Contraction and extension faults in the southern Beaverhead Mountains, Idaho and Montana

Abstract

Geologic mapping of 340 square miles (884 square kilometers) in the southern Beaverhead Mountains demonstrates that the area is a segment of the Mesozoic to early Tertiary Cordilleran thrust belt, and a northward continuation of the Idaho-Wyoming thrust salient. Five thrust plates are bounded by major west-dipping, low-angle folded thrusts that juxtapose older strata over younger, and are characterized by east-verging concentric folds, frontal ramp anticlines, and transverse tear faults. The five thrust plates, each with a distinctive stratigraphic sequence, are, in descending structural order: Hawley Creek, Fritz Creek, Cabin, Medicine Lodge, and Tendoy. The Cabin thrust plate locally includes Precambrian (Archean(?)) basement crystalline rocks along its leading edge. These crystalline rocks probably were part of an older Precambrian block uplift cut by the eastward directed Cabin thrust. Extension faults with youngest movements ranging in age from early Eocene to Holocene offset the five thrust plates. The Divide Creek extension fault zone, along which middle Paleozoic stratigraphic section is deleted, probably is of early Eocene age. Miocene to Holocene basin-range faults offset older structures, and loci of the younger faults have shifted westward through time. Thrusts and thrust plates identified in the study area also make up a large part of the central and northern Beaverhead Mountains. The Cabin thrust plate is the most extensive of these. IX The redefined Cabin thrust is more than 125 miles (200 kilometers) long, has two major lateral ramps, has Archean(?) crystalline rocks along a 48-mile (75-kilometer) segment of its leading edge in central parts of the mountains, and Middle Proterozoic Yellowjacket Formation and overlying Lemhi Group rocks in northern parts. The Cabin plate includes Archean(?) through Lower Triassic rocks in the central Beaverheads. In the northern Beaverheads, rocks of the Yellowjacket Formation and the Lemhi Group have been thrust over Belt Supergroup rocks creating a structural culmination of Proterozoic rocks in east-central Idaho. The foreland that was overridden by the easterly transported Cabin and Medicine Lodge thrust plates was deformed by northwest to southeast directed reverse faults that probably were formed and partly eroded before the thrust plates arrived in Late Cretaceous (Maestrichtian(?)) time.

Cordilleran thrust belt and faulted foreland in the Beaverhead Mountains, Idaho and Montana

“…Rocks of the redefined Cabin thrust plate extend from the Snake River Plain northwestward for about 200 km to the Tertiary and Cretaceous plutonic rocks of the Idaho batholith complex near Lost Trail Pass (Plate 2) and may continue northward (Skipp, 1987). This definition is based on the reinterpretation of several previously mapped structures in the central and northern Beaverhead Mountains.…”

“…13,14A); this fault has the characteristics of a detachment fault within the Cabin plate rather than a major thrust. The Hawley Creek thrust is the western boundary thrust of the west-dipping Cabin plate in the northern Beaverhead Mountains (Skipp, 1987).…”

Geology and mineral resources of the North-Central Idaho Sagebrush Focal Area: Chapter C in <em>Mineral resources of the Sagebrush Focal Areas of Idaho, Montana, Nevada, Oregon, Utah, and Wyoming</em>

“…Other more detailed digital maps that are used where possible include 1:250,000-scale maps (Fisher and others, 1992;Wilson and Skipp, 1994; and 1:100,000-scale maps for local areas (Link and others, 1995;Lund and others, 2003;Kuntz and others, 2007;Skipp and Kuntz, 2009;and Othberg and others, 2012). In special cases, more detailed, non-digital geologic maps (for example Skipp, 1984Skipp, , 1985Skipp and others, 1984;Worl and others, 1991) are used to inform the interpretations presented here.…”

“…A few areas with detailed mapping show large, upright to northeast-overturned folds as much as 10 km long others, 1983, 1989;Skipp, 1985;Skipp and Kuntz, 2009;Tysdal, 2002). These folds are associated with axial planar cleavage in rocks with siliciclastic components, including some zones of phyllite (Skipp, 1985;Lund and others, 2003).…”

“…The only digital geologic map available is the statewide compilation (Lewis and others, 2012), but more detailed mapping in the Italian Peaks Wilderness Study Area to the north (Skipp and others, 1983;Skipp, 1984) indicates a series of east-vergent thrust faults and related folds, which are both upright and overturned, that can be extended southward into the South Tip Beaverhead tract. These Cretaceous contractional structures were disrupted by Cenozoic normal faults that both reactivated thrust faults and offset them (Skipp, 1985). No Cretaceous or Eocene plutons are mapped in the southern Beaverhead Range and none are inferred from geophysical interpretation.…”