Site U1439

Abstract

<p>The Izu-Bonin-Mariana (IBM) fore arc is believed to carry one of the best records worldwide of processes associated with the initiation of subduction. If current models are correct, subduction began with a period of rapid rollback and sinking of a newly subducting Pacific plate, continued through a transitional period of reorganization, and ended with the stable trench-parallel subduction that we see today. In geological terms, it evolved from seafloor spreading through proto-arc volcanism to normal arc volcanism. Reconstruction of the IBM volcanic stratigraphy is key to understanding and dating this evolutionary sequence. At the base of the volcanic section is a distinctive magma type known as fore-arc basalt (FAB). This FAB is overlain by lavas with compositions that are transitional between FAB and boninite, then by boninite lavas themselves, and finally by members of the tholeiitic and calc-alkaline series typical of normal island arcs (Reagan et al., 2010). This stratigraphy is speculative, however, having been pieced together from a series of partial sections that are typically a considerable distance apart. Site U1439 provides an important test of the stratigraphy of the middle part of this sequence, namely the period that records the transition from spreading to arc volcanism. It thus records the birth of an island arc.</p>