Structural evolution of the Lewis plate in Glacier National Park, Montana: Implications for regional tectonic development

Abstract

Detailed geologic mapping in southern Glacier National Park, Montana, reveals four episodes of deformation in the hanging wall of the Lewis thrust. (1) Pre-Lewis thrust structures include west- and east-dipping imbricate thrusts, conjugate contraction faults, and west- and east-directed bedding-parallel faults. Although these structures are truncated from below by the Lewis thrust, their development was kinematically compatible with the emplacement of the Lewis plate. Thus, they may have formed during early stages of the emplacement of the Lewis plate. (2) Syn-Lewis thrust structures include the Late Cretaceous-early Tertiary Lewis thrust, west-dipping duplexes, east-dipping normal faults, and the Akamina syncline, a broad fold that lies directly west of the Lewis thrust and extends northwestward for about 120 km from southern Glacier Park, western Montana, to southeastern British Columbia and southwestern Alberta, Canada. The development of the duplexes and the normal faults may have been related to east-verging simple-shear deformation during emplacement of the Lewis plate. The formation of the segment of the Akamina syncline in the study area was the consequence of development of the duplexes in the Lewis plate, because strata above the duplexes are concordant with the syncline. The syncline is, however, disconcordant with the Lewis thrust. This observation contrasts strongly with the well-established concordant relationship between the Lewis thrust and the Akamina syncline in its hanging wall in Canada, about 100 km north of the study area. We propose that the formation of the Akamina syncline on a regional scale was related to the development of duplexes and imbricate thrusts at two structural levels, one above and one below the Lewis thrust. During the development of these duplexes, the Lewis thrust transferred horizontal shortening laterally along the strike of regional compressional structures from its footwall in the Paleozoic-Mesozoic strata to its hanging wall in the Proterozoic strata. We speculate that development of the broad-fold belt, a major structure in the fold-and-thrust belt in the southern Canadian Rocky Mountains and western Montana, was related to duplex formation at deep structural levels below the folds. (3) Post-Lewis thrust contractional structures include a high-angle reverse fault that cuts the Lewis thrust and strikes N70°W, which is about 30°-40° more to the west than the average strike of the syn- Lewis thrust structures. The development of this fault represents a change in compressional direction after emplacement of the Lewis plate. (4) Post-Lewis thrust extensional structures include southwest-dipping normal faults. These faults truncate the post-Lewis thrust reverse fault and are part of the Eocene-Oligocene Rocky Mountain trench normal fault system.