Rheologic controls on the dynamics of slab detachment

Abstract

The detachment of subducted lithosphere (slabs) has been proposed in a number of locations as the cause for observed volcanism and regional tectonic evolution. For example, along-strike progression of uplift in the Adriatic Sea and west-to-east progression of volcanism in the Trans-Mexican Volcanic Belt may be caused by propagating detachment of subducted slabs. An understanding of the dynamics of slab detachment is necessary to relate the timing of detachment to surface observations. We demonstrate using two-dimensional (2D) numerical models that the timing, occurrence, and surface effects resulting from the detachment of subducted lithosphere are strongly dependent upon the stiffness (i.e. stress supported viscously) of the lithosphere and rheology of the upper mantle. In particular, in models with a Newtonian upper mantle where plastic yielding is the only weakening process within the slab, detachment does not occur. Models accounting for non-Newtonian upper mantle deformation demonstrate two modes of detachment: (1) slow and deep detachment of strong slabs (500 MPa or greater maximum yield strength) controlled by heating of the slab, and (2) fast and shallow detachment of weaker slabs (300 MPa) controlled by yielding within the slab interior. For slabs with a maximum yield strength of 500 MPa, the time to detachment increases with slab age (for ages ranging from 40 to 120 My), indicating that detachment time is controlled by the integrated stiffness of the slab, which limits the sinking rate. However, the detachment time for weaker slabs (300 MPa) is independent of age because weaker slabs sink rapidly, and the time to detachment is limited by the rate of flow of the surrounding mantle.