User:Macwilliams3/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 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.

Geolithology
The region is dominated by Mesozoic-to-Cretaceous aged rocks which make up an uplifted subduction zone accretionary wedge called the Franciscan Complex. The complex is composed of Coastal, Central, and Eastern belts with distinct geological units. Within these areas on shore, quaternary sediments & sandstones, as well as volcanics can be found. 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. Comprised of individual beds that are often discontinuous, seeing large bodies of rocks with differing lithology in contact with one another is quite common. A metamorphic gradient crosses the Coastal, Central, and Eastern Belts seeing an increase in metamorphic grade going from west to east. Facies observed in the Coastal Belt are metamorphosed to zeolite while we see progression of grade to blueschist/greenschist facies in the Eastern Belt implying an increase in pressure as well. Offshore, the area consists of basaltic oceanic floor overlain with varying thickness of sedimentation. These ranges are from 0 metres to as much as 1500 metres, increasing as towards the shore.