Scarp analysis of the Centennial Normal Fault, Beaverhead County, Montana and Fremont County, Idaho

Abstract

The Centennial Mountains are an east-west trending mountain range in southwest Montana. The Centennial Mountains are bound on the south by the Eastern Snake River Plane, the north-trending Madison Range and fault on the east and the Centennial Valley on the north. The Centennial normal fault offsets the Centennial Mountains on the north down-dropping the Centennial Valley. Approximately 3000 meters of offset along the Centennial normal fault creates the Centennial Mountains. The present Centennial Mountains are subdivided into two stratigraphically different blocks by the Odell Creek normal fault. The eastern Centennial Mountains are interpreted as the upthrown block of the Odell Creek normal fault exposing Precambrian and Paleozoic rock along the northern face of the range. The western Centennial Mountains are interpreted as the downthrown block of the Odell Creek normal fault exposing Cretaceous and younger rocks. Both eastern and western segments of the Centennial Mountains are then offset along the range bounding Centennial normal fault. Offset along the Centennial normal fault started approximately 2.1 Ma as evidenced by the displacement of the 2.1 Ma Huckleberry Ridge tuff. It is believed that prior to the emplacement of the 2.1 Ma Huckleberry Ridge tuff, the Centennial Mountains had minimum to no surface relief. The majority of offset along the Centennial normal fault has occurred with in the late Pleistocene with estimated slip rates of 0.65-0.82 mm/yr. The late Pleistocene surface offsets along the Centennial Mountains have an average of 9.1-9.6 meters with similar offset seen along the eastern and western segments.