SUBDUCTION INITATION AND TERRANE ACCRETION

Terrane accretion can be accomplished by strike-slip faulting or by partial subduction of a buoyant block so that subduction fails, accreting the collided block to the continent. If relative plate motion continues after terrane accretion, this must be accommodated by formation of a new subduction zone, which can lead to accretion of another terrane. Both subduction and strike-slip modes of terrane accretion can be followed by either strike-slip faulting or formation of a new subduction zone. We can use what we have learned from studying subduction initiation (SI) globally to make more realistic models of terrane accretion and associated subduction initiation. SI mechanisms can be divided into those induced by existing relative plate motions (induced nucleation of a subduction zone, INSZ) and those that occur spontaneously because of lithospheric body forces adjacent to sufficiently large lithospheric weaknesses (spontaneous nucleation of a subduction zone, SNSZ). There are two main types of INSZ: polarity reversal and transference. Polarity reversal happens readily, as demonstrated by formation of a new, N-dipping subduction zone on the S side of the Solomon arc after partial subduction of the Ontong Java plateau caused the S-dipping Vitiaz subduction zone to fail in Miocene time. Transference happens when a new subduction zone forms away from the collided block and is difficult, as demonstrated by the fact that a new subduction still has not formed in the Indian Ocean south of India. There are no convincing Cenozoic examples of transference SI. Transform margin collapse SNSZ is increasingly well-documented for the Eocene Izu-Bonin-Mariana arc and may also have been important for generating the many Late Jurassic ophiolites in the western Cordillera from which long-lived Cretaceous convergent plate margin magmatism evolved. Plume-induced SI may be responsible for accretion of Siletzia and formation of the modern Cascadia subducton zone in Eocene time. Both SNSZ modes are accompanied by voluminous magmatism and formation of ophiolites in proto-forearcs. Reconstructions of terrane displacement and consolidation should consider SI origins of ophiolites and keep in mind likely vs. unlikely modes of SI.