CRETACEOUS(?) SYNCONTRACTIONAL EXTENSION IN THE SEVIER OROGEN, SOUTHWESTERN MONTANA
Footwall rocks of the Georgetown thrust are more complexly deformed and contain a ductilely strained, tectonically attenuated stratigraphy with a Missoula Group section as thin as 200 feet. The stratigraphy has been thinned both by broad, diffuse zones of ductile shear and by distinct, nearly bedding-parallel faults that omit section. Both types of structures appear to have formed simultaneously and are attributed to extension. Repetition of one of the faults, the Sawed Cabin detachment, by a thrust, indicates that a convergent tectonic setting persisted. The structures were tightly folded by at least two subsequent fold events and intruded by granitic plutons that place a minimum age of Late Cretaceous on the postulated extensional features. However, at this latitude foreland thrusting is known to have continued until 58 Ma. The fault zone previously mapped as the Cretaceous Georgetown thrust is actually comprised of Tertiary high-angle faults that cut the structures described above. Although the zone does juxtapose the hanging wall and footwall of the thrust, the original thrust geometries are not preserved.