TRANSCONTINENTAL GEOLOGIC CROSS SECTION OF THE NORTH AMERICAN PLATE NEAR 36° LATITUDE, PART II: ATLANTIC OCEAN CRUST TO THE MID-CONTINENT

The American Geological Institute Geologic Cross Section of the North American Plate, in memory of Marcus E. Milling, has produced a technical cross section across the continent for researchers, and will produce a geologic section useful for K-12 education, the general public, and introductory college geology classes, to provide better understanding of the tectonic processes that produced southern half of our continent. The technical product consists of the cross section at a 4:1 vertical exaggeration to 150 km, and a section at 1:1 that portrays the geology to depths of 250 km, with an oblique DEM, adding a 3D component. The E segment extends westward from 67° to 99° W, from Atlantic Ocean crust E of the Blake Spur magnetic anomaly (BSMA) across the East Coast magnetic anomaly (ECMA) and modern continental margin; across the Appalachians, the Nashville dome, Mississippi Embayment (ME) and into the Mid-Continent. The modern continental margin records the Mesozoic rift-to-drift transition following breakup of Pangea, which includes the enigmatic BSMA (abandoned ridge segment?) and ECMA (mafic intrusions formed by decompression melting as Africa separated from Laurentia?); the southern Appalachians 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 cratonic stability despite major Phanerozoic tectonic events along its southern and eastern margins. Relatively thin Phanerozoic cover characterizes the continental interior, with local thickening across the ME and Reelfoot rift. The E section also crosses the New Madrid and East Tennessee seismic zones, the two most active in the eastern U.S. Recorded here is a history and crustal formation processes spanning almost 2 Ga, with normal-thickness crust (30-40 km) beneath the E segment, except beneath the topographically high southern Appalachians (~50 km), suggesting the eastern U.S. highlands may be explained by local isostatic imbalance.