Catastrophic initiation of subduction following forced convergence across fracture zones 1 Chad E. Hall a;  , Michael Gurnis a , Maria Sdrolias b , Luc L. Lavier a , R. Dietmar Mu «ller b a Seismological Laboratory, California Institute of Technology, Pasadena, CA 91125, USA b Division of Geology and Geophysics, School of Geosciences, The University of Sydney, Sydney, NSW 2006, Australia Received 20 December 2002; received in revised form 22 April 2003; accepted 29 April 2003 Abstract Although the formation of subduction zones plays a central role in plate evolution, the processes and geological settings that lead to the initiation of subduction are poorly understood. Using a visco-elastoplastic model, we show that a fracture zone could be converted into a self-sustaining subduction zone after approximately 100 km of convergence. Modeled initiation is accompanied by rapid extension of the over-riding plate and explains the inferred catastrophic boninitic volcanism associated with Eocene initiation of the Izu-Bonin-Mariana (IBM) subduction zone. Using global plate reconstructions, we suggest that IBM nucleation was associated with a change in plate motion between 55 and 45 Ma. We estimate that the forces resisting IBM subduction initiation were substantially smaller than available driving forces. ß 2003 Elsevier Science B.V. All rights reserved. Keywords: subduction; initiation; fracture zones; Bonin Arc; Izu Arc; Mariana Trough 1. Introduction The formation of new subduction zones is a fundamental, yet poorly understood, process in the normal evolution of tectonic plates. Processes that lead to the demise of subduction zones, such as the subduction of buoyant material (i.e. conti- nental or young oceanic lithosphere), must ulti- mately result in the formation of new subduction zones. Several questions regarding subduction ini- tiation are unanswered: what are the kinematic and dynamic relations between plate motions and the formation of new subduction zones? Do changes in plate motion precede or follow incipi- ent subduction? What tectonic settings are most favorable for the nucleation of new trenches? What are the short term tectonic and dynamic consequences of incipient subduction? In looking for answers to these questions, there are two stumbling blocks which have inhibited progress, one theoretical and one empirical. First, while events such as the opening and closure of 0012-821X/03/$ ^ see front matter ß 2003 Elsevier Science B.V. All rights reserved. doi:10.1016/S0012-821X(03)00242-5 * Corresponding author. Tel./Fax: +1-626-395-3861. E-mail address: chall@gps.caltech.edu (C.E. Hall). 1 Supplementary data associated with this article can be found at doi:10.1016/S0012-821X(03)00242-5 Earth and Planetary Science Letters 212 (2003) 15^30 R Available online at www.sciencedirect.com www.elsevier.com/locate/epsl