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

Abstract Sulawesi and Borneo are tectonically complex islands with multistage subduction histories stretching back through the Cenozoic. Seismic studies have played an important role in helping to unravel this history, with spatial distributions of earthquakes tracking actively subducting slabs. In contrast, old or relict aseismic slabs are illuminated not by earthquakes within the slab, but rather by seismic waves that transmit through the slab and reveal them as regions of anomalously high velocity in the mantle via tomographic imaging. We use the relative arrival times of teleseismic body‐waves from 1,359 unique teleseismic earthquakes between 2019 and 2023, recorded by a network of 134 land and ocean bottom seismometers in the study region (corresponding to 72,760 relative arrival times), to tomographically image the P wave velocity structure beneath Sulawesi and the Makassar Strait, which lies between eastern Borneo and Sulawesi. The resulting velocity model robustly uncovers a tabular high velocity anomaly at depths between ∼300 and 700 km dipping NW beneath the Makassar Strait. By comparing tomographic models to plate tectonic reconstructions, we connect this anomaly to the hypothesized NW‐Celebes slab, which actively subducted beneath northern Borneo between ∼21 and 9 Ma. Geochemical evidence from contemporaneous volcanics are consistent with slab break‐off between ∼13 and 9 Ma, which coincides with the cessation of subduction. Comparison of our findings to pre‐existing geodynamic modeling suggests that sinking of the broken‐off NW Celebes slab may have helped to initiate S Celebes subduction via subduction polarity reversal.