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The Palaeo-bathymetry of Base Aptian Salt Deposition on the Northern Angolan Rifted Margin and the Composition of Underlying Basement

Author(s): Leanne Cowie Earth and Ocean Sciences, University of Liverpool, Liverpool, L69 3BX, United Kingdom
R. M. Angelo Earth and Ocean Sciences, University of Liverpool, Liverpool, L69 3BX, United Kingdom
Nick J. Kusznir Earth and Ocean Sciences, University of Liverpool, Liverpool, L69 3BX, United Kingdom
Gianreto Manatschal IPGS-CNRS-EOST, Université de Strasbourg, Strasbourg, France
Brian Horn ION Geophysical Houston, TX, U.S.A.

The bathymetric datum with respect to global sea level for Aptian salt deposition on the deep-water Angolan rifted margin and the composition of underlying basement are hotly debated. Hyper-extended continental crust, oceanic crust and exhumed serpentinised mantle have been proposed to underlie the Aptian salt. Some models propose that the salt was deposited in an isolated ocean basin in which local sea level was between 2 km and 3 km below global level. Quantitative analysis of deep seismic reflection and gravity anomaly data together with reverse post-breakup subsidence modelling has been used to investigate ocean-continent transition structure, continent-ocean boundary location crustal type and the palaeo-bathymetry of Aptian salt deposition. The analysis has been applied to the ION-GXT CS1-2400 deep long-offset seismic reflection profile and the P3 and P7+11 seismic cross sections of Moulin (2005) and Contrucci et al. (2004) offshore northern Angola.

The palaeo-bathymetry of base Aptian salt deposition has been examined using 20 reverse post-breakup subsidence modelling consisting of the sequential flexural isostatic backstripping of the post-breakup sedimentary sequences, decompaction of remaining sedimentary units and reverse modelling of post-breakup lithosphere thermal subsidence. The reverse modelling of post-breakup lithosphere thermal subsidence requires knowledge of the continental lithosphere stretching factor (beta), which is determined from gravity anomaly inversion.

Our analysis predicts that Aptian salt was deposited between approximately 0.2 and 0.6 km below global sea level, and that the inner proximal salt subsided by post-rift (post-tectonic) thermal subsidence alone. In contrast, the predicted water-loaded bathymetries of the more distal base salt at breakup time are much greater (ranging between 1 km and 3 km) implying that the base outer distal salt was deposited pre-breakup and underwent additional syn-tectonic crustal-thinning subsidence. The predicted bathymetries of the first unequivocal oceanic crust at breakup are approximately 2.5km, as expected for newly formed oceanic crust of 'normal' thickness.

Gravity inversion to give Moho depth and crustal thickness, RDA (residual depth anomaly) analysis to identify departures from oceanic bathymetry and subsidence analysis shows that the distal Aptian salt is underlain by hyper-extended continental crust rather than exhumed mantle or oceanic FIXME text ends unexpectedly


Title:
The Palaeo-bathymetry of Base Aptian Salt Deposition on the Northern Angolan Rifted Margin and the Composition of Underlying Basement
Type:
Oral
Origin:
Academia
Day:
2
Session:
1
Daily sequence no.:
07
Lead author last name:
Cowie
Lead author first name:
Leanne
Affiliation(s):
Earth and Ocean Sciences, University of Liverpool, Liverpool, L69 3BX
Country:
United Kingdom
Abstract status:
Abstract text/title needs fixing
UID:
41