Numerical simulations of deformation and dynamics of horizontally lying slabs

Abstract

SUMMARY Recent results of seismic tomography have revealed that configurations of subducted slabs are varied; some lie horizontally on the boundary between the upper and the lower mantle, and others penetrate into the lower mantle. In this study, to understand the physical mechanism of deformation and dynamics of subducted slabs in the mantle, a 2-D numerical model was constructed. For this purpose, the slabs were treated so as to move freely in an incompressible fluid model by removing a guide right after its bending at a depth of 660 km and at a depth of 400 km in the upper mantle. Incorporating density and viscosity jumps and a Clapeyron slope for a γ → Pv + Mw phase transformation of olivine obtained from high-pressure and hightemperature experiments at a depth of 660 km into the model, we investigate temperature and velocity distributions, state of stress and viscosity distribution in relation to the formation of horizontally lying slabs at a depth of 660 km. When the slabs are treated to move freely just after their bending at the 660 km phase boundary, the slabs temporarily appear to lie horizontally on the phase boundary. However, the slabs penetrate the phase boundary and eventually sink into the lower mantle. The length of the horizontally lying portion of the slab becomes longer as descending velocity increases; the lengths are about 1500 km, 1200 km, and 800 km for subducting velocities of 8 cm yr −1 , 4 cm yr −1 ,a nd 2c m yr −1 , respectively. When a viscosity jump of 10 to 100 times the viscosity for the upper mantle is given at the 660 km phase boundary, the length of the horizontally lying portion of the slab becomes shorter because of friction between the slab and the underlying lower mantle. For all the models in which the guide was removed at a depth of 400 km, the slabs penetrate into the lower mantle without lying horizontally on the phase boundary. These results indicate that it is difficult for the slabs to bend at the γ → Pv + Mw phase boundary, and to form the configuration of the horizontally lying slabs even if the slabs descend with a small dip angle. The calculated maximum principal compressive stress axes are oriented horizontally within the horizontally lying portion of the slabs, which is inconsistent with seismologically observed vertical directions of the P-axes there. In order to explain the observed configurations and state of stress within the horizontally lying slabs, it would be necessary to investigate the effects of a complicated viscosity and composition structure in the mantle in more detail.