OUT-OF-SEQUENCE AND FOLDED SHORT THRUSTS PRODUCED BY PROTRACTED FORELAND-THRUST-BELT INTERACTIONS, SW MONTANA

New mapping in the SW Montana reentrant of the Cordilleran fold-and-thrust belt in the Tendoy and Beaverhead mountains shows that it is very different in its structure and sedimentology from most thrust belts. (1) Most frontal thrusts are out-of-sequence in their final stages of evolution; (2) the most forward thrusts have sinuous traces because they were folded about NE-trending foreland structures before being overridden by out-of-sequence thrusts; (3) crystalline and metamorphic basement rocks are prominent in frontal emergent thrust sheets; (4) thrust faults are short; (5) lateral linkages between thrust faults are obscure and debated; (6) the spacing between thrust faults is small; and (7) some “slide blocks” of Mississippian rocks in the synorogenic deposits of the Tendoy Mountains are actually intact bedrock bounded below by a thrust fault that sliced off the uplifted nose of the Blacktail-Snowcrest uplift. We explain these unusual structural features as a response to a ~20 m.y. interaction between the Cordilleran thrust belt and Laramide-style structures in the foreland. The foreland style deformation persisted at least into Late Cretaceous to Early Tertiary time based on the presence large NE-trending overturned folds in the Red Butte Formation of the Beaverhead Group, the youngest synorogenic deposits of the thrust belt. Thus significant Laramide deformation postdates the older Blacktail-Snowcrest uplift. The reentrant also has anomalous sedimentologic characteristics. These include (1) little coarse detritus shed from nearby thrust sheets; (2) voluminous coarse conglomerates in the western foreland basin that were derived from foreland uplifts or quartzite-bearing thrust sheets in the western thrust belt; (3) significant along strike changes in stratigraphy and provenance of syntectonic deposits in the foredeep due to segmentation by basement uplifts, and (4) presence of quartzite clast conglomerates from distant thrust sheets in the earliest to latest syntectonic deposits of the reentrant. A long history of Laramide deformation within the toe of the thrust wedge destabilized this segment of the thrust belt and resulted in an unusually complex segment of the Cordilleran thrust belt.