KINEMATIC RECONSTRUCTION OF THE SEVIER THRUST BELT, IN SOUTHWEST MONTANA USING THE SYNOROGENIC BEAVERHEAD GROUP

The Sevier thrust belt formed from late Jurassic to Eocene time. Subsequent extension (ca. 49-20 Ma) dissected the thrust belt leaving isolated exposures of deformed rock and rendering a kinematic interpretation difficult. This is particularly problematic in southwest Montana where extensional grabens are well developed leaving thrust belt rocks in the horst blocks. Near Bannack State Park, the Beaverhead Group is a thrust belt derived syntectonic package of sediments that is Late Cretaceous to lower Tertiary in age. The Beaverhead conglomerate is unique in the fact that it is interfingered with volcanic rocks, which can be dated using 40Ar/39Ar dating to help determine the timing of deposition. Because the Beaverhead Group is syntectonic, clast-counts, point-counts, and paleocurrent data in this unit provide an idea as to what types of rocks were involved in the thrust belt and the origin of the rocks. Systematic changes, either upsection or downsection, yield insight into the evolution of the source terrane. Thermochonologic data provide a constraint as to when each unit within the Beaverhead Group was deposited. Because the Sevier thrust belt has been extended in this region, the field and thermochronologic data used in conjunction can help build a kinematic model which details how the Sevier thrust belt evolved in this part of the Southwest Montana reentrant.