CATASTROPHIC EMPLACEMENT OF THE HEART MOUNTAIN BLOCK SLIDE: ROLE OF DIKING AND BASAL FLUIDIZATION

The Heart Mountain slide block of northwestern Wyoming and southwestern Montana is one of the world's largest slides encompassing an area of over 3,400 km2 and traversing at least 45 km of open terrain. This structure was emplaced in the mid-Eocene and slid on a slope of only 2°. The mode of emplacement has been debated for over one hundred years. Previous disputes have centered on the rate of emplacement (hours vs. thousands to millions of years) and on the involvement or non-involvement of volcanic rocks found at the slide's basal contact. Recently several authors, including ourselves, have suggested that emplacement was catastrophic and that the volcanic rocks above the basal contact were demonstratively involved in the movement phase. Key to any catastrophic model is understanding how to overcome the basal friction resisting movement. Models assuming rapid emplacement have generally involved either injection of volcanic gases or earthquake-generated vibrations. Here we propose a two-stage model, which explains how the slide block first initiated movement and how that movement was sustained for several hours while the upper plate began to break up. In our model, extensive intrusion of dikes into the 2 to 4 km thick overlying mass of Paleozoic sedimentary rocks and volcanic rocks of the Absaroka Volcanics created conditions where water trapped with the Big Horn Dolomite became overpressured. The overpressuring is the result of the thermal coefficient of expansion of the waters and the reduced pore space caused from the Skempton effect. The high fluid pressures produced a lowering of basal friction coefficient. An episode of extensive dike intrusion and concomitant earthquakes (à la Walter Bucher, 1947) is a likely initiator of movement. Once sufficient sliding velocity is achieved, the dynamic fluidization of basal granular material supersedes pore fluid pressure as the cause of reduced friction as fluids escape into the extending upper plate. This mechanism may be applicable to other slide masses originating on the flanks of volcanic edifices.