Modeling the evolution of the lower crust with laboratory derived rheological laws under an intraplate strike slip fault

Abstract:

The earth’s crust can be divided into the brittle upper crust and the ductile lower crust based on the deformation mechanism. Observations shows heterogeneities in the lower crust are associated with fault zones.

One of the candidate mechanisms of strain concentration is shear heating in the lower crust, which is considered by theoretical studies for interplate faults [e.g. Thatcher & England 1998, Takeuchi & Fialko 2012].

On the other hand, almost no studies has been done for intraplate faults, which are generally much immature than interplate faults and characterized by their finite lengths and slow displacement rates.

To understand the structural characteristics in the lower crust and its temporal evolution in a geological time scale, we conduct a 2-D numerical experiment on the intraplate strike slip fault.

The lower crust is modeled as a 20km thick viscous layer overlain by rigid upper crust that has a steady relative motion across a vertical strike slip fault. Strain rate in the lower crust is assumed to be a sum of dislocation creep and diffusion creep components, each of which flows the experimental flow laws. The geothermal gradient is assumed to be 25K/km. We have tested different total velocity on the model. For intraplate fault, the total velocity is less than 1mm/yr, and for comparison, we use 30mm/yr for interplate faults.

Results show that at a low slip rate condition, dislocation creep dominates in the shear zone near the intraplate fault’s deeper extension while diffusion creep dominates outside the shear zone. This result is different from the case of interplate faults, where dislocation creep dominates the whole region. Because of the power law effect of dislocation creep, the effective viscosity in the shear zone under intraplate faults is much higher than that under the interplate fault, therefore, shear zone under intraplate faults will have a much higher viscosity and lower shear stress than the intraplate fault. Viscosity contract between inside and outside of the shear zone is smaller under an intraplate situation than in the interplate one, and smaller viscosity difference will result in a wider shear zone.