Libby thrust belt and adjacent structures - new factors to consider in thrust tectonics of northwestern Montana
About 40 mi (65 km) west of the Rocky Mountain trench and at least 9 mi (15 km) above the sole detachment of the Rocky Mountain thrust belt is a zone of Cretaceous-Tertiary thrust faults up to 25 mi (40 km) wide in middle Proterozoic and Cambrian rocks. This zone (the Libby thrust belt) extends northward from the Lewis and Clark line to the northwest corner of Montana. Within the Libby thrust belt is a series of complex ramps, horsts, splays, and folds that accommodate a tectonic shortening of about 6.2 mi (10 km). Backsliding has occurred on some listric thrust faults, and middle Tertiary(.) extensional horst-and-graben faults offset or join most thrust faults. On the east, the lead thrust ramps up onto the broad open Purcell anticlinorium. On the west, the Libby thrust belt is overridden in the north by the lead thrust of the Yaak plate (whose central part is the broad, open Sylvanite anticline), and in the south, it is overridden by the Moyie thrust (which trends northwest and also overrides the west edge of the Yaak plate). Geologic cross sections suggest that the Belt rocks have overridden the Cambrian at shallow depths only and that Cambrian and younger Phanerozoic strata probably do not occur at greater depths beneath and west of the Purcell anticlinorium. This interpretation differs significantly from interpretations that suggest intercalation of major wedges of Paleozoic and Belt rocks at depth in this same area.
- Research Organization:
- Geological Survey, Denver, CO
- OSTI ID:
- 6146108
- Report Number(s):
- CONF-8506201-
- Journal Information:
- Am. Assoc. Pet. Geol., Bull.; (United States), Vol. 69:5; Conference: AAPG Rocky Mountain Section meeting, Denver, CO, USA, 2 Jun 1985
- Country of Publication:
- United States
- Language:
- English
Similar Records
Geology of north end of the Pioneer Mountains, Beaverhead County, Montana
Mesozoic and Cenozoic structures of Pocatello region: transition from thrust belt to hinterland, southeast Idaho
Related Subjects
MONTANA
GEOLOGIC HISTORY
GEOLOGIC STRUCTURES
TECTONICS
ANTICLINES
CAMBRIAN PERIOD
GEOLOGIC FAULTS
PETROLEUM GEOLOGY
TERTIARY PERIOD
CENOZOIC ERA
FEDERAL REGION VIII
GEOLOGIC AGES
GEOLOGIC FRACTURES
GEOLOGY
NORTH AMERICA
PALEOZOIC ERA
USA
020200* - Petroleum- Reserves
Geology
& Exploration