Early arc volcanism and the ophiolite problem: a perspective from drilling in the western Pacific

Marine geologic studies and Deep Sea Drilling Project (DSDP) and Ocean Drilling Program (ODP) transects of the intraoceanic Izu-Bonin-Mariana (IBM) forearcs have generated several important new ideas about the origin and evolution of these terranes. The first, and most striking, is the recognition that the initial phases of volcanism in these subduction zones developed nearly synchronously in the middle to late Eocene over a zone up to 300 km wide and thousands of kilometers long. This early, or “infant,” arc volcanism was characterized by the eruption of very depleted boninitic and arc tholeiitic lava compositions and occurred in extensional environments (as evidenced by dikes on Chichi-jima in the Bonin Islands). This infant arc volcanism was built on, or displaced, the pre-existing oceanic crust and had igneous production rates much higher than those of mature arcs—eruption rates on the order of those in slow-spreading ridges. This initial arc volcanism is unlike that developed during the “normal” or mature phases of arc activity and is a plausible mechanism for developing supra-subduction zone ophiolites. In fact, the duration of volcanism, volcanic and plutonic rock compositions, and structural setting in the IBM forearc are virtually identical to those in the Troodos ophiolite. The exposure of Eocene arc basement immediately adjacent to the axis of the Mariana-Bonin Trench requires some subduction erosion since the Eocene. However, if the Eocene basement did form by unusual, voluminous volcanism in an extensional environment, the required amount of erosion may only be 20 to 50 km; the vertical tectonics of the forearc indicate there has been little erosion of the forearc since the Oligocene. There is also clear evidence of minor, episodic, post-Eocene incorporation of older Pacific plate sediments and crustal fragments into the outer forearc. These forearcs are structurally and compositionally complex, and are clearly direct products of subduction, rather than just trapped pieces of oceanic crust. They have geological and geochemical similarities to supra-subduction zone ophiolites like those in Troodos and Oman and represent a previously unappreciated type of crustal construction. They are, in fact, the closest crustal analog to supra-subduction zone ophiolites that has been found in any modern geologic environmen