Field studies of the Heart Mountain detachment and overlying volcanic rocks reveal that “autochthonous” Eocene volcanic rocks previously reported as postdating faulting are tectonically emplaced and, in many places, far traveled. Evidence of tectonic deformation of these volcanic rocks is documented in areas where volcanic rocks directly overlie the detachment, as well as at structurally higher levels within the allochthon. Such evidence includes shearing, brecciation, and faulting of volcanic rocks, and tilting and truncation of volcanic strata. This widespread deformation casts doubt concerning earlier suggestions that volcanic rocks were deposited upon the detachment and upon numerous detached slide blocks after detachment faulting. In light of these new data, the upper plate of the Heart Mountain detachment is interpreted as having been a single, continuous allochthon composed largely of volcanic rocks, rather than as having consisted of numerous separate slide blocks as was previously envisioned. Crosscutting relationships between dikes and faults within the allochthon suggest that allochthon emplacement occurred coeval with volcanism and, contrary to earlier suggestions, need not have been catastrophic.

The mechanism of emplacement of the allochthon, which once may have covered >3,400 km2, is viewed in terms of gravity-induced spreading on the flanks of an active volcanic field. Kinematic data indicate that transport of the allochthon was locally directed to the north, northeast, east, and southeast, generally away from active volcanoes, as well as downslope toward the Bighorn Basin. Transport was accompanied by variably directed extension of the allochthon, accommodated by normal, oblique-normal, and strike-slip faults, tilting of fault-bounded blocks within the allochthon, and dike intrusion.

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