Mantle Transition Zone Structure Beneath the Sunda–Java Trench and Christmas Island: Implications for Deep Dehydration and Petit-Spot Volcanism

Petit-spot volcanism is a type of intraplate volcanism that commonly occurs near the outer rise in subducting plates. Although deformation of the incoming plate is considered important for its development, the deeper mantle processes contributing to this volcanism remain poorly understood. Here, we use more than 1500 broadband seismograms containing long-period SS precursors to investigate the mantle transition zone (MTZ) beneath the Sunda–Java subduction system. Beneath the northern Sunda–Java region, the 660 km discontinuity (d660) reaches ~690 km depth, resulting in an MTZ thicker than 270 km, consistent with the thermal influence of the subducted Indo-Australian slab. The associated weakening of S660S amplitudes may reflect hydration or other factors affecting the lower MTZ. South of the trench, the MTZ beneath most of the Christmas Island Seamount Province is relatively uniform, with a thickness of ~251 km. Beneath Christmas Island, however, the 410 km discontinuity (d410) is depressed to ~420–425 km, whereas d660 remains near 670–675 km, producing a localized thinner MTZ of ~245–250 km. This structure is compatible with localized slab-related mantle processes and may reflect thermal or compositional heterogeneity near the top of the MTZ. Although our observations do not directly resolve volatile transport or melt generation, they provide new constraints on the deep-mantle environment associated with rejuvenated petit-spot volcanism at Christmas Island.