M.K. Reagan , J.A. Pearce , K. Petronotis , R. Almeev , A.A. Avery , C. Carvallo , T. Chapman , G.L. Christeson , E.C. Ferré , M. Godard , D.E. Heaton , M. Kirchenbaur , W. Kurz , S. Kutterolf , H.Y. Li , Y. Li , K. Michibayashi , S. Morgan , W.R. Nelson , J. Prytulak , M. Python , A.H.F. Robertson , J.G. Ryan , W.W. Sager , T. Sakuyama , J.W. Shervais , K. Shimizu , S.A. Whattam
29 September 2015
According to the hypothesis being tested, the Izu-Bonin-Mariana (IBM) fore arc formed during the period of seafloor spreading that accompanied the rapid rollback and sinking of the newly subducting plate immediately following subduction initiation. According to the related hypothesis of Reagan et al., the deepest volcanic rocks should primarily be fore-arc basalt (FAB), a distinctive type of volcanic rock recovered during dredging and submersible sampling of the IBM fore arc. Both hypotheses predict that FAB is underlain by sheeted dikes, as these are characteristic of crust accreted during seafloor spreading at all but the slowest spreading rates. Reconstruction of the IBM volcanic stratigraphy also provides evidence for the inference that the FAB should be overlain by the initial products of arc volcanism, specifically by (1) Lavas with compositions that are transitional between FAB and boninite; (2) Boninite lavas themselves; and finally, (3) Members of the tholeiitic and calc-alkaline series typical of “normal” island arcs. Drilling at this site provides an important test of the lower part of this sequence. Although the stated aim of drilling at Site U1439 was to core the upper (boninitic) part of the fore-arc volcanic sequence, the aim of drilling at Site U1440 was to core the lower basaltic part of the sequence.