Metamorphic Core Complex Dichotomy in the North American Cordillera Explained by Buoyant Upwelling in Variably Thick Crust

Abstract

Metamorphic core complexes (MCC) in the North American Cordillera exhibit a strong dichotomy. Those in the north formed in a thickened orogenic plateau during Paleogene Farallon subduction, are widely spaced (~200 km), and young SW. Conversely, those in the south formed in thinner crust, are closely spaced (~50 km), developed during the Oligocene-Miocene transition to regional transtension, and young NW. Synthesis of magmatism and cooling ages, modeling, and plate reconstructions demonstrate that MCCs could have initiated as buoyant domes driven by lower-crust heating caused by asthenospheric upwelling after Farallon slab rollback. These domes were later exhumed by Miocene extension. The widely spaced Paleogene hinterland domal upwellings and associated mylonites were temporally decoupled from Miocene detachments, manifesting a two-stage development. The closely spaced Oligocene-Miocene foreland MCCs show almost synchronized doming and detachment faulting. The spacing dichotomy of