Multiple reactivation of a collisional boundary: An example from southwestern Montana

Abstract

Structural elements and stratigraphy of the central Snowcrest Range, southwestern Montana, reflect multiple reactivation of what was probably a late Archean and/or Early Proterozoic collisional margin. The oldest structural element is a N54°E, 41°NW foliation in metamorphic basement rocks, on the western margin of the range, which is inferred to have coincided with establishment of the NE-trending margin of the range as an important zone of crustal weakness (Snowcrest-Greenhorn lineament). The second major structural event is reflected in upper Paleozoic strata. A substantially thicker section of Mississippian through Triassic rocks was deposited in a crustal depression (Snowcrest trough) on the northwest side of the lineament. The lineament was probably a NW-dipping normal fault zone during late Paleozoic time, reflecting crustal extension across the earlier boundary. During the Late Cretaceous, E-W shortening across the lineament produced a system of right oblique-slip, NW-dipping thrusts (Snowcrest-Greenhorn thrust system). The earlier Snowcrest trough facies were juxtaposed against thinner shelf facies during this basement-involved thrusting. Foliation-parallel brittle fractures probably formed in basement rocks at this time. Beginning in the Miocene, extension across the lineament produced a NW-dipping listric normal fault system (Sage Creek system) along the lineament, which partially accounts for existing basin-range topography. The normal faults are inferred to merge at depth into the Snowcrest-Greenhorn thrust system.KeywordsShear ZoneLate CretaceousNormal FaultGeological SocietyForeland BasinThese keywords were added by machine and not by the authors. This process is experimental and the keywords may be updated as the learning algorithm improves.