The Farallon Plate was an ancient oceanic plate. It formed one of the three main plates of Panthalassa, alongside the Phoenix Plate and Izanagi 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 centre 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 completely subducted under the southwestern part of the North American Plate. The remains of the Farallon Plate are the Juan de Fuca, Explorer and Gorda Plates, 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 crustal material rifted off from other distant plates and accreting them to the North American Plate.
These fragments from elsewhere are called terranes (sometimes, "exotic" terranes). Much of western North America is composed of these accreted terranes.
The understanding of the Farallon Plate is rapidly evolving as details from seismic tomography provide improved details of the submerged remnants. Since the North American west coast shows a convoluted structure, significant work has been required to resolve the complexity. In 2013 a new and more nuanced explanation emerged, proposing two additional now-subducted plates which would account for some of the complexity.
As data accumulated, a common view developed that one large oceanic plate, the Farallon plate, acted as a conveyor belt, conveying terranes to North America's west coast, where they accreted. As the continent overran the subducting Farallon plate, the denser plate became subducted into the mantle below the continent. When the plates converged, the dense oceanic plate sank into the mantle to form a slab below the lighter continent.
As of 2013[update], it is generally accepted that the western quarter of North America consists of accreted terrane accumulated over the past 200 million years as a result of the oceanic Farallon plate moving terranes onto the continental margin as it subducts under the continent. However this simple model was unable to explain many terrane complexities, and is inconsistent with seismic tomographic images of subducting slabs penetrating the lower-mantle. In April 2013 Sigloch and Mihalynuk noted that under North America these subducting slabs formed massive, essentially vertical walls of 800 km to 2,000 km deep and 400–600 km wide, forming "slab walls". One such large "slab wall" runs from north-west Canada to the eastern U.S. and extends to Central America; this "slab wall" had traditionally been associated with the subducting Farallon plate. Sigloch and Mihalynuk proposed that the Farallon should be partitioned into Northern Farallon, Angayucham, Mezcalera and Southern Farallon segments based on recent tomographic models. Under this model, the North American continent overrides a series of subduction trenches and incorporates microcontinents (similar to those in the modern-day Indonesian Archipelago) as it moves west in the following sequence:
When the final archipelago, the Siletzia archipelago, lodged as a terrane, the associated trench stepped west as the terrane accreted, converting an intra-oceanic subduction trench into the current Cascadia subduction zone and creating a slab window.