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. Pre-volcanic extension (?) may reflect gravitational collapse of the Sevier thrust belt beginning in the lateMesozoic. The second episode of extension occurred in middle Eocene time on northwestdipping syn-volcanic normal faults. Syn-volcanic faults can be attributed to extension of the Challis volcanic arc, and typically accommodate less than I km of dip-slip separation. The third generation of normal faulting occurred on the low-angle, south-southwest- and west-dipping Lemhi Pass and Maiden Peak fault systems, respectively. Slip may have initiated during the waning phases of Challis volcanism, but these late to post-volcanic normal faults probably reflect continued gravitational collapse of the Sevier thrust belt, because they generally parallel contractional structures in the region. Late to post-volcanic normal faults accommodate the bulk of extensional strain in the study area, and are responsible for the deposition and preservation of the majority of basin-fill deposits in the HPB. 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.
Spatial and temporal relationships between magmatism and extension suggest that large magnitude extension in the HPB (episodes I and 3) was not associated with magmatism, and that extension in this portion of the Basin and Range province initiated due to gravitational instabilities imparted on the crust during the Sevier orogeny
