The history of subduction beneath western North America
For the past several hundred millions of years oceanic crust has been subducting along the western coast of North America. For the most part the subducting material has comprised the easterly moving Farallon and Kula Plates rather than the more northerly moving Pacific Plate. A spreading-ridge boundary existed between the Farallon/Kula Plates and the Pacific Plate. The boundary between the Farallon and Kula Plates may have been largely a transform boundary or some combination of transform and spreading boundaries. By around 40 m.y. ago all of the Kula Plate plus the Kula-Farallon and Kula-Pacific boundaries, as well as most of the Farallon Plate, had been consumed beneath North America.
The Juan de Fuca Plate, which is a remnant of the northern part of the Farallon Plate, is now subducting in an easterly direction beneath North America (from northern California to Vancouver Island). Similarly, the Cocos Plate, which is a remnant of the southern part of the Farallon Plate, is subducting beneath Mexico. The Pacific Plate continues to move towards the north and is subducting beneath the Alaskan part of the North American Plate
This history of the plates in this area has been determined from studies of current plate motion, and the structural, topographic and volcanic characteristics of western North America (Atwater, 1970), as well as the seismic characteristics of the lower part of the North America Plate and the upper part of the underlying mantle. This work has shown that the Farallon Plate subducted at a very shallow angle for a long period, in contrast with the steep angle (ca 45°) which is characteristic of the Juan de Fuca and Cocos Plates, and of most other subducting plates. This shallow subduction is believed to be responsible for the topographic characteristics of the western third of the United States and much of Mexico, including a huge area of continental uplift extending as far east as Colorado (the eastern limit of the Rocky Mountains). The Basin and Range province of Nevada and surrounding states also lies within this region. This area experienced strong compression when the subudcting slab was moving nearly horizontally, and was then affected by as much as 150 km of extension as that slab started to sink down into the mantle. (click here for a larger version of the map to the left - 500 kb)
In a recent paper published in Nature geologists from Princeton University and the University of Texas have used seismic tomography to assess the characteristics of the mantle up to 1500 km deep beneath North America in order to evaluate the history of the Farallon and Kula Plates. One of the objectives of this research was to determine the likely north-south position of the Farallon-Kula boundary - either near southern Mexico, or northern California.
The models tested by these authors show that it is most likely that the Farallon-Kula boundary was situated well to the north - near to northern California at around 70 m.y. ago. Because of the persistent northerly movement of the Pacific Plate it is probable that this boundary moved north in the ensuing millions of years, such that it was north of Vancouver Island by the time the Kula plate eventually disappeared beneath North America. The differences in the geological character of the Canadian and American parts of the Rock Mountains can probably be attributed to the differences in behaviour of the subducting Farallon and Kula Plates. It appears likely that the Kula Plate subducted at a relatively steep angle, so that the Canadian Rockies are primarily comprised of thrusted sedimentary sheets with relatively little contribution of continental uplift, while the American Rockies are characterized by significant continental uplift in response to the shallow subduction of the Farallon Plate.
Atwater, T., Implications of plate tectonics for the Cenozoic tectonic evolution of western North America, Geol. Society of America Bulletin, V. 81, p. 3513-3556 (1970)
Bunge, H-P, Grand, S., Mesozoic plate-motion history below the northeast Pacific Ocean from seismic images of the subducted Farallon slab, Nature, V. 405, p. 337-340. (May 2000)
Steven Earle, 2000. Return to Earth Science News