The Nazca Plate
The Nazca plate is an oceanic tectonic plate in the southeastern Pacific Ocean that shares both convergent and divergent boundaries, corners multiple triple junctions, contains three seamount chains, overrides four hotspots, and is responsible for the creation of the Andean orogeny (Figure 1). Over the last half-century, it has garnered much attention from the scientific and political community due to the hazards it poses to the populated west coast of South America. With oblique subduction underneath the South American plate, this active convergent margin is the longest subduction zone in the world, stretching 7500 km (Klotz et al., 2001) and produced the largest earthquake ever recoded on earth, the M 9.5 Valdivia earthquake (Kanamori & Cipar, 1976).
Relatively young, the Nazca plate is a remnant of its precursor, the Farallon plate, which was created during the break-up of Pangea in the late Jurassic (Lonsdale, 2005; Sigloch & Mihalynuk, 2013). Consequently, during significant inter-plate spreading around ~23 Ma, the Farallon plate was split into two pieces, creating the current Nazca and Cocos plates (Lonsdale, 2005; Searle, 1989)(Video 1). This coincides markedly with increases in the plate convergence rate of the South American-Nazca boundary ~26 Ma and the increase and change in direction of divergence of the East Pacific Rise ~25 Ma, both perhaps major driving forces in the Farallon breakup (Sempere, et al., 1990, Wilson, 1996, Goff & Cochran 1996). Furthermore, the plate contains three major seamount chains (Easter Seamount Chain, Nazca Ridge, Carnegie Ridge) all thought to be the result of upwelling from mantle plumes. Interestingly, the continuous ridge joining the Nazca Ridge and Easter Seamount Chain, made up of moderately alkalic basalts, exhibits a clear bend dated to ~23 Ma. This bend is evidence for a significant directional shift in the motion of the Nazca plate around the time of the Farralon breakup (Figure 2)(Ray et al., 2012).
Relatively young, the Nazca plate is a remnant of its precursor, the Farallon plate, which was created during the break-up of Pangea in the late Jurassic (Lonsdale, 2005; Sigloch & Mihalynuk, 2013). Consequently, during significant inter-plate spreading around ~23 Ma, the Farallon plate was split into two pieces, creating the current Nazca and Cocos plates (Lonsdale, 2005; Searle, 1989)(Video 1). This coincides markedly with increases in the plate convergence rate of the South American-Nazca boundary ~26 Ma and the increase and change in direction of divergence of the East Pacific Rise ~25 Ma, both perhaps major driving forces in the Farallon breakup (Sempere, et al., 1990, Wilson, 1996, Goff & Cochran 1996). Furthermore, the plate contains three major seamount chains (Easter Seamount Chain, Nazca Ridge, Carnegie Ridge) all thought to be the result of upwelling from mantle plumes. Interestingly, the continuous ridge joining the Nazca Ridge and Easter Seamount Chain, made up of moderately alkalic basalts, exhibits a clear bend dated to ~23 Ma. This bend is evidence for a significant directional shift in the motion of the Nazca plate around the time of the Farralon breakup (Figure 2)(Ray et al., 2012).
Video 1. Wax tank modelling of the break-up of the Farallon plate into the Cocos and Nazca plates (Stoddard, P.R., Northern Illinois University).
Figure 2. Image of the change in direction located between the Nazca Ridge and the Easter Seamount Chain (Ray et al., 2012)
Today, plate motions around the Nazca plate include convergence towards the east with the South-American plate and divergence with the Antarctic, Pacific and Cocos Plates. Towards the east, the Nazca plate is subducting beneath the South American plate at the fastest current subduction rate on the planet, approximately 61+/- 3mm/yr (Norabueana et al., 1999). Towards the west, the East Pacific rise exhibits the fastest current spreading rates on the planet, ranging from 120 +/- 3 mm/yr near the Galapagos triple junction to 145+/-4 mm/yr near the Chile Triple junction (Norabuena et al., 1999; De Mets et al., 2010). To the south, the Nazca plate is diverging from the Antarctic plate at rates of 50 mm/yr +/- 1mm (DeMets et al., 2010). Towards the north the Nazca plate is diverging from the Cocos plate at a rate of 62-42 mm/yr.
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Notably, there are two “flat-slab” subduction zones between 2°-15°S and 28°-33°30’S that and are distinguished by the lack of late Miocene to Holocene volcanic activity (Gregory-Wodzicki, 2000)(Figure 3). Spatial correlations between flat-slab segments and the subduction of the overthickened crust of two hotspot-derived oceanic ridges suggest buoyancy to be the primary factor controlling subduction style (Gutscher, 2002). This is observed by the 2-40 km lithoospheric ‘sag’ between two relative highs at 5°S and 13°S that are present at the two former oceanic ridges (Gutscher et al., 1999)(Figure 4).
Lastly, significant deceleration of the plate motion has been observed on Nazca-Pacific and Nazca-South America plate boundaries since ~20 Ma. Although there is no consensus on the mechanism for such a deceleration, some theories propose that the aforementioned flat-slab subduction could be responsible (Martinod et al., 2010), while others theorize the loading exerted by the overriding Andes could be slowing down subduction |
(Iaffaldano et al., 2006). Moreover, a recent study suggests the main force decelerating the motion could be the penetration of the Nazca tip into the mantle transition zone (Quinteros & Sobolev, 2014)(Video 2). As stated, more persuasive evidence will need to be found to definitively explain the deceleration of the Nazca plate.
References
DeMets, C., Gordon, R. G., & Argus, D. F. (2010). Geologically current plate motions. Geophysical Journal International, 181(1), 1-80.
Goff, J. A., and J. R. Cochran, The Bauer Scarp ridge jump: a complex tectonic sequence revealed in satellite altimetry, Earth and Planetary Sci. Lett., 141, 21-33, 1996.
Gregory-Wodzicki, ,K.M. (2000). Uplift history of the central and northern andes: A review. Bulletin of the Geological Society of America, 112(7), 1091-1105.
Gutscher, ,M.A., Olivet, J. L., Aslanian, D., Eissen, J. P., & Maury, R. (1999). The "lost inca plateau"; cause of flat subduction beneath peru? Earth and Planetary Science Letters, 171(3), 335-341.
Gutscher, M. (2002). Andean subduction styles and their effect on thermal structure and interplate coupling. Journal of South American Earth Sciences, 15(1), 3-10.
Iaffaldano, G., Bunge, H., & Dixon, T. H. (2006). Feedback between mountain belt growth and plate convergence. Geology (Boulder), 34(10), 893-896.
Kanamori, H., & Cipar, J. J. (1974). Focal process of the great chilean earthquake may 22, 1960. Physics of the Earth and Planetary Interiors, 9(2), 128-136.
Klotz, J., Khazaradze, G., Angermann, D., Reigber, C., Perdomo, R., & Cifuentes, O. (2001). Earthquake cycle dominates contemporary crustal deformation in central and southern andes. Earth and Planetary Science Letters, 193(3-4), 437-446.
Lonsdale, P. (2005). Creation of the cocos and nazca plates by fission of the farallon plate. Tectonophysics, 404(3-4), 237-264.
Martinod, J., Espurt, N., Guillaume, B., Husson, L., & Roperch, P. (2010). Horizontal subduction zones, convergence velocity and the building of the andes. Geophysical Research Abstracts, 12, EGU2010-8666-3.
Norabuena, E. O., Dixon, T. H., Stein, S., & Harrison, C. G. A. (1999). Decelerating nazca-south america and nazca-pacific plate motions. Geophysical Research Letters, 26(22), 3405-3408.
Pardo-Casas, F., & Molnar, P. (1987). Relative motion of the nazca (farallon) and south american plates since late cretaceous time.Tectonics, 6(3), 233-248.
Quinteros, J., & Sobolev, S. V. (2012). Why has the nazca plate slowed since the neogene? Geology (Boulder), 41(1), 31-34.
Ray, J. S., Mahoney, J. J., Duncan, R. A., Ray, J., Wessel, P., & Naar, D. F. (2012). Chronology and geochemistry of lavas from the nazca ridge and easter seamount chain; an approximately 30 myr hotspot record. Journal of Petrology, 53(7), 1417-1448.
Searle, R. C. (1989). Location and segmentation of the cocos-nazca spreading centre west of 95 degrees W. Marine Geophysical Researches, 11(1), 15-26.
Sempere, T., Herail, G., Oller, J., Bonhomme, M.G. (1990). Late Oligocene-early Miocene major tectonic crisis and related basins in Bolivia. Geology (Boulder), 18(10), 946-949.
Sigloch, K., & Mihalynuk, M. G. (2013). Intra-oceanic subduction shaped the assembly of cordilleran north america. Nature (London), 496(7443), 50-56.
Wilson, D. S. (1996). Fastest known spreading on the miocene cocos-pacific plate boundary. Geophysical Research Letters,23(21), 3003-3006.
Goff, J. A., and J. R. Cochran, The Bauer Scarp ridge jump: a complex tectonic sequence revealed in satellite altimetry, Earth and Planetary Sci. Lett., 141, 21-33, 1996.
Gregory-Wodzicki, ,K.M. (2000). Uplift history of the central and northern andes: A review. Bulletin of the Geological Society of America, 112(7), 1091-1105.
Gutscher, ,M.A., Olivet, J. L., Aslanian, D., Eissen, J. P., & Maury, R. (1999). The "lost inca plateau"; cause of flat subduction beneath peru? Earth and Planetary Science Letters, 171(3), 335-341.
Gutscher, M. (2002). Andean subduction styles and their effect on thermal structure and interplate coupling. Journal of South American Earth Sciences, 15(1), 3-10.
Iaffaldano, G., Bunge, H., & Dixon, T. H. (2006). Feedback between mountain belt growth and plate convergence. Geology (Boulder), 34(10), 893-896.
Kanamori, H., & Cipar, J. J. (1974). Focal process of the great chilean earthquake may 22, 1960. Physics of the Earth and Planetary Interiors, 9(2), 128-136.
Klotz, J., Khazaradze, G., Angermann, D., Reigber, C., Perdomo, R., & Cifuentes, O. (2001). Earthquake cycle dominates contemporary crustal deformation in central and southern andes. Earth and Planetary Science Letters, 193(3-4), 437-446.
Lonsdale, P. (2005). Creation of the cocos and nazca plates by fission of the farallon plate. Tectonophysics, 404(3-4), 237-264.
Martinod, J., Espurt, N., Guillaume, B., Husson, L., & Roperch, P. (2010). Horizontal subduction zones, convergence velocity and the building of the andes. Geophysical Research Abstracts, 12, EGU2010-8666-3.
Norabuena, E. O., Dixon, T. H., Stein, S., & Harrison, C. G. A. (1999). Decelerating nazca-south america and nazca-pacific plate motions. Geophysical Research Letters, 26(22), 3405-3408.
Pardo-Casas, F., & Molnar, P. (1987). Relative motion of the nazca (farallon) and south american plates since late cretaceous time.Tectonics, 6(3), 233-248.
Quinteros, J., & Sobolev, S. V. (2012). Why has the nazca plate slowed since the neogene? Geology (Boulder), 41(1), 31-34.
Ray, J. S., Mahoney, J. J., Duncan, R. A., Ray, J., Wessel, P., & Naar, D. F. (2012). Chronology and geochemistry of lavas from the nazca ridge and easter seamount chain; an approximately 30 myr hotspot record. Journal of Petrology, 53(7), 1417-1448.
Searle, R. C. (1989). Location and segmentation of the cocos-nazca spreading centre west of 95 degrees W. Marine Geophysical Researches, 11(1), 15-26.
Sempere, T., Herail, G., Oller, J., Bonhomme, M.G. (1990). Late Oligocene-early Miocene major tectonic crisis and related basins in Bolivia. Geology (Boulder), 18(10), 946-949.
Sigloch, K., & Mihalynuk, M. G. (2013). Intra-oceanic subduction shaped the assembly of cordilleran north america. Nature (London), 496(7443), 50-56.
Wilson, D. S. (1996). Fastest known spreading on the miocene cocos-pacific plate boundary. Geophysical Research Letters,23(21), 3003-3006.