Abstract

Four phases in the tectonic evolution of the Montana Cordillera are recorded by remnants of paleovalleys and their lithologically distinctive gravel deposits. (1) Strike-parallel paleovalleys and alluvial plains marked the culmination of overthrusting in the Rocky Mountain fold-thrust belt in Late Cretaceous-early Eocene time, and fed the Beaverhead-Harebell-Pinon mega-fan of the Wyoming foreland basin. (2) Upon cessation of thrusting, the Cordillera experienced widespread erosional denudation, volcanism, and extension, with emergence of metamorphic core complexes and gravitational collapse of a regional rift system by middle Eocene time. A large, north-draining paleovalley appears to have followed the axis of a major Eocene-early Miocene rift system from central Idaho north to the Rocky Mountain trench. (3) A significant period of tectonism coincided with the outbreak of the middle Miocene Columbia River flood basalt, and a radiating system of new graben valleys diverted flow from the Eocene-early Miocene rift system in southwest Montana. (4) By late Miocene, extensional faults on the shoulder of the Yellowstone hotspot cross-cut middle Miocene paleovalleys and shunted streams into new graben valleys in southwest Montana. Many extensional faults in southwestern Montana are still active. However, widely-preserved paleovalley strath-terraces suggest that central-western Montana, near Missoula, has been tectonically stable since middle Miocene time.