Reassessing metamorphic core complexes in the North American Cordillera

Abstract

Continental metamorphic core complexes (MCCs) are widely distributed on Earth, primarily exposed at convergent plate margins. These structural systems involve the exhumation of metamorphosed, commonly migmatitic, middle-lower crust rocks along high-strain mylonitic shear zones to the brittle upper crust. However, the extent to which the brittle faults, ductile shear zones, and syn-kinematic igneous rocks are genetically and kinematically related within MCCs remains inadequately understood. To address this, we synthesize structural, geochronological, thermochronological, sedimentological, and petrological observations from MCCs distributed across the strike-length of the North American Cordillera. We show that Paleogene ductile MCC exhumation tracked via medium-to-high temperature thermochronology (i.e., biotite and muscovite ⁴⁰Ar/³⁹Ar dates) and cross-cutting relationships youngs toward the central Cordillera latitudes (∼37°N), whereas predominately Miocene brittle faulting and syn-kinematic basin formation youngs northward from the south. Ductile MCC exhumation age trends parallel magmatic sweeps associated with rollback of the Farallon slab, whereas brittle extensional faulting trends correlate with the northward migration of the Mendocino triple junction and the initiation of Basin and Range extension. In light of these observations, we argue that MCCs in the North American Cordillera were not the result of a single phase of extension, but rather reflect two decoupled stages including an early phase of buoyant diapirism followed by a later phase of lithospheric extension. The diapirism ocurred as a Rayleigh-Taylor instability due to crustal melting driven by asthenospheric influx during slab rollback and coupled magmatism. Later, lithospheric extension was caused by regional gravitational relaxation due to change of plate boundary kinematics. This synthesis demonstrates the complexities of MCC generation and highlights the need for better dating constraints for both brittle and ductile structures to make improved interpretations of MCCs globally.