Site U1440

Abstract

According to the Stern and Bloomer (1992) 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. (2010), 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 Lavas with compositions that are transitional between FAB and boninite; Boninite lavas themselves; and finally, Members of the tholeiitic and calc-alkaline series typical of “normal” island arcs (Ishizuka et al., 2011). Drilling at this site provides an important test of the lower part of this sequence.