# When plumes tickle continents

Eric Debayle

► To cite this version:

Eric Debayle. When plumes tickle continents. Nature Geoscience, 2018, 11 (3), pp.150-151.
10.1038/s41561-018-0074-z. hal-02327176

HAL Id: hal-02327176

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GEODYNAMICS

# When plumes tickle continents

Continental stability may be linked to a shallow, buoyant mantle layer, and the deepest craton roots can be destabilized and removed by mantle plumes.

Eric Debayle

Cratons form the core of continents. Seismic images¹ show that these old and stable Precambrian cores are underlain by high-velocity roots or keels that can extend to depths greater than 200 km. These keels are probably composed of cold, dry and highly viscous upper mantle rocks²,³. Analysis of peridotite xenoliths, mantle rocks erupted at the surface, indicate that these keels formed early in the history of cratons¹. The great antiquity of keels implies that the deep roots of the continents are stable and have largely been preserved from the erosive influence of mantle convection since their formation. Writing in *Nature Geoscience*, however, Hu et al.⁵ report that the deepest cratonic lithosphere can be removed when perturbed by mantle upwellings, challenging the classical view of stable cratonic roots. They propose that the

long-term stability of cratons is due to the shallower buoyant lithosphere only.

If continental roots are cold, they should in principle be denser than the surrounding mantle at the same depth and sink. However, analysis of xenolith samples reveals that cratonic roots are depleted in dense basaltic components compared to the surrounding mantle. So, the excess density due to cooler temperatures could be cancelled out by the light composition⁶, providing a way to steady the highly viscous and cold but depleted keels beneath cratons over billions of years. This classical view of keel stability has, however, been refined as a number of studies revealed that the lithosphere beneath cratons may be stratified⁷ and the mantle root of many cratons has been removed or modified⁸. If the roots beneath cratons can be destabilized, it is then unclear how

and why cratons remain unchanged for billions of years.

Hu et al.⁵ propose that the answer to this question lies in the buoyancy of the shallow cratonic lithosphere. They use a combination of topography, gravity and seismic data to analyse the structure of cratons in South America and Africa, which once formed the western part of the supercontinent Gondwana. Some of these cratons display widespread high surface topography of 1 km or more. The researchers demonstrate that the high topography is supported by buoyant lithospheric heterogeneities: the crust is too thin to support the high topography, and the dynamic topography created by the convective sub-lithospheric mantle extends beneath both elevated and lower areas, and thus cannot be the cause either.

<figure><img src="image_2.png" /></figure>

**Fig. 1** | Schematic illustration of the present-day lithosphere structure for South American and African cratons. Hu et al.⁵ use a variety of data sets to argue that the stability of these cratons relies on the shallow and buoyant upper lithosphere. They suggest that the dense roots (blue) of some cratons and eventually some parts of the lower lithosphere were removed by Cenozoic mantle plumes and replaced with more buoyant mantle. The delaminated roots sank and became trapped in the mantle transition zone. The lost cratonic roots are then restored to their original positions (green layer). Patterns of Cenozoic mantle flow (seismic anisotropy, dark half-barbed arrows) are frozen in the restored lithosphere and in some parts of the lower lithosphere. Figure adapted from ref. ⁵, Macmillan Publishers Ltd.
Additionally, xenolith data show that temperature variations are only moderate at less than 150 K, so thermal buoyancy cannot be responsible for the raised topography. Instead, the uplift must be sustained by compositional heterogeneities in the shallow lithosphere.

Seismic images reveal anomalous regions in the mantle transition zone, where seismic waves travel unusually fast. Hu and colleagues propose that these anomalies represent remnants of cratonic roots that have been removed by mantle plume activity and then sunk into the mantle. Sedimentary deposits constrain the timing of craton uplift to the Cretaceous, so the researchers reconstruct the trajectory of southern Atlantic hotspots, thought to be caused by mantle plumes, back to the Early Cretaceous. They find that Cretaceous-aged hotspot tracks indeed coincide with regions of high topography in both South America and Africa. This correlation supports the idea that Cretaceous mantle plumes could have triggered delamination of a high-density layer, enriched in garnet-peridotite at the base of the cratonic lithosphere, at depths greater than 200 km. If correct, tomographic and heat flow data imply that the thermal perturbation caused by plume-lithosphere interaction during the Cretaceous has now largely disappeared.

The authors demonstrate that delaminated cratonic roots can be thermally restored back to their original positions over a period of 90 million years (Fig. 1). Importantly, the restored lithosphere has

lower density than the original delaminated lithosphere, so that cratons that were once delaminated have increased their buoyancy and maintained high topography. Patterns of seismic anisotropy in the restored cratonic roots align fairly well with the Cenozoic patterns of mantle flow in the lower lithosphere beneath the high-topography cratons, adding support to the authors' model of root delamination and Cenozoic restoration.

The multiple high-velocity anomalies observed in the mantle transition zone were previously interpreted as small-scale mantle convection at the edge of cratons⁹, but it now seems the anomalies might instead be delaminated parts of cratonic roots. The anomalies do not perturb the patterns of seismic anisotropy, so they are probably neutrally buoyant and stay trapped in the transition zone.

The variety of observations used in the research each has their own limitations (one may, for example, argue that the interpretation of anisotropy is at the limit of current models resolution). However, taken together the observations provide a set of constraints for a novel scenario of cratonic evolution. It is important to now test the proposed scenario. Mapping simultaneously seismic attenuation and velocity may help disentangle the thermal or compositional origin of lithospheric heterogeneities, and therefore distinguish intact from restored lithosphere. We must also check whether this model applies elsewhere. A number of observations

suggest that cratons affected by subduction have experienced a different story.

Hu et al.⁵ propose that plume-induced delamination can destabilize a high-density layer located at the base of cratons and eventually some parts of the lower lithosphere. The buoyancy of the shallower lithosphere ensures the long-term stability of cratons, whereas the delaminated root would be thermally restored in a relatively short period of time. This view of cratonic evolution may explain how cratonic cores remain stable in the light of an emerging picture of remarkably layered and complex cratonic roots.

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### Eric Debayle

Laboratoire de Géologie de Lyon: Terre, Planètes, Environnement, Université Lyon 1, Ecole Normale Supérieure de Lyon and CNRS, Villeurbanne, France. e-mail: Eric.Debayle@ens-lyon.fr

Published online: 19 February 2018
https://doi.org/10.1038/s41561-018-0074-z

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