Mantle thermal structure and active upwelling during continental breakup in the North Atlantic

We have addressed the problem of subduction initiation with a solid-mechanical and fluiddynamical approach, using a finite-element method. The setup has been extended by a ratesensitive coupling at the bottom of a semi-brittle lithosphere and a fully coupled thermomechanical 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.