Geophysical, geological, and petrologic data in southwestern China have been integrated in order to characterize magmatic underplating associated with the Late Permian Emeishan large igneous province (LIP; ca. 260 Ma). Seismic reflection and refraction reveals a heterogeneous crustal structure with high-velocity layers or bodies in the upper crust (6.0–6.6 km/s), lower crust (7.1–7.8 km/s), and upper mantle (8.3–8.6 km/s). These seismically anomalous bodies are all confined in the inner zone of the prevolcanic domal structure, but are generally absent in the intermediate and outer zones. There is a decreasing trend in crustal thickness from the inner zone (>60 km, with a ∼20-km-thick high-velocity lower crust, or HVLC) via the intermediate zone (∼45 km) to the outer zone (<40 km). Because the domal uplift immediately preceding eruption of the Emeishan basalts was unambiguously related to a mantle plume, such a configuration highlights a genetic relationship between the formation of the high-velocity crust and the mantle plume that led to the eruption of the Emeishan basalts. It is proposed that the HVLC may have resulted from magmatic underplating associated with the Emeishan volcanism, whereby the fast mantle represents the residues left after extensive melt extraction from the plume head. Magmatic underplating can also account for the prolonged crustal uplift that formed the Chuandian “old land” in southwestern China. Petrologic modeling further suggests that the HVLC may represent fractionated cumulates from picritic melts and that the Emeishan basalts represent residual melts after polybaric fractionations. This relationship allows a reestimation of the volume of Emeishan magmas, which is as much as 3.8 × 10⁶ km³, typical of plume-generated LIPs in the world.