FORELAND BASEMENT INVOLVEMENT IN SINISTRAL TRANSPRESSION ALONG THE LEWIS AND CLARK SHEAR ZONE, MONTANA-WYOMING ROCKY MOUNTAINS

The origin and structural evolution of the foreland basement uplifts of Wyoming intrigued Professor Donald Blackstone throughout his long and prolific career. Implications from the perspective of the Montana Rockies suggest that some of the basement uplifts of northern Wyoming and adjacent Montana may have resulted from sinistral transpression along the Lewis and Clark line (LCL) shear zone. The LCL initiated along the Helena embayment of the Mesoproterozoic Belt-Purcell basin of Montana. Northeast of the LCL, the basin appears to have undergone a significant component of clockwise rotation about an Euler pole near Helena during late Cretaceous and Paleocene Laramide thrusting; northeastward displacement of the leading edge of the Belt-Purcell Supergroup increases from a few km near Helena to >250 km in Canada. Tectonic balance required basement blocks south of the Helena embayment to occupy the space vacated by the Belt-Purcell rocks, effectively closing the old Belt-Purcell rift. Kinematics required those basement rocks to rotate clockwise to transfer rotational displacement across the LCL. They rotated, however, about separate Euler poles. The eccentric rotations resulted in sinistral-transpressive shear and produced a profound belt of southeast-trending en-echelon folds on the northeast side of the LCL. These folds face either northeast or southwest and represent flower structures extruded along the LCL. On the southwest side of the LCL, Laramide foreland basement uplifts define a second zone of en-echelon, southeast-trending structures; these include the ancestral Blacktail-Snowcrest arch, Beartooth and Bighorn Mountains, and possibly the Black Hills. Like the fold structures on the northeast side of the LCL, these structures face either northeast or southwest. The longer wavelength of the basement array may result from its greater layer thickness.