Basement Control of Recurrent Faulting, Central Montana

Abstract

A system of basement faults has been mapped in Fergus, Petroleum, Garfield and northern Rosebud Counties, Montana, U.S.A. The faults dip steeply and strike east-west, ENE-WSW, and ESE-WNW. These faults have been reactivated repeatedly through geologic time, most recently during the Laramide orogeny in early Tertiary time. They are expressed at the surface as monoclines and asymmetrical anticlines, and as belts of en echelon faults and domes indicative of oblique-slip displacement.

The faults are believed to have originated as normal faults in a Proterozoic failed rift, the Montana aulacogen, which extended eastward from the edge of the North American craton in western Montana. Grabens within the aulacogen were filled with sediments of the Belt Supergroup. Some of these faults were reactivated as reverse faults during Late Cambrian time; locally, normal displacement occurred on the Cat Creek fault in latest Cambrian or Early Ordovician time. A second episode of reverse faulting took place near the end of the Devonian Period in response to the Antler orogeny west of Montana. Faults were inactive during the Mississippian Period, although previously-uplifted blocks gradually subsided late in the Mississippian. Widespread normal faulting, with displacements as large as 430 m, occurred between Middle Pennsylvanian and Middle Jurassic time. The area was generally stable through Late Jurassic and Cretaceous time. Laramide deformation may have commenced late in the Cretaceous Period, but most of the activity occurred early in the Tertiary. Laramide compressional stresses, oriented northeast-southwest, induced reverse and left-lateral strike-slip displacements on basement faults.

Phanerozoic tectonic history of central Montana is similar to that reported on structures farther east in Montana and North Dakota. However, not all faults were active during each episode of deformation. Some periods of activity within the craton can be related to events at continental margins, but others cannot.