TRANSCONTINENTAL GEOLOGIC CROSS SECTION OF THE NORTH AMERICAN PLATE NEAR 36° LATITUDE, PART I: WESTERN U.S. FROM THE PACIFIC OCEANIC CRUST TO THE MID-CONTINENT

The American Geological Institute, in dedication to Marcus E. Milling, is producing a geologic cross section across the southern United States near latitude 36°. The cross section extends from the eastern to the western margin of the North American plate and will provide an up-to-date working model for the structure of the North American plate and its interactions with the upper mantle asthenosphere. The cross section will be in two sheets (Eastern and Western U.S.). Each will consist of an annotated 1:2M-scale geological cross section useful for K-12, the general public, introductory college geology classes, and researchers. An oblique DEM at the top of the cross section will add a 3D component. The DEM will overlie a cross section with 4:1 vertical exaggeration that will portray the geology to depths of 150 km which, in turn will overlie a 1:1 cross section extending to 250 km. The western half of the cross section extends from the coastal Franciscan accretionary complex, across the Salinia forearc microplate, the San Andreas fault, the Cretaceous forearc basin, the Sierra Nevada Mesozoic magmatic arc, the eastern California shear zone of the Basin and Range province, the Colorado Plateau, the Rio Grand rift, the Rocky Mountains, and the western Great Plains. In this region, plate boundary deformation is inducing and interacting with a complex intraplate deformational field in a 1000-km-wide uplifted orogenic plateau that extends as far east as the Great Plains. The scientific “punchline” for this part of the cross section is that the western North American plate is dynamically uplifting and tectonically active because of interactions with flowing mantle near its base. Both halves of the cross section emphasize that North America provides a field laboratory for understanding the structure and evolution of continental plates. The continent is profoundly segmented because of its >4 billion year history, and has been built progressively by collision of continental fragments and oceanic terranes to the existing continental nucleus. The overall theme for North America (and for all continents) involves a history where active plate tectonic processes are superimposed on a heterogeneous existing structure developed during their billion-year evolution.