TRANSCONTINENTAL GEOLOGIC AND GEOPHYSICAL CROSS SECTION I: EASTERN SEGMENT

The concept of constructing a deep crustal-upper mantle transcontinental cross section was conceived during the mid-1990s by Marcus E. Milling, and organized by William R. Muehlberger. We have produced E and W segments of a geologic and geophysical section near latitude 36° through North American continental crust and mantle from Atlantic to Pacific Ocean crust, providing a better understanding of crustal tectonic processes.

The E segment extends west from 67° to 99° W. Eastern U.S. crust records two complete Wilson cycles and the beginnings of a third: from formation of Rodinia to formation and breakup of Pangea; the modern continental margin records the Mesozoic rift-to-drift transition. The southern Appalachians were constructed via three Paleozoic accretionary and collisional orogenies on the Laurentian margin formed on Grenvilian crust following Rodinia breakup. The Mid-Continent records southward-younging Proterozoic Mazatzal and Yavapai arcs and granite-rhyolite provinces that were added to the craton. The generally thin Phanerozoic cover and thinner crust locally thickens across the Mississippi Embayment and Mississippi Valley graben. The eastern section also crosses the New Madrid and eastern Tennessee seismic zones, the two most active in the eastern U.S.

The section thus records a history of crust formation spanning almost 2 b.y., with normal-thickness crust beneath the eastern segment, except beneath the topographically high southern Appalachians where ~50-km crustal thickness exceeds Airy isostasy predictions by ~10 km. Crustal seismic tomography images high-velocity lower crust beneath the eastern segment, particularly in areas of thicker crust, consistent with denser composition partially balancing the buoyancy of increased crustal thickness. Mantle tomography provides a proxy for thermal structure and images the highest velocity and presumbaly coldest upper mantle beneath the Laurentian interior, and more moderate mantle velocities beneath the Appalachians and passive margin. An exception is the Early Cambrian Mississippi Valley graben, where upper mantle velocities are relatively low despite ~0.5 b.y. since rifting, suggesting that modern mantle heterogeneity may be linked to ongoing seismicity in the region.