A triple junction is a point where three tectonic plates intersect. A few kilometers offshore of Cape Mendocino, in northern California, is the Mendocino Triple Junction, at which the North American Plate, the Pacific Plate and the Gorda Plate (the southern-most part of the Juan de Fuca Plate, a fragment of the now broken-up Farallon Plate) meet. It is studied extensively by geoscientists because of its close link to the San Andreas Fault system, its important seismic activity (frequent earthquakes) and high crustal deformation rates and extent. Approximately 80 magnitude 3 or higher earthquakes of per year have been detected in the area since 1983 (Oppenheimer).
The Mendocino Triple Junction (MTJ) joins three active boundaries: extending North, the Cascadia subduction zone, where the Gorda plate (oceanic plate) is subducting under the North American Plate (continental plate); extending South, the San Andreas Transform Fault and to the West, the Mendocino Transform Fault. It is therefore classified as FFT (Fault-Fault-Trench).
The Cascadia subduction zone, where subduction happens at a rate of 36–40 mm/year in a N55ºE direction (Riddihough, 1984), is accompanied by the Gorda Ridge at the western edge of the plate, a mid-ocean ridge where mantle upwelling occurs, creating new oceanic crust. The Pacific and Gorda plates converge at a rate of 5 cm/year at N115ºE (Oppenheimer). The Gorda plate is not very big, only about 45 000 km2, and the age of its rocks ranges from 0 to 7 Ma (Chaytor, 2004) while the Pacific Plate in that region is aged 25-27 Ma (Gulick, 2001). The rocks close to the MTJ are mainly sandstones, shales, cherts, metagreywackes, melange, mafic volcanics and blueschist and eclogite facies metamorphic rocks, all from an exhumed accretionary wedge, the Fransiscan Complex (Furlong, 2004).
The MTJ is migrating north, in conjunction with the south-migrating Rivera triple junction, at the other end of the San Andreas fault system. This movement can be explained with the history of the MTJ, dating back to approximately 30 Ma.
The Mendocino Triple Junction (MTJ) joins three active boundaries: extending North, the Cascadia subduction zone, where the Gorda plate (oceanic plate) is subducting under the North American Plate (continental plate); extending South, the San Andreas Transform Fault and to the West, the Mendocino Transform Fault. It is therefore classified as FFT (Fault-Fault-Trench).
The Cascadia subduction zone, where subduction happens at a rate of 36–40 mm/year in a N55ºE direction (Riddihough, 1984), is accompanied by the Gorda Ridge at the western edge of the plate, a mid-ocean ridge where mantle upwelling occurs, creating new oceanic crust. The Pacific and Gorda plates converge at a rate of 5 cm/year at N115ºE (Oppenheimer). The Gorda plate is not very big, only about 45 000 km2, and the age of its rocks ranges from 0 to 7 Ma (Chaytor, 2004) while the Pacific Plate in that region is aged 25-27 Ma (Gulick, 2001). The rocks close to the MTJ are mainly sandstones, shales, cherts, metagreywackes, melange, mafic volcanics and blueschist and eclogite facies metamorphic rocks, all from an exhumed accretionary wedge, the Fransiscan Complex (Furlong, 2004).
The MTJ is migrating north, in conjunction with the south-migrating Rivera triple junction, at the other end of the San Andreas fault system. This movement can be explained with the history of the MTJ, dating back to approximately 30 Ma.
Formation of the MTJ
The above figure (from Furlong and Schwartz, 2004) shows the disposition of the three plates at three stages of development of the Mendocino triple junction. 30 million years ago, the divergent boundary separating the Pacific and Farallon plate (East Pacific Rise) was subducted under the North American plate, creating the MTJ. Since its formation, it follows the movement direction of the Pacific plate, dragging the Mendocino Transform fault along with it. The “slab window” represented in white on the diagram is the result of the East Pacific rise continuing to spread after its subduction. This creates a void, or a gap, that fills up with upwelling material unable to cool because it is no longer exposed on the ocean floor.
The above figure (from Furlong and Schwartz, 2004) shows the disposition of the three plates at three stages of development of the Mendocino triple junction. 30 million years ago, the divergent boundary separating the Pacific and Farallon plate (East Pacific Rise) was subducted under the North American plate, creating the MTJ. Since its formation, it follows the movement direction of the Pacific plate, dragging the Mendocino Transform fault along with it. The “slab window” represented in white on the diagram is the result of the East Pacific rise continuing to spread after its subduction. This creates a void, or a gap, that fills up with upwelling material unable to cool because it is no longer exposed on the ocean floor.
This animation made by Tanya Atwater shows the same process of the Farallon plate subduction creating two triple junctions moving apart from each other along the San Andreas Fault System. The lighter shades of blue indicate younger rocks created at divergent plate boundaries.
Deformation
A concentration of active plate boundaries at a single point will inevitably produce abundant crustal deformation. The crust North of the MTJ is being thickened and the crust south of the MTJ is being thinned by a mechanism referred to as the “Mendocino Crustal Conveyor” (Furlong, 2003), in which material upwelling in the slab window slowly cools and accretes to both the Gorda Plate and the North American Plate, inducing deformation (folding, faulting) on the North American coast when the mantle material is entrained with the North-migrating Gorda plate (Furlong, 1999). This is called viscous coupling and, while still being debated, is thought to be a significant way of creating continental crust (by accretion). The related process of thickening-thinning happens over timescales of less than 5 my (Furlong, 1999).
Deformation
A concentration of active plate boundaries at a single point will inevitably produce abundant crustal deformation. The crust North of the MTJ is being thickened and the crust south of the MTJ is being thinned by a mechanism referred to as the “Mendocino Crustal Conveyor” (Furlong, 2003), in which material upwelling in the slab window slowly cools and accretes to both the Gorda Plate and the North American Plate, inducing deformation (folding, faulting) on the North American coast when the mantle material is entrained with the North-migrating Gorda plate (Furlong, 1999). This is called viscous coupling and, while still being debated, is thought to be a significant way of creating continental crust (by accretion). The related process of thickening-thinning happens over timescales of less than 5 my (Furlong, 1999).
Deformation also happens inside the Gorda plate. Compressed North-South by the migrating MTJ, existing normal faults in the plate have been re-activated as sinistral transform faults and major seismicity is taking place as a result. The North American and Pacific plates being much bigger, older and thus nehaving more rigidly than the Gorda plate, it acts as a buffer, and therefore breaks along its edges near the triple junction and rotates clockwise (Gulick, 2001).
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This figure from (Gulick, 2001) depicts the magnetic anomalies on the Gorda and Pacific plates as grey strands. The scale to the right indicates the ages of these anomalies, starting at 35 Ma, coinciding with the MTJ formation. This indicates that the collision of the Farallon plate with North America marked an abrupt rise in deformation in this region. The bottom image shows a concentration of major seismic events near the MTJ, particularly along the Mendocino transform fault, as well as the location of the reactivated faults.
Sources
-Chaytor, JD, Goldfinger, C, Dziak, RP, Fox, CG. “Active deformation of the Gorda plate: Constraining deformation models with new geophysical data” (2004). Geology 32 (4): pp. 353-356. DOI: 10.1130/G20178.1
-Dickinson, W.R., Snyder, W.S. “Geometry of triple junctions related to San Andreas Transform”.(1979) Journal of Geophysical Research, vol. 84 no.B2.
-Eakin C, Obrebski M, Allen R, Boyarko D,Brudzinski M, Porritt R. (2010). “Seismic anisotropy beneath Cascadia and the Mendocino triple junction: interaction of the subducting slab with mantle flow”
Earth Planet. Sci. Lett., 297 (2010), pp. 627–632
-Furlong, K. P., Lock, J ,Guzofski, C ,Whitlock J., Benz H. (2003) “The Mendocino Crustal Conveyor: Making and Breaking the California Crust”, International Geology Review, 45:9, 767-779, DOI: 10.2747/0020-6814.45.9.767
-Furlong, K.P., Govers, R. “ Ephemeral crustal thickening at a triple junction: The Mendocino crustal conveyor” (1999). Geology 27, pp.127-130, DOI: 10.1130/0091-7613(1999)027<0127:ECTAAT>2.3.CO;2
-Furlong, K.P., Schwartz, S. Y., “INFLUENCE OF THE MENDOCINO TRIPLE JUNCTION ON THE TECTONICS OF COASTAL CALIFORNIA” (2004).Annu. Rev. Earth Planet. Sci. 32:403–33 doi: 10.1146/annurev.earth.32.101802.120252
-Gulick, S.P.S.; Meltzer, A.S.; Henstock, T.J.; Levander, A. (2001). "Internal deformation of the southern Gorda plate: Fragmentation of a weak plate near the Mendocino triple junction". Geology 29 (8): 691–694. doi:10.1130/0091-7613(2001)029<0691:idotsg>2.0.co;2.
-Dengler, L., Carver G., McPherson, R,.”Sources of North Coast Seismicity” .California Geology, March/April 1992
Oppenheimer, D."Mendocino Triple Junction Offshore Northern California". USGS.http://woodshole.er.usgs.gov/operations/obs/rmobs_pub/html/mendocino.html. Webpage visited on 29/03/2015
Sources
-Chaytor, JD, Goldfinger, C, Dziak, RP, Fox, CG. “Active deformation of the Gorda plate: Constraining deformation models with new geophysical data” (2004). Geology 32 (4): pp. 353-356. DOI: 10.1130/G20178.1
-Dickinson, W.R., Snyder, W.S. “Geometry of triple junctions related to San Andreas Transform”.(1979) Journal of Geophysical Research, vol. 84 no.B2.
-Eakin C, Obrebski M, Allen R, Boyarko D,Brudzinski M, Porritt R. (2010). “Seismic anisotropy beneath Cascadia and the Mendocino triple junction: interaction of the subducting slab with mantle flow”
Earth Planet. Sci. Lett., 297 (2010), pp. 627–632
-Furlong, K. P., Lock, J ,Guzofski, C ,Whitlock J., Benz H. (2003) “The Mendocino Crustal Conveyor: Making and Breaking the California Crust”, International Geology Review, 45:9, 767-779, DOI: 10.2747/0020-6814.45.9.767
-Furlong, K.P., Govers, R. “ Ephemeral crustal thickening at a triple junction: The Mendocino crustal conveyor” (1999). Geology 27, pp.127-130, DOI: 10.1130/0091-7613(1999)027<0127:ECTAAT>2.3.CO;2
-Furlong, K.P., Schwartz, S. Y., “INFLUENCE OF THE MENDOCINO TRIPLE JUNCTION ON THE TECTONICS OF COASTAL CALIFORNIA” (2004).Annu. Rev. Earth Planet. Sci. 32:403–33 doi: 10.1146/annurev.earth.32.101802.120252
-Gulick, S.P.S.; Meltzer, A.S.; Henstock, T.J.; Levander, A. (2001). "Internal deformation of the southern Gorda plate: Fragmentation of a weak plate near the Mendocino triple junction". Geology 29 (8): 691–694. doi:10.1130/0091-7613(2001)029<0691:idotsg>2.0.co;2.
-Dengler, L., Carver G., McPherson, R,.”Sources of North Coast Seismicity” .California Geology, March/April 1992
Oppenheimer, D."Mendocino Triple Junction Offshore Northern California". USGS.http://woodshole.er.usgs.gov/operations/obs/rmobs_pub/html/mendocino.html. Webpage visited on 29/03/2015