ABSTRACT

The exposed faults on the northeastern flank of the Beartooth Mountains display a reversal of vergence, with southwest-dipping thrust faults to the southeast and northeast-dipping thrust faults to the northwest. The surface exposures in the northwest exhibit complex thin-skinned and thick-skinned deformation. Published structural models of the Beartooth uplift do not adequately explain this reversal in vergence and the complex thin-skinned deformation.

The proposed structural model for the northwestern Beartooth Mountains indicates that the northeast-dipping faults define wedges of basement. These wedges are back-thrust over the mam Beartooth block, due to motion on a blind master fault dipping to the southwest, paralleling the exposed master thrust in the southeastern part of the uplift. The basement wedge formation was probably synchronous with thin-skinned deformation including thrust duplexes and ramp structures which help fill strain incompatibilities or space problems between basement blocks. These complex structural geometries do not require any major folding of basement. The balancing of thick-skinned deformation and resultant thin-skinned deformation give shortening estimates of 13 kilometers in the upper crust. This is compatible with the estimate of 12 kilometers of Precambrian overhang in the southeastern Beartooth Mountains by Bonini and Kinard (1983).

The subsidence of the Paradise Valley to the southwest of the Beartooth Mountains postdates the Beartooth thrusting, but predates horizontal Miocene strata. Models involving pure dip-slip motion are difficult to reconcile with the variation of throw along strike, with almost none at the southern end, 6.1 kilometers in the center and 1.76 kilometers at the northern termination of the valley. The rhomboid shape of the Paradise Valley is consistent with left lateral strike-slip motion, suggesting formation in a rhomb-shaped pull-apart basin of late Eocene age.