Field-based Analysis of Diapiric and Channel Flow of Partially Molten Crust

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Mountain building processes (orogenesis) are characterized by crustal thickening, abundant partial melting, and the formation of continental plateaus. Geodynamic models show that partially molten crust redistributes mass and heat in the orogen by flowing laterally in a channel or vertically in diapirs. Although channel flow and diapiric flow are first-order mechanisms controlling the evolution of orogens, field-testing of these models is lagging. This project is concentrating on geologic studies of two exhumed migmatite terrains, the Okanogan-Kettle dome (North American Cordillera), and the Naxos dome (Aegean metamorphic core complex). In these terrains, migmatite structures and fabrics have preserved the memory of channel/diapiric flow and the extent to which the partially molten crust interacts with the upper crust prior to and during exhumation of the channel and the domes. The project's methodology includes the determination of the geometry and kinematics of flow in migmatites using both field mapping and the analysis of anisotropy of magnetic susceptibility. Thermobarometric and textural analyses of metamorphic assemblages reconstructs the pressure and temperature histories of migmatites, which are distinct for channel and diapiric flow. Geochronologic analyses, using in-situ dating (SHRIMP facilty) of zircon grains crystallized in leucosomes of known relationship to deformation, date the flow of partially molten crust. In addition, dating of monazite of known relationship to metamorphic assemblages offers temporal constraints on the pressure-temperature path. The timing of cooling of these rocks, approached by argon and fission-track geochronologic methods, evaluates the genetic relationship between channel/diapiric flow and exhumation. This integrated methodology applied to two Tertiary orogenic segments evaluates the role of the partially molten crust during orogeny and the relationship between crustal flow and plate geodynamics.