Rheologic Controls on the Dynamic Evolution of Slab in the Upper Mantle.

Magali I. Billen, Department of Geology, University of California, Davis.
Greg Hirth, Department of Geology and Geophysics, Wood Hole Oceanographic Institute.

Subduction of tectonic plates is characterized by long-lived subduction zones, asymmetric subduction and slab dip angles of 25–80° in the upper mantle. Several mechanisms proposed to explain the variation in observed dip include largescale mantle flow, trench roll-back, and interaction of the slab with the transition zone. Previous dynamic models of subduction that include only Newtonian viscosity and moderately strong slabs generally fail to predict subduction angles less than 60–90° at shallow depths (10–300 km). We find that the observed characteristics of subduction are reproduced by viscous flow models, in which the rheologic structure is consistent with experimentally determined flow laws for Newtonian and non-Newtonian visco-plastic deformation of olivine. The properties of the models required to match the observed characteristics of slabs are: non-Newtonian viscosity in the mantle producing a weak mantle wedge (10¹⁸–10¹⁹ Pa s), a stiff slab interior (10²⁵ Pa s) limited by a plastic yield criterion and a weak plate boundary shear zone (10²⁰–10²¹ Pa s). The shallow slab dip reaches a minimum of 25–30° for high convergence rates and a stiff slab, without trench roll-back or relative motion of the entire lithosphere with respect to the mantle, suggesting these other mechanisms are not the primary controls on slab geometry. The deep slab dip (350–650 km) decreases as the slab penetrates the stiffer (x10), Newtonian viscosity lower mantle, eventually stabilizing the upper mantle slab geometry.