ANATOMY OF THE IZU-BONIN-MARIANA ARC SYSTEM: RESULTS FROM INTERNATIONAL OCEAN DISCOVERY PROGRAM EXPEDITIONS 352 AND 366

Geologic processes at convergent plate margins control geochemical cycling, seismicity, and deep biosphere activity in subduction zones. The Izu-Bonin-Mariana convergent margin system provides the environment where igneous forearc lithosphere formed due to subduction initiation, and a natural process brings materials from great depths directly to the surface.

International Ocean Discovery Program (IODP) Expedition 352 obtained sedimentary-volcaniclastic successions and extensional structures that allow the reconstruction of the evolution of the Izu-Bonin forearc by shipboard core data, seismic reflection images, and calcite vein microstructures. The oldest recorded biostratigraphic ages within the sediment basins (Late Eocene to Early Oligocene) imply a ~15 million years hiatus between the formation of the igneous basement from 52 to 50 Ma and the onset of sedimentation.

At the forearc upslope sites extension lead to the formation of asymmetric, fault-bound basins with stretch of ~16%-19% at strain rates of ~1.58 x 10^-16s^-1 to 4.62 x 10^-16 s^-1. Sites closer to the trench are characterized by symmetric bsins bound by conjugate normal faults with stretch of ~3.66 at strain rates of 9.66 x 10^-15s^-1 to 3.31 x 10^-15s^-1. Upper plate extension was triggered by incipient Pacific Plate rollback. Extension was accommodated by normal faulting with syntectonic sedimentation during Late Eocene to Early Oligocene times. Towards the backarc, extension was assisted by magmatism with related spreading at ~25 Ma, so that parts of the arc and rear arc were dismembered from the forearc.

IODP Expedition 366 recovered cores from three serpentinite mud volcanoes at increasing distances from the Mariana trench subduction zone. The cores consist of serpentinite mud containing lithic clasts derived from the underlying forearc lithosphere, and from the subducting Pacific Plate. Additionally, in situ pelagic sediments and volcanic ash deposits underlying the serpentinite mud volcanoes were recovered. Recycled materials from the subducted slab include fault rocks, metamorphosed pelagic sediments, diagenetic shallow water reef assemblages, and metavolcanic rocks. The recycled materials are interpreted to be parts of subducted Pacific plate seamounts of diverse age.