Earth science

The slippery base of a tectonic plate

High-resolution imaging of the base of the Pacific plate as it descends beneath New Zealand discloses a 10-kilometre-thick channel that decouples the plate from underlying upper mantle. See Letter p.85

In the theory of plate tectonics, the outer shell of the Earth, known as the lithosphere, consists of several rigid plates, which move relative to each other over the weaker, flowing asthenosphere. The bottom of the lithosphere, the lithosphere–asthenosphere boundary (LAB), is fundamental to our understanding of how plate tectonics works, although an exact understanding of the mechanism that gives the plates their rigidity and defines their thickness remains elusive and widely debated. On page 85 of this issue, Stern et al.1 describe how they have used reflected seismic waves generated by explosive sources in steel-cased boreholes to image the Pacific plate as it descends beneath New Zealand. They find a LAB that is less than 1 kilometre thick at the top of a 10-km-thick channel, in which slow seismic velocities may require the presence of water or melt (Fig. 1). The authors suggest that the thin channel decouples the lithosphere from the asthenosphere and allows plate tectonics to take place. The existence of such a localized channel probably has implications for the driving forces of plate tectonics and mantle dynamics.

Figure 1: A model of the subducting Pacific plate.
figure1

Stern et al.1 used seismic waves that travelled deep into the Earth and were reflected back to the surface to image the Pacific plate as it subducts beneath New Zealand. The seismic data reveal a plate that is 73 kilometres thick, significantly less than that predicted by conventional conductive-cooling models of the oceanic lithosphere (dashed line). The base of the plate, the lithosphere–asthenosphere boundary (LAB), is sharp (less than 1 km thick) and is underlain by a 10-km-thick water- or melt-rich channel that decouples the plate from the underlying flowing asthenosphere. The extent to which the channel continues beneath the ocean plate and/or varies in depth has yet to be determined, as indicated by the question mark. The arrows show the directions of motion in the different Earth layers. The plate may be relatively dehydrated, possibly as a result of melting beneath the mid-ocean ridge. The depth extent of the melt triangle would determine the thickness of the dehydrated layer (dotted line). Melt or water may travel to the channel from greater depths. The mechanism by which a 10-km-thick channel might form is unclear.

Plate tectonics has been a fundamental tenet of Earth science for almost 50 years. It is the foundation of modern Earth science, and provides a framework for our understanding of the formation of the continents and ocean basins, and the evolution of the planet. However, questions remain, such as, where is the base of a plate and what makes a plate 'plate-like'? There are many proxies used to estimate the depth and nature of the base of tectonic plates, but so far no consensus has been reached. The transition from the rigid lithosphere to the flowing asthenosphere has classically been defined by temperature. Temperature has a large effect on the viscosity of rocks — their ability to flow.

If temperature alone were the sole mechanism governing the definition of the plate, then we would expect a gradual transition from the lithosphere to the asthenosphere. However, in the past decade the high resolution provided by imaging techniques based on measurements of seismic waves scattered from the base of the lithosphere has revealed that the transition from the lithosphere to the asthenosphere is sharp2. This suggests that another mechanism such as the presence of water or melt must exist in the asthenosphere, which would weaken it3,4, and thus necessarily define the LAB2.

Stern et al. used data from explosive-source seismic waves that travelled deep into the Earth and were reflected back to the surface, where they were recorded by seismometers in New Zealand. The seismic waves vibrated at high frequency, allowing the authors to image the low-seismic-velocity channel and also to deduce the thickness of the LAB. The deduced LAB thickness is one of the tightest constraints so far on the transition from the lithosphere to the asthenosphere. Similarly significant is the reported thickness of the 120-million-year-old Pacific plate at 73 ± 1 km, much thinner than predicted by the classic thermal model of conductive cooling of the oceanic lithosphere with time5. Finally, an increase in seismic-wave velocity about 10 km deeper and parallel to the LAB is interpreted as the base of a decoupling channel.

If the deduced LAB represents the base of the plate, the plate's thinness may explain the enigmatic observed lack of subsidence for sea floor that is more than 70 million years old6. However, whether or not it is the plate base depends on the mechanism responsible for the authors' observations. A seismic-velocity discontinuity imaged beneath the Pacific plate at similar depths is sometimes interpreted as anisotropic fabric7,8,9,10,11 — the directional dependence of seismic-wave velocity. A purely anisotropic interpretation for the observed seismic-velocity discontinuity would not necessarily equate it with the LAB because anisotropic fabric could be frozen into the plate from a previous episode of deformation. Although not impossible here, an exotic anisotropic fabric would probably be required that may not be consistent with typical notions of horizontal fast directions in the lithosphere (see, for example, ref. 12).

Therefore, the authors are left with the hypothesis that water or melt is present in the channel, which would weaken the mantle3,4 and define the base of the plate2. An increase in hydration13 with depth could be related to the shallow dehydration that occurs during plate formation at a mid-ocean ridge14 (Fig. 1), whereas melt could be caused by complex mantle flow from subduction tectonics and/or melt ponding15. Further investigation is needed to find the origin of any existing melt, because normal oceanic lithosphere is predicted to be cold at a depth of 73 km and so is not necessarily predicted to melt. In this case, a steady supply of melt from greater depths in the mantle to the base of the plate would be required, given that the melt might travel up along the base of the plate.

The very existence of the channel itself is more of an enigma. How and why channelization would occur over a 10-km depth range is not known. Perhaps water availability from phase transformations16 or melt ponding15 occurs over a certain depth range. It could be specific to locations at which plate motions deviate from mantle flow, as is the case off both New Zealand and Costa Rica17, where a similar channel was reported18. Overall, channels offer an explanation for some of the elusive nature of the LAB. Narrow channels would be nearly imperceptible in seismic imaging methods that rely on low-frequency waves, which might explain intermittent and discrepant LAB detection among methods10. For a full understanding of such channels, we need better constraints on where they exist.

However, global channel imaging may prove difficult. Anisotropy is probably important at the LAB, and may bias results if it is not properly considered. In addition, high seismic-wave frequencies are needed to distinguish fine-scale channels, although studies such as those of Stern and colleagues are not feasible at a global scale. Finally, what are the implications of these channels for the coupling of the plates to the underlying asthenosphere and the driving forces of plate tectonics? Tackling these questions will require incorporating tight seismic constraints with laboratory experiments and geodynamical modelling.Footnote 1

Notes

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Correspondence to Catherine A. Rychert.

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Rychert, C. The slippery base of a tectonic plate. Nature 518, 39–40 (2015). https://doi.org/10.1038/518039a

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