The Along-Atrike Pattern of Magmatism During Breakup in the Southern South Atlantic: Evidence From Numerical Modelling and Deep Seismic Reflection Data Sets

Abstract

The Argentinian and Uruguayan passive margins are classic examples of volcanic margins and continue to be a focus for hydrocarbon exploration. Whilst our understanding of rift formation and evolution has improved in recent years there remains a fundamental debate on the relative roles of deep processes, and in particular mantle plumes, compared to inherited crustal structure. An understanding of both the magmatism during rifting and the geodynamics of margin formation will be essential for exploration success in the South Atlantic. We document the distribution of magmatism in the southern segment of the South Atlantic, where we have mapped the volcanic seaward dipping reflector sequences (SDRs) and high velocity lower crust reflectivity (HVLC) using recently acquired, long-offset, commercial seismic reflection data that images to 40 km depth. From this mapping we can make estimates of the volume of syn-rift magmatism expressed as both magmatic intrusions (HVLC) and extrusive lava flows (SDRs). Previous work, based on closely spaced, academic seismic reflection data imaging to 8-10 s TWT, has suggested that rather than a systematic increase in the volumes of the SDRs towards the ancestral Tristan da Cunha hot-spot, SDR thicknesses vary within ∼500-km long segments defined by transfer zones which may have acted as rift propagation barriers during Cretaceous rifting. We test this hypothesis with our new mapping and numerical models of melt production during lithospheric extension. We have compiled observations of the along-strike variation in initial ocean crustal thickness as a measure of the thermal structure immediately post-breakup. We show that along both conjugate sides there is a systematic decrease in oceanic crust production with distance away from the ancestral Tristan hotspot impact point, averaging 1.4 km/1000 km. This observation can be matched with numerical models with a 200 km thick hot later beneath the lithosphere, where the excess temperature reduces from 200°C to 50°C over a distance of 1500 km, southwards away from the plume. These results suggest that a simple plume concept is appropriate for these margins to a first order, albeit with modification from plate thickness. However our mapping suggests that at a more local scale there are significant along-margin thickness variations in the SDR packages at length-scales of 100-200 km, which suggests additional controls on the rift-related magmatism of the margin.

AAPG Datapages/Search and Discovery Article #90259 ©2016 AAPG Annual Convention and Exhibition, Calgary, Alberta, Canada, June 19-22, 2016