The Mendocino Triple Junction (MTJ) is the point where the Gorda plate, the North American plate, and the Pacific plate meet, in northern California (see triple junction). The Mendocino Triple Junction is the location of a change in the broad 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. 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 Fault, and a divergent boundary at the Gorda Ridge.

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. At this point, removal of the subducting Gorda lithosphere from beneath North America causes asthenospheric upwelling. 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 centred 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.

A triple junction is the point where the boundaries of three tectonic plates meet. At the triple junction each of the three boundaries will be one of 3 types - a ridge (R), trench (T) or transform fault (F) - and triple junctions can be described according to the types of plate margin that meet at them (e.g. Transform-Transform-Trench, Ridge-Ridge-Ridge, or abbreviated F-F-T, R-R-R). Of the many possible types of triple junction only a few are stable through time ('stable' in this context means that the geometrical configuration of the triple junction will not change through geologic time).

History

The first scientific paper detailing the triple junction concept was published in 1969 by W. Jason Morgan, Dan McKenzie, and Tanya Atwater. The term had traditionally been used for the intersection of three divergent boundaries or spreading ridges. These three divergent boundaries ideally meet at near 120° angles.

In plate tectonics theory during the breakup of a continent, three divergent boundaries form, radiating out from a central point (the triple junction). One of these divergent plate boundaries fails (see aulacogen) and the other two continue spreading to form an ocean. The opening of the south Atlantic Ocean started at the south of the South American and African continents, reaching a triple junction in the present Gulf of Guinea, from where it continued to the west. The NE-trending Benue Trough is the failed arm of this junction.