ALBERT

Description: The high velocity lower crust HVLC (Vp 〉 7km/s) together with seaward dipping reflectors (SDRs) and continental flood basalts are specific characteristics of volcanic rifted margins. The nature and origin of the HVLC is still under discussion. Here we provide a comprehensive study of the deep crustal structure of the South Atlantic rifted margins in which we focus on the HVLC. We also assess the size and variations along and across the margins. Two new and five existing refraction lines complemented by gravity models cover the area between the Rio Grande Rise - Walvis Ridge to the Falkland Agulhas Fracture Zone. Three seismic lines on the South American margin outline the change from a non-magmatic margin (lacking seaward dipping reflectors) in the south to a well-developed volcanic rifted margin off Uruguay in the north. While the HVLC exhibit a consistent increase in the cross-sectional area along both margins from South to North, we can observe a major asymmetry across the margins. The African margin reveals about two-three times thicker and four times more voluminous HVLC than the South American margin. The distribution of the HVLC stands in a sharp contrast to the one of Etendeka-Paraná flood basalt provinces, which shows the opposite asymmetry. Also the spatial position of the HVLC with regard to the inner SDRs varies consistently from south-to-north along the margins. A simple extrusive/intrusive relationship SDRs and HVLC is questioned. Further it provides evidence for the formation of the HVLC during different times in the rifting and break-up process. We conclude that the HVLC is predominantly a magmatic feature that is related to break-up. Melt generation scenarios based on variations in thickness and average Vp suggest that the greater thickness of HVLC on the African margin is due to active upwelling combined with elevated mantle potential temperatures while the model predicts passive upwelling and a thick lithospheric lid for the South American HVLC. This contrast in upwelling rate and lithospheric thickness can be explained by a model of asymmetric rifting with a simple shear dominated extension. Our estimates for the volume of HVLC bodies imply a total magma production about 4 x 106 km3 on the rifted margins of the South Atlantic.