Subduction of the Woodlark spreading center, Solomon Islands: Slab window extent, and subduction of the Ontong-Java Plateau

The active spreading ridge between the Woodlark and Australia plates is currently subducting northward beneath the Solomon Islands. We determined the shape and extent of the upper mantle slab window that has developed as a result of subduction of this ridge, the Woodlark spreading center. Seafloor magnetic anomalies on the Woodlark and Australia plates, formed at the Woodlark spreading center, indicate the geometry of now-subducted conjugates of the spreading center, both spreading segments and transform faults. Our synthesis of active seismicity, marine geophysical surveys, and on-land geological studies shows that the portion of the Solomon Islands above the Woodlark spreading center subduction region currently acts as a microplate. Thus, we determined relative velocities of the three plates involved at their triple junction and then constructed the boundaries of the Woodlark slab window accordingly. To first order, the slab window extends from ~157.0°W to at least 160.0°W and coincides with the southwestward-subducting slab of Ontong-Java Plateau lithosphere (i.e., Pacific plate) beneath the Solomon Islands block. We infer that heat advected into the slab window upper mantle by ridge subduction lowers mantle viscosity there to ~2 × 10¹⁸ Pa.s, thus facilitating southwestward subduction and incipient subduction of the thick crust and lithosphere of the Ontong-Java Plateau beneath the Solomon Islands block.