Subduction of oceanic and continental sediments into the mantle is fundamental to the geochemical evolution of Earth. Using thermal-mechanical models, we examine the dynamics of sediments that are subducted below continental lithosphere. Owing to their low density relative to the mantle, model sediments detach from the subducting plate at ∼100 km depth. With ongoing subduction, a subhorizontal sediment plume develops and intrudes the continental lithosphere. This occurs for a wide range of sediment densities and rheologies, suggesting that sediment detachment may be important for regions where the subducted sediment thickness is larger than ∼350 m. In these areas, a reservoir of sediments may be found in the shallow backarc mantle. In contrast to models of sediment transport to the deep mantle, the detachment model predicts chemical and mechanical interactions between the sediments and backarc mantle lithosphere and a shallow sediment source for arc and backarc magmas.