Slab Morphology Research Articles

Effects of relative plate motion on the deep structure and penetration depth of slabs below the Izu-Bonin and Mariana island arcs

An increasing number of seismological studies indicate that slabs of subducted lithosphere penetrate the Earth's lower mantle below some island arcs but are deflected, or, rather, laid down, in the transition zone below others. Recent numerical simulations of mantle flow also advocate a hybrid form of mantle convection, with intermittent layering. We present a multi-disciplinary analysis of slab morphology and mantle dynamics in which we account explicitly for the history of subduction below specific island arcs in an attempt to understand what controls lateral variations in slab morphology and penetration depth. Central in our discussion are the Izu-Bonin and Mariana subduction zones. We argue that the differences in the tectonic evolution of these subduction zones—in particular the amount and rate of trench migration—can explain why the slab of subducted oceanic lithosphere seems to be (at least temporarily) stagnant in the Earth's transition zone below the Izu-Bonin arc but penetrates into the lower mantle below the Mariana arc. We briefly speculate on the applicability of our model of the temporal and spatial evolution of slab morphology to other subduction zones. Although further investigation is necessary, our tentative model shows the potential for interpreting seismic images of slab structure by accounting for the plate-tectonic history of the subduction zones involved. We therefore hope that the ideas outlined here will stimulate and direct new research initiatives.

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°S to 25°S at 500–700 km depth indicate that the axis of maximum shortening lies within 15° of the slab's downdip azimuth and plunge. The discrepancy between shortening and slab dip may be larger in northernmost Tonga (17°S–18°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.