Cenozoic tectonic and paleogeographic evolution of the Horse Prairie half-graben, southwest Montana

The Horse Prairie basin (HPB) of southwestern Montana is a complex, east-dipping half-graben that contains three angular unconformity-bounded sequences of Tertiary lacustrine, paludal, and fluvial sediments overlying middle Eocene volcanic rocks. The basin is near the eastern edge of the Cordilleran thrust belt, and represents the western half of a larger Paleogene rift basin. Geologic mapping within the Everson Creek and Bannock Pass 7.5 minute quadrangles indicates that five temporally and geometrically distinct episodes of extension characterize the late Mesozoic (?) to Cenozoic tectonic evolution of the upper HPB.;The first episode of extension occurred prior to emplacement of middle Eocene volcanic rocks on an enigmatic, low-angle, southeast-dipping fault. The second episode of extension occurred in middle Eocene time on northwest-dipping syn-volcanic normal faults. The third generation of normal faulting occurred on the low-angle, south-southwest- and west-dipping Lemhi Pass and Maiden Peak fault systems, respectively. Two episodes of middle Miocene and younger extension also occur within the study area; however, structural and basin analysis indicates the HPB experienced only minor extension in the last 17 m.y. (Abstract shortened by UMI.).