5th Swiss Geoscience Meeting, Geneva 2007
Numerical modelling of slab breakoff dynamics
Cyrill Baumann*,**, Taras V. Gerya*
* Institute of Physics, University of Zurich (cbauman@erdw.ethz.ch)
* 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.