Catastrophic emplacement of the Heart Mountain block slide, Wyoming and Montana, USA

Abstract

The mechanism that allowed many tens of km of movement of the enormous block slide fl oored by the rootless Heart Mountain detachment fault in NW Wyoming has long been a puzzle. Carbonate-rich microbreccia that is widespread along the fault and in dikes in the upper plate contains accreted grains indistinguishable from those observed as fallout from volcanic eruption clouds (accretionary lapilli) and impact ejecta clouds and in intrusive diatremes. In these settings and also in industrial processing, accreted grains form when particles in a turbulent gaseous suspension containing limited water adhere to a nucleating grain or to each other. Elongate grains in thick microbreccia have strong but diverse shape-preferred orientations unlike those reported from other fault rocks but instead suggestive of turbulent fl ow, and the microbreccia contains layering and other features of sedimentary character that appear to record deposition from suspension rather than frictional processes along a fault. We suggest that frictional heating led to dissociation of carbonate rock along the fault, producing supercritical CO 2 as the suspending medium. High CO 2 pressure drastically reduced friction along the fault and allowed continuation of catastrophic movement, probably initiated by a volcanic or phreatomagmatic explosion, resulting in very large displacement on a lowdipping surface. Earlier slower sliding may have occurred but fi nal emplacement was rapid (minutes) and spectacular.

Episodic Dissolution, Precipitation, and Slip along the Heart Mountain Detachment, Wyoming

“…Previous studies discussing the presence of banded grains similar to those in figures 7 and 8 interpret them as having formed during suspension within a pressurized fluid at the base of the detachment, constituting evidence of catastrophic slip (Hughes 1970;Beutner and Craven 1996;Beutner and Gerbi 2005;Craddock et al 2009Craddock et al , 2012Anders et al 2010). In the discussion of the processes that might have created the banded grains, the analogues of volcanic accretionary lapilli and impact ejecta were invoked, with an interpreted fluid of volcanic gas preferred for Heart Mountain (e.g., Beutner and Gerbi 2005).…”

“…In the discussion of the processes that might have created the banded grains, the analogues of volcanic accretionary lapilli and impact ejecta were invoked, with an interpreted fluid of volcanic gas preferred for Heart Mountain (e.g., Beutner and Gerbi 2005). However, in these ana- logues the clasts with accretionary rims make up a very high percentage of the rock-at least 90% (see Beutner and Gerbi 2005, their figures 8-10)-and the remaining 10% of material is the matrix, not uncoated grains. Later authors adopted the same mechanism of suspension in a fluid to explain the presence of "accreted grains," albeit with an interpreted CO 2 fluid, formed from massive decomposition of carbonate during frictional heating (e.g., Anders et al 2010).…”

“…At present, there is thus apparent consensus that the allochthon is a continuous, extended mass in which large volumes of volcanics are involved in faulting and that, given its extant area of exposure of 12100 km 2 , it constitutes perhaps the largest known subaerial landslide in the geological record (Beutner and Gerbi 2005;Beutner and Hauge 2009;Goren et al 2010;Craddock et al 2012;Anders et al 2013). However, opinion remains sharply divided on the kinematic question of the whether emplacement was catastrophic Goren et al 2010;Craddock et al 2012) or, at least in part, gradual (Hauge 1985(Hauge , 1990Beutner and Hauge 2009), limiting progress toward understanding the dynamics of slip.…”

“…Given these facts, any mechanical conception of the Heart Mountain detachment as forming catastrophically has at its disposal a broad palette of extraordinary circumstances affecting an area of thousands of square kilometers (e.g., fluid overpressure, magmatic devolatilization, seismic accelerations, bolide impact) that need to operate only for as little as a few seconds prior to emplacement rather than a million years or more (e.g., Hughes 1970;Pierce 1973Pierce , 1980Voight 1973;Melosh 1983;Sales 1983;Beutner and Gerbi 2005;Aharonov and Anders 2006;Craddock et al 2009Craddock et al , 2012Anders et al 2010Anders et al , 2013. The gradual emplacement hypothesis, on the other hand, Figure 2.…”

Observations from the Basin and Range Province (western United States) pertinent to the interpretation of regional detachment faults

“…Yarnold & Lombard 1989;Shaller 1991;Beutner & Craven 1996;Anders et al 2000), is thought to be associated with fluidization, which serves to explain the long runouts of such features (Shaller 1991;Anders et al 2000;Beutner & Gerbi 2005).…”