Brian Horn

Evolution of seaward-dipping reflectors at the onset of oceanic crust formation at volcanic passive margins: Insights from the South Atlantic

Seaward-dipping reflectors (SDRs) have long been recognized as a ubiquitous feature of volcanic passive margins, yet their evolution is much debated, and even the subject of the nature of the underlying crust is contentious. This uncertainty significantly restricts our understanding of continental breakup and ocean basin–forming processes. Using high-fidelity reflection data from offshore Argentina, we observe that the crust containing the SDRs has similarities to oceanic crust, albeit with a larger proportion of extrusive volcanics, variably interbedded with sediments. Densities derived from gravity modeling are compatible with the presence of magmatic crust beneath the outer SDRs. When these SDR packages are restored to synemplacement geometry we observe that they thicken into the basin axis with a nonfaulted, diffuse termination, which we associate with dikes intruding into initially horizontal volcanics. Our model for SDR formation invokes progressive rotation of these horizontal volcanics by subsidence driven by isostasy in the center of the evolving SDR depocenter as continental lithosphere is replaced by more dense oceanic lithosphere. The entire system records the migration of >10-km-thick new magmatic crust away from a rapidly subsiding but subaerial incipient spreading center at rates typical of slow oceanic spreading processes. Our model for new magmatic crust can explain SDR formation on magma-rich margins globally, but the estimated crustal thickness requires elevated mantle temperatures for their formation.

Structure of the ocean–continent transition, location of the continent–ocean boundary and magmatic type of the northern Angolan margin from integrated quantitative analysis of deep seismic reflection and gravity anomaly data

Abstract The crustal structure and distribution of crustal types on the northern Angolan rifted continental margin have been the subject of much debate. Hyper-extended continental crust, oceanic crust and exhumed serpentinized mantle have all been proposed to underlie the Aptian salt and the underlying sag sequence. Quantitative analysis of deep seismic reflection and gravity anomaly data, together with reverse post-break-up subsidence modelling, have been used to investigate the ocean–continent transition structure, the location of the continent–ocean boundary, the crustal type and the palaeobathymetry of Aptian salt deposition. Gravity inversion methods (used to give the depth to the Moho and the crustal thickness), residual depth anomaly analysis (used to identify departures from oceanic bathymetry) and subsidence analysis have all shown that the distal Aptian salt is underlain by hyper-extended continental crust rather than exhumed mantle or oceanic crust. We propose that the Aptian salt was deposited c. 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, whereas outer distal salt formation was synrift, prior to break-up, resulting in additional tectonic subsidence. Our analysis argues against Aptian salt deposition on the Angolan margin in a 2–3 km deep isolated ocean basin and supports salt deposition on hyper-extended continental crust formed by diachronous rifting migrating from east to west and culminating in the late Aptian.

The palaeo-bathymetry of base Aptian salt deposition on the northern Angolan rifted margin: constraints from flexural back-stripping and reverse post-break-up thermal subsidence modelling

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.

Evidence for magma entrapment below oceanic crust from deep seismic reflections in the Western Somali Basin

Our understanding of melt generation, migration, and extraction in the Earth’s mantle beneath mid-oceanic ridges is mostly derived from geodynamic numerical models constrained by geological and geophysical observations at sea and field investigations of ophiolites, and is therefore restricted to the oceanic crust and the shallow part of the mantle. Here we use a >200-km-long, deep seismic reflection section to image with high resolution the sub-oceanic lithosphere within the Western Somali Basin (offshore eastern Africa) where spreading ceased at ca. 120 Ma. The location of the failed spreading axis is inferred from both seismic data and gravity data. Several groups of strong reflections are imaged to depths of >30 km below the top of the oceanic crust. We interpret the deepest reflectors, within the mantle, as resulting from frozen melt bodies which may be relicts of a paleo–melt channel system located at the base of the lithosphere and formerly feeding the failed ridge axis. Other reflectors within the mantle may correspond to melt bodies injected into major shear zones along the Davie fracture zone. Another group of reflectors, located below a 8–5-km-thick oceanic crust, is interpreted as marking a fossil melt-rich crust-mantle transition zone as much as 3 km thick. This interpretation implies an inefficient extraction of melt out of the mantle, which is favored by the combination of a slow spreading rate and a high magma budget.

Introduction to special section: Salt basin model building, imaging, and interpretation

Salt basins such as the Gulf of Mexico, Brazil, and West Africa have proven to be prolific areas for hydrocarbon discoveries. In these basins the salt usually presents a high velocity contrast, with fast salt juxtaposed against slower sediments creating substantial imaging challenges, making