149 Y. Ogawa et al. (eds.), Accretionary Prisms and Convergent Margin Tectonics in the Northwest Pacific Basin, Modern Approaches in Solid Earth Sciences 8, DOI 10.1007/978-90-481-8885-7_7, © Springer Science+Business Media B.V. 2011 Abstract Despite its rapid intermediate-spreading rate, the Parece Vela Basin (PVB) shows unusual characteristics that indicate a depressed magmatic budget, such as the occurrence of numerous oceanic core complexes (OCCs) and rugged terrain, exposing abundant peridotites and gabbros. Based on the geologic interpre- tations of crust with analogous features on global mid-ocean ridges, we propose three possible mechanisms that can account for these unusual characteristics: (1) presence of a cold and/or refractory mantle domain, (2) declining spreading rate during the later phase of the second-stage spreading of the PVB, and (3) a transform sand- wich effect. Recent numerical modeling for formation of OCC suggests that there is a minimum as well as a maximum magmatic supply necessary to produce long- lived detachment fault. In the western PVB, a cold and/or refractory mantle domain inhibited a large amount of mantle melting within an intermediate-spreading ridge, attaining the limited window of the condition of magma supply demonstrated in the numerical model in an otherwise robust magmatic environment. In the central PVB, a transform sandwich effect and/or declining spreading rate inhibited a large amount of mantle melting within an intermediate-spreading ridge, also attaining the limited window of the condition of magma supply demonstrated in the numerical model in an otherwise robust magmatic environment. Y. Ohara (*) Hydrographic and Oceanographic Department of Japan, Tokyo 104-0045, Japan and Institute for Research on Earth Evolution, Japan Agency for Marine-Earth Science and Technology, 2-15 Natsushima-cho, Yokosuka 237-0061, Japan e-mail: yasuhiko.ohara@gmail.com K. Okino Division of Ocean-Earth System Science, Atmosphere and Ocean Research Institute, The University of Tokyo, 5-1-5 Kashiwanoha, Kashiwa-shi, Chiba 277-8564, Japan e-mail: okino@aori.u-tokyo.ac.jp J.E. Snow Department of Earth and Atmospheric Sciences, University of Houston, Houston, TX 77204, USA e-mail: jesnow@uh.edu Tectonics of Unusual Crustal Accretion in the Parece Vela Basin Yasuhiko Ohara, Kyoko Okino, and Jonathan E. Snow ! 2 3 4 5 6 7 8 9 !0 !! !2 !3 !4 !5 !6 !7 !8 !9 20 2!