RUSSIAN JOURNAL OF EARTH SCIENCES VOL. 7, ES3001, doi:10.2205/2005ES000179, 2005
[42] The formation and evolution of the continental lithosphere can be easily conceived by analyzing the above modeling results obtained for the evolution of the mantle floating continents system. As seen from Figure 1, two continents in the form of foreign plates were placed onto the mantle at the initial time moment. Later, in the process of evolution (see Figure 2 and Figure 4), significant changes took place in the mantle due to the effect of the continents and, in particular, cold (and thereby highly viscous) stagnant regions arose in the vicinities of the continents. Each of these regions moves together with its continent.
[43] The distribution of the dimensionless temperature is shown in color in Figures 1, 2, 3, and 4.
With an overall temperature difference of about 4000 K, the temperature
shown by green is about 1200 K. Assuming that this value corresponds to the
lower boundary of the lithosphere, the thickness of the continental lithosphere
in such a simplified model should amount to a few hundred kilometers. As noted
above, a higher intensity of thermal convection should result in thinner thermal
boundary layers and, therefore, in a thinner continental lithosphere. Conversion
based on the theory of parametric convection shows that with Ra
10
6 the thickness of the oceanic lithosphere is about 50 km and that of the continental
lithosphere is on the order of 200 km (400 km with roots).
[44] Figure 5 shows mantle flow velocities against the mantle viscosity distribution (blue and red correspond to high and low viscosity values).
[45] The numerical experiments presented in this paper show that the continental lithosphere varies with time in thickness and configuration. The continental lithosphere thickens when a continent moves into a colder mantle region and thins when it is in a hotter region. Since the mantle temperature significantly varies over the length of the continent, the shape of the continental lithosphere should also change. In particular, the lithosphere thins where it overlies an approaching hot plume and deep continental roots develop above anomalously cold mantle regions.
[46] The continental lithosphere is seen most distinctly in Figure 4, showing the situation after 300 Myr. By this time, the continents diverged for a distance of about 10,000 km and an ocean with a system of hot mantle upwellings formed between them, where the mantle heat flux is anomalously high. A cold, highly viscous continental lithosphere grew under each continent. It has an irregular shape, is somewhat thinned above ascending plumes, and has deep roots above descending cold flows. Thus, in terms of a fairly simple model, a continental lithosphere arises in a self-consistent way, without any additional assumptions. A physical interpretation of this phenomenon consists in that continents are permanently drawn by descending mantle flows into the coldest mantle regions. As a result, highly viscous zones arise under the continents and promote the formation of the continental lithosphere (see Animation 3 for more details of the evolution of mantle convection with floating continents and varying viscosity).
[47] Oceanic lithospheric plates resemble ice floes temporarily freezing to a floating body on its sides. The continental lithosphere forms when mantle material is frozen to the floating continent from below. The lower boundary of the lithosphere is defined by the partial melting point of mantle material (about 1200o C). Therefore, the continental roots can be compared to icy outgrowths whose interior is a residue from smelting of the crust.
[48] However, the oceanic lithosphere basically differs from ice. As distinct from floating floes, cooling oceanic lithospheric plates become heavier than the underlying mantle, sink into the mantle, and gradually mix with its material. Likewise, the primary continental lithosphere could have mixed in the mantle. However, due to a low temperature and a high viscosity, mantle flow velocities under a continent are lower than horizontal velocities of oceanic plates. Therefore the continental lithosphere has a chance to be reworked by repeatedly arising plumes. Moreover, it is permanently subjected to metamorphic transformations promoted by fluid flows. As a result, the continental lithosphere becomes lighter and rigid and is "welded'' to the continent. Note also that an older lithosphere is stronger. Possibly, it is a strong lithospheric substrate of large continents that prevents them from being crushed by oceanic plates.
[49] For convenience of analysis, the continental lithosphere can be conventionally subdivided into two parts (although no boundary separates them): the upper older (metamorphized and lighter) part and the lower younger part that differs from the underlying mantle by a higher viscosity (due to a lower temperature). Whereas the older subcrustal continental lithosphere is more stable, the lower younger continental lithosphere permanently varies. In higher temperature regions, it softens and can be carried away by mantle flows. This can account for great variations in the thickness of continental lithosphere observed both between different continents and within a continent.
[50] We should note that the results presented in our paper do not take into account processes of transformations and transportation of chemical elements in the continental crust that lead to its strengthening. A floating continent includes not only the crust but also the oldest strong stable part of the continental lithosphere. Numerical experiments show the origination and evolution of the lower younger continental lithosphere beneath continents. The continental lithosphere first automatically arises as an anomalously cold mantle layer frozen to the base of a continent and moves together with the continent due to an anomalously high viscosity. However, with a temperature drop by about 200 K, the viscosity of the continental lithosphere increases by only two orders. This is insufficient in order to completely inhibit the mixing of the continental lithosphere with mantle flows. In the real Earth, the formation of the continental lithosphere is accompanied by material differentiation.
[51] The mixing of the lithosphere with the mantle is also hindered by buoyancy. The higher viscosity of the cold lithosphere gives it a chance to "dry'' and become strong to such a degree that, along with continental roots, it remains attached to floating continents for a few billion years.
Citation: 2005), Evolution of mantle plumes and uplift of continents during the Pangea breakup, Russ. J. Earth Sci., 7, ES3001, doi:10.2205/2005ES000179.
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