Integrated gravity, magnetic, and seismic constraints on continent-ocean boundary geophysical characteristics at volcanic passive margins

Volcanic passive margins constitute more than half of the global length of passive margins, with seaward-dipping reflectors (SDRs) being a hallmark feature of their ocean-continent transition zones. During the development of volcanic passive margins, significant amounts of mantle-derived magmas participate in the continental rifting process, representing a critical phase in the evolution of the crust-mantle system, which is crucial for understanding the Earth's internal dynamics. However, the specific mechanisms governing the transition from continental to oceanic crust and the precise delineation of the continent-ocean boundary remain contentious issues within the scientific community. This study focuses on two pairs of selected conjugate volcanic passive margins: the Uruguay-Namibia margins in the South Atlantic and the Laxmi Basin margins off western India. By analyzing multi-channel seismic reflection profiles, magnetic anomalies, free-air gravity anomalies, Bouguer gravity anomalies, and vertical gravity gradients, we conducted a detailed analysis of the structure and gravity-magnetic characteristics of the ocean-continent transition zone. Our findings reveal that, although the crustal structures of the conjugate margins display asymmetric development along the spreading center, their gravity and magnetic anomaly patterns exhibit axial symmetry from the ocean-continent transition zone to the spreading center. Both seismic and gravity-magnetic anomaly data indicate that the crustal structure, density, material composition, and magma upwelling mechanisms of the outer SDRs differ from those of typical oceanic crust. We propose that following the maturation of the outer SDRs, there was a significant shift in the magma upwelling pattern, with the transition from continental to oceanic crust occurring within adjacent thickened crustal regions.