The Relation Between Tractions and Strain Rate at the Base of the Lithosphere: Key to Understanding Cratonic Stability
Abstract
Archean cratons are the oldest regions on Earth and have undergone little deformation since their formation. Their stability has remained an outstanding question in the geodynamics community. In this study, we try to understand how horizontal tractions and strain rates generated by mantle flow could be related to the stability of cratons. We compute tractions and strain rates at the base of the lithosphere and observe how uniquely cratons deform under the convective stresses exerted by the mantle. We use the finite element code CitcomS to solve the mantle convection equations and calculate tractions and strain rates at the base of the lithosphere. The flow in our models is driven by density anomalies within the mantle and is modulated by viscosity variations, both radial and lateral. Lateral viscosity variations are introduced by implementing temperature dependent viscosity above 300 km depth. Moreover, we introduce neutrally buoyant, thick (300 km) and highly viscous cratons in our models. We test different asthenosphere viscosities (0.01-1 times the viscosity of the upper mantle) as well as different craton viscosities (10-1000 times more viscous than the ambient layer). We find that, despite higher tractions under these thick cratons, deformation is slowest beneath them. On the other hand, thin and weak mid-oceanic ridges deform much faster despite very low traction amplitudes underneath them. This inverse relation between tractions and strain rate is obtained because the effective viscosity of the lithosphere, which resists deformation due to convective shearing, increases almost linearly with the square of the thickness of the lithosphere. This increment of effective viscosity is due to the free slip boundary condition in our models, which mimics the plate tectonic response of the Earth and allows stress transmission throughout the strong lithosphere without small scale local deformation. It is probably the cratons' enhanced thickness, aided by their higher effective viscosity, that has primarily resisted deformation due to mantle shear, stabilizing them since the Archean.
- Publication:
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AGU Fall Meeting Abstracts
- Pub Date:
- December 2018
- Bibcode:
- 2018AGUFMDI51B0020P
- Keywords:
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- 1031 Subduction zone processes;
- GEOCHEMISTRYDE: 8031 Rheology: crust and lithosphere;
- STRUCTURAL GEOLOGYDE: 8120 Dynamics of lithosphere and mantle: general;
- TECTONOPHYSICSDE: 8124 Earth's interior: composition and state;
- TECTONOPHYSICS