Abstract

Ramp mechanisms associated with decoupling of a 6-7-km-thick basement slab may have been responsible for progressively more and more horizontal components of thrusting of the northeast corner of the Beartooth uplift near Red Lodge, Montana. As part of a nearly right-angle corner of the uplift, two apparent tear faults bound a 7-km-long block of Laramide mountain-front structures. New roadcuts and a deep well through basement refine geometry of range overthrusting and show that these apparent tear faults are really pivoting normal faults that cut frontal thrust structures on either side of an uplifted corner flap. A ship's prow analogy of late-stage horizontal thrust motion is proposed with the "bow wave" causing uplift and rotation of the corner flap. Volumetric adjustments associated with late-stage stuffing of basin material beneath frontal thrusts plus deeper duplexing of basement beneath the uplift helped define final details of range geometry, a mechanism probably applicable elsewhere in the middle Rocky Mountains.

These mechanisms of basement deformation at the northeast corner contrast with those of the western Beartooth uplift where strong, dense rocks associated with the Precambrian mafic Stillwater Complex precluded detachment of a basement slab and created a different style of structural underthrusting and frontal rotation. Eastward escape tectonics around the Stillwater obstacle caused later stages of the Beartooth uplift to change thrust direction from north-northeast to east, helping form the Red Lodge corner and to create the east-directed thrusts that separate the uplift from the adjacent Bighorn basin.

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