CORE COMPLEX METAMORPHISM AND EXHUMATION IN THE NORTHERN U.S. NORTH AMERICAN CORDILLERA (Invited Presentation)

Metamorphic core complexes in the northern U.S. Rockies are located inboard of the North American craton margin, within the Sevier-Laramide hinterland, and are commonly cored by high-grade migmatitic rocks that were exhumed in the Paleocene-Eocene while Farallon-North American plate convergence was still occurring. This coincidence with the Sevier-Laramide orogenic belt suggests gravitational collapse as the primary mechanism for core complex development. However, core complex evolution is also affected by other processes such as the degree of crustal anatexis and regional magmatism. The northern Rockies core complexes vary by size, exhumation depth of footwall rocks, degree of partial melting, volume of associated magmatism, and timing of magmatism relative to onset of exhumation. For example, magmatism preceded exhumation in the Bitterroot complex (MT/ID), whereas in the Priest River and Clearwater complexes (WA/ID), magmatism post-dates exhumation by as much as 10 million years. These core complexes also share several similarities including the timing of prograde metamorphism (85-60 Ma), onset of exhumation (59-50 Ma), and decompression-related magmatism (54-48 Ma). Peak metamorphic pressures decrease from the most deeply exhumed rocks in the North American Cordillera (Priest River, ~11-12 kbar) to shallower levels of exhumation to the south (Anaconda (MT), ~3-4 kbar). Thus the crust exhumed by these core complexes records protracted, complex histories of metamorphism, deformation, crustal anatexis and exhumation, providing evidence for the duration and conditions of orogenic processes, from crustal thickening through collapse. These histories coincide largely with respect to timing of process, but vary considerably in the levels of exhumation, degree and volume of partial melting and associated magmatism, and peak metamorphic temperatures prior to core complex exhumation.