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 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. This region is 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. 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. 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.
Slab-window model and triple junction migration
The passage of the MTJ causes mantle material to flow into the region vacated by the Gorda plate. Once this hot mantle material is south of the triple junction, it will cool, stiffen, and accrete to adjacent lithosphere, eventually welding to it and moving along with it, analogous to the motion of a conveyor belt. Lower crust-upper mantle viscous coupling plays a dominant role in converting accretionary margin materials into continent-like crust. Researchers were able to demonstrate that in this 'conveyor belt' mechanism, the crust is first thickened north of the triple junction, and after passage, the crust is thinned south of the triple junction. In this way, as the MTJ migrates, there is production of a basal conveyor belt beneath North America that transports material from the south to the north. This is consistent with an observed pattern of anomalous crustal structure determined by local-earthquake crustal tomography in the region.
Geolithology
The region is dominated by Mesozoic-to-Cretaceous aged rocks which make up an uplifted subduction zone accretionary wedge called the Franciscan Complex. This unit is made up of sandstones, shales, cherts, metagraywackes, melanges, as well as mafic volcanics, and is mostly metamorphosed to blueschist and eclogite facies.
Thermal regime
The spatial distribution of heat flow in the vicinity of the MTJ is similar to what would be expected in a subduction environment. That is, heat flow is low above the subducting Gorda slab, between 40 and 50 mW/m2. South of the MTJ, heat flow through the California coast is higher, around 90 mW/m2. The distance south of the MTJ over which heat flow increases gives an indication of the timing of development of the heat flow anomaly. The observed surface heat flow doubles over a distance of ~200 km, corresponding to a timeframe of migration of 4–5 Ma. It is also consistent with a source for the anomaly, thought to be asthenospheric mantle material, emplaced at shallow depths of 15–25 km, i.e. in the slab window. This rise of the heat flow anomaly time implies that there is probably only a thin crustal lid above the slab window.