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Modeling Plume Head - Continental Lithosphere Interaction Using a Tectonically Realistic Lithosphere

E. Burov1 & L. Guillou-Frottier2

1deceased

2Service des Ressources Minérales, Bureau de Recherches Géologiques et Minières, Orléans, France; l.guillou-frottier@brgm.fr

Introduction

Current debates on the existence of mantle plumes originate substantially from the interpretations of supposed signatures of plume-induced surface topography that are compared with the predictions of conventional fluid-dynamic models of plume-lithosphere interactions. However, conventional models are poorly suited to predict surface evolution, specifically in case of continental lithosphere. In general, the boundary conditions imposed in such models correspond to a fixed upper surface, and the lithosphere is represented by a simple viscous mono-layer with a rigid top. More realistically, the plate surface is free and its deformation is controlled by the elastic–brittle–ductile rheological properties and the contrasting layered structure of thick continental lithosphere. Effective surface topographical evolution computed from conventional models thus provides only a rough estimate based on simple isostatic assumptions (Figure 1).

Figure 1. Sketches of plume-lithosphere interactions: (a) a “conventional” plume impinges on a single-layer viscous lithosphere resulting in a single, large-wavelength topographic signature ; (b) when realistic lithosphere rheology (brittle – elastic –ductile) and multi-layer structure are considered, several wavelengths of surface topographic undulations are expected.

Modelling

In order to make more meaningful comparisions of observations and models, we introduce a thermo-mechanical model of plume-lithosphere interaction that incorporates “realistic” continental lithosphere. This model includes: (1) a natural free-surface boundary condition, (2) an explicit elastic–viscous–plastic (EVP) rheology, and (3) a stratified continental lithosphere structure.

Our numerical experiments assume a non-Newtonian mantle plume that initiated at a depth of 650 km. The plume rises to the base of a stratified brittle–elastic–ductile continental lithosphere. The vertical rheological profile and the structure of the lithosphere are varied from “young juvenile” lithosphere to “old, craton-like” lithosphere. A separate set of experiments was conducted to test the influence of lateral variations in lithospheric structure such as inherited blocks, cratons and intraplate boundaries.

The surface amplitude and wavelength predicted by our experiments differ from the predictions of conventional models (Figure 2):

  • the amplitudes of the predicted surface uplift are reduced by as much as 50%;
  • the initial largest wavelength of uplift is 20-30% greater;
  • there are additionally prominent secondary harmonics of surface deformation that correspond to small “tectonic” wavelengths (from 50-100 km to 300-500 km).

The thick continental lithosphere thus works as a frequency modulator of the plume impact. Due to its flexural resistance it increases the largest wavelengths produced by the plume head and generates smaller surface wavelengths associated with crust-mantle decoupling and intraplate instabilities. Attenuation of long surface wavelengths (> 500 km) occurs in the case where weak, ductile lower crust separates the upper crust from the mantle and accommodates the mantle domal uplift produced by the plume. The initial surface reaction to the plume ascent is a regional domal uplift. However, in most cases it is followed by surface subsidence (Figure 2) resulting from differential crustal thinning produced both by the mantle uplift and by gravity-driven flow in the lower crust. The mantle uplift may be rapidly suppressed by negative Rayleigh-Taylor instabilities that develop in the mantle lithosphere. For these reasons, plumes may not always produce detectable large-scale topography. Instead, they produce easily detectable alternating, small- or medium-scale features normally attributed to regional tectonics. Large-scale, mono-harmonic deformation, predicted by conventional models, develops only in the case of strong oceanic or very cold, thick continental lithosphere.

 

Figure 2. Results of numerical experiments on plume-lithosphere interaction beneath a “realistic” continental lithosphere. The plume Rayleigh number is 106. Surface topography shows three types of signature at different phases of evolution. A single topography wavelength is only observed when the plume head is still en route to the bottom of the lithosphere. When the plume head impinges on the base of the lithosphere, surface subsidence is observed above the center of the plume head, as well as above each of its edges (case at 2.4 Myr). The final stages show more irregular topographic signatures, while the initial plume head is flattened and secondary plumes have ascended. Click here for enlargement.

Results

As can be seen, the important differences from the predictions of conventional models are related to plate bending, mechanical decoupling of crustal and mantle layers and tension-compression instabilities that produce transient topographic signatures such as uplift and subsidence at a large (> 500 km) and small scales (300-400 km, 200-300 km and 50-100 km). Four main differences from the conventional viscous models are noteworthy (Figure 2):

  1. the appearance of additional short- and medium-intermediate wavelengths of deformation (alternating basins and uplifts), which do not necessarily correlate with the size of the plume head;
  2. poly-phase deformation related to visco-elastic relaxation and progressive reduction of the resistance of lithospheric layers as the plume head flattens horizontally;
  3. reduced impact of plume ascent on the amplitude of surface elevations; and
  4. a concentration of plume-related extension in the mantle lithosphere, with little effect on the decoupled crust; in the case of strong mechanical decoupling the crust may not even “feel” the plume.

Experiments also suggest that the plate surface reacts to the plume ascent well before its final emplacement.

The basin-scale uplifts and subsidences result from several processes:

  • interplay between the rheological layers within the lithosphere;
  • visco-elastic relaxation (Maxwell time for the lithosphere may reach 10 Myr);
  • plume-induced Rayleigh-Taylor instabilities in the mantle.

These small-scale undulations are superimposed on lithospheric-scale wavelengths, as it can be verified in east Africa (Figure 3a), where an underlying mantle plume would behave differently in the northern and southern parts (the Main Ethiopian rift and the Tanzania areas). Spectral analyses of surface topography in east Africa show two dominant wavelengths (at 200-400 km and 30-80 km), reflecting lithospheric and crustal scale instabilities. Similar analyses on numerical topography confirm this bimodal character (Figure 3b).

a) Data profiles

b) Model predictions

Figure 3. Comparison of a) real topography and amplitude spectra profiles across the Ethiopian rift and Tanzanian craton (Africa), with b) topography and amplitude spectra predicted by the model. The model reproduces the same three characteristic wavelengths (approx. 300, 100 and 50 km) as observed.

The instabilities that develop at the top and the sides of the plume head provide a mechanism for crustal delamination. In the case of a plume rising below “normal” lithosphere bounding a craton, there is important lateral flow of material from the plume head to the base of the craton. This suggests a new mechanism for crustal growth, in which surface magmatism is not required. Lithospheric faulting at the craton edges and the enhanced magmatic activity can explain such geological events as apparent plume-related metallogenic crises in Archeen West Africa and Australia.

The continental Moho presents a strong density and rheology barrier to upward propagation of plume material. The role of the competent mechanical “core” of the crust may be as important as that of the mantle lithosphere, particularly for early Archean lithosphere or lithosphere younger than 300 Ma. Thus, crustal deformation interferes with mantle deformation resulting in complex periodic subsidence/uplift patterns. Surface evolution often begins with a central uplift, followed by subsidence accompanied by formation of series of smaller basins.

Closing remarks

The new EVP model infers that the plume model can explain complex “tectonic” processes such as polyphase rifting, lower-crustal delamination, large-scale faulting, crustal growth, etc. Distinct topographic wavelengths or temporally spaced events observed, for example, in the East African Rift system, as well as over the French Massif Central and the Pannonian-Carpathian system (Ed: see also Carpathians page), can be explained by a single plume impinging at the base of the continental lithosphere, i.e. without the necessity of evoking any complex asthenospheric upwelling or additional tectonic events. Plume-lithosphere interactions can explain a number of other key phenomena such as the simultaneous occurrence of a climax of extension of young plates/segments and a climax of compression in surrounding belts. This scenario is consistent with differential mantle lithosphere thinning of the younger plate and is compatible with inferences from seismic tomography.

Reference

This page is a summary of work described in detail in:

See also:

last updated 7th September, 2005
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