The bathymetric datum with respect to global sea level for Aptian salt deposition in the South Atlantic is hotly debated. Some models propose that the salt was deposited in an isolated ocean basin in which local sea level was between 2 and 3 km below the global level. In this study, we use reverse post-break-up subsidence modelling to determine the palaeo-bathymetry of base Aptian salt deposition on the Angolan rifted continental margin. The reverse post-break-up subsidence modelling consists of the sequential flexural isostatic back-stripping of the post-break-up sedimentary sequences, decompaction of remaining sedimentary units and reverse modelling of post-break-up lithosphere thermal subsidence. The reverse modelling of post-break-up lithosphere thermal subsidence is carried out in 2D and requires knowledge of the continental lithosphere stretching factor (β), which is determined from gravity anomaly inversion. The analysis has been applied to the ION-GXT CS1-2400 deep long-offset seismic reflection profile, and two seismic cross-sections (P3 and P7+11) from offshore northern Angola. Reverse post-break-up subsidence modelling restores the proximal autochthonous base salt to between 0.2 and 0.6 km below global sea level at the time of break-up. In contrast, the predicted water-loaded bathymetries of the more distal base salt restored to break-up time are much greater between 2 and 3 km. The predicted bathymetries of the first unequivocal oceanic crust at break-up are approximately 2.5 km, as expected for newly formed oceanic crust of ‘normal’ thickness. Several interpretations of these results are possible. Our preferred interpretation is that all Aptian salt on the northern Angola rifted continental margin was deposited between 0.2 and 0.6 km beneath global sea level, and that the proximal salt subsided by post-rift (post-tectonic) thermal subsidence alone; while the distal salt formed during late syn-rift, when the underlying crust was actively thinning, resulting in additional tectonic subsidence (followed by post-rift thermal subsidence). An alternative interpretation is that the distal salt is para-autochthonous and moved downslope into much deeper water during and just after break-up. We do not believe that a deep isolated ocean basin, with a local sea level 2–3 km beneath that of the global sea level, as has been proposed, is required to explain the Aptian salt deposition on the northern Angolan rifted continental margin.
<p>&#160; &#160; The Valencia Trough is commonly included as part of the set of western Mediterranean Cenozoic extensional basins that formed in relation with the Tethyan oceanic slab rollback during the latest Oligocene to early Miocene. It lies in a complex tectonic setting between the Gulf of Lions to the North-West, the Catalan Coastal Range and the Iberian chain to the West, the Balearic promontory to the East and the Betic orogenic system to the South. This rifting period is coeval with or directly followed by the development of the external Betics fold and thrust belts at the southern tip of the Valencia Trough. Recent investigations suggest that the Valencia Trough is segmented into two main domains exhibiting different geological and geophysical characteristics between its northeastern and southwestern parts. The presence of numerous Cenozoic normal faults and the well-studied subsidence pattern evolution of the NE part of the Valencia Trough suggest that it mainly formed coevally with the rifting of Gulf of Lion. However, if a significant post-Oligocene subsidence is also evidenced in its SW part; fewer Cenozoic rift structures are observed suggesting that the subsidence pattern likely results from the interference of different processes.</p><p>&#160; &#160; In this presentation, we quantify the post-Oligocene subsidence history of the SW part of the Valencia Trough with the aim of evaluating the potential mechanisms explaining this apparent subsidence discrepancy. We analyzed the spatial and temporal distribution of the post-Oligocene subsidence using the interpretation of a dense grid of high-quality multi-channel seismic profiles, also integrating drill-hole results and velocity information from expanding spread profiles (ESP). We used the mapping of the main unconformities, especially the so-called Oligocene unconformity, to perform a 3D flexural backstripping, which permits the prediction of the post-Oligocene water-loaded subsidence. Our results confirm that the post-Oligocene subsidence of the SW part of the Valencia Trough cannot be explained by the rifting of the Gulf of Lions. Previous works already showed that the extreme crustal thinning observed to the SW is related to a previous Mesozoic rift event. Here, we further highlight that if few Cenozoic extensional structures are observed, they can be interpreted as gravitational features rooting at the regionally identified Upper Triassic evaporite level. Backstripping results combined with the mapping of the first sediments deposited on top of the Oligocene unconformity show that they are largely controlled by the shape of Betic front with a possible additional effect of preserved Mesozoic structures. At larger scale, we compare the mechanisms accounting for the origin and subsidence at the SW part of the Valencia Trough with those responsible for the subsidence of its NE part and the Gulf of Lions.</p>
Rifted margins are often classified as magma-poor or magma-rich based on a magmatic budget interpretation from seismic reflection data. The southern segment of the East Indian rifted margin is often regarded as a type-example of a magma-poor margin displaying exhumed mantle. However, in its southern segment, 9 km thick transitional crust, previously interpreted as magmatic crust, separates the exhumed mantle from thin oceanic crust. Such thick transitional crust is atypical for a magma-poor margin, so we investigate its likely formation and potential implications for the evolution of magma-poor margins. Using an integrated set of geophysical techniques alongside seismic reflection data, we test the existence of exhumed mantle and the composition of the transitional crust. These geophysical techniques consist of gravity inversion, residual depth anomaly analysis, flexural subsidence analysis and joint inversion of gravity and seismic data. We apply these methods to high-quality seismic reflection data (ION line INE1-1000) on the southern segment of the East Indian rifted margin and test a series of geological scenarios for the margin structure using our integrated quantitative analysis. Of these, our quantitative analysis, seismic observations and the regional plate kinematic history support a structure consisting of thinned continental crust inboard of exhumed, serpentinized mantle followed by thick (∼9 km) magmatic crust transitioning into thin oceanic crust (∼5 km). The juxtaposition of exhumed mantle and thick magmatic crust is explained by the occurrence of a jump in seafloor spreading during the Early Cretaceous formation of the south-east Indian Ocean. The final rifted margin structure contains characteristics of both magma-poor and magma-rich rifted margins resulting from two distinct rift events with different magmatic budgets. The investigation of the East Indian rifted margin structure and evolution shows the importance of incorporating the plate kinematic history and quantitative validation of seismic interpretation into the analysis. Classifying the East Indian margin as a typical magma-poor rifted margin is misleading causing us to question the use of end-member terminology to describe rifted margins.
The Rockall Trough is a large, deep-water, sedimentary basin, offshore west of Ireland. However, relatively little is known about the geological development of the basin. This study uses gravitational field data to investigate the broad crustal structure of the area. This will give valuable information on the formation of the Rockall Trough.
P194 Wide Angle Converted Shear Wave Analysis of Volcanic Rifted Continental Margins J.D. Eccles* (University of Cambridge) R.S. White (University of Cambridge) P.A.F. Christie (Schlumberger Cambridge Research) N.J. Kusznir (University of Liverpool) A.M. Roberts (Badley Geoscience) D. Healy (University of Liverpool) R. Spitzer (University of Cambridge) A. Chappell (University of Liverpool) R. Fletcher (University of Liverpool) N. Hurst (University of Liverpool) Z. Lunnon (University of Cambridge) C.J. Parkin (University of Cambridge) A.W. Roberts (University of Cambridge) L.K. Smith (University of Cambridge) & V.J. Tymms (University of Liverpool) SUMMARY High-quality wide-angle seismic data have been acquired with a low frequency seismic
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