Along Strike Variation of Magmatic Addition on the Austral South Atlantic Rifted Margins

We use a new approach to quantify magmatic addition on the S. American and African rifted margins of the S. Atlantic south of the Florianopolis Fracture Zone. At magma-rich and magma-normal rifted margins, decompression melting starts before the continental crust is thinned to zero thickness. This results in a crustal “sandwich” of volcanics underlain by thinned continental crust, underlain by magmatically intruded continental basement and mantle. Usually all that can be imaged seismically is the top and base of extrusive volcanics and the seismic Moho, with no reliable indication of the quantities of remaining continental crust and magmatic addition. While the individual thicknesses of remaining continental crust and magmatic addition cannot be geophysically determined, their combined isostatic response controls margin bathymetry. We show using a simple isostatically balanced rifted margin model for thermally re-equilibrated lithosphere that the TWTT of first-proximal-volcanics provides a proxy for quantifying the total magmatic addition on a rifted margin, and distinguishing magma-rich from magma-normal rifted margins.  The model predicts that the TWTT of first-proximal-volcanics correlates inversely with the timing of first magmatism with respect to crustal thinning.

We measure the TWTT of first-proximal-volcanics for a set of deep long-offset seismic reflection lines. The TWTT of first-proximal-volcanics for the highly magmatic northern Pelotas and  conjugate Namibian margins is between 1 and 2 s; these margin segments have SDRs with thicknesses in excess of 15 km. Southwards, the TWTT of first-proximal-volcanics increases to between 6 and 7 s, corresponding to a magma-normal margin type. Despite this large north to south variation in first-proximal-volcanics TWTT, the interval TWTT of first oceanic crust varies little between 2.4 and 2.0 s from north to south, corresponding to normal thickness oceanic crust. Application of the Warner 10 s Moho TWTT rule for thermally equilibrated lithosphere indicates  that decompression melting starts when crustal basement interval TWTT is between 8-9 s in the magma-rich north and at 3-4 s TWTT (β = 3) in the magma-normal south. Margin volcanic width, measured between first-proximal-volcanics and the landward limit of oceanic crust (LALOC) is approximately 300 km in the north, decreasing southwards to approximately 50 km width, and correlates inversely with the TWTT of first-proximal-volcanics. TWTT measurements of first-proximal-volcanics show that the very magma-rich margin type is restricted to the north adjacent to the Florianopolis Fracture Zone and rapidly decreases southwards to magma-normal in less than 300 km.

Our TWTT measurements and comparison with the simple isostatic margin model predictions indicate that magma-rich margins are explained by timing advance of decompression melting with respect to crustal thinning rather than melt magnitude increase. This together with the very rapid along-strike Pelotas margin decrease in magmatic addition, and the relatively normal thickness of first oceanic crust is difficult to reconcile with magma-rich margin formation due to mantle plume elevated temperature. Our observations are more consistent with magma-rich margin formation by rifting and decompression melting of inherited locally enriched mantle.