MAGNETOTELLURIC INSIGHTS INTO THE LITHOSPHERIC ARCHITECTURE AND GEODYNAMIC HISTORY OF THE APPALACHIAN DOMAIN (Invited Presentation)

EarthScope magnetotelluric (MT) data have provided valuable new insights into the lithospheric architecture and tectonic history of the Appalachian domain. Imaged geoelectric structures are interpreted to represent the full tectonic history of this region, from initial Grenvillian orogenesis, through Paleozoic terrane accretion and early Mesozoic rifting, to the modern geodynamic evolution of the passive margin. Many crustal high-conductivity anomalies are discontinuous along strike and are interpreted to be the local signature of Paleozoic collision and Mesozoic extension. However, two of the most significant geoelectric structures are continuous along the grain of the orogen for >500 km and are interpreted to temporally bookend the bulk of Appalachian orogenesis. The first of these is a margin-parallel conductive structure that extends from the mid-crust into the uppermost mantle and that follows the trace of the New York-Alabama magnetic lineament. With constraints from potential fields and radiogenic isotope data, this conductor is interpreted to represent a major Grenville-aged suture. This structure may represent the main boundary along which Grenville terranes were accreted onto Laurentia, and it also likely accommodated significant margin-parallel deformation during Grenvillian orogenesis. Furthermore, as a probable zone of lithospheric weakness, the structure appears to have localized post-rifting lithospheric modification along the North American passive margin. The other major geoelectric feature is a large block of highly resistive lithosphere beneath the Piedmont and Coastal Plain physiographic provinces in the southeastern US. Here, MT data require that lithospheric temperatures (<1330°C) persist to 200 km depth and therefore indicate that a thick, coherent block of thermal lithosphere underlies this region. Seismic results are consistent with this conclusion. This lithospheric domain is interpreted to be the result of major mantle melting and depletion associated with formation of the Central Atlantic Magmatic Province (CAMP). Preliminary MT images of the northeastern US suggest that this thick mantle root extends along the entirety of the eastern North American margin. CAMP is therefore inferred to have been a key event that has shaped the modern Appalachian lithosphere.