Broadband Seismological Imaging of Flat-Slab Subduction and its Long-Term Impact on Lithospheric Structure and Processes

Abstract

In subduction zones, the dip of the downgoing oceanic lithosphere has a profound impact on the nature and extent of deformation as well as the generation of melt. In ~10% of subduction zones, the downgoing slab assumes a low-angle, or horizontal geometry, referred to as flat-slab subduction. The focus of this work is to better understand both the driving forces and impacts of flat-slab subduction on the Earth's lithosphere and asthenosphere. This is accomplished by focusing on three areas impacted by flat-slab subduction. The first area is the Pampean region of central Argentina and Chile, a modern flat-slab subduction zone. In this region, we invert Rayleigh-wave-dispersion data to produce a 3D shear velocity model. The flat slab is visible within the upper mantle as a high-velocity body containing low-velocity pockets that dissipate inboard from the trench. We interpret these velocities in the context of slab hydration and argue that the subducting Nazca plate is initially hydrated at the trench and dewaters as it subducts. The second area is southern California, which was impacted by Laramide flat-slab subduction. In this area, we use receiver functions to locate and parameterize anisotropy within the crust. Results show a persistent NE-SW oriented layer of lower crustal anisotropy. We conclude that this layer consists of schists that were emplaced during Laramide flat-slab subduction and have remained largely intact since. The final component of this work is a study of the Colorado Plateau in which we use ambient-noise tomography and receiver functions to study lithospheric structure. Results show fast crust, a complicated Moho and intact Laramide features throughout the crust beneath the Colorado Plateau while slower crust with a sharp Moho is observed along its margins. Based on these observations, published tomographic data and the volcanic and uplift history of the region, we argue that delamination of the lower crust has occurred beneath the Marysvale volcanic field. This process was driven by the gravitational instability of a dense mafic root that formed during mid-Tertiary magmatism related to the rollback of the Farallon flat slab.