Mendocino Triple Junction

The Mendocino Triple Junction (MTJ) is the point where the Gorda plate, the North American plate, and the Pacific plate meet, in the Pacific Ocean near Cape Mendocino in northern California. This triple junction is the location of a change in the broad tectonic plate motions which dominate the west coast of North America, linking convergence of the northern Cascadia subduction zone and translation of the southern San Andreas Fault system.^[1] This region can be characterized by transform fault movement, the San Andreas also by transform strike slip movement, and the Cascadia subduction zone by a convergent plate boundary subduction movement. The Gorda plate is subducting, towards N50ºE, under the North American plate at 2.5–3 cm/yr, and is simultaneously converging obliquely against the Pacific plate at a rate of 5 cm/yr in the direction N115ºE. The accommodation of this plate configuration results in a transform boundary along the Mendocino Fracture Zone, and a divergent boundary at the Gorda Ridge.^[2][3] This area is tectonically active historically and today. The Cascadia subduction zone is known to be capable of producing megathrust earthquakes on the order of MW 9.0.

Due to the relative plate motions, the triple junction has been migrating northwards for the past 25–30 million years, and assuming rigid plates, the geometry requires that a void, called slab window, develop southeast of the MTJ.^[4] At this point, removal of the subducting Gorda lithosphere from beneath North America causes asthenospheric upwelling.^[3] This instigates different tectonic processes, which include surficial uplift, crustal deformation, intense seismic activity, high heat flow, and even the extrusion of volcanic rocks. This activity is centered on the current triple junction position, but evidence for its migration is found in the geology all along the California coast, starting as far south as Los Angeles.^[4]