Complex Pre-Lewis Thrust Deformation, Southeastern Glacier Park, Montana

Yin, An

A complex structural association, the frontal zone, was mapped along the eastern edge of the presently preserved Lewis thrust sheet in southeastern Glacier National Park, Montana. This structural association was developed by multiple episodes of faulting and folding. In contrast to most structures in the Lewis thrust sheet, structures in the frontal zone are truncated by the underlying Lewis thrust, suggesting that the evolution of the frontal zone predates the Lewis thrust in the study area. Although the absolute age for the development of the frontal zone is not known, the trends of folds and faults are compatible kinematically with structures related to evolution of the Lewis thrust system, suggesting the zone was related to emplacement of the Lewis thrust sheet. Kinematic reconstruction of the frontal zone indicates that its formation was characterized by (1) conjugate contraction faulting, (2) west-directed, bedding-parallel faulting, and (3) top-to-the-east simple shear that rotated primary, east-dipping contraction faults to apparent west-dipping normal faults.