CENOZOIC PLATE KINEMATICS IN THE EASTERN PACIFIC AND ALONG THE WESTERN RIMS OF THE AMERICAS

In Post-Pangaean times (about 180 Ma to present) the western rim of the Americas has been dominated by subduction. Vast areas of oceanic lithosphere (equivalent to more than half the surface of the planet) have been subducted beneath the rims of the Americas.

Since 80 Ma, the material subducted beneath the Americas has primarily been that of the Farallon plate. The Pacific plate is the largest plate in the present day world, but in the late Cretaceous, it was relatively small while the Farallon was a huge plate, filling the eastern Pacific. As the Pacific plate grew, the Farallon plate became steadily narrower and younger. About 55 Ma, it began to break up, spawning first the Vancouver (Juan de Fuca) plate, then the Guadalupe, Cocos, and Nazca plates (29-20 Ma) plus numerous smaller, short-lived plates. In the northeast Pacific, most of the Farallon plate was completely subducted, so that today the Pacific plate nearly fills that ocean basin. Farther south, the older configuration continues, with the Farallon-derived Cocos and Nazca plates still being created and subducted.

Other oceanic plates, the Kula and Aluk plates, once lay to the north and south of the Farallon-Pacific plate pair. The boundaries among these and the main plates formed triple junctions both at sea and at their intersections with the land, and altered the characteristics of the subducting slabs in interesting ways.

Many Cordilleran tectonic events have been tied to interactions among the oceanic plates, their triple junctions, and the land. These include the origin and development of the San Andreas and Queen Charlotte transform systems in North America, complex fault and volcanic patterns in central America and northern South America, and the origins of massive basalts in southern Argentina, to name a few. They include possible insertion and capture of Pacific-realm crust in the Caribbean and Bering seas. In North America, peculiarities in the subducting oceanic plate have been invoked as a cause for the late Cretaceous - early Cenozoic Laramide Orogeny (Rocky Mountains) and subsequent patterns of volcanism, uplift and crustal breakup. Similar peculiarities in the Nazca plate are used to explain modern anomalies in the present day South American subduction zones and their overlying Andean geology.