Laboratory experiments on the structure of subducted lithosphere

Abstract

Mantle seismic tomography images indicate a variety of shapes for slabs of oceanic lithosphere subducted into the transition zone. In the vicinity of the 660 km discontinuity both horizontally deflected slabs and nearly vertical slabs have been inferred at subduction zones on the Pacific and Indian plates. Here we determine experimentally the behavior of a cold, negatively buoyant and highly viscous slab incident on a fluid interface with a viscosity and a density increase. The experiments are scaled to mantle conditions, with realistic subduction Peclet number, ratio of trench migration to subduction velocities, and physical properties of the interface. A wide range of slab deformation styles is observed when slight changes in the parameters are imposed. Sinking slab, stagnant slab, spreading slab, sinking pile and stagnant pile represent the five different modes identified. We show that these deformation modes are governed by two velocity ratios, which characterize the horizontal and the vertical components of the slab velocity near the interface. Slab penetration into the lower layer takes the form of folded slab piles for large values of the velocity ratios, and stagnant behavior is promoted by rapid retrograde trench motion. The sensitivity of slab behavior to transition zone density and viscosity changes and to trench migration can account for the coexistence of different modes of deformation in subducted lithosphere.