Abstract

Backthrusts in the Rocky Mountain Thrust Belt are being recognized more and more frequently as geologists delve into the structural evolution of the area. As used in this paper, the term backthrust refers to any thrust fault in the Cordilleran system which moved a package of sedimentary rocks westward over younger rocks.

Backthrusting occurred at several different times, from middle Jurassic through late Tertiary. There are known examples throughout the Thrust Belt from Canada to Nevada. Backthrusts seem to fall into three categories: low-angle faults ahead of the leading edge of the thrust system but antithetic to it and going to bedding plane in sediments of the foreland basin; high-angle reverse faults which are usually located just behind major thrust traces; and thin, shallow detachment masses which are associated with listric normal-faulted valleys.

It is believed that all three types of backthrusts are related to basement arches and ramps in lower thrust plates, and can be used to indicate where these structures are located.

Backthrusts are significant to exploration because they may both conceal and indicate important features beneath them, and because they themselves may contribute to forming hydrocarbon traps.