Laramide Orogeny Lab Report

The chemists' main objection focused on two isotopes of the element neodymium. By measuring the relative abundances of the isotopes in a rock, geochemists can tell whether the rock originated in the mantle lithosphere -- the slice of the North American jelly sandwich that, according to Bird, the Farallon Plate had dragged away. When the chemists checked the neodymium, however, the rocks proclaimed that the mantle lithosphere is very much intact. The evidence was compelling; many of Bird's colleagues abandoned his version of the Laramide orogeny. "Peter is pretty much a party of one at this point," Humphreys says. His model "is very well reasoned, but it hasn't held up with the observations."
To replace the Bird model, geologists have devised new theories that downplay or dismiss shallow subduction as an engine for the Laramide orogeny. One theory, proposed late in 1996 by Craig Jones, …show more content…
For example, the Great Basin, where vertical forces outweigh the horizontal forces, is extending -- tearing itself apart fast enough to push the Sierra Nevada mountains toward the Pacific Ocean about a half inch every year. In contrast, in the coastal ranges of California, where the gravitational potential energy is low, horizontal forces are compressing the crust.
It struck Jones that a sudden drop in gravitational potential energy might have given the crust enough of a jolt to create the Laramide ranges. His scenario goes like this: 141 million years ago, during the Cretaceous Period, western North America from the Arctic to the Gulf of Mexico was covered by a shallow sea that some geologists have dubbed the Western Interior Seaway. Dead marine organisms rained down on the seafloor, forming thick sediments that ultimately became the Pierre Shale, one of the sedimentary rock layers that the American Rockies punched up