Large-scale geometry of Montana thrust belt
Regional plunge of a structural culmination in the Sawtooth Mountains, combined with seismic profiles and borehole data, show that the imbricated faults of the Montana Disturbed belt gather upward to merge into a duplex beneath a large thrust plate dominated by the Precambrian Belt Supergroup. The duplex formed after deposition of the Paleocene St. Mary River Formation. This thrust plate overlies the Lewis, Hoadley, El dorado, Steinbach, and related thrust faults and forms the main ranges of the Montana Rocky Mountains. The plate is shaped like a northeasterly tapering wedge; it is 3 to 4 km (10,000 to 13,000 ft) thick at its leading edge, but thickens to more than 25 km (15 mi) to the southwest. At its leading edge, the plate carries lower parts of the Belt supergroup, including the Greyson, Empire, and Spokane formations. These units and overlying parts of the Belt Supergroup thicken dramatically westward within the plate, and the older, very thick, metamorphosed Prichard Formation emerges along the Purcell and related anticlinoria. The plate forms a south-facing monocline along the Lewis and Clark line. The monocline is corrugated into a series of southeast-plunging en echelon folds of Late Cretaceous age and merges with the southern terminus of the Purcell anticlinorium. Along these folds, metamorphosed Belt strata of the plate plunge systematically beneath Paleozoic and Mesozoic rocks that are preserved in a regional structural depression at the foot of the monocline. Geometric constructions based on areal geology suggest that the monocline has 25 km (15 mi) of structural relief and overlies a major footwall ramp that continues northward beneath the Purcell anticlinorium. This footwall ramp corresponds to the depositional locus for the easterly tapering belt strata of the plate, suggesting a total northeasterly displacement of about 50 km (30 mi) for the plate.
- Research Organization:
- Univ. of Montana, Missoula
- OSTI ID:
- 7230065
- Report Number(s):
- CONF-8609129-
- Journal Information:
- Am. Assoc. Pet. Geol., Bull.; (United States), Vol. 70:8; Conference: AAPG Rocky Mountain Section meeting, Casper, WY, USA, 7 Sep 1986
- Country of Publication:
- United States
- Language:
- English
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