The role of mantle temperature and lithospheric thickness during initial oceanic crust production: numerical modelling constraints from the southern South Atlantic

Abstract:

Evidence from seaward dipping reflector distributions has recently suggested that segmentation plays a major role in the pattern of volcanism during breakup, particularly in the South Atlantic. At a larger scale, variations in mantle temperature and lithosphere thickness can enhance or reduce volcanism. To understand what generates along strike variation of volcanism at conjugate margins, we measure the thickness of earliest oceanic crust in the South Atlantic, south of the Walvis and Rio Grande ridges. We use data from 29 published wide-angle and multichannel seismic profiles and at least 14 unpublished multichannel seismic profiles. A strong linear trend between initial oceanic crustal thickness and distance from hotspot centres, defined as the commencement of Walvis and Rio Grande ridges, with a regression coefficient of 0.7, is observed. At 450km south of the Walvis Ridge, earliest oceanic crustal thickness is found to be 11.7km. This reduces to 7.0km in the south at a distance of 1,420km. Such a linear trend suggests rift segmentation plays a secondary role on volcanism during breakup. To explore the cause of this trend, we use a 2D numerical model of extension capable of predicting the volume and composition of melt generated by decompressional melting during extension to steady state seafloor spreading. We explore the effect of both mantle temperature and lithosphere thickness on melt production with a thermal anomaly (hot layer) 100km thick located below the lithosphere with an excess temperature of 50-200°C, and lithospheric thickness ranging from 125-140km, covering the thickness range estimated from tomographic studies. By focusing on a set of key seismic profiles, we show a reduction in hot layer temperature is needed in order to match observed oceanic crustal thickness, even when the effect of north to south variations in lithosphere thickness are included. This model implies that the observed oceanic thickness requires the influence of a hot layer up to 1,500km away from the hotspot centre, with temperatures decreasing from 200 to 75°C from north to south. This decrease in mantle temperature, also seen in published thermobarometry results, suggests that temperature is the primary control of volcanism during breakup of the southern South Atlantic.