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The Galápagos Islands are the Result of a Mantle Plume

Dennis Geist1 & Karen Harpp2

1Dept. Geological Sciences, University of Idaho, Moscow, ID 83844-3022, U.S.A. dgeist@uidaho.edu
2Dept. Geology, Colgate University, 13 Oak Drive, Hamilton, NY 13346, U.S.A. kharpp@mail.colgate.edu

Introduction

The prevailing view is that the Galápagos Islands are due to a plume currently beneath the easterly moving Nazca Plate. The Galápagos plume is thought to lie about 250 km south of the Galápagos Spreading Center (GSC), beneath the western islands of Fernandina and Isabela, and it is thought that there has been extensive interaction between the two tectonic entities.  In the past, the GSC has jumped south and migrated north, so the hotspot has produced variable amounts of material on both plates through time.  The plume is thought to contribute material to GSC lavas, and lavas from the Galápagos hotspot contain an unusually large component of upper mantle material.

1.  The plume model

Figure 1: Map prepared by Bill Chadwick of NOAA

The bathymetry of the Galapagos region shows several tectonic provinces (Figure 1).  From south to north, some of the features are:

  1. The main Galapagos archipelago. The hotspot is thought to lie underneath the volcanically-active islands of Fernandina and Isabela (the equant, westernmost island and the large, J-shaped island); the eastern islands have much less volcanic activity.
  2. The Carnegie Ridge, which is thought to be the hotspot trace on the Nazca plate.  The prominent saddle at 85.5°W is attributed to the southern migration of the Galapagos Spreading Center; the hotspot was under the Cocos plate at the time the saddle was produced;
  3. The Wolf-Darwin Lineament (WDL) and northern islands are thought to be produced by a combination of high heat flow due to flow of plume material toward the ridge and unusually strong deviatoric stresses in the lithosphere owing to oblique spreading across the 90.5° transform;
  4. The Galapagos Spreading Center (GSC), the boundary between the Nazca plate to the south and the Cocos plate to the north. It is an intermediate-spreading ridge (about 58 km/my, roughly N-S spreading).  The bathymetric effects of the hotspot are clear; and
  5. The Cocos Ridge, the trace of excess crust produced by the hotspot, but on the Cocos plate.

2. Isotopes indicate a mantle plume

Figure 2

The istopic evidence has been interpreted as mixing between a heterogeneous plume source (dominantly PLUME, with domains of WD and FLO) and DUM. PLUME, WD, FLO and DUM are compositional endmembers, as identified by principal component analysis of isotopic and trace element data on all Galapagos Island and seamount data (Harpp & White, 2001). Note the substantial overlap beteen island lavas and those from the GSC, which has been interpreted as two-way mixing between Galápagos plume material and depleted asthenosphere.

Figure 3

The isotopic characteristics of the Galápagos Islands and seamounts show a regular zonation (Figure 3, left panel). The DUM component is concentrated in the center of the archipelago and increases eastward.  This has been attributed to torroidal mixing between plume and upper mantle material, and shearing of the plume by the Nazca plate (Figure 3, right panel). The occurrence of FLO in the south and WD in the north is attributed to zoning of heterogeneities in the plume.

Helium isotope ratios show a different pattern: the most "plume-like" signal is in the center of the archipelago and in the FLO-bearing islands to the south (Figure 4).  The GSC, despite being contaminated with plume Sr, Nd, and Pb, lies within even the most conservative estimates of MORB 3He/4He.


Figure 4

3. Age  Progression Indicates a Fixed-Source Hotspot

The Galápagos Islands and seamounts along the Carnegie Ridges (Sinton et al., 1996) show a progression of ages (Figure 5).  Many of the ages fall within the estimate of Gripp & Gordon (1990) of 37 km/Myr for the absolute motion of the Nazca plate.  More of the ages fit the fixed-source hypothesis if 3 million years of activity on an individual volcano is added.  Field study of some Galápagos volcanoes indicate that such long durations of volcanism do take place.  The ages suggest that velocity of the Nazca plate may have decreased at about 9 Ma.

Figure 5

4. Subsidence patterns indicate history of ridge migration over a fixed plume

Figure 6

These plots show the cross sectional areas of the two hotspot traces against the square root of distance from the leading edge of the hotspot.  The decrease in elevation of the Carnegie Ridge is much faster than predicted by conductive cooling models ("Normal Subsidence"). For lithosphere out to about 625 km (17 Ma), this is attributed to the northward drift of the GSC away from hotspot, so progressively more crust is produced on the Nazca plate and less on the Cocos plate.

In contrast, the Cocos ridge shows no regular subsidence.  This is because the GSC has been drifting north, and the hotspot has had a progressively smaller effect on the Cocos plate (i.e. the initial elevation of the Cocos ridge has decreased for the past 17 million years).  The "Cocos Island Event" is thought to have resulted from excess magmatism from an abandoned rift produced by the southward stepping of the GSC (Harpp et al., 2003).

5. Transition-zone thinning and seismic tomography show a mantle plume

S-wave arrival time tomography indicates a steeply-dipping, sharply-bounded slow zone (anomalies of about -2% relative to PREM) extending to at least 400 km depth, beneath Fernandina and Isabela islands in the western archipelago (Toomey et al., 2001). Because the aperature of the array is only about 400 km, larger depths cannot be imaged.

The transition zone is 18 ± 8 km thinner just to the west of the archipelago than the regional average (which is not significantly different from PREM) (Hooft et al., 2003). This corresponds to a temperature anomaly of +130 ± 60 K, and could be explained by a plume that pierces the bottom and top of the transition zone. The thin transition zone coincides precisely with the low-velocity zone in the upper mantle indicated by the tomography.

... and for the alternative view ...

References

  • Sinton, C.W., Christie, D.M. & Duncan, R.A., (1996), Geochronology of Galapagos seamounts, J. Geophys. Res., 101, 13,689-13,700.
  • Gripp, A.E. & Gordon, R.G., (1990), Current plate velocities relative to the hotspots incorporating the NUVEL-1 global plate motion model, Geophys. Res. Lett., 17, 1109-1112.
  • Harpp, K. S., White, W. M. (2001). Tracing a mantle plume; isotopic and trace element variations of Galápagos seamounts. Geochemistry, Geophysics, Geosystems, 2, 46. DOI 2000GC000137.
  • Harpp, K. S.; Fornari, D. J.; Geist, D. J.; Kurz, M. D., Genovesa Submarine Ridge: A manifestation of plume-ridge interaction in the northern Galápagos Islands, (2003), Geochemistry, Geophysics, Geosystems, 4, DOI 10.1029/2003GC000531.
  • Hooft, E.E. & Toomey, D.R., Anomalously thin transition zone beneath the Galápagos Hotspot, Earth Planet. Sci. Lett., in press.
  • Toomey, D.R., Hooft, E.E., Solomon, S.C., James, D.E., and Hall, M.L., (2001), Upper mantle structure beneath the Galápagos archipelago from body wave data, Eos Trans. AGU, 82
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