Dynamics of slab tear faults: Insights from numerical modelling Alina J. Hale a, ⁎, Klaus-D. Gottschaldt b, 1 , Gideon Rosenbaum c , Laurent Bourgouin b , Matthieu Bauchy b , Hans Mühlhaus b a The School of Geosciences, The University of Sydney, Australia b ESSCC, School of Earth Sciences, The University of Queensland, Australia c School of Earth Sciences, The University of Queensland, Australia abstract article info Article history: Received 22 January 2009 Received in revised form 1 May 2009 Accepted 20 May 2009 Available online 24 May 2009 Keywords: Subduction Tear resistance Computational modeling FEM Tear resistance at the edge of a slab is an important parameter controlling the evolution of subduction zones. However, compared with other subduction parameters such as plate strength, plate viscosity, plate thickness and trench width, the dynamics of tearing are poorly understood. Here we obtain a first-order understanding of the dynamics and morphology of subduction zones to resistance during tear propagation, by developing and using a novel computational modelling technique for subducting slabs, with side boundaries described by visco-plastic weak zones, developing into tear faults. Our 3D model is based upon a visco-plastic slab that sinks into the less dense mantle, generating poloidal and toroidal flows. The asthenospheric mantle field is static and only develops flow due to the subducting slab. We use the finite element code eScript/Finley and the level set method to describe the lithosphere to solve this fluid dynamics problem. Our results show the importance of tear resistance for the speed of trench migration and for shaping the final geometry of subduction systems. We show that slab tearing along a weak layer can result in a relatively straight slab hinge shape, while increasing the strength in the weak layer results in the curvature of the hinge increasing substantially. High tear resistance at the slab edges may hinder rollback to the extent that the slab becomes stretched and recumbently folded at the base of the domain. Tear resistance also controls whether the subducting lithosphere can experience accelerating rollback velocities or a constant rollback velocity. © 2009 Elsevier B.V. All rights reserved. 1. Introduction The lifecycle of subduction zones typically involves subduction initiation, accelerated sinking of the slab tip through the upper mantle, interaction with the 660-km discontinuity, and steady subduction until the arrival of buoyant lithosphere that prevents further subduction (Funiciello et al., 2003). During this lifecycle, most subduction hinges migrate with respect to the lower mantle, typically in a direction opposite to the dip of subduction (Garfunkel et al., 1986; Royden, 1993; Schellart, 2008; Schellart et al., 2008). This process, known as subduction rollback, is affected by the interaction of the slab with induced or background asthenospheric mantle flow (Dvorkin et al., 1993; Schellart, 2004a). Subduction rollback also plays a crucial role in the development of back-arc extensional basins, in particular when the velocity of subduction rollback exceeds the velocity of plate convergence (Dewey, 1980). Uniform subduction rollback and its associated mantle flow are relatively well understood and has been simulated in numerous experimental and numerical studies (Funiciello et al., 2003; Schellart, 2004a; Morra et al., 2006; Piromallo et al., 2006; Stegman et al., 2006; Schellart et al., 2007). However, much less is known about the dynamic response to non-uniform rollback velocities along the length of the subduction system. Such responses include the progressive curvature of subduction zones (e.g. Morra et al., 2006; Schellart et al., 2007) and the development of vertical slab tear faults (Govers and Wortel, 2005; Rosenbaum et al., 2008). The latter would propagate horizontally provided that subduction continues and the lithospheric strength is less than the slab strength. Otherwise subduction may stall or the slab may break off. In this paper, we numerically model the evolution of a subducting slab with side boundaries controlled by tear faults. In published subduction models (e.g. Morra et al., 2006; Stegman et al., 2006; Schellart et al., 2007), tear resistance at the slab boundaries is implicitly neglected allowing free slab propagation. However, slab propagation is likely to be significantly affected by any resistance to tearing. The instantaneous lithospheric response to a subduction edge has been considered by Govers and Wortel (2005), however their models neglect the dynamics of trench migration and tear propagation. In the following models we address this issue, showing how tear resistance at a propagating fault affects trench migration and the geometry of subduction. Tectonophysics 483 (2010) 58–70 ⁎ Corresponding author. E-mail address: a.hale@usyd.edu.au (A.J. Hale). 1 Now at: Deutsches Zentrum für Luft-und Raumfahrt, Institut für Physik der Atmosphäre, Oberpfaffenhofen, Germany. 0040-1951/$ – see front matter © 2009 Elsevier B.V. All rights reserved. doi:10.1016/j.tecto.2009.05.019 Contents lists available at ScienceDirect Tectonophysics journal homepage: www.elsevier.com/locate/tecto