Cenozoic Subduction Polarity Reversal Within the Celebes Sea Inferred from Teleseismic Tomography

Sulawesi and Borneo are tectonically complex islands in South East Asia that were assembled from numerous crustal fragments throughout the Cenozoic. Past P wave tomography studies of this region have used land stations and hence image seismic velocity structure primarily beneath the two islands. By adding data from Ocean Bottom Seismometers deployed between 2019 and 2020 in the Makassar Strait, which lies between Sulawesi and Borneo, and incorporating core-going converted P phases with steeper ray paths (PKP and PKIKP), we illuminate the mantle beneath this offshore region to depths of around 800 km. The new tomographic velocity model robustly images a high-velocity north-west dipping tabular anomaly at depths between ~300 and ~660 km beneath the strait, which is interpreted as an aseismic ancient slab. By combining a slab age-depth relationship with a plate tectonic reconstruction, we uncover the palaeosubduction boundary responsible for this slab, thereby providing conclusive evidence for the previously hypothesised north-westward subduction of the Celebes Sea beneath Northern Borneo around ~15 Ma (plus/minus a few million years). Following slab break-off and northward plate migration of Sulawesi, sinking of this northwest Celebes Sea slab may have contributed to the initiation of subduction of the Celebes Sea southwards beneath Northern Sulawesi, which today is confidently imaged by a Benioff zone in addition to seismic tomography.