MOUNTAIN BUILDING: THE OROGENIC EVOLUTION OF MONTANA

The Cretaceous to Paleogene orogenic history of Montana is complex and unique compared to other areas of the Cordilleran foreland to the north and south. This is the result of a variety of tectonic infl uences that include Precambrian basement terranes, recurrent reactivation of Precambrian fault systems, the enormous thickness and widespread extent of the Mesoproterozoic Belt Supergroup, overlapping contractional and extensional structural regimes, invasion of the plate margin magmatic arc into the actively deforming foreland, and complicated plate boundary interactions involving the Farallon, Kula (or the proposed Resurrection), and North American plates. The lithospheric foundation of Montana is a cryptic collage of Archean and Paleoproterozoic terranes or blocks, with both lateral and vertical variability in composition and rheological strength. The Paleoproterozoic Big Sky orogenic belt, described earlier in the literature as the northeast-trending Great Falls Tectonic Zone, has had a direct infl uence on subsequent Proterozoic and Phanerozoic tectonic events. Laramide deformation in Montana is not restricted to the southwestern part of the State, but instead steps down in structural relief through central Montana to the Canadian border. Laramide deformation is subdivided into three domains, from the exposed basement-cored arches of southwest Montana (Domain I), to the structurally inverted central Montana platform (Domain II), to low-amplitude basement arches beneath the plains of north-central Montana (Domain II). Similarly, the Sevier fold–thrust belt of western Montana can be subdivided into structural domains that include northwest Montana, the Lewis and Clark Tectonic Zone, the Helena structural salient, and the southwest Montana recess that also incorporates numerous plutons. Thrust sheets within the Helena Salient and southwest Montana recess were intruded by a host of largely synkinematic felsic plutons of Late Cretaceous to Paleogene age, most notably the Boulder and Pioneer Batholiths, which were often emplaced in ramp-top structural positions where accommodation space is created within thrust sheets. Some component of synconvergent channel fl ow may have occurred within the “Bitterroot to Boulder Batholith magmatic corridor” in western Montana, leading to super-critical taper of the orogenic wedge during Maastrichtian–early Paleocene time. Lithospheric shortening involving Laramide and Sevier styles of deformation, coupled with voluminous magmatic activity within the evolving orogenic wedge, ultimately led to the development of an orogenic plateau, herein named Montanaplano, a northern topographic extension of Nevadaplano in the hinterland of the Sevier orogen farther southwest. The eastern slopes of Montanaplano were likely impacted by a seasonal monsoon from the Western Interior Seaway, resulting in high rates of erosion of poorly consolidated volcanic and volcaniclastic rocks and deposition of one of the thickest foreland basin deposits along the Sevier orogenic front (Livingston Group). Collapse and extension of Montanaplano began ~52 Ma, coincident with the onset of widespread Challis–Absaroka–Kamloops magmatism and extension in the Bitterroot and Anaconda metamorphic core complexes. Huge displacement, east-dipping detachment faults formed on these core complexes, extending to depths of 10–20 km and resulting in the exhumation of rocks from the middle crust. Extensional detachment faults are linked to transverse strike-slip faults in the Lewis and Clark Tectonic Zone, the latter forming an accommodation zone between core complexes in Montana and those to the northwest in Idaho and northeastern Washington, resulting in dextral transtension distributed across the northern U.S. Rocky Mountains. The change at ~52 Ma from regional shortening to transtension and collapse was caused by plate boundary processes involving northward migration of the Kula/Resurrection triple junction with the Farallon and North American plates, in turn resulting in decoupling of Farallon fl at-slab subduction and progressive southwestward rollback and/or slab removal beneath the Rocky Mountain foreland. The change in the regional stress/strain fi eld at ~52 Ma is refl ected by the length and distribution of radial dikes in the Crazy Mountains Basin.