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

Slip on shallowly dipping detachments is one of the longest-debated puzzles in structural geology. The Eocene Heart Mountain detachment in northwestern Wyoming is among the largest, best-studied examples of such an enigmatic feature. Extant exposures of the upper plate of the detachment, the Heart Mountain allochthon, form an elongate, internally coherent, extended mass comprising Paleozoic carbonate strata and overlying Eocene Absaroka volcanics. The allochthon is at least 70 km long, with apparent slip of as much as 45 kilometers. At present, the base of the allochthon is regionally subhorizontal, with local dips rarely exceeding a few degrees. Given its highly unfavorable orientation for either coulombic failure or continued slip, it would seem likely that a viscous mechanism, where failure may occur under relatively low ratios of shear stress to normal stress, is needed to explain how such low-angle faults are able to form.

Most recent conceptions of the emplacement of the Heart Mountain allochthon as a catastrophic event, occurring within a single day. However, we have observed evidence of both cyclic and long-duration, fault-related deformation, including cross-cutting clastic dikes and overprinting relationships involving brecciation, cementation, veining, and pressure solution. In particular, textures within and around distinctive banded grains (“accreted grains” of previous workers) suggest their formation via the relatively slow, fluid-related processes.

The only known potential mechanism to facilitate viscous deformation under upper crustal conditions is pressure solution creep. We propose that the Heart Mountain detachment began to form via heterogeneous, perhaps highly localized, pressure solution creep along discrete patches of the future detachment surface. The loading induced by these patches could serve to rotate the principal stress directions locally, and thereby trigger brittle failure on the low-angle surface. Most of the slip along the detachment need not be accommodated by pressure solution creep for this mechanism to apply. An inclined orientation for the maximum principal stress direction is supported at one locality by the orientations of microfractures and possibly by en echelon clastic dikes near the fault plane.