The Galapagos Microplate
The Galapagos Microplate (GMP) is a tiny tectonic plate west of the Galapagos Islands in the eastern equatorial Pacific Ocean (Searle et al., 1985). It is famous for the intriguing bathymetry resulting from its triple junction, which occurs when three plate tectonic boundaries meet each other (Searle et al., 1985). As the site of the confluence of the Cocos-Nazca, Pacific-Nazca and Pacific-Cocos spreading centres, the GMP is characterized by a ridge-ridge-ridge (RRR) triple junction (Searle et al., 1985). Another notable feature of the GMP region is that it was the site of the discovery of the first black smoker and hydrothermal vent (Tao et al., 2011).
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The rocks composing the GMP are classic basaltic pillow lavas exhibiting typical mid-ocean ridge basalt (MORB) compositions (Lonsdale et al., 1992). Analyses of rare earth elements along with moderately incompatible and relatively immobile elements suggest that the mantle source for the rocks of the GMP is heterogeneous (Lonsdale et al., 1992). The idea of a heterogeneous source for the rocks of the GMP is further supported by Pb isotopic analyses, since the observed isotopic variation of these rocks represents nearly the entire range of variation for the MORB of the East Pacific Rise (EPR) as a whole (Cumming et al., 1995). Plate motion measurements resulted in the discovery that the Cocos Plate and Nazca Plate are both separating from the Pacific Plate at about 140 mm/year, the Cocos in a direction of 083° and Nazca at 100°, while the Cocos Plate is separating from the Nazca Plate at about 41 mm/year in a direction slightly east of north (Searle et al., 1985).
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Development of the GMP likely began with the isolation of oceanic crust generated at the EPR ~1 myr ago, which was initiated with the growth of a prominent seamount adjacent to the Pacific-Nazca spreading centre (Lonsdale, 1988). This configuration then led to a zone of weakness and magma upwelling between the seamount and the adjacent EPR, forming a short east-west-trending spreading centre (Lonsdale, 1988). Over time, this spreading centre propagated northeastwards to become the localized Nazca-Galapagos spreading centre (Lonsdale, 1988). The northern boundary is more poorly defined, though part of it seems to have formed by the “incipient rift”, a localized spreading rift east of and orthogonal to the EPR (Lonsdale et al., 1992).
Based on sparse bathymetry data, it was first proposed that the GMP was rotating clockwise at 6°/myr (Lonsdale, 1988) which was eventually adjusted to 22°/myr by the application of an edge-driven microplate model (Klein et al., 2005). Researchers reached the pioneering conclusion that the Galapagos triple junction region consists of two counter-rotating microplates distributing strain around the triple junction, each of which being treated as a rigid block driven by drag on its edge (Klein et al., 2005). In 2009 however, more detailed mapping of the GMP led to a different conclusion driven by data collected in Hess Deep and Dietz Deep, narrow rift valleys being opened between the Cocos-Nazca spreading ridge and Galapagos-Nazca spreading ridge, respectively (Smith et al., 2009). It was found that most of the core of the microplate shows north-south abyssal hills produced at the EPR and suggests that the microplate is not rotating and has not rotated for much of its history (Smith et al., 2009). Interestingly, it appears as though a section of seafloor in the northeast part of the microplate has been rotated, which indicates that before ~1 myr ago the kinematics of the region was different (Smith et al., 2009). |
The following link leads to the NOAA Ocean Explorer website, where you can follow the log entries of an expedition to the Galapagos Spreading Centre, a fascinating region where mid-ocean ridge meets hotspot: http://oceanexplorer.noaa.gov/explorations/05galapagos/welcome.html
Scripps Institution of Oceanography's profile of Peter Lonsdale, who spearheaded much of the early research of the Galapagos Microplate: http://www.siosearch.org/?page_id=31 |
There are still several outstanding questions regarding the characteristics of the GMP and its triple junction. For example, why did the Cocos-Nazca Rift increase its propagation rate toward the EPR ~1.5 myr ago (Searle et al., 1985). Furthermore, what is the origin of the rotated section of crust in the northeast of the GMP, and since it is not rotating with high angular velocity, how is extension on its southern boundary transferred to the Cocos-Nazca Rift (Smith et al., 2013)? The unique setting of the GMP inevitably lends itself to further study, as it enables a better understanding of how plates deform internally near plate boundaries as well as the relationship between this deformation and upwelling mantle material (Smith et al., 2013).
The following videos discuss the study of the Hess Deep Rift to gain a better understanding of crustal accretion processes:
References
Cumming, G. L., Krstic, D., Puchelt, H. (1995). Pb, Sr, and Nd Isotopic Systematics of Rocks from the Galapagos Microplate, Canadian Journal of Earth Sciences, 32, 508-515.
Klein, E. M., Smith, D. K., Wiliams, C. M., Schouten, H. (2005). Counter-Rotating Microplates at the Galapagos Triple Junction, Nature, 433, 855-858.
Lonsdale, P. (1988). Structural Pattern of the Galapagos Microplate and Evolution of the Galapagos Triple Junctions, Journal of Geophysical Research, 93(B11), 13551-13574.
Lonsdale, P., Blum, N., Puchelt, H. (1992). The RRR Triple Junction at the Southern End of the Pacific-Cocos East Pacific Rise, Earth and Planetary Science Letters, 109, 73-85.
Searle, R. C., Francheteau, J. (1985). Morphology and Tectonics of the Galapagos Triple Junction, Marine Geophysical Researches, 8, 95-129.
Smith, D. K., Schouten, H., Cann, J. R., Zhu, W., Montesi, L. G., Mitchell, G. A. (2009). The Galapagos Microplate Revealed, American Geophysical Union Fall Meeting 2009.
Smith, D. K., Schouten, H., Montesi, L., Zhu, W. (2013). The Recent History of the Galapagos Triple Junction Preserved on the Pacific Plate, Earth and Planetary Science Letters, 371-372, 6-15.
Tao, C., Li, H., Wu, G., Su, X., Zhang, G. (2011). First Hydrothermal Active Vent Discovered on the Galapagos Microplate, American Geophysical Union Fall Meeting 2011.
Cumming, G. L., Krstic, D., Puchelt, H. (1995). Pb, Sr, and Nd Isotopic Systematics of Rocks from the Galapagos Microplate, Canadian Journal of Earth Sciences, 32, 508-515.
Klein, E. M., Smith, D. K., Wiliams, C. M., Schouten, H. (2005). Counter-Rotating Microplates at the Galapagos Triple Junction, Nature, 433, 855-858.
Lonsdale, P. (1988). Structural Pattern of the Galapagos Microplate and Evolution of the Galapagos Triple Junctions, Journal of Geophysical Research, 93(B11), 13551-13574.
Lonsdale, P., Blum, N., Puchelt, H. (1992). The RRR Triple Junction at the Southern End of the Pacific-Cocos East Pacific Rise, Earth and Planetary Science Letters, 109, 73-85.
Searle, R. C., Francheteau, J. (1985). Morphology and Tectonics of the Galapagos Triple Junction, Marine Geophysical Researches, 8, 95-129.
Smith, D. K., Schouten, H., Cann, J. R., Zhu, W., Montesi, L. G., Mitchell, G. A. (2009). The Galapagos Microplate Revealed, American Geophysical Union Fall Meeting 2009.
Smith, D. K., Schouten, H., Montesi, L., Zhu, W. (2013). The Recent History of the Galapagos Triple Junction Preserved on the Pacific Plate, Earth and Planetary Science Letters, 371-372, 6-15.
Tao, C., Li, H., Wu, G., Su, X., Zhang, G. (2011). First Hydrothermal Active Vent Discovered on the Galapagos Microplate, American Geophysical Union Fall Meeting 2011.