Understanding the dynamics and evolution of cratons

Abstract

The earth is the only rocky planet in the solar system that exhibits plate tectonics. One of the basic tenets of plate-tectonics is that it recycles the lithosphere within a few hundred million years. No oceanic lithosphere, which actively participates in the plate-tectonic process, is more than 250 million years old. Even most continental lithosphere age peaks around 1.5 billion years. However, several geochemical studies indicate that some parts of the continental lithosphere existing today are more than 3 billion years old (Ga). Such rock records are not more than 5% of the total volume of the earth. Further studies have shown that by 3 Ga, around 65-70% of the present-day continental lithosphere was already formed, of which most has been destroyed during the earth's geodynamic evolution. This 5% of the oldest rock record of the earth's lithosphere, known as cratons, remains controversial for their long-term survival. The reasons behind cratons’ stability have been considered as one of the grand challenges in geodynamics.

In my PhD thesis, I have developed 3-D spherical earth-like models to understand the dynamics and evolution of the cratons to investigate the rationale of their prolonged survival for more than ~3 Ga, which is unusual for any other non-cratonic lithosphere. I have constructed both instantaneous and time-dependent numerical models to quantify the stresses and strain-rates originating from density-driven mantle convection. These convective stresses control deformation. Hence, at first, I have calculated how stresses and stain-rates vary with lithospheric thickness and viscosity. Results show that cratons being highly viscous and thicker than the average lithosphere, can resist the deformation by convective stresses. I attribute large thickness and high viscosity as the primary reasons for cratons' long-term survival. From these instantaneous models I estimate possible combinations of craton and asthenosphere viscosity that could support cratons' long-term survival. To verify this estimate of viscosity, I also develop time-dependent mantle convection models. Here I reconstruct the present-day location of cratons till 409 Ma and drive mantle convection from 409 Ma to the present-day using reconstructed plate velocities as boundary condition. I obtain similar results as in the instantaneous models, i.e., a craton needs to be at least 100 times more viscous than the surroundings, and the asthenosphere should not be weaker than 10^20 Pa-s in order to support cratons' long-term survival.

However, all cratons do not remain immortal. In certain geological scenarios, they may get partially or fully destroyed. Recent discovery of the mid-lithospheric discontinuity (MLD) underneath most cratons has shown that cratons may get delaminated along weaker MLDs. I test this hypothesis in 3-D spherical instantaneous models. I find that, indeed, in the presence of a weak mid-lithospheric discontinuity, strain-rates increase up to 40 times within cratons, which can make them more prone to delamination. Also, I attempt to resolve the controversy regarding the viscosity of MLDs using SHmax directions predicted from my models. Apart from MLDs, plume-induced weakening can also be an important reason for craton destruction. The eruption of the Reunion plume at 65 Ma underneath the Indian craton could have made the craton weak and thin. I investigate this idea in 3-D spherical time-dependent models. I show that the plume-induced weakening could have reduced the thickness of the Indian craton if viscosity is strongly temperature dependent. I find up to ~130 km of reduction of the Indian cratonic root in one of my models. Moreover, my results suggest that plume could lubricate the lithosphere-asthenosphere boundary. This lubrication could be a key factor for the acceleration of the Indian plate since 65 Ma.