CRETACEOUS(?) SYNCONTRACTIONAL EXTENSION IN THE SEVIER OROGEN, SOUTHWESTERN MONTANA

New 1:24,000-scale mapping in the Pintler Range of southwestern Montana suggests that extensional features developed synchronously with regional crustal contraction in the Late Cretaceous. The Kelly Lake 7 1/2' quadrangle lies within the hinterland of the Sevier thrust belt and also in the footwall of the Eocene(?) Anaconda metamorphic core complex. The northeast-striking Cretaceous Georgetown thrust bisects the quadrangle, with hanging wall rocks to the northwest and footwall rocks to the southeast. The hanging wall consists of a 10,000-foot-thick section of Proterozoic Missoula Group sediments intruded by Late Cretaceous plutons and deformed into open, upright folds. It also contains a major low-angle normal fault, the Shadow Lake detachment. This detachment is also folded and intruded by the 73 Ma Sapphire batholith.

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.