Tectonic Mechanisms Indicated by Structural Framework of Central Beaverhead Range, Idaho–Montana
Published:January 01, 1968
Robert Schölten, L.D. Ramspott, 1968. "Tectonic Mechanisms Indicated by Structural Framework of Central Beaverhead Range, Idaho–Montana", Tectonic Mechanisms Indicated by Structural Framework of Central Beaverhead Range, Idaho–Montana, Robert Scholten, L. D. Ramspott
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The central Beaverhead Range forms the Continental Divide along the Idaho-Montana border north of the Snake River Plain. This general region is located at, or near, the eastern edge of the former Rocky Mountain miogeosyncline. Pre-Laramide sediments consist of Belt and lower Paleozoic quartzitic sandstones and upper Paleozoic carbonates, separated by Upper Devonian to Lower Mississippian dolomite, shale, and shaly limestone. Mesozoic sediments were eroded in post-Laramide time.
Sedimentary processes were influenced by faulting during Late Pre-cambrian, Late or post-Ordovician, and Middle to Late Devonian time, and by moderate folding (Skull Canyon Disturbance) between Late Pre-cambrian and Middle Ordovician time.
The Beaverhead Pluton, a leucocratic granite-syenite body, was emplaced during Late Ordovician or Early Silurian time at shallow depth, as indicated by miarolitic cavities, aplite, granophyre, and the hyper-solvus (perthitic) nature of most of the pluton.
The Laramide structural framework can be divided into an infra-level and a supra-level, which behaved disharmonically with respect to each other, the detachment horizon being the mid-Paleozoic shaly rocks. Early Laramide compressive stress, oriented NE.–SW., caused major arching, superposed folding, and faulting in both levels. Sub-vertical faults, and thrusts growing steeper with depth, later became avenues of mafic intrusion. The pluton was raised into a faulted dome and carried north-eastward, creating plan-curvature of earlier folds and faults. In the supra-level, gravitational gliding and cascading of Carboniferous carbonates took place down the flanks of the early arches and radially away from the pluton dome. Continued compression caused reactivation of earlier faults.
Post-orogenic NE.–SW. extension resulted in mid-Tertiary to Recent normal range front faulting and in north-south, left-lateral wrenching, as interpreted from regional oroclinal bends and from local curvature of fold axes. Folding of the Tertiary sediments in the down-dropped blocks could have resulted from intermittent regional NE.–SW. compressive spasms, or from local accommodation of the beds to a narrowing space between converging faults. Crustal downwarping in the Snake River region caused a southeast plunge of all tectonic axes and an extensional stress in that direction which was resolved along northeast-trending normal faults and, in nearby areas, along north-trending, right-lateral wrench faults.