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The Mendocino Triple Junction (MTJ) is the point where the
Gorda plate The Gorda Plate, located beneath the Pacific Ocean off the coast of northern California, is one of the northern remnants of the Farallon Plate. It is sometimes referred to (by, for example, publications from the USGS Earthquake Hazards Program) as ...
, the North American plate, and the
Pacific plate The Pacific Plate is an oceanic tectonic plate that lies beneath the Pacific Ocean. At , it is the largest tectonic plate. The plate first came into existence 190 million years ago, at the triple junction between the Farallon, Phoenix, and Iza ...
meet, in the Pacific Ocean near
Cape Mendocino Cape Mendocino ( Spanish: ''Cabo Mendocino'', meaning "Cape of Mendoza"), which is located approximately north of San Francisco, is located on the Lost Coast entirely within Humboldt County, California, United States. At 124° 24' 34" W longitu ...
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 The Cascadia subduction zone is a convergent plate boundary that stretches from northern Vancouver Island in Canada to Northern California in the United States. It is a very long, sloping subduction zone where the Explorer, Juan de Fuca, and ...
and translation of the southern
San Andreas Fault The San Andreas Fault is a continental transform fault that extends roughly through California. It forms the tectonic boundary between the Pacific Plate and the North American Plate, and its motion is right-lateral strike-slip (horizontal). ...
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 Fracture Zone The Mendocino Fracture Zone is a fracture zone and transform boundary over 4000 km (2500 miles) long, starting off the coast of Cape Mendocino in far northern California. It runs westward from a triple junction with the San Andreas Fault ...
, and a divergent boundary at the
Gorda Ridge The Gorda Ridge (41°36'19.6"N 127°22'03.1"W), aka ''Gorda Ridges'' tectonic spreading center, is located roughly off the northern coast of California and southern Oregon. Running NE – SW it is roughly in length.Carey, Stein, & Rona. (1990). ...
. 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 A mantle is a piece of clothing, a type of cloak. Several other meanings are derived from that. Mantle may refer to: *Mantle (clothing), a cloak-like garment worn mainly by women as fashionable outerwear **Mantle (vesture), an Eastern Orthodox ve ...
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 An accretionary wedge or accretionary prism forms from sediments accreted onto the non- subducting tectonic plate at a convergent plate boundary. Most of the material in the accretionary wedge consists of marine sediments scraped off from the ...
called the
Franciscan Complex The Franciscan Complex or Franciscan Assemblage is a geologic term for a late Mesozoic terrane of heterogeneous rocks found throughout the California Coast Ranges, and particularly on the San Francisco Peninsula. It was named by geologist Andrew ...
. This unit is made up of sandstones, shales, cherts, metagraywackes, melanges, as well as mafic volcanics, and is mostly metamorphosed to
blueschist Blueschist (), also called glaucophane schist, is a metavolcanic rock that forms by the metamorphism of basalt and rocks with similar composition at high pressures and low temperatures (), approximately corresponding to a depth of . The blue c ...
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 Subduction is a geological process in which the oceanic lithosphere is recycled into the Earth's mantle at convergent boundaries. Where the oceanic lithosphere of a tectonic plate converges with the less dense lithosphere of a second plate, ...
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.


Genesis of volcanics

The presence of hot asthenospheric mantle at shallow levels beneath the western margin of North America is likely to generate melt and cause magmatism. Accordingly, a sequence of volcanoes in the wake of the MTJ passing were activated; this magmatism likely leads to the intrusion of
pluton In geology, an igneous intrusion (or intrusive body or simply intrusion) is a body of intrusive igneous rock that forms by crystallization of magma slowly cooling below the surface of the Earth. Intrusions have a wide variety of forms and com ...
s within the overlying crust in the region. An example of volcanic bodies that formed by magma upwelling and solidification are the Nine Sisters, located between Morro Bay and
San Luis Obispo San Luis Obispo (; Spanish language, Spanish for "Louis of Toulouse, St. Louis the Bishop", ; Chumashan languages, Chumash: ''tiłhini'') is a city and county seat of San Luis Obispo County, California, San Luis Obispo County, in the U.S. state ...
in California. The source of the material which flows into the slab window is a matter of debate, specifically whether it is derived directly from the underlying mantle, or from the mantle wedge to the east. It turns out that the chemistry of erupted
basalt Basalt (; ) is an aphanitic (fine-grained) extrusive igneous rock formed from the rapid cooling of low-viscosity lava rich in magnesium and iron (mafic lava) exposed at or very near the surface of a rocky planet or moon. More than 90% of a ...
s associated with the MTJ are typical of mantle wedge–derived melts, characterized by enrichment in the large-ion lithophile elements and depletion in the high-field-strength elements. In general, mantle wedge-derived melts are relatively more hydrous, have lower viscosity and temperatures than melts derived from sub-slab mantle.


Seismicity

Most of the seismicity near the MTJ is offshore, concentrated along the Mendocino Transform Fault. That having been said, seismicity is also distributed within the Gorda plate itself, owing to its small size, young age, and relatively thin lithosphere. Oblique convergence of the Gorda plate towards the Pacific plate causes intense north–south compression, and induces anomalously strong internal deformation in the former, giving rise to the
Gorda Deformation Zone Gorda may refer to: * Gorda, California, United States, a small town on the Pacific Ocean * Gorda Plate, a tectonic plate located beneath the Pacific Ocean near northern California * Cayo Gorda, a small island of Honduras in Caribbean Sea * Gord ...
(GDZ) and resulting in abundant seismicity. Motion along the Mendocino Transform Fault (MTF) is right-lateral on E-W oriented, vertically dipping planes. Within the portion of North American crust overlying the Gorda slab, motion on faults is reverse, and in April 1992, a M = 7.1 earthquake ruptured the southern portion of the Cascadia subduction zone. Similar to the general seismicity patterns in the region, the majority of the aftershocks for this event had vertical strike-slip motions and were located within the Gorda plate, or on the MTF at depths between 23 and 35 km. None of the aftershocks were consistent with northeast underthrusting of the Gorda plate beneath North America, as was the case in the main event. This set of earthquake geometries implies a stress field characterized by N-NW, horizontal principal compressive stress; this is consistent with the orientation of compression in the GDZ northwest of the MTJ.


Notes


References

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Further reading

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External links


Mendocino Triple Junction Offshore Northern California
United States Geological Survey
Where the San Andreas Fault ends
Berkeley Seismological Laboratory The Berkeley Seismological Laboratory (BSL) is a research lab at the Department of Geology at the University of California, Berkeley. It was created from the Berkeley Seismographic Stations, a site on the Berkeley campus where Worldwide Standard S ...
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