ABSTRACT

The thrust belt in southwest and west-central Montana and in east-central Idaho includes at least five major thrust plates. Each of these major plates is characterized by a distinctive sequence of sedimentary rocks and by distinctive patterns of folding and imbricate thrust faulting within a plate. Most of the plates have a basal decollement.

The plates in southwest Montana and in east-central Idaho include the Medicine Lodge and Grasshopper plates, and a frontal zone of folds and thrust faults that fringes the major thrust plates on the east. This frontal zone is the leading part of the thrust belt in southwest Montana.

The thrust plates in west-central Montana are the Rattlesnake and Sapphire plates and the Elkhorn thrust zone. The Rattlesnake plate is located north of Missoula, Montana, and the Sapphire plate extends from the Idaho batholith to the Boulder batholith. The Elkhorn thrust zone is a lobate zone of thrusts present east of the Sapphire plate. The Rattlesnake and Sapphire plates probably are underlain by basal decollement zones, whereas the Elkhorn thrust zone probably does not have a decollement at its base. The Sapphire plate is subdivided into three subplates: (1) Rock Creek subplate, (2) Garnet Range subplate, and (3) Flint Creek subplate. The Flint Creek subplate may be equivalent to the Grasshopper plate present to the south, and the Garnet Range subplate may be equivalent, in part, to the frontal thrust and fold zone present to the south.

Thrust faulting probably occurred in east-central Idaho between 75 and 100 m.y. ago, and rocks may have been transported as much as 160 km to the east on the Medicine Lodge decollement. In southwest and west-central Montana thrust faulting occurred about 74 to 75 m.y. ago and parts of the formerly continuous Belt Basin and Belt seaway were transported a minimum of 40-70 km to the east.