Determination of ocean continent transition structure, continent ocean boundary location and magmatic type at rifted continental margins

• Main Author: Cowie, Leanne
• Published: University of Liverpool 2014
• Subjects: 550
• Online Access: http://ethos.bl.uk/OrderDetails.do?uin=uk.bl.ethos.664370

Knowledge of ocean continent transition (OCT) structure, continent ocean boundary (COB) location and magmatic type are of critical importance for understanding rifted continental margin formation and evolution, and in evaluating petroleum systems in deep-water frontier oil and gas exploration. A suite of quantitative analytical techniques have been developed in order to determine the structure of the OCT, the location of the COB and magmatic type at rifted continental margins; these techniques include the use of gravity anomaly inversion, residual depth anomaly (RDA) and subsidence analysis. Gravity anomaly inversion, incorporating a lithosphere thermal gravity anomaly correction, has been used to determine Moho depth, crustal basement thickness and continental lithosphere thinning. RDA analysis has been used to investigate OCT bathymetric anomalies with respect to expected oceanic bathymetries, and subsidence analysis has been used to determine the distribution of continental lithosphere thinning. These techniques have been applied to the Iberian, Gulf of Aden, northern Angolan, and south-eastern Brazilian rifted continental margins. Integrated quantitative analysis results have been validated using ODP well data and magnetic anomalies along the Iberian rifted continental margin. Results identify a clearly defined zone of exhumed mantle on the Iberian profiles, which is also observed in the ODP well data. In addition to the quantitative analysis techniques, a methodology for the joint inversion of deep long offset seismic data and gravity anomaly data has been developed in order to further constrain the OCT structure and COB location. The joint inversion method solves for coincident seismic and gravity Moho in the time domain and calculates the lateral variations in crustal basement densities and velocities along profile. Integrated quantitative analysis along the northern Angolan profile suggests that exhumed mantle, corresponding to a magma-poor margin, is absent beneath the allochthonous salt and that the thickness of earliest oceanic crust is approximately 7km. Integrated quantitative analysis along the south-eastern Brazilian profile predicts that the earliest oceanic crust is between 7km and 8km thick and that there is no evidence of exhumed mantle as suggested by Zalán et al. (2011). The results from the integrated quantitative analysis have also been used together with post-breakup subsidence modelling to determine the palaeo-bathymetry of the base Loeme salt along northern Angolan profiles. The proximal autochthonous base salt restores to near sea level, but not the distal allochthonous salt. Our interpretation is that the distal salt formed during the late syn-rift while the crust under it was being actively thinned, resulting in additional tectonic subsidence. Residual topography, which we attribute to mantle dynamic topography, has been determined using RDA analysis, with corrections for sediment loading and crustal basement thickness variations. Residual topography of -650m is measured along the Iberian Abyssal Plain; +400m in the Gulf of Aden, and +700m along the northern Angolan margin.