Geologic Cross Section of the North American Plate near 36° Latitude, Part II: Atlantic Ocean Crust to 99° W Longitude

American Geological Institute Geologic Cross Section of the North American Plate (GeCSNAP) in memory of Marcus E. Milling will produce an annotated 1:1M-scale technical cross section across the continent useful to researchers, and a 1:2M-scale geological cross section useful for K-12, the general public, and introductory college geology classes to provide greater understanding of the complex sequence of tectonic events that produced our continent. The visual product will consist of an oblique DEM, adding a 3D component, along with the cross section at 4:1 vertical exaggeration that will portray the geology to depths of 250 km. The E segment of the TGCS extends from Atlantic Ocean crust E of the Blake Plateau (29 degrees 30' N, 75 degrees 45' W) NW across the modern continental margin to the Cumberland Plateau (36 degrees N 84 degrees 30' W), normal to Appalachian strike, then follows 36 degrees N westward to 99 degrees W. The E segment crosses the modern continental margin, which records the Mesozoic rift-to-drift sequence following breakup of Pangea; the southern Appalachian collage of terranes accreted through three Paleozoic orogenies to the Neoproterozoic-early Paleozoic Laurentian margin, and the subsurface Grenville front, recording two complete Wilson cycles; then crosses much of the Mid-Continent, which records accretion of the mid-Proterozoic Mazatzal and Yavapai Arcs and late plutons. The Mid-Continent component illustrates the cratonic stability despite major Phanerozoic tectonic events along its southern and eastern margin. Relatively thin Phanerozoic cover characterizes the continental interior, with local thickening across the Reelfoot Rift. The section also crosses the two most active seismic zones in the Eastern U.S.: New Madrid and East Tennessee. Crustal thickness appears normal (30-40 km) beneath the E segment, except beneath the topographically high parts of the southern Appalachians, where thickness reaches approximately 50 km. This thickness suggests the eastern U.S. highlands may be accounted for by isostatic imbalance.