The morphology and dynamics of subducting lithosphere

Residual sphere images from deep earthquakes not only detect the presence of slabassociated velocity anomalies but can characterize the morphology and flow of lithosphere subducted into the lower mantle. Observations of seismic strain rate from deep-focus earthquake moment tensor data provide complementary insight into deformation of the slab due to resistance encountered at the 650-km discontinuity. We have compared travel times from deep events in the Kuril and Mariana arcs with the seismic velocity anomalies implied by kinematical models that thicken the slab perpendicular to its plane by reducing the vertical velocity of the flow with depth. We assume that the details of the deformation (whether the slab buckles, imbricates, fragments, etc.) are averaged out along the ray paths, and hence our models constrain the scale, not the mode, of slab thickening. For all models examined, the data require that slab material extends to depths of 900-1000 km. The deep event travel times are best fit by undeformed models, but the ability of the residual sphere method to resolve slab thickness is limited by ray bending effects. Although the Mariana times are consistent with advective thickening factors of 5 or more, factors larger than 3 are ruled out by the Kuril data. Global tomographic models and regional studies which delineate highvelocity anomalies in the lower mantle beneath zones of Cenozoic subduction are consistent with our results, as is recent work on pulse distortion by slab gradients. Residual sphere analysis was also applied to 25 intermediateand deep-focus earthquakes in the Tonga arc, mapping variations in slab strike and dip, depth extent and deep lateral advection at all latitudes along the arc. Similarly to the northwest Pacific, the Tonga slab is required to extend below the deepest events to depths of 800-900 km, and the data are consistent with, and in some cases prefer, advective thickening of the deep lithosphere perpendicular to the slab plane of factors of three or more. Considerable variation in slab structure occurs along the arc, and the match that the observed seismicity provides to detailed undulations in slab strike and dip is strong evidence that residual spheres do reflect nearsource heterogeneity. Travel times in southern Tonga (21.5* S27' S) require that the slab bends to a steeper dip at depth, but residuals in central (190 S21.5* S) and northern (130 S190 S) Tonga do not. Southern Tonga also contains a cusp in along-arc structure and seismicity whose surface expression in the trench topography corresponds to the intersection of the arc with the Louisville ridge (25' S). Northern Tonga is the locus of considerable complexity. The relationship of seismicity to inferred slab morphology may be explained by the collision of the subducting Tonga lithosphere with a remnant lithospheric fragment, and the southward shear of deep material relative to shallow structure. Evidence for slab deformation parallel as well as perpendicular to the arc also appears in average strain rate vectors constructed by summing observed moment tensor solutions from the Harvard CMT Catalogue. Assuming that the source data accurately reflect overall slab flow, compression of the deep slab parallel to its dip is balanced by thickening in the cross-strike and shear in the along-strike directions. The axis of maximum shortening deviates significantly more than 200 from down-dip only in northernmost Tonga where seismicity anf travel time data suggest disrupted subduction. Strain rate data indicate that the proportion of along-arc strain rate decreases south to north. Since estimates of scalar strain rate are constant with latitude, the degree of advective thickening perpendicular to the arc reaches a maximum in southern Tonga, assuming that aseismic flow and seismic deformation have the same strain rate geometry. If along-arc flow occurs largely as southward shear of the deeper lithosphere, vector descriptions of strain rate may require thickening factors of at least 1.5 in central Tonga. A substantially greater degree of deformation would be largely aseismic. Thickening factors significantly greater than one may not be necessary to match strain rate data from the arcs of the northwest Pacific where scalar values of strain rate are substantially smaller than in the deep Tonga slab. Aseismic extension of subducting lithosphere into the lower mantle argues against rigorously stratified convection, but a discontinuity in mantle properties, such as an increase in viscosity, is plausible if it merely resists the descending slabs without acting as a complete barrier to flow. Thesis committee: Thomas H. Jordan, Thesis supervisor Kenneth C. Creager, University of Washington, Seattle Marcia K. McNutt Jason Phipps Morgan I dedicate this thesis to my grandfather, George Bogdanovich Kistiakowsky