Young lava fields on the Cretaceous Pacific Plate in the Japan Trench: Non-hotspot volcanism?

The northwestern part of the Pacific Plate is comprised of Early Cretaceous abyssal oceanic lithosphere and Early to Late Cretaceous seamounts. Until recently, no present-day volcanic activity had been definitively documented on the cool, thick, and old Cretaceous lithosphere; however, Hirano et al. (2001) reported the presence of anomalously young alkali-basalt lavas (5.95±0.31 Ma) on the subducting, ∼130 Ma Pacific Plate. The trench-oceanward slope is characterized by trench-parallel normal faults, resulting from bending of the subducting Pacific Plate. Some hummock structures named the Kaiko Knolls can also be observed on the faulted abyssal plain using seabeam sonar bathymetric mapping. The Kaiko Knolls hummocks and some of the horst and graben fault walls are recognized in the seabeam sonar data by the presence of ocean floor with high acoustic intensity. The newly discovered lava fields include all hummocks in the Kaiko Knolls as well as the underlying sheet flow. The distinct WNW-ESE alignments of knolls are perpendicular to hinge lines of bending plate of the trench and outer-rise system. Composition of the dredged lavas shows the garnet presence in the source because the residual garnet buffered Al₂O₃ contents with degrees of partial melting and lowered HREE contents. Hirano et al. (2004) demonstrated that the olivine xenocrysts in this rock were entrained from the uppermost mantle. Volcanic eruption occurred ∼600 km ESE off the northern Japan Trench based on the radiometric age and the present absolute motion of the Pacific Plate. Morphological and petrological evidences show that the magma has been brought to the surface along some fissures, which can be interpreted along the direction of the maximum horizontal compression caused by the stress in the downwarping Pacific Plate at eastern edge of the outer-rise.