A fossil subduction system preserved beneath Sri Lanka

The long-term fate of lithosphere subducted during supercontinent assembly remains uncertain. Sri Lanka, located near the center of Gondwana and underlain by Precambrian basement, provides an ideal setting in which to search for remnants of ancient subduction. Using teleseismic tomography, we imaged upper-mantle velocity structure beneath Sri Lanka to test whether oceanic lithosphere subducted during Gondwana assembly has been preserved at depth. Our results revealed previously unrecognized mantle structure consistent with ancient subduction. A steep, coherent high-velocity body extends from ~100 to ~200 km depth, rooted near the Highland-Vijayan shear-thrust boundary and extending westward under the Wanni Complex. A localized uppermost-mantle low-velocity anomaly overlies its shallow portion, consistent with a fossil serpentinized mantle wedge above a former subduction zone. The geometry, amplitude, and depth continuity of the high-velocity anomaly suggest a relic oceanic slab emplaced during late Precambrian−Cambrian convergence along the Highland−southern India margin, marking a major suture associated with East Gondwana assembly. Coexisting slab and mantle-wedge signatures imply limited mantle reworking beneath central Gondwana since the early Paleozoic. These results provide the first seismic evidence for a fossil subduction system beneath Sri Lanka and new constraints on East Gondwana assembly.