Seismic Structure of the Izu Arc-Backarc System

Abstract

Arc-backarc systems are inherently shaped by subduction, representing an essential window into processes acting in the Earth's interior such as the recycling of subducted slabs. Furthermore, they are setting where new crust is formed and are believed to be sites where juvenile continental crust emerges. We present a seismic refraction and wide-angle velocity model across the Izu arc-backarc system, and use its characteristic features to constrain geochemically and petrologically different compartments, revealing processes governing crustal formation overlying subduction zones. Our result delineates the Izu arc with a maximum thickness of ∼20 km and the Shikoku Basin with thicknesses of ∼7 to 11 km. In the volcanic arc, the middle crust of the felsic to intermediate tonalitic layer (6.0–6.5 km/s) is remarkably thicker beneath the basalt-dominated area than in the rhyolite-dominated area, indicating that basaltic volcanism is indispensable in the transformation process from arc to continental crust. However, rhyolitic volcanism may relate to the juvenile stage of arc evolution or the remelting of middle crust due to the insufficient supply of basaltic magma from the mantle. The mafic restite and cumulates, which used to be part of the arc crustal material, are delaminated and foundered into the mantle, forming extremely low mantle velocities (<7.5 km/s). In the Shikoku Basin, our result supports a fertile mantle source with passive upwelling and normal temperature during the opening process, but the lack of high velocity in the lower crust rules out hydrous melts entrained from the subducting slab or anomalous mantle trapped during subduction zone reconfiguration.