7.08 - Mantle Downwellings and the Fate of Subducting Slabs: Constraints from Seismology, Geoid Topography, Geochemistry, and Petrology

Abstract

The incorporation of subduction zones presents one of the most challenging aspects of mantle convection modeling, as the asymmetric nature of subduction is not a natural result of convection of a creeping viscous fluid. As both observations and models of subduction zones continue to improve, some long-standing questions are being answered and new questions are being raised. Seismic imaging is providing an increasingly greater resolution and the emerging images of subduction zones reveal a complex, three-dimensional (3-D) structure with some slabs lying flat at the base of the transition zone, slab dip changing along the length of an arc, and bends, kinks, tears, and windows in slabs. Shear-wave-splitting observations are being interpreted as complex flow patterns with significant slab-parallel components of motion. Geochemistry and arc petrology also change along the length of many arcs and in some cases these changes cannot be correlated with the traditionally recognized controls on slab thermal structure (e.g., incoming plate age, slab dip, and convergence rate). Thus subduction zones appear to be more complex with a significant amount of 3-D structure as opposed to the traditional, 2-D tabular view of slabs.