Deformation of subducted oceanic lithosphere

Abstract

The deformation of subducted oceanic lithosphere is revealed by detailed studies of the distribution of seismicity in Benioff zones and by the increasingly well-resolved images of the upper mantle obtained by seismic P-wave tomography. By using numerical experiments based on a simplified dynamical model of a subducted lithospheric slab, we test the idea that the observed deformation of the subducted slab results from a balance between internal buoyancy forces and the viscous stresses associated with deformation. The simplified model assumes that the lithosphere has uniform physical properties, it is denser and much more viscous than the upper mantle, and its penetration below 670 km depth is resisted by a step-like density increase at that level. The primary determinant of the style of deformation is the buoyancy number, F, a ratio of buoyancy stress generated by the mass anomaly in the slab to the stress associated with viscous deformation at a strain rate U0/L, defined in terms of subduction rate U0 and slab thickness L. For a small buoyancy number, deformation of the slab is dominated by viscous flexure. For large F, down-dip extension of the slab appears to dominate, and a buckling instability of the slab is observed if there is resistance to penetration of the 670 km level. The transition value of F at which this change in behaviour is observed is about 0.05 for constant viscosity, and about 0.2 for stress-dependent viscosity with a stress versus strain-rate exponent of n = 3. With n = 3, a boudinage-type instability of the slab is observed for large F. Penetration of the slab below the 670 km level depends on the density contrast between slab and lower mantle. Resisted only by the density difference, the slab may initially penetrate several hundred kilometres below the 670 km level (for plausible density parameters) before a buckling instability causes this part of the slab to rotate and ascend. Comparison of the deforming slab geometry and stress field with seismicity distribution and tomographic images from the Tonga subduction zone suggests that the effective buoyancy number F for this slab is approximately equal to the transition value that we determined experimentally. We use this constraint to estimate average rheological parameters for the Tonga slab which, when compared with published creep deformation laws for olivine, are consistent with the average temperature of the slab being about 0.4 to 0.45 times the melting temperature, based on constitutive laws for olivine. At shallow depths (<100km) these temperatures correspond to around 590°C for wet olivine or around 700°C for dry olivine.