Fig. 2. CCP stacking of receiver functions. Both the 410 km discontinuity (a, b) and the 660 km discontinuity (c, d) deepen by approximately 15 and 20 km (blue circles) and represent a vestige of an upwelling mantle plume in the NCC. Left panels: results corrected using the AK135 1-D velocity model (a, c). Right panels: results corrected using the 3-D velocity model (b, d) inferred tomography (He, 2020).