Cenozoic Tectonics and Paleogeography

In the  Cenozoic, the Earth’s tectonic plates moved into more familiar places, with the biggest change being the closing of the Tethys Sea with collisions such as the Alps, Zagros, and Himalaya. This collision started about 57 million years ago and continues today. Maybe the most significant tectonic feature that occurred in the  Cenozoic of North America was the conversion of the west coast of California from a convergent boundary subduction zone to a  transform boundary.  Subduction off the western United States coast, which had occurred throughout the Mesozoic, had continued in the Cenozoic.

After the Sevier Orogeny in the late Mesozoic, a subsequent orogeny called the Laramide Orogeny occurred in the early Cenozoic. The Laramide was thick-skinned, different than the Sevier Orogeny. It involved deeper crustal rocks and produced bulges that would become mountain ranges like the Rockies, Black Hills, Wind River Range, Uinta Mountains, and the San Rafael Swell. Instead of descending directly into the mantle, the subducting plate shallowed out. It moved eastward beneath the continental plate affecting the overlying continent hundreds of miles east of the continental margin and building high mountains. This occurred because the subducting plate was so young and near the spreading center, and the density of the plate was therefore low, and subduction was hindered.

As the mid-ocean ridge itself started to subduct, the relative motion changed. Subduction caused a relative convergence between the subducting Farallon plate and the North American plate. On the other side of the mid-ocean ridge from the Farallon plate was the Pacific plate, which was moving away from the North American plate. Thus, as the subduction zone consumed the mid-ocean ridge, the relative movement became a transform boundary instead of convergent (View the video below to understand better how this occurred), becoming the San Andreas Fault System. As the San Andreas grew, it caused east-west directed extensional forces to spread over the western United States, creating the Basin and Range province (where we are in Arizona!). These forces built a system of mountains and valleys in the Western United States.

The transform fault switched its position over the last 18 million years, twisting the mountains around Los Angeles. New faults in the southeastern California deserts may become a future San Andreas-style fault. During this switch from subduction to transform, the nearly horizontal Farallon slab began to sink into the mantle. This caused magmatism as the subducting slab sank, allowing asthenosphere material to rise around it. This event is called the Oligocene ignimbrite flare-up, one of the most significant periods of volcanism ever, including the largest single confirmed eruption, the 5000 cubic kilometer Fish Canyon Tuff.