Cenozoic Pull-apart Basins in Southwest Montana
RUPPEL, EDWARD T., Montana Bureau of Mines and Geology, Butte, MT
Faults and fault zones bounding the mountain ranges of southwest Montana commonly have been described as normal faults, and the region has been considered to be a northern extension of the Basin and Range. New geologic mapping suggests, however, that Cenozoic movements along most of the zones of steep faults in southwest Montana and in east-central Idaho have been strike-slip, and the intermontane basins appear to be pull-aparts. The principal fault zones trend about north, northwest, east, and north-northeast; the north-trending zones are Cenozoic in age, but the others are of Archean ancestry and are rooted in basement rocks. These faults break the region into rhomboidal mountain blocks separated by broad basins with parallel sides. The basins are as much as 5000 m deep, and their f oors are deeply indented by centers of subsidence where they are crossed by major fault zones. The basins are floored by Archean or Proterozoic rocks and are filled with tuffaceous sedimentary rocks of late Oligocene to late Miocene age. The Big Hole basin and the smaller basins in upper Grasshopper Creek and Horse Prairie are interpreted to be pull-aparts
between zones of east-trending right-lateral faults. The cratonic basins farther east in southwest Montana are interpreted to be basement-floored openings between mountain blocks that have been separated by subcrustal flow to the northwest. The interpretations suggest that significant accumulations of oil or gas are not likely to be found in this region.
AAPG Search and Discovery Article #91010©1991 AAPG Rocky Mountain Section Meeting, Billings, Montana, July 28-31, 1991 (2009)