QUARTZ C-AXIS STUDIES FROM A SHEATH FOLD IN METAMORPHOSED PRECAMBRIAN ROCKS, TOBACCO ROOT MOUNTAINS, MT

C-axis orientations in samples taken from the limbs and noses of a single sheath fold located in the Tobacco Root Mountains of MT constrain the kinematics involved in the creation of this sheath fold and, by extrapolation, of sheath folds in general. The three-dimensional surface of the fold, well exposed in outcrop, consists of upper and lower tongue-shaped portions and a thinned middle region which give the impression of a flattened-hourglass. The hinge angle of the fold is 64 degrees; its cross-sectional ellipticity is 0.22; the exposed length along the central symmetry axis of the fold is 4.93 m, and the maximum exposed width is 4.14 mm, yielding a length-to-width ratio of 1.19:1.

Five rock types are present in the fold: a biotite-garnet-feldspar-quartz-kyanite-schist, a garnet-quartz-feldspar granulite, a quartz-feldspar-garnet gneiss, and two amphibolites that differ in feldspar content. Most samples analyzed are garnet-quartz-feldspar granulite. Units range in thickness from about 5 cm to about 1 m and are consistently much thicker in the hinges than in the limbs.

The sheath fold folds both foliation and compositional layering but not mineral lineation, which is constant in orientation on the limbs and hinges. The outcrop occurs in a pervasively-folded domain whose fabrics and folds are consistent with those of the sheath fold, all of which are believed to have formed during an upper amphibolite facies deformation event that is best dated at 1775 Ma.