Deep structure of the western South African passive margin — Results of a combined approach of seismic, gravity and isostatic investigations

Abstract

The passive margin of the South Atlantic shows typical features of a rifted volcanic continental margin, encompassing seaward dipping reflectors, continental flood basalts and high-velocity/density lower crust at the continentocean transition, probably emplaced during initial seafloor spreading in the Early Cretaceous.The Springbok profile offshore western South Africa is a combined transect of reflection and refraction seismic data. This paper addresses the analysis of the seismic velocity structure in combination with gravity modelling and isostatic modelling to unravel the crustal structure of the passive continental margin from different perspectives.The velocity modelling revealed a segmentation of the margin into three distinct parts of continental, transitional and oceanic crust. As observed at many volcanic margins, the lower crust is characterised by a zone of high velocities with up to 7.4 km/s. The conjunction with gravity modelling affirms the existence of this body and at the same time substantiated its high densities, found to be 3100 kg/m³. Both approaches identified the body to have a thickness of about 10 km. Yet, the gravity modelling predicted the transition between the highdensity body towards less dense material farther west than initially anticipated from velocity modelling and confirmed this density gradient to be a prerequisite to reproduce the observed gravity signal. 2Finally, isostatic modelling was applied to predict average crustal densities if the margin was isostatically balanced. The results imply isostatic equilibrium over large parts of the profile, smaller deviations are supposed to be compensated regionally. The calculated load distribution along the profile implies that all pressures are hydrostatic beneath a depth of 45 km.3

Asymmetry of high-velocity lower crust on the South Atlantic rifted margins and implications for the interplay of magmatism and tectonics in continental breakup

“…The fourth traverse is located at about 30 • S (Fig. 1, line 7) and the seismic velocity profile derived by Hirsch et al (2009) shows a well-developed body of HVLC below SDRs. Finally, it is worth mentioning for reference that Stankiewicz et al (2008) published a seismic velocity profile ( Fig.…”

“…In contrast to the reasonable spatial coverage of seismic reflection data, wide-angle seismic lines are few, especially on the South American margin, which motivated the new studies reported below. To some extent, gaps in the seismic coverage can be compensated for by regional gravity interpretations (e.g., Blaich et al, 2009Blaich et al, , 2011Dragoi-Stavar and Hall, 2009;Maystrenko et al, 2013;Franke et al, 2006;Hirsch et al, 2009). …”

Rifted margins: Ductile deformation, boudinage, continentward-dipping normal faults and the role of the weak lower crust

“…the case of a volcanic margin such as the Uruguay one, the weakness of the crust could be related to the high heat flow related to the emerging plume and/or to the important syn-tectonic underplating and impregnation of the lower crust by magmatic melts (Franke et al, 2006;Hirsch et al, 2009). On the conjugate margin of Namibia/South Africa, high seismic velocities (up to >7 km/s) also point to a gabbroic composition of the lower crust (Fromm et al, 2015).…”

Deep Crustal Profile Across the Southern Karoo Basin and Beattie Magnetic Anomaly, South Africa: An Integrated Interpretation With Tectonic Implications

“…Namaqua-Natal metamorphic terranes (NNMB): R = Richtersveld; G = Gordonia; B= Bushmanland of the Namaqua province; N = Natal province. Black = Green and Durrheim (1990), Mahanyele et al (2004), Hirsch et al (2008); Lines on the South coast: light grey line = onland seismic refraction line of Stankiewicz et al (2007;2008) and the longer onshore-offshore seismic refraction line (Stankiewicz et al 2008;Parsiegla et al 2007;; black line perpendicular to CFB front = IyA-The regions with complex reflectivity patterns cross-cut by the high conductivity anomalies in the mid-crust (Figures 7 and 9) could also be interpreted as ore bodies, since ore minerals such as Pyrrhotite and other iron sulphides have a high conductivity. Impedance spectroscopy on samples from boreholes SA-1/66 and KW-1/67, proved high conductivity in the Wh/PA Formations, associated with pyrite and chalcopyrite (Branch et al, 2007).…”