THRUSTING, MAGMATISM, AND COLLAPSE OF THE SEVIER OROGENIC WEDGE, WESTERN MONTANA, USA

The Sevier orogenic belt in the Northern Rocky Mountains (Montana and Idaho) formed in response to complex, overlapping tectono-magmatic processes during Late Cretaceous-Paleogene time. The Cordilleran magmatic arc migrated more than 400 km inboard (eastward), between ca. 90 Ma in the west (Idaho-Bitterroot batholith) and ca. 60 Ma in the east (eastern Helena salient) with large-volume felsic plutons intruding and their associated volcanic fields erupting within the thrust belt. As evidenced by a spectrum of pre-, syn-, and post-kinematic relations among plutons and fold-thrust structures in country rocks, the in-board sweep of magmatism significantly altered the geometry and kinematic development of the evolving orogenic wedge. Most notably, epizonal intrusion of the late Campanian-Maastrichtian Boulder batholith and coeval eruption of the Elkhorn Mountains volcanic field occurred along the advancing thrust-front, creating a thick (15-20 km) igneous and structural culmination. The locus of magmatism and spatial geometry of the evolving thrust wedge appear to have been strongly influenced by reactivated Mesoproterozoic faults. Extreme magmatic inflation of the orogenic wedge, over a relatively short time interval (76-70 Ma) facilitated supercritical taper resulting in: 1) out-of-sequence thrusting of the inner foreland thrust system (Eldorado-Lombard thrust); 2) rapid eastward propagation of the toe of the thrust wedge to form the Helena salient; 3) erosion of the volcanic wedge-top and deposition of one of the thickest (>3 km) Campanian-Maastrichtian foreland basin sequences in the western United States (Livingston Group and Fort Union Formation); and 4) Paleogene collapse of the thickened structural-magmatic wedge and development of “stacked” metamorphic core complexes in western Montana and Idaho. Other consequences of supercritical taper include the widespread development of pressure-solution cleavage and large duplex systems in frontal thrust sheets, features not seen in frontal thrusts of the Wyoming-Idaho salient to the south. It has been suggested that similar tectono-magmatic conditions existed in the central Andes of Bolivia south of 17.5o S.