SEISMIC STRAIN RATE AND DEEP SLAB DEFORMATION IN TONGA

Seismic strain rates estimated from volume-averaged moment tensor data are combined with the strike, dip and extent of the subducting lithosphere obtained from residual sphere analysis and seismicity to map deformation in the deep Tonga slab. Tonga strain rate orientation tensors, constructed from the Harvard Centroid Moment-tensor catalog, are consistent with slab deformation models in which the lithosphere encounters resistance to penetration at the 650-km discontinuity. Those which sample lithospheric volumes from 18-degrees-S to 25-degrees-S at 500-700 km depth indicate that the axis of maximum shortening lies within 15-degrees of the slab's downdip azimuth and plunge. The discrepancy between shortening and slab dip may be larger in northernmost Tonga (17-degrees-S-18-degrees-S) where slab morphology is less well constrained, but overall the slab is compressing roughly parallel to its trajectory. Tonga strain rate orientation tensors also indicate horizontal strain both parallel and perpendicular to the strike of the slab. The proportion of along-arc strain rate decreases from northern to southern Tonga. The scalar rate of seismic strain peaks at certain latitudes, but its values averaged over northern and central Tonga and southern Tonga are roughly equal. Strain rate data may be related (nonuniquely) to constraints on slab structure and finite strain inferred from seismicity and travel time data. Assuming that aseismic and seismic flow have the same geometry, and that the total rate of strain is constant along the arc, the degree of cross-strike advective thickening increases to the south, while along-arc strain decreases. If along-arc strain rate exclusively produces southward shear of the deeper lithosphere, and the amount of shear is estimated from the offset during subduction of the northern edge of the slab and the Louisville Ridge, thickening of at least 50% is required in central Tonga. This lower bound improves the resolution of minimum slab width provided by travel time observations. If 50% thickening occurs, an upper bound on the estimated rate of seismic moment release is large enough to account for scalar strain rate in the seismogenic core of the slab. Larger thickening factors or flow in the outer regions of the slab require aseismic deformation. Assuming that the observed travel time residuals are the result of a single, coherent slab, a synthesis of seismicity, strain rate and travel time data indicate a slab structure in northern Tonga which curves sharply to the west at the base of the upper mantle, but which strikes roughly to the north at shallower depths and in the lower mantle. Such a contortion in the deep lithosphere is consistent with the subduction zone configuration prior to 8 m.y. and the tectonic history of the region.