Clark Fork Lineament, West-Central Montana: A Model for Wrench Anticlines

SHEPARD, WARREN, Infinity Exploration, Inc., Billings, MT

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 wrench anticlines are upright and trend at 45 degrees to the lineament near Helena; but westerly they progressively flatten (rotate counterclockwise) toward the lineament, so that near Missoula, the anticlines are quasi-parallel to the lineament and have axial planes that have become overturned toward the northeast.

The youngest rocks involved in wrench folding are Blackleaf Formation shales deposited in the Cretaceous seaway about 90 Ma. Ensuing left-lateral strike-slip movement on the ancient basement fault occurred in response to Laramide subduction and terrane accretion. Clark Fork cover rocks then formed wrench anticlines about 85 to 81 Ma in response to basement strike-slip movement. 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 3000 km3 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.

AAPG Search and Discovery Article #91010©1991 AAPG Rocky Mountain Section Meeting, Billings, Montana, July 28-31, 1991 (2009)