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Initiation of Rayleigh–Taylor Instabilities in Intra-Cratonic Settings

Weronika Gorczykaa, Bruce Hobbsa,b, Taras Geryac

aUniversity of Western Australia (UWA), Australia

bAustralian Commonwealth Scientific and Research Organisation (CSIRO), Australia

cSwiss Federal Institute of Technology Zürich (ETHZ), Switzerland

weronika.gorczk@uwa.edu.au, bruce.hobbs@csiro.au, taras.gerya@erdw.ethz.ch

 


This webpage is a summary of Gorczyk, Weronika, Bruce Hobbs, Taras Gerya, Initiation of Rayleigh–Taylor instabilities in intra-cratonic settings, Tectonophysics, 514-517, 146-155, 2012.


 

Summary

Delamination of the continental mantle lithosphere can initiate as a result of:

  1. Mechanical shortening and thickening of the lithosphere (Houseman & Molnar 1997; Conrad 2000);
  2. Density variations between the lithosphere and asthenosphere, resulting from thermal contraction (Jull & Kelemen 2001), or density increases as a result of metamorphic processes (Elkins-Tanton 2007); and
  3. Combinations of the above processes coupled with lateral variations in the plastic strength of the lithosphere.

Numerical technique

The work described in this webpage was done using the software I2ELVIS developed by Taras Gerya (Gerya & Yuen, 2003; Gerya & Yuen, 2007). I2ELVIS is based on finite differences schemes and marker-in-cell techniques combined with a multigrid approach. The code is designed to model the behaviour of rocks with realistic complex elasto-visco-plastic rheology, and toaccount for changes in topography due to erosion-sedimentation processes and changes in the physical properties of the rocks due to phase transformations.

The Petrological-Thermochemical Mode

Mineral phase transformations, such as dehydration reactions and melting, can affect the physical properties of rocks during tectonic processes. The petrological-thermomechanical numerical modeling approach incorporates, with all in situ rock properties, effective density, isobaric heat capacity, thermal expansion, and adiabatic and latent heating as well as equilibrium water and melt content. All these rock-, melt- and fluid-properties are calculated for the Lagrangian rock markers at every time step, based on Gibbs free energy minimization (Connolly, 2005) as a function of the local pressure, temperature and rock composition. In particular, the in situ rock density is interpolated for every marker at each time step from look-up density tables (in P-T space) precomputed with the PERPLE_X program for four rock compositions. To simulate the migration of water released by dehydration process, we use independently moving rock and fluid markers (Gorczyk et al., 2007). A fluid marker corresponding to a particular water amount is generated and moves upward until it reaches a lithology that assimilates water, and can account for water transport.

How can Rayleigh–Taylor instabilities be triggered in intra-plate settings?

Studies of mantle lithosphere underneath old continents (O'Reilly & Griffin 1996; Begg et al. 2009; Griffin et al. 2009) indicate that the SCLMs underneath these continents are composed of blocks with different thermal and compositional characteristics. This is probably a result of strong depletion or extensive metasomatism during tectonic processes such as subduction and/or accretion. Figure 1 illustrates the state of continental crust after 20 Ma of compression at a rate of 2 cm/a.

Modeling program runs with a homogenous lithosphere do not develop any strain localization or mechanical thickening. In contrast, program runs with a weak zone introduced (which may have been inherited from the amalgamation of lithospheric blocks, or may be a post-collisional feature) lead to immediate localization of strain and thickening of the lithosphere. Rayleigh-Taylor instabilities may develop later as a consequence.

 

Figure 1. Results of four models with different initial geometries: (a) initial geometry and bulk composition, (b) second strain rate invariant after 20 Ma of compression at rate of 2 cm/a, and (c) temperature distribution after 20 Ma.

 

Topographic response to the classical Rayleigh-Taylor instability

The ductile detachment of a portion of the mantle lithosphere (a Rayleigh-Taylor instability) may result in opposing topographic responses arising from (1) intra-continental orogeny, and (2) intra-continental basin formation, as follows.

  1. Surface deflection above the dripping “blob” results from lithospheric extension after detachment of the downwelling, as has been described by a number of authors (McKenzie, 1977; Houseman & Molnar, 1997; Conrad, 2000; Elkins-Tanton & Hager, 2000; Pysklywec & Beaumont, 2004; Le Pourhiet et al., 2006; Elkins-Tanton, 2007).
  2. Intra-continental orogeny above delaminating mantle lithosphere occurs not only because the mantle lithosphere is thickened, but also because of buoyant continental crust  (Neil & Houseman 1999).

In the numerical experiments illustrated in Figures 2 & 3, both of the above phenomena are confirmed. In the models, topography is influenced by the thickness of the continental crust above the perturbation. In cases where the initial perturbation is only imposed on the mantle lithosphere (Figure 2, column a), the sinking of the perturbation depresses surface topography at the axis of symmetry, resulting in basin subsidence directly above the instability. This effect is illustrated in Figure 3a where the initial depression of surface topography is followed by long-lasting relaxation, leading to extension within the lithosphere after delamination.

Conversely, when an initial thickness perturbation (a crustal root) is also introduced into the less-dense continental crust (a continental root), the topographic response differs drastically (Figures 2b & 3b). The dynamic evolution of the topography can be described in three stages:

  1. The initial stage is characterized by uplift (250 m) at the axis of the perturbation as a result of isostatic equilibration within the crust;
  2. In the second stage, strong subsidence (100 m) approximately centred on the axis of symmetry occurs, which leads to the formation of basins on the sides of the elevated plateau above the instability. At the same time, within the plateau area, subtle subsidence takes place in response to downward displacements arising from the dripping blob;
  3. After detachment of the downwelling material, isostatic rebound of the mantle lithosphere takes place. During this final stage, the base of the continental root heats up, causing melting of the lower continental crust. This crustal melting triggers igneous intrusions, volcanic activity at the surface and the formation of mountain belts. After the main uplift in the axial area, further subsidence (~ 2 km) takes place, expressed as the development of basins on the sides of the plateau.

 

Figure 2. Dynamic evolution of the lithosphere in two simulations representing different initial geometries: (a) Initial strength perturbation imposed only on the mantle lithosphere, down to 160 km. The initial thickness of the continent is kept constant across the model. (b) Initial thickness applied to the mantle lithosphere, down to 160 km, together with a continental root reaching down 80 km. Each time-frame is illustrated by two figures: (i) the upper image represents the second strain rate invariant) with σ1 v.s σ3 as black crosses, long axis corresponds to σ3 and short σ1; (ii) the lower image shows the composition profile of deformed lithosphere after lapse of indicated time period.

 

Figure 3. Evolution of topography above the developing instability corresponding to the program runs shown in Figure 2: (a) Program run with initial strength perturbation imposed only on the mantle lithosphere; and (b) program run with the initial strength perturbation imposed on the whole lithosphere but with an additional continental root. The initial peaks in topography on the sides of instability occur as a result of initial equilibration of the topography. Later in the simulation, peaks form above the downwelling, corresponding to intrusion of magma into the crust, in addition to mountain-building processes resulting from the deep lithospheric detachment. Click here or on figure for enlargement.

 

Melting and the Rayleigh–Taylor instability

Melting is triggered during delamination in the following ways.

  1. Delamination and sinking of large portions of lower continental crust allow the influx of asthenosphere from the low velocity zone. This produces the melts that form the source of continental flood basalts (Lustrino, 2005).
  2. Sinking instabilities may devolatilize (similar to a descending slab in a subduction zone), and may themselves also melt, or they may carry volatiles to depth, depending upon their sinking rate (Elkins-Tanton, 2007).
  3. Crystalline plasticity could provide a mechanism for narrow zones of thinning and upwelling. This could facilitate decompression, and related volcanism far from plate boundaries (Harig et al., 2010).

In all three of the above models, melting of the mantle lithosphere arises from decompression melting, lowering of the mantle solidus by the introduction of fluids from hydrated and detached material, or hydrous heterogeneities that remain long after subduction. Although the numerical simulations shown in Figure 2b incorporate melting processes, no melting of mantle lithosphere/asthenosphere occurs as a response to delamitation, as dry olivine rheology for the mantle is used. Thus, no hydrous melting is possible. In addition, no decompression melting of mantle lithosphere and no wholesale detachment of continental lithosphere occur. On the other hand, the simulations described here suggest that after the detachment and sinking of cold, dense materials, elastic rebound of the remaining lithosphere occurs. In response, the Moho shallows, pressure decreases, and temperature increases in the previously thickened part of the crust. In response, melting of lower crust at the base of Moho is expected. This may result in additional, extensive volcanic activity as well as extensive granitic intrusions, depending on the bulk composition of the lower crust. Dry olivine rheology for the mantle is used, so no hydrous melting is possible. In addition, no decompression melting of mantle lithosphere and no wholesale detachment of continental lithosphere occur. On the other hand, the simulations described here suggest that after the detachment and sinking of cold, dense materials, elastic rebound of the remaining lithosphere occurs. In response, the Moho shallows, pressure decreases, and temperature increases in the previously thickened part of the crust. In response, melting of lower crust at the base of Moho is expected. This may result in additional, extensive volcanic activity as well as extensive granitic intrusions, depending on the bulk composition of the lower crust.

References

last updated 6th February, 2012

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