Farallon plate

The Farallon plate was an ancient oceanic tectonic plate. It formed one of the three main plates of Panthalassa, alongside the Izanagi plate and the Phoenix plate, which were connected by a triple junction. The Farallon plate began subducting under the west coast of the North American plate—then located in modern Utah—as Pangaea broke apart and after the formation of the Pacific plate at the center of the triple junction during the Early Jurassic. It is named for the Farallon Islands, which are located just west of San Francisco, California.

Over time, the central part of the Farallon plate was subducted under the southwestern part of the North American plate. The remains of the Farallon plate are the Explorer, Gorda, and Juan de Fuca plates, which are subducting under the northern part of the North American plate; the Cocos plate subducting under Central America; and the Nazca plate subducting under the South American plate.

The Farallon plate is also responsible for transporting old island arcs and various fragments of continental crust, which had rifted off of other distant plates. These fragments from elsewhere are called terranes (sometimes, "exotic" terranes). During the subduction of the Farallon plate, it accreted these island arcs and terranes to the North American plate. Much of western North America is composed of these accreted terranes.

As an ancient tectonic plate, the Farallon plate must be studied using methods that allow researchers to see deep beneath the Earth's surface. The understanding of the Farallon plate has evolved as details from seismic tomography provide improved details of the submerged remnants. Since the North American west coast has a convoluted structure, significant work has been required to resolve the complexity.

Seismic tomography can be used to image the remainder of the subducted plate because it is still "cold," meaning that it has not reached thermal equilibrium with the mantle. This is important for the use of tomography because seismic waves have different velocities in materials of different temperatures, so the Farallon slab appears as a velocity anomaly on the tomography model.

Multiple studies show that the subduction of the Farallon plate was characterized by a period of "flat-slab subduction," which is the subduction of a plate at a relatively shallow angle to the overriding crust (in this case, North America). This phenomenon accounts for the far-inland orogenesis of the Rocky Mountains and other ranges in North America which are much farther from the convergent plate boundary than is typical of a subduction-generated orogeny.

Significant deformation of the slab also occurred due to this flat subduction phenomenon, which has been imaged by seismic tomography. There is a concentration of velocity anomalies in the tomography that is thicker than the slab itself should be, indicating that folding and deformation occurred beneath the surface during subduction. In other words, more of the slab should be in the lower mantle, but the deformation has caused it to remain shallower, in the upper mantle.

Multiple hypotheses have been proposed to explain this shallow subduction angle and resulting deformation. Some studies suggest that the faster movement of the North American plate caused the slab to flatten, resulting in slab rollback. Another cause of flat slab subduction may be slab buoyancy, a characteristic influenced by the presence of oceanic plateaus (or oceanic flood basalts). In addition to influencing slab buoyancy, some oceanic plateaus may have also become accreted to North America.