Deformation of the Continental Lithosphere at the Margins of the North American Craton: Constraints from Seismic Anisotropy

Abstract:

Earth's continents exhibit striking properties, including relatively thick and low-density crust and a strong, thick, long-lived mantle lithosphere. Major questions related to the formation, stability, evolution, and dynamics of cratonic lithosphere remain unanswered. One promising avenue for understanding the stability of cratonic lithosphere through geologic time is to understand how their margins are deformed via tectonic processes such as orogenesis and rifting. Here we present results of several recent and ongoing studies which aim to constrain past lithospheric deformation along the eastern margin of the North American craton. Each of these studies focuses on constraining seismic anisotropy, or the directional dependence of seismic wavespeeds, in the lithospheric upper mantle. Because there is a causative link between upper mantle deformation and the resulting seismic anisotropy, studies of anisotropic structure in the upper mantle beneath continental interiors can shed light on the deformation processes associated with past tectonic events. The recent explosion in the availability of seismic data in the eastern United States, largely due to the EarthScope initiative, has enabled detailed studies of lithospheric deformation using anisotropic receiver function (RF) analysis and SKS splitting analysis. A comparison of lithospheric structure inferred from RFs for stations located to the east of the Grenville deformation front with those located within the cratonic interior argues for extensive deformation of the lithosphere during the formation and/or breakup of Rodinia. The pattern of fast SKS splitting directions measured at USArray Transportable Array (TA) stations shows clear evidence for a specific lithospheric anisotropy signature at stations beneath the Appalachian Mountains, indicating strong, coherent lithospheric deformation associated with Appalachian orogenesis. The Mid-Atlantic Geophysical Integrative Collaboration (MAGIC) experiment, a linear array of 28 broadband seismometers that is currrently deployed, is providing new insights into the structure and dynamics of the southeastern US continental margin from the Coastal Plain to the continental interior.