Abstract The tectonic evolution of Sulawesi is shaped by complex subduction processes, yet the geometry and extent of its slabs remain debated. Using teleseismic P‐wave tomography, we present a new 3‐D model of the lithosphere and underlying upper mantle beneath Sulawesi, based on passive seismic data from 89 seismic stations. Our results reveal three distinct slabs beneath North Sulawesi: a south‐dipping Celebes Sea slab, a westward‐subducting Sangihe slab, and a north‐dipping structure that likely represents the Sula slab. We also identify a prominent low‐velocity anomaly beneath the East Arm of Sulawesi (∼200–500 km depth), which we interpret as the inflow of deep mantle required as extension occurred. Furthermore, a separate low‐velocity anomaly beneath the Tambarana Fault suggests it to be a major lithospheric‐scale structure that extends down into the upper mantle. These findings provide new constraints on subduction dynamics and highlight the role of deep mantle processes in driving surface magmatism.

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