Numerical modelling of spontaneous slab breakoff dynamics during continental collision
Published:January 01, 2010
Cyrill Baumann, Taras V. Gerya, James A. D. Connolly, 2010. "Numerical modelling of spontaneous slab breakoff dynamics during continental collision", Advances in Interpretation of Geological Processes: Refinement of Multi-scale Data and Integration in Numerical Modelling, M. I. Spalla, A. M. Marotta, G. Gosso
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Slab detachment or breakoff is directly associated with phenomena like morphological orogenesis, occurrence of earthquakes and magmatism. At depth the detachment process is slow and characterized by viscous rheolgy, whereas closer to the surface the process is relatively fast and plastic. Using a 2D mantle model 1500 km deep and 4000 km wide we investigated, with finite-difference and marker-in-cell numerical techniques, the impact of slab age, convergence rate and phase transitions on the viscous mode of slab detachment. In contrast to previous studies exploring simplified breakoff models in which the blockage responsible for inducing breakoff is kinematically prescribed, we constructed a fully dynamic coupled petrological–thermomechanical model of viscous slab breakoff. In this model, forced subduction of a 700 km-long oceanic plate was followed by collision of two continental plates and spontaneous slab blocking resulting from the buoyancy of the continental crust once it had been subducted to a depth of 100–124 km. Typically, five phases of model development can be distinguished: (a) oceanic slab subduction and bending; (b) continental collision initiation followed by the spontaneous slab blocking, thermal relaxation and unbending – in experiments with old oceanic plates in this phase slab roll-back occurs; (c) slab stretching and necking; (d) slab breakoff and accelerated sinking; and (e) post-breakoff relaxation.
Our experiments confirm a correlation between slab age and the time of spontaneous viscous breakoff as previously identified in simplified breakoff models. The results also demonstrate a non-linear dependence of the duration of the breakoff event on slab age: a positive correlation being characteristic of young (<50 Ma) slabs while for older slabs the correlation is negative. The increasing duration of the breakoff with slab age in young slabs is attributed to the slab thermal thickness, which increases both the slab thermal relaxation time and duration of the necking process. In older slabs this tendency is counteracted by negative slab buoyancy, which generate higher stresses that facilitate slab necking and breakoff. A prediction from our breakoff models is that the olivine–wadsleyite transition plays an important role in localizing viscous slab breakoff at depths of 410–510 km due to the buoyancy effects of the transition.
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Advances in Interpretation of Geological Processes: Refinement of Multi-scale Data and Integration in Numerical Modelling
Iterative comparison of analytical results and natural observations with predictions of numerical models improves interpretation of geological processes. Further refinements derive from wide-angle comparison of results from various scales of study. In this volume, advances from field, laboratory and modelling approaches to tectonic evolution – from the lithosphere to the rock scale – are compared. Constructive use is made of apparently discrepant or non-consistent results from analytical or methodological approaches in processing field or laboratory data, P–T estimates, absolute or relative age determinations of tectonic events, tectonic unit size in crustal scale deformation, grain-scale deformation processes, various modelling approaches, and numerical techniques. Advances in geodynamic modelling critically depend on new insights into grain- and subgrain-scale deformation processes. Conversely, quantitative models help to identify which rheological laws and parameters exert the strongest control on multi-scale deformation up to lithosphere and upper mantle scale.