A MAP-SCALE SHEATH FOLD IN THE TOBACCO ROOT MOUNTAINS, MONTANA AND ITS ROLE IN THE EVOLUTION OF THE 1.78-1.73 GA BIG SKY OROGEN
Mineral lineations have a consistent NNE trend across all parts of the sheath fold. Where exposed, the axis of the sheath fold is oblique to this lineation, suggesting that the paraboloidal nose of the sheath fold has a hinge line angle of 70 degrees and closes downward. Outcrop-scale fold axes are dispersed in the regionally N-dipping plane of foliation (which parallels the hinge plane of the sheath fold) with a weak cluster parallel to the mineral lineation. Outcrop-scale S and Z folds are systematically arranged on opposite sides of the mineral lineation and allow determination of sense of shear along the mafic schist/quartzofeldspathic gneiss contact around the sheath fold. Quartz c-axes have crystallographic preferred orientations (CPOs) reflecting both basal < a > and prism [ c ] slip (consistent with high T metamorphism during folding) and also indicate non-coaxial deformation at sites along the mafic schist/quartzofeldspathic gneiss contact.
Both outcrop-scale asymmetric fold patterns and quartz c-axis CPOs demonstrate a vergence reversal between the upper and lower limbs of the map-scale sheath fold. The mafic schists in the core of the fold moved down and to the north relative to the surrounding gneisses. This suggests a component of overall channel flow rather than, or in addition to simple shear during convergence, maximum crustal thickening, and high T metamorphism in the lower levels of the Big Sky orogen.