ABSTRACT

Two basic concepts pertaining to the Heart Mountain fault have recently been challenged (Hauge, 1982a, 1982b, 1982c, 1983a, 1983b, 1985); one, that there was tectonic denudation, and two, that volcanic rock of the Wapiti Formation was deposited on the exposed fault surface. We believe that tectonic denudation occurred as a consequence of the upper plate having broken up into numerous blocks that separated as movement progressed along a nearly horizontal fault surface, thus leaving the fault surface exposed between blocks. Volcanic rocks of the Wapiti Formation were then deposited on the exposed fault surface. Documentation of the validity of this concept depends in part upon recognizing two formations of Eocene volcanic rocks. The older Cathedral Cliffs Formation, together with Paleozoic carbonate rocks, is part of the upper plate of the Heart Mountain fault and moved with it, whereas the younger Wapiti Formation was deposited on the fault surface after movement had ceased.

In the alternate interpretation recently advanced, subdivisions of the Absaroka Volcanic Supergroup, of which the Cathedral Cliffs and Wapiti Formations are units, are not recognized and it is concluded that, "During Heart Mountain faulting, extension of the once-continuous slab of Palezoic sedimentary rock was accompanied by the formation of ten or more graben now composed dominantly of Absaroka volcanic rocks" (Hauge, 1982b, p. 178). This interpretation further proposed that the volcanic rock infilling took place while the separating blocks of Palezoic strata were still moving and that the basal part of the volcanic rock between these blocks is in fault contact rather than depositional contact with the strata beneath the Heart Mountain fault. More recently, Hauge (1985) has suggested a gravity-spreading origin of the Heart Mountain allochthon, in which the mechanism of emplacement of the allochthon, which once may have covered more than 3,400 km², is viewed in terms of gravity-induced spreading on the flanks of an active volcanic field.

Geologic evidence that supports our contention that the Wapiti Formation is younger than the Heart Mountain fault and was not fault-emplaced include: (1) A break-away fault cuts the Paleozoic carbonate rocks and Cathedral Cliffs Formation, but is overlain by several thousand feet of unbroken Wapiti Formation; (2) The volume of Wapiti Formation filling the spaces between allochthon blocks is much too great to have been allochthonous as part of the upper plate; (3) The fault breccia at the base of the upper plate carbonate blocks is composed entirely of carbonate fault breccia, with no volcanic contribution; (4) small blocks have been transported from the upper part of the allochthon to the fault surface; (5) Wapiti volcanic rocks with chilled borders are in tightly bonded or "frozen" contact with upper plate Paleozoic rocks; (6) Faults present in the upper plate blocks do not enter the overlying Wapiti Formation; and (7) absence of volcanic fault breccia where volcanic rocks overlie carbonate fault breccia.