Asymmetric lithospheric instability facilitated by shear modulus contrast: Implications for shear zones

Viscoplasticity has been considered to be a dominant element in causing the nucleation of shear instability leading to lithospheric weakening. Here, we propose that a simple contrast in shear moduli may be sufficient for explaining the fast timescale asymmetric shear instability in a bimaterial setting. Not much attention has been paid to heterogeneous elasticity in geodynamical modelling because it is dominant only for short timescales. Up to now, no studies have been made on asymmetric shear instability induced by elastic modulus contrast. Thermal-mechanical numerical simulations based on high-resolution (from 0.4 km × 0.4 km to 0.2 km × 0.4 km meshes) finite-element methods were performed to understand the effects of shear modulus contrast on inducing asymmetric instabilities. Strain-rate and stress-dependent rheology are used with a wide range of activation energy 0-850 kJ mol ^-1 for all models. Numerical results with enough shear modulus contrast show asymmetric shear instability, which is generated around the interface and then propagates across the interface. Two parts of the lithosphere with different shear moduli (stiff for higher and soft for lower shear modulus lithospheres), which are simply connected to each other without a pre-defined weak zone, were compressed at a constant rate of 2 cm yr ^-1. Having different shear modulus is justified by chemical heterogeneity of geological minerals and their pressure-temperature dependence. To explore the dynamical effects generated by the contrast in the elastic modulus, the shear modulus of the soft lithosphere is fixed at 32 GPa, whereas that of stiff lithosphere is increased systematically from 32 up to 640 GPa. We also examined the role of activation energy (0-850 kJ mol ^-1) on the geometrical pattern and the initiation time of asymmetric shear localization. The shear modulus contrast has to be close to two for triggering asymmetric shear instability and is found to be by far a more important controlling factor in causing shear instability than activation energy of the creep law. The instability develops rapidly between 250 000 and 500 000 yr after deformation begins, and thermal weakening in the shear zone is greater, when a stronger shear modulus contrast is prescribed. Our work suggests that initiation of lithosphere-scale asymmetric instability would be faster than previous considerations. Our finding stresses that naturally occurring shear modulus contrast has also important impact on many geological problems related to bimaterial instability.