Colliding Channels and Double Domes in Metamorphic Core Complexes

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Mountain building involves thickening of continental crust and is typically followed by extensional collapse and the formation of metamorphic core complexes that are cored by migmatite domes (rocks that were once partially molten). This project tests the hypothesis that extension in the upper crust triggers two channels of partially molten crust that converge beneath the zone of extension. These "colliding channels" generate contraction structures and upward flow of deep crust into two subdomes ("double dome") that are separated by a steep high strain zone. This study combines structural geology, metamorphic petrology, geochronology, and numerical modeling to test the hypothesis that colliding channels explain many features of gneiss/migmatite domes, such as the complex internal geometries displayed by domes relative to mantling rocks, the common occurrence of "nappe-like" structures that may be erroneously interpreted to indicate contraction prior to extension, and strain variations in domes as a function of depth, where contraction at depth may be coeval with extension at shallower levels. The field-based research component focuses on the Montagne Noire (Massif Central, France) double dome, where pressure-temperature-time-deformation history of dome rocks and overlying units are evaluated. Modeling (2D and 3D) explores the conditions that favor, modify, or limit colliding channels and double domes; modeling also quantifies pressure-temperature paths at all points of double domes structures for comparison with field-based data. Modeling in 3D examines how the obliquity of upper crustal extension and the lateral changes in crustal composition/thickness affect the development of colliding channels and double domes. This project addresses how deep-seated rocks are exhumed toward the Earth's surface during the collapse of mountain belts, how partially molten rocks that crystallize at high temperature are juxtaposed with non-metamorphic rocks, and how lateral and vertical flow of partially molten crust accommodates mass and heat transfer in the continental crust. The methodology integrates the fields of structural and metamorphic geology, geophysics, geochronology, and geodynamic modeling. This project explores the internal dynamics of gneiss/migmatite domes and offers a new concept of crustal deformation in which contraction structures at depth are coeval with extension structures in the shallow crust. This work has the potential to reconcile enigmatic relations within and around gneiss domes and to provide a new three-dimensional analysis of the processes that exhume the deep crust. In addition to the intellectual merit of the project, it is fostering international collaboration between researchers in the U.S., Australia, and France and is contributing to the training and mentoring of a Ph.D. student and an undergraduate student at the University of Minnesota. Results of the research will be presented at professional society meetings and the peer-reviewed literature and will also be presented as part of a modeling short course for geoscience professionals. The research is supported by the NSF EAR Tectonics Program and by the NSF Office of International Science and Engineering.