Possibly, nothing you will read here is true.
In geology, my main interest lies in the southern Appalachian Blue Ridge and Piedmont. Mindful that "the present is key to the past," I have tried to understand tectonics of Paleozoic Eastern North America in part by studying more-recent events in Western North America. In mid-1997, having read a few of John McPhee's books (Basin and Range, Rising from the Plains, Assembling California, The Control of Nature), numerous references on Georgia's geology, and a few USGS papers, I passed way too much time with feet on my desk and a Sam Adams in my hand, staring at the U.S. geologic map and AAPG's Tectonic Map of North America, absorbing the crazy patterns of Western North America's geology. Follows the result, a personal hypothesis of events in the most recent 30 million years.
Among this area's odd features, three large capes project westward into the Pacific Ocean. These three promontories -- Cape Mendocino in northern California, Point Arguello near Los Angeles, and a cape in central Baja California -- align with major east-west fracture zones between strips of oceanic crust generated at the now-extinct Pacific Spreading Center. The two northern capes, Mendocino and Arguello, mark zones where the San Andreas fault sidesteps its northwesterly course from the Gulf of California. In these zones, faults trend west rather than northwest, and crust appears chaotically sheared and broken. North of Point Arguello / Los Angeles, the Sierra Nevada's southern tail bends westward. At Cape Mendocino, a 50-mile gap displaces Sierra Nevada granites eastward from similar Klamath granites.
East of the Sierra Nevada, north-south ranges of the rhythmically fractured Basin and Range wriggle like "an army of caterpillars marching south to Mexico." North of the Basin and Range, the Columbia Plateau has been inundated by floods of basaltic lava having chemical characteristics similar to upwelling mantle at mid-ocean ridges. At the northeast corner of the Basin and Range, the volcanic track of the Snake River Plain trends northeast, terminating -- for now -- at the Yellowstone Caldera. In Arizona and northern New Mexico, similar recent volcanics dot a still-rifting landscape. Between these volcanic tracks, the Colorado Plateau and the Rockies platform have uplifted in relatively recent time, causing the Colorado River to excavate the Grand Canyon. Some geologists interpret these these northern and southern volcanics as effects of "hot spots" or concentrated plumes of molten rock arising from deep in Earth's mantle.
What an interesting mess! Read on . . .
| Illustrated narrative of hypothesized events Early version Comments and caveats Visual Index of illustrations |