Normal thickness of the upper mantle transition zone in NE Brazil does not favour mantle plumes as origin for intraplate Cenozoic volcanism

We have investigated variations in transition zone thickness under the Borborema Province of NE Brazil by migrating and stacking teleseismic P-wave receiver functions at 32 seismic stations in the region. The Borborema Province represents the western portion of a larger Neoproterozoic mobile belt that occupied much of northern Gondwana, where extensional processes in the Mesozoic lead to the formation of a number of intracontinental basins and ultimately continental breakup. Episodes of intraplate volcanism and uplift marked the evolution of the Province during the Cenozoic, but it is unclear whether those episodes originated from shallow or deep-seated magmatic sources. On one hand, the elliptical shape of the uplifted area, the stress pattern of the Cenozoic deformation and the time overlap between uplift and volcanism suggest doming from thermal activation due to a deep-seated mantle plume. On the other hand, geochronological dates of volcanic bodies in the Province are better understood if resulting from lithospheric erosion by a shallow, small-scale convection cell. Large temperature anomalies are expected to be associated with mantle upwellings, and constraints on the depth extent of the upwellings can be obtained from transition zone thickness. Thinning of the transition zone with respect to its nominal 250 km value is considered diagnostic for positive temperature anomalies, while thickening is considered diagnostic for negative anomalies. Our results show that transition zone thickness is normal, around 250 km, throughout the Province and suggest that thermal perturbations-if present-are confined to the upper mantle. We argue that our results are consistent with a local, shallow magmatic source for the Cenozoic intraplate volcanism of the Borborema Province, although other proposed scenarios-such as channeling of upwelling plume material along lithospheric thin spots-cannot be ruled out with our analysis.