Because an oceanic plate colliding with a continental plate will usually be subducted and recycled into the deep mantle, a fossil oceanic plate after the closure of an ancient ocean has rarely been imaged in the subcontinental lithospheric mantle. This has led to a long-standing debate about the fate of subducted ocean plates. The problem can be addressed by imaging the lithosphere in a continental accretion zone with past ocean subduction. We present a study using long-period magnetotelluric data that reveals a large shallow-mantle conductor in a Phanerozoic accretion area in northwestern Xinjiang, China. This conductor extends >300 km laterally at depths from 120 to 220 km and resembles a segment of a fossil oceanic plate. The reduced resistivity is ascribed to the volatile-bearing metasomatic minerals, based on its relatively fertile nature and low temperature. Our results demonstrate that an oceanic plate can be trapped in continental lithosphere, underscoring the significance of oceanic plate subduction to continental accretion, and shedding new light on our understanding of continental formation and evolution.