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

Distinctive mafic schists in the Tobacco Root Mountains lie in the core of a partly-occluded, map scale sheath fold that is mantled by quartzofeldspathic gneisses. The ellipsoidal core of the sheath fold has exposed dimensions of 10 x 4 km. Quartzofeldspathic gneisses in the Tobacco Root Mountains experienced previous episodes of tectonism, but this sheath fold developed between 1.78 and 1.73 Ga, concurrent with fabric development, mesoscopic folding and metamorphism at about 10 GPa and 750-800 degrees C, during the Big Sky orogeny.

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 (CPO’s) 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 CPO’s 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.