STRATIGRAPHIC AND STRUCTURAL STUDY OF MESOPROTEROZOIC STRATA IN ALLAN MOUNTAIN QUADRANGLE, EAST-CENTRAL ID

Allan Mountain quadrangle, on the Idaho-Montana border northwest of Salmon, contains two distinct packages of Mesoproterozoic metamorphosed strata. The Allan Mountain Thrust fault, striking roughly N60W dipping >30°SW, separates the two packages. A fine-grained, plagioclase-rich feldspathic arenite to siltite-argillite lies in the hangingwall southwest of the fault. This unit has traditionally been correlated to the Gunsight Formation of the Lemhi Group. It has a dominant detrital zircon age spectrum peak at 1757 Ma and a bulk rock ε_Nd(i) of +1.1. Northeast of the fault lies a sequence of coarse to very-fine grained alkali feldspar–rich feldspathic arenites, correlated on the existing Salmon National Forest geologic map to the upper Missoula Group. Interbedded argillite within the coarse facies produced a bulk rock ε_Nd(i) value of -3.0, and quartzite from the same location produced a detrital zircon age spectra with a major peak at 1737 Ma, but with no significant Archean population.

At least four phases of deformation are present in the quadrangle. The earliest phase produced folded rocks in the hanging wall of the Allan Mountain Thrust, although the fold geometry is indeterminate. The second deformation is characterized by outcrop- and map-scale tight folds throughout the quadrangle, whose hinges define a great circle distribution oriented N26W 70SW. This distribution is interpreted to be the F2 axial plane and indicates ENE-directed D2 shortening. The third phase is represented by movement along the Allan Mountain Thrust as a map-scale, gentle, hanging wall syncline. The D3 syncline is interpreted to reflect a concealed bend from flat to ramp in the underlying thrust. During the fourth phase, all rocks were tilted 30° NE based on attitudes of Oligocene volcanic rocks immediately SE of the map area.