The Farallon Trench was a subduction related tectonic formation located off the coast of the western California continental margin during the late to mid Cenozoic era, around 50 miles southeast of modern-day Monterey Bay.
The earliest record of subhorizontal subduction of the Farallon slab is the extinguishing of magmatism in the Sierra Nevada batholith of California roughly 85 Ma.
[7] As the Farallon Plate subducted below the California continental margin an accretionary wedge was formed in the trench, which yielded unique rock types as a result of regional metamorphism.
The formation of Franciscan Melange and blueschist units along paleo-coastlines resulted from this subduction and are direct evidence of the Farallon Plate's past existence.
[4] As the plate falls apart, not only is compressional stress relieved, but significant back-slip along the old subduction zone is also possible, perhaps bringing blueschist rapidly upward from 20- to 30-km depths,[4] where it can be observed along the California coast to this day.
Since the North American coast shows an extremely complicated geologic structure, intensive work has been required to understand the complexity of this system.