University of Montana, Maureen and Mike Mansfield Library
Abstract
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 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 40 km-wide shear zone. 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, imprinting a compressional fabric in its aureole of E-W striking cleavage planes and top-to-the-SW rotated porphyroblasts. 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. Maximum pressure and temperature of contact metamorphism is estimated at 2.8 kbar and 650°C from thermodynamic modeling of andalusite-sillimanite bearing hornfels. Magmatic stoping led to downward shear being applied to rock of the contact aureole, creating a down-dip stretching lineation and layer-parallel boudins. Stratigraphic units thin to ~1/2 of their normal thickness within the aureole due to metamorphism as well as shear associated with stoping. Emplacement was followed by a second phase of transpression that began after 76 Ma and ended during the early Cenozoic, when the Lewis and Clark Line underwent dextral transtensile shear evidenced by a newly-mapped right-lateral extensional accommodation zone within the field area
