Subduction zones: observations and geodynamic models

This review of subduction and geodynamic models is organized around three central questions: (1) Why is subduction asymmetric? (2) Are subducted slabs strong or weak? (3) How do subducted slabs interact with phase transformations, changes in mantle rheology, and possibly chemical boundaries in the mantle? Based on laboratory measurements of the temperature dependence of olivine, one would conclude that the core of a subducting slab is at least 10,000 times more viscous than ambient mantle; however, there are a number of complementary but independent observations that suggest that slabs are much weaker than this. Slabs undergo significant deformation in the upper mantle and may thicken to twice their original width by the time they reach the base of the transition zone. The lack of a clear correlation between the observed dip angle of deep slabs and plate velocity, rate of trench migration, and slab age in modem subduction zones is consistent with hypothesis that subduction is a time-dependent phenomenon. Both tank and numerical convection experiments with plates conclude that subduction is not a steady phenomenon, but that slabs bend, thicken, stretch, and change dip through time. This is at odds with the assumptions used in steady-state slab thermal models, where slab deformation is not considered. (C) 2001 Elsevier Science B.V. All rights reserved.