Structural Geology of the Beartooth Mountains, Montana and Wyoming

Abstract

The Beartooth Mountains—an 80 by 40 mile elevated crystal block of Precambrian crystalline rocks—are in the Middle Rocky Mountains east of the great north-south belt of overthrusts. The Beartooth Mountains are bounded on the east by the Bighorn Basin (average structural relief, 15,000 feet); on the northeast and north by the Nye-Bowler lineament (along which movement has been dominantly left-lateral); and on the north by the Crazy Mountain syncline (average structural relief of at least 10,000 feet). The southwest side (Yellowstone-Absaroka Mountains) is mostly covered by thick Tertiary volcanic rocks. Detailed mapping, supplemented by results of earlier workers, provides an integrated geologic picture of the tectonic development of the block. Much of the tectonic history is recorded in the structures developed in the Paleozoic and Mesozoic sedimentary rocks exposed around the edge of the mountains. The sedimentary rocks played a relatively passive role in the dynamic rise of the block during Laramide time, as they draped, folded, broke, and slid along the edge of the block. A thrust plane(s) dipping west-southwest is the major structural feature between the Clarks Fork of the Yellowstone and Nye, Montana. Near Red Lodge numerous tear faults displace the upper plate of the thrust as much as 10,000 feet. From Nye to Livingston, Montana, the major bounding structure is a north-northeast-dipping thrust plane(s) terminating in several left-lateral tear faults. Near Gardiner a high-angle thrust plane dipping northeast bounds part of the southwest Beartooth. Several structural mechanisms apparently operated during deformation: (1) Uplifting, tilting, and depressing of large crustal blocks. These commonly form ramps from the Beartooth massif to adjacent basins; during uplift smaller blocks were rotated in the opposite sense. (2) Vertical raising of the northwest and southeast corners. Here, the general lack of horizontal displacement relative to the adjacent basins limits the total horizontal displacement of the block. (3) Thrust faulting. Most of the frontal thrusts show moderate dips. Imbrication is developed particularly at the unconfined northeast corner. The major thrust steepens with structural depth to the south. The horizontal thrust displacement is not directly measurable at most places but is limited by the lack of horizontal movement at the two fixed corners of the block. (4) Tear faulting. Several major tear faults (displacements up to 10,000 feet) bound large keystone-shaped blocks near Red Lodge. (5) Lateral shearing. During the late stage of uplift, horizontal movement toward the unconfined Red Lodge corner created major lateral shears, associated with imbrication, along the north and east sides, with folding along vertical axes and consequent shortening of the sedimentary section. Tectonic development is attributed to (1) horizontal compressive forces, responsible for early crustal block movements and subcrustal plastic shifting, and (2) vertical forces of fluid pressures from the continued transfer of subcrustal material which probably helped raise the block. Uplift ceased when one or both of these forces terminated.