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AAPG Bulletin

Abstract


Volume: 70 (1986)

Issue: 4. (April)

First Page: 360

Last Page: 376

Title: Thrust Faults and Back Thrusts in Madison Range of Southwestern Montana Foreland

Author(s): Russell G. Tysdal (2)

Abstract:

In the Rocky Mountain foreland of southwestern Montana, a zone of Late Cretaceous thrust faults, named the Hilgard fault system, extends along the west side of the Madison Range from Hebgen Lake northward for about 50 mi (80 km). The thrust faults are steep at their leading edges but flatten westward beneath the associated plates, where they commonly dip 25°-30°. Structural lows and highs are apparent beneath the Beaver Creek plate, the major thrust sheet of the system, and correlate with salients and reentrants of the plate. The Beaver Creek plate consists primarily of Archean metamorphic rocks, but Phanerozoic strata are preserved along the northern part of the plate's leading edge. Only the forward part of the plate is preserved in the Madison Range because C nozoic normal faults of the Madison Range fault system dropped much of the plate beneath the Madison Valley on the west.

The Kirkwood plate lies east of and beneath the Beaver Creek plate, and contains structures of the eastern part of the Hilgard system. The central segment of the leading edge of the Kirkwood plate is not completely detached from underlying strata. The leading edge of the northern part of the plate, defined by the Cache Creek fault, is flanked on the west by an associated anticline. The southern end of the Cache Creek fault is an eastward-dipping back thrust, which abruptly steepens on the west adjacent to the anticline. Both the fault and the anticline are believed to have formed above a concealed detachment fault. The southern part of the Kirkwood plate displays structures interpreted to represent displacement transfer from the Beaver Creek plate.

Back thrusts are associated with both thrust plates and played a key role in accommodating strain associated with folding the strata at the leading edges of the plates. The back thrusts project upward from the main thrust fault beneath each plate. They dip eastward at moderate angles in their lower segments, but steepen at structurally higher levels; they become overturned locally and dip westward in their uppermost segments.

The northern part of the study area contains the northwest-trending Buck Creek reverse fault, which dips steeply to the northeast. Movement on this structure ceased before the north-trending thrust faults of the Hilgard system were emplaced.

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