A LABORATORY INVESTIGATION OF EFFECTS OF TRENCH MIGRATION ON THE DESCENT OF SUBDUCTED SLABS

Abstract

A laboratory investigation of viscous slabs subducted from a migrating trench reveals a range of possible behaviours. The slab dip in a uniform mantle is found to be steady or oscillatory, depending on the rates of descent and trench migration. In addition, density and viscosity interfaces are used to model the increased resistance to sinking of stiff slabs through the seismic discontinuity at a depth of about 660 km in Earth's mantle. If the slab is denser than the lower layer and its dip in the upper layer is steady, it can continue to descend through the lower layer in a tabular form and in either a steady or oscillatory manner, or it can be laid horizontally on the interface for a distance before descending in a chain of diapirs resulting from gravitational instability at the interface. If the slab dip in the upper layer is unstable, the slab sinks into the lower layer in a chain of large diapirs at a spacing determined by the frequency of oscillations set by instability of the slab within the upper layer. The style of penetration depends on the trench migration speed and the ratio of sinking velocities in the two layers. Estimates for mantle slabs indicate that they may range across the major regime transitions. The experimental system provides a gross simplification of mantle conditions but the results are a testimony to the possibility of a range of complex behaviour of subducted lithosphere. They also indicate that the relationship between the structural features in the lower mantle revealed by seismic imaging and present-day tectonic processes at the surface may not be obvious. Tomographic images of western Pacific subduction zones and data for the migration rates of associated trenches suggest a dependence of slab behaviour on migration rate similar to that seen in the experiments.