Clark Fork lineament, west-central Montana: A model for wrench anticlines
- Infinity Exploration, Inc., Billings, MT (United States)
The Clark Fork lineament is a 150-km-long left-lateral wrench fault zone developed between Missoula and Helena, Montana. It is part of the extensive Montana Laramide wrench system that includes classic wrench faults like Cat Creek, Lake Basin, and Nye-Bowler. The Clark Fork is remarkable in that it consists of a dazzling array of en echelon wrench anticlines. To better define it as a model for wrench anticlines, field mapping, gravity, and compilation were undertaken. Fourteen wrench anticlines occur between Missoula and Helena. Generally, they are symmetrical, south plunging, about 5 km wide, and 13-35 km long. They are arranged en echelon along the Clark Fork lineament forming a series of wrench anticline umbrellas over a basement fault. The anticlines are age-constrained by cross-cutting plutons of the Garnet stocks, the Elkhorn Mountain volcanics, and the Boulder batholith, all dating about 80 to 78 Ma. Following folding, a south-facing monocline formed along the entire length of the Clark Fork lineament, increasing from zero throw to about 5 km westward. The approximately 70 Ma Sapphire block, a 3,000 km{sup 3} thrust sheet from the southwest, was emplaced against the lineament and tectonically loaded the south side of the Clark Fork wrench zone, adding to (or causing) down-to-the-south, tilt-to-the-west movement. Wrench anticlines adjacent to the Sapphire block margin were squeezed toward the northeast by the block and structurally modified.
- OSTI ID:
- 5694627
- Report Number(s):
- CONF-9107109-; CODEN: AABUD
- Journal Information:
- AAPG Bulletin (American Association of Petroleum Geologists); (United States), Vol. 75:6; Conference: American Association of Petroleum Geologists (AAPG) Rocky Mountain Section meeting, Billings, MT (United States), 28-31 Jul 1991; ISSN 0149-1423
- Country of Publication:
- United States
- Language:
- English
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