Seismic evidence for a slab window beneath central Sumatra

Subducting slabs generally inhibit mantle exchange between ocean basins and marginal seas, yet the origin of Indian Ocean MORB-type mantle signatures in the southwestern South China Sea remains debated. Here, we image mantle transition zone (MTZ) structure beneath Sumatra using receiver functions that account for non-planar wavefronts. Stacking 34,255 records from 197 stations reveals widespread MTZ thickening beneath central and southern Sumatra, accompanied by pronounced along-strike heterogeneity. In central Sumatra, MTZ thickening in the middle segment is primarily associated with a depressed 660-km discontinuity, whereas adjacent segments are characterized by predominant uplift of the 410-km discontinuity (d410), indicating a laterally discontinuous slab. This inference is further supported by a gap in slab-related velocity anomalies near d410, localized MTZ thinning in the forearc, and contrasting discontinuity signatures, including a sharp d410 and weak 520-km phases in the middle segment versus stronger or locally split 520-km signals and broadened d410 phases in adjacent segments. These observations favor the presence of a slab window beneath central Sumatra, likely representing a fossil slab breakoff event initiated during the late Early to Middle Miocene (∼20–15 Ma). Numerical modeling further suggests that along-trench slab age variations can trigger such non-collisional slab breakoff, facilitating mantle material exchange between the Indian Ocean and the South China Sea.