Gravity modeling of the ocean‐continent transition along the South Atlantic margins

Abstract

Gravity, magnetic, and seismic data have been used to examine changes in crustal structure of conjugate portions of the South Atlantic volcanic margins south of the Walvis Ridge‐Rio Grande Rise hot spot tracks. We have constructed 18 seismically constrained crustal‐scale gravity models of the ocean‐continent transition: 12 across the African margin and 6 across the South American margin. We attribute changes in character of the gravity anomalies to variations in crustal thickness, sedimentation, and magmatic underplating. To investigate variations in the lower crust and upper mantle, we have removed the relatively well‐constrained contributions to the gravity field from shallow sources. The resulting residual anomalies delineate distinct crustal domains associated with crustal thinning and regions of underplating that correspond well with those independently identified from limited seismic refraction data. Near the hot spot tracks, we find underplated crust to be symmetrically distributed with a horizontal extent of ∼240 km for each margin. Further south, however, the underplated region is asymmetric with significantly more beneath the African than the corresponding South American margin. Residual gravity anomalies indicate that the width of underplated crust decreases from 240 km to ∼150 km at 33°S on the African side and to ∼100 km at 38°S on the South American side. The wider underplated region on the African side suggests an asymmetric rifting mechanism in which greater melts accumulate beneath an upper plate margin. Our results are consistent with recently proposed asymmetric rifting models that involve strain softening of a coupled, cold frictional upper lithosphere at low rifting velocities. Such models also predict greater symmetry for areas with more heat such as those associated with the hot spot.