Distributed deformation in the subducting lithosphere at Tonga

Abstract

SUMMARY In this paper we attempt to apply techniques that have recently been developed to describe distributed deformation on the continents to distributed deformation in subducting lithosphere slabs. We chose a part of the Tonga slab for this study because it has a simple, approximately planar, shape and high seismicity. We then used the spatial distribution of seismic strain rates, based on earthquake centroid-moment-tensor solutions in the interval 1977-1994, to recover a velocity field that describes the seismic deformation in the plane of the slab below a depth of 100 km. Between 100 and ˜450 km depth the seismic deformation is dominated by down-dip shortening and slab thickening. Below ˜450 km the down-dip shortening seen in the earthquakes is still important, but it is absorbed roughly equally by along-strike extension and by thickening. There is little evidence of along-strike shear at depth. We have more confidence in the pattern of strain rates and velocities that we obtain than in their absolute values. Nevertheless, the rates of down-dip shortening accounted for by seismicity are probably less than half those needed if the whole down-dip component of Pacific-Australia plate convergence is absorbed by shortening in the upper mantle. The style of deformation at the base of the slab is complex and, unlike many regions of distributed continental tectonics, is not easily represented by simple patterns of faulting.