STRUCTURE OF THE CRUST AND MANTLE LITHOSPHERE ACROSS THE CENTRAL APPALACHIANS AND IMPLICATIONS FOR TOPOGRAPHIC EVOLUTION AND EOCENE VOLCANISM

The geological record of the Appalachian region records over 1 Ga of complex tectonic history, but major questions related to its history, evolution, and preservation remain unanswered. Specifically, it is not well known how the geological architecture and topography at the surface relate to the deeper structure of the crust and mantle lithosphere. Furthermore, the evolution and persistence of Appalachian topography remains a fundamental outstanding problem in the study of landscape evolution. Here we present results from the MAGIC experiment, a two-year deployment of 28 broadband seismic stations across the mid-Atlantic Appalachians as part of the EarthScope USArray Flexible Array. The array traverses several physiographic provinces, including the Atlantic Piedmont, Blue Ridge, Appalachian Valley and Ridge, and Appalachian Plateau, and passes through a region around Harrisonburg, VA that experienced basaltic volcanism during the Eocene. Analysis of P-to-S and S-to-P converted seismic waves reveals evidence for significant variability in crustal and lithospheric structure across the array. Specifically, we image a sharp increase in the depth of the Moho beneath the eastern Piedmont and Valley and Ridge regions, near the location of the central Virginia seismic zone and the epicenter of the 2011 Mineral, VA earthquake. Our results suggest that the Appalachian Mountains are overcompensated isostatically, suggesting that the lower crust is denser and stronger than typical continental crust and may have undergone incremental garnet growth after orogenesis. Our investigations of lithospheric thickness reveal relatively thin lithosphere beneath the Appalachian Mountains, suggesting that lithosphere has been removed; the thinnest lithosphere coincides with the location of Eocene volcanism. This observation is consistent with a lithospheric delamination model for the Harrisonburg volcanics, and suggests significant post-rifting modification of the deep structure of the passive margin in this region.