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5th Swiss Geoscience Meeting, Geneva 2007 Numerical modelling of slab breakoff dynamics Cyrill Baumann*,**, Taras V. Gerya* * Institute of Physics, University of Zurich ([email protected]) * Department of Earth Sciences, ETH - Zürich Slab detachment or breakoff is appreciated as an important geological process and directly associated with phenomena like morphologic orogenesis, occurrence of earthquakes and magmatism. Using a 2-D mantle model 1500 km deep and 4000 km wide, we have investigated with a 2-D finite-difference and marker-in-cell numerical technique the impact of slab age, convergence rate and phase transitions onto this complex geological process. In contrast to previous studies we constructed fully dynamic coupled mineralogicalthermomechanical model where forced subduction of a 700 km long oceanic plate is followed by continental collision and spontaneous slab blocking by strong positive buoyancy of subducted continental crust. In general four phases of model development can be distinguished in most experiments: (1) subduction of the oceanic plate associated with its bending, (2) continental collision associated with slab blocking, thermal relaxation and unbending (in experiments with old oceanic plate slab roll back associated with opening of asthenospheric window below the collision zone also occurs), (3) slab necking and (4) slab detachment and sinking (breakoff). Our experiments show strong positive correlation between the oceanic slab age and the time of breakoff which is in line with previous thermomechanical studies. On the other hand the dependence of duration of breakoff from the slab age is nonlinear: positive correlation is characteristic for relatively young (<50 Myr) slabs while for old (>50 Myr) slabs the correlation is negative. An important new prediction from our study is that the olivine/spinel transition in the mantle plays an important role in localizing depth of the breakoff in the interval 410-510 km due to the strong negative buoyancy effects induced by this transition.