART MOUNTAIN DETACHMENT FAULT, NW WYOMING

The Heart Mountain detachment fault (HMD) has been a puzzle for over 100 years. The volume of this huge semi-coherent rockslide far exceeds any other known subaerial rockslide. The scale and unusual characteristics of the fault, e.g. low slope, lack of or limited deformation of the footwall, remarkable carbonate fault rock (microbreccia) containing sedimentary structures, and contemporaneous volcanism, have led to numerous hypotheses, none widely accepted, to explain its movement. Accreted grains within microbreccia along the fault represent the long-sought "smoking gun" for defining conditions along the HMD during movement.

We have found these grains at 8 localities along the fault spanning >50 km in the displacement direction. First recognized at White Mountain and attributed to volcanic processes (Hughes, 1970; Beutner and Craven, 1996), they form by accretion of fine particles to a core grain and each other. They are indistinguishable from accreted grains found in deposits from volcanic (accretionary and armored lapilli) and impact ejecta clouds (Ries, Chicxulub, Alamo) and in the diatreme facies of kimberlites and similar rocks. In all of these settings, formation of the accreted grains in a gaseous suspension has been observed or invoked. Experiments and observations seem to further limit conditions to a dispersed gaseous suspension within which electrostatic forces and condensed H₂O or other liquid provide adhesion and contemporaneous cementation, required for survival of accreted grains, occurs. Their presence at the base of the Palisades rockslide (Anders, et al., 2000) demonstrates that volcanism is not necessary; rather, the common feature is the presence of carbonate along the slide surface. Dissociation of carbonate by frictional heating provides CO₂ for suspension and gas pressure to produce the "hovercraft" effect (which Hughes attributed to volcanic gas), and chemically active lime for cementation of microbreccia and accreted grains. The formation and lack of subsequent deformation of accreted grains indicate that mechanisms of friction reduction such as acoustic or dynamical fluidization, which involve energetic grain-grain interactions, were not significant, and that gas suspension was maintained until movement ceased, i.e., movement was catastrophic.