Lithospheric stress-states predicted from long-term tectonic models: Influence of rheology and possible application to Taiwan

The long-term stress-state of the lithosphere results from a combination of boundary forces, surface loads and tractions from mantle flow. Here, we have employed a numerical technique which allows for the self-consistent treatment of the interaction between the lithosphere and mantle and takes into account the deformation of the free-surface. We have used this to study lithospheric dynamics in order to gain insights on the present-day conditions of stress and deformation of the lithosphere. Our numerical approach takes the observed topography and far-field plate velocities as input and computes the resultant deformation as a function of the rheological stratification. Our model application is inspired by Taiwan tectonics. Stress data and earthquake focal mechanisms indicate that the upper crust in Taiwan may be predominantly under compression. The recent Pingtung earthquake in 12/26/2006, however, suggests that the sub-Moho mantle lithosphere under southern Taiwan may be under extension. If this earthquake indeed represents the local state of stress this raises the question as to why extension and compression can occur simultaneously in this region. For addressing this issue we have performed a number of numerical simulations, in which we studied the effects of rheological stratification, boundary conditions and geometry on the stress-state and lithospheric dynamics. We have employed both layered viscosity models and models in which we use laboratory-based, non-Newtonian and temperature-dependent creep laws. Results show that the rheological stratification plays an important role in lithospheric deformation. The relatively hot lithospheric thermal state in Taiwan makes it likely that part of the lower crust is relatively weak, which results in decoupling from surface-near deformation with deformation at depth. Bending of a subducting slab underneath Taiwan causes an extensional stress-state in the sub-Moho mantle lithosphere whereas the collision of the Philippine Sea plate with the Eurasian margin is responsible for a compressional state of stress in the crust. It is demonstrated that models with a weak lower crust are also in reasonable agreement with available GPS data as well as with constraints on uplift rates from long-term geodynamic modelling studies.