TECTONITE FABRIC IN THE CONTACT AUREOLE OF A SYNKINEMATIC STOCK, LEWIS AND CLARK LINE TRANSPRESSIONAL SHEAR ZONE, GARNET MINING DISTRICT, MONTANA

The Lewis and Clark line experienced sinistral transpressive shear during the Late Cretaceous-late Paleocene Laramide orogeny, along the boundary between the massive Lewis-Eldorado-Hoadley (LEH) thrust slab to the NE and the Sapphire and Lombard thrust slabs to the SW. The transpression extruded SE trending, en echelon flower structures along a 20 km-wide shear zone. Deeper rocks were strongly strained with vertical stretching lineations, whilst shallow levels folded into box-folds with limbs overturned to either the NE or SW. Vertical cleavage zones flattened upwards into reverse faults with a few km of throw. Late Cretaceous satellite stocks of the Boulder batholith intruded the shear zone and interfered with folding and faulting. One of these, the 83-Ma Garnet stock, invaded a narrow NE-trending fracture zone that straddled the shear zone at deeper levels in the Proterozoic Belt Supergroup but mushroomed in a SE-trending, SW-verging syncline in the Paleozoic section. The stock and its family of sills did not cross the axial plane of the neighboring anticline, and the contact aureole was generally confined to the syncline and the steep limb of the anticline-syncline pair. Fabric elements within the contact aureole indicate SW-directed shear, with strain increasing toward the stock. Stratigraphic units thin to 1/3 of their normal thickness within the aureole due to metamorphism and shear. Cordierite porphyroblasts are flattened and rotated with sigmoidal inclusion trails, andalusite crystals align along the down-dip stretching direction, and biotite occupies the NE-dipping cleavage. Gold-bearing quartz veins in the Garnet mining district lie along NE-dipping reverse faults that pass into the axial planes of kink folds.