Weak zone formation for initiating subduction from thermo-mechanical feedback of low-temperature plasticity

We have addressed the problem of subduction initiation with a solid-mechanical and fluid-dynamical approach, using a finite-element method. The setup has been extended by a rate-sensitive coupling at the bottom of a semi-brittle lithosphere and a fully coupled thermo-mechanical model. The central element of our model is a broad asymmetric sedimentary loading function at the passive continental margin, which grows with time to 15 km. Two fundamentally different modes of shear zone formation have been found depending on the rheology of the creep layer. Mode 1: For cases of low or absent yield stress in the creep layer only, the semi-brittle top develops a weak zone, while the rate-sensitive layer acts as a decoupling shear zone. Mode 2: For cases with a yield strength in the creep layer (strain rates above 10(-15) s(-1) after yielding), the entire mechanical lithosphere fails on a major shear zone; mode 1 fails to model subduction. initiation, while mode 2 creates a weak, major shear zone that severs through the entire lithosphere. (C) 2001 Published by Elsevier Science B.V.