Four elements of the long-enigmatic Heart Mountain detachment are the focus of this synthesis of new and earlier work. First, the geometry of the detachment system is more completely defined by our interpretation that the nearby South Fork thrust was the contractional toe of the Heart Mountain detachment during its early movement. Second, emplacement of the allochthon included phases with both catastrophic and slow rates of movement, driven by gravitational instability of active volcanoes above a dipping, pre-volcanic substrate. Third, initiation of displacement resulted from reduction of basal friction by elevated fluid pressure along the basal detachment, presumably beneath a critically stressed Coulomb wedge. Fourth, maintenance of low basal friction, to allow displacements of the allochthon in excess of 30 kilometers, was aided by endogenic formation of a gas suspension along the basal detachment. Thus, a unique combination of conditions and processes led to formation of the world's largest-known subaerial, detached extensional system. It represents a worst-case scenario for the magnitude of destruction resulting from sector collapse of an active subaerial volcano.

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