Dominant Creep Mechanism and the Descending Lithosphere

Abstract

Summary The descending of the lithospheric slab into the asthenosphere is governed by gravitational body forces generated by thermal volume contraction and by phase boundary elevation in the colder slab. The dynamics of sinking depends on the rate controlling deformation mechanism in the lithospheric slab and in the adjacent mantle; it is a function of the thermal conditions, the state of shear stress, the grain size, and the elastic moduli. A finite element initial strain, incremental procedure has been used to model the sinking of the lithospheric slab into the asthenosphere. Both diffusional flow and power law creep have been incorporated as dominant deformation mechanisms separately. Computed principal stresses and maximum shear stresses are in good agreement with observations of seismicity and focal mechanisms in those models which involved power law creep mechanisms with a power of n= 3. Similarly, computed creep displacements and velocities support the view that the power law creep mechanism is the dominant mechanism in the mantle region of slab subduction. The resistance of the mantle to a slab sinking with a velocity of 8 cm/yr requires a stress-strain rate equation equivalent to the one based on experimental and theoretical results, involving constants, however, which render strain rates five orders of magnitude smaller than those in experiments. A corresponding strain rate map of the upper mantle based on the computed shear stress suggests a maximum rate of 3 × 10−14s−1 in the asthenosphere and an average rate of 10−19s−1 in the mesosphere, both taken at 100 bars shear stress. Translation into effective viscosities leads to a reasonable agreement with viscosities derived from glacio-isostatic rebound data.