Subcontinental Lithospheric Mantle Strength Modulates Crustal Deformation: An Example From The North American Cordillera

Abstract:

The Basin and Range province of the North American Cordillera preserves a record of Mississippian to Present crustal deformation and magmatism. Analyzing the geochronology and geochemistry of magmatism throughout this period allows evaluation of the first order rheologic evolution of the lithospheric mantle, and the controls its rheology may exert on spatial patterns of deformation in the crust. In this study, we integrate U-Pb zircon geochronology and whole rock geochemistry from a suite of Jurassic, Cretaceous and Eocene plutons in northeastern Nevada, with that from existing literature to track the relative strength of the lithospheric mantle through time. These results are compared to structural analysis of the kinematics and magnitude of Mesozoic shortening and Cenozoic extension in the eastern Basin and Range based on our detailed field observations and 1:24k geologic quadrangle mapping. In eastern Nevada, low magnitude thrusting was immediately followed by emplacement of Late Jurassic granodiorite plutons and lamprophyre dikes likely sourced from enriched lithospheric mantle. Widespread Late Jurassic magmatism across Nevada may have operated to deplete the subcontinental lithospheric mantle effectively enhancing its viscosity, or strength. This may explain the ~250 km eastward jump in retroarc shortening during the Late Jurassic-Early Cretaceous. Metasomatism during Late Cretaceous-Paleogene flat-slab subduction likely enriched the subcontinental mantle, which would have reduced its viscosity. Eocene-Oligocene monzogranite and granodiorite plutons in eastern Nevada originated from basaltic melts emplaced in the lower crust during slab rollback and asthenospheric upwelling. This magmatism weakened the crust promoting localized extension in the metamorphic core complexes, which was succeeded by widespread Basin and Range extension in the Oligocene-Miocene above chemically and thermally weakened lithospheric mantle. We argue modification of the subcontinental lithospheric mantle modulates the expression of crustal deformation.