R. A. Edwards

The crustal structure across the transform continental margin off Ghana, eastern equatorial Atlantic

Forward modeling of a suite of wide-angle seismic lines across the transform continental margin at the eastern end of the Romanche fracture zone off Ghana has shown the transition from continental to oceanic crust to be confined to a narrow, 6 to 11-km-wide zone located at the foot of the steep continental slope. The structure of the adjacent oceanic and continental crusts has also been resolved. These results are confirmed and enhanced by gravity and magnetic models. The crust of the ocean-continent transition zone is characterized by high velocities (5.8–7.3 km s−1), a high density (3.10 Mg m−3), and high magnetizations (1.1–1.25 A m−1). This rules out a purely continental origin for the zone and suggests that it may be formed of basic igneous rocks intruded when the hot oceanic spreading center migrated along the margin. The oceanic crust within 70 km of the margin is abnormally thin compared to normal Atlantic oceanic crust and shows an average thickness of just 4.4 km. We suggest that the region of abnormally thin oceanic crust is the result of a reduced magma supply due to a combination of closely spaced fracture zones and the conductive loss of heat from the upwelling oceanic mantle in small basins surrounded, on at least three sides, by cold continental lithosphere. The preferred seismic model also shows a new fracture zone, 30 km southeast of the ocean-continent transition, which is characterized by low velocities in the upper crust, a lens-shaped layer with velocities of 7.2–7.4 km s−1 at the base of the crust, and a crustal thickness of just 3.4 km. The continental crust appears largely unaffected by the proximity of the adjacent oceanic crust. There is no evidence for underplating of the continental crust adjacent to the ocean-continent transition zone.

Crustal structure of the NE Rockall Trough from wide‐angle seismic data modeling

Two wide-angle seismic lines located in the northern Rockall Trough were acquired in May 2000. One line (line E) crosses the trough from the continental shelf off Lewis to normal oceanic crust west of Lousy Bank in NW-SE direction. The other line (line D) intersects with line E, crosses the Wyville-Thomson Ridge in a SW-NE direction and ends in the Faeroe-Shetland Basin. Sonobuoy data and expanding spread profiles acquired in the same area have been remodeled. Analysis of the seismic data using travel times and amplitudes reveals an up to 5 km thick sedimentary basin including an up to 1.5 km thick basaltic layer which is present in most of the trough. Further conclusions of this study are that the Rockall Trough is underlain by highly stretched continental crust of ~13 km thickness. The crust thickens to ~24 km beneath Lousy Bank, which is interpreted to be of continental nature. Beneath the Hebrides continental shelf a three-layer continental crust of 26 km is modeled. An up to 12 km thick high-velocity layer is observed underneath the ocean-continent boundary and is interpreted as magmatic underplating resulting from excess volcanism during rifting. No evidence for an underplate layer could be distinguished beneath the trough area. Modeling of the structure of the Wyville-Thomson Ridge revealed no existing igneous core of the ridge confirming existing theories, that it is a compressional structure.

Abrupt transition from magma-starved to magma-rich rifting in the eastern Black Sea

The amount of magmatism that accompanies the extension and rupture of the continental lithosphere varies dramatically at rifts and margins around the world. Based on widely spaced geophysical transects, some margins are known to preserve a transition from magmatically robust to magmatically starved rifting along strike, but the nature of the transition is unknown. Wide-angle seismic data from the Black Sea provide the first direct observations of such a transition and show that it is abrupt, occurring over only ~20–30 km, and coincides with a transform fault. This abrupt transition cannot be explained solely by gradual along-margin variations in mantle properties, since these would be expected to result in a smooth transition from magma-poor to magma-rich rifting over hundreds of kilometers. We suggest that the abruptness of the transition results from the development of three-dimensional (3-D) melt migration due to along-strike variations in extension and thus the thickness of the lithosphere at the time of rifting. Localized magmatic addition attributed to melt focusing has been observed in modern mid-ocean ridges and active rift environments, but here we show that such processes can also produce abrupt along-strike changes from magma-poor to magma-rich rifting.

Wide-angle seismic data reveal sedimentary and crustal structure of the Eastern Black Sea

Wide-angle seismic data from the Eastern Black Sea have been used to determine the geological structure of the sediments, the entire crust, and upper mantle. Data were acquired using a combination of ocean-bottom seismometers (OBS), land seismometers, and a marine air-gun source, providing refracted and reflected energy recorded to offsets in excess of 100 km.

Seismic data reveal eastern Black Sea basin structure

Rifted continental margins are formed by progressive extension of the lithosphere. The development of these margins plays an integral role in the plate tectonic cycle, and an understanding of the extensional process underpins much hydrocarbon exploration. A key issue is whether the lithosphere extends uniformly, or whether extension varies with depth. Crustal extension may be determined using seismic techniques. Lithospheric extension may be inferred from the waterloaded subsidence history, determined from the pattern of sedimentation during and after rifting. Unfortunately, however, many rifted margins are sediment-starved, so the subsidence history is poorly known. To test whether extension varies between the crust and the mantle, a major seismic experiment was conducted in February–March 2005 in the eastern Black Sea Basin (Figure 1), a deep basin where the subsidence history is recorded by a thick, post-rift sedimentary sequence. The seismic data from the experiment indicate the presence of a thick, low-velocity zone, possibly representing overpressured sediments. They also indicate that the basement and Moho in the center of the basin are both several kilometers shallower than previously inferred. These initial observations may have considerable impact on thermal models of the petroleum system in the basin. Understanding the thermal history of potential source rocks is key to reducing hydrocarbon exploration risk. The experiment, which involved collaboration between university groups in the United Kingdom, Ireland, and Turkey, and BP and Turkish Petroleum (TPAO), formed part of a larger project that also is using deep seismic reflection and other geophysical data held by the industry partners to determine the subsidence history and hence the strain evolution of the basin.

From Continental Hyperextension to Seafloor Spreading: New Insights on the Porcupine Basin from Wide-angle Seismic Data

Key Points: - New analysis of wide-angle seismic data from the southern Porcupine Basin. - Evidence for presence of oceanic crust in the southern Porcupine Basin. - Jurassic rifting propagated from south to north, resulting in non-uniform strain when rifting stopped. The deep structure and sedimentary record of rift basins provide an important insight into understanding the geological processes involved in lithospheric extension. We investigate the crustal structure and large‐scale sedimentary architecture of the southern Porcupine Basin, offshore Ireland along three wide‐angle seismic profiles, supplemented by thirteen selected seismic reflection profiles. The seismic velocity and crustal geometry models obtained by joint refraction and reflection travel‐time inversion clearly image the deep structure of the basin. Our results suggest the presence of three distinct crustal domains along the rifting axis: (a) continental crust becoming progressively hyperextended from north to south through the basin, (b) a transitional zone of uncertain nature and (c) a 7‐8 km thick zone of oceanic crust. The latter is overlain by a ~ 8 km compacted Upper Paleozoic‐Mesozoic succession and ~ 2 km of Cenozoic strata. Due to the lack of clear magnetic anomalies and in the absence of well control, the precise age of interpreted oceanic crust is unknown. However, we can determine an age range of Late Jurassic to Late Cretaceous from the regional context. We propose a northward‐propagating rifting process in the Porcupine Basin, resulting in variations in strain along the rift axis.

Converted PS-wave Velocity Structure from Full Waveform Inversion Reveal High Pore Pressures in the Eastern Black Sea

We model an approximately 100 km southeast-northwest transect in the eastern part of the EBSB, where 15 ocean bottom seismometers were placed between the coast and the centre of the basin. The data are of excellent quality and we were able to analyse both previously indentified P-wave reflectors and some new reflectors found during the processing of this model.Previous analysis has revealed the presence of a widespread low-velocity zone within the basin fill, which coincides with the Maikop formation, an important sedimentary sequence of this region due to its potential as a hydrocarbon source. This low-velocity zone is interpreted as due to excess pore pressures. Our detailed S wave model extends to 3.5 km below the seabed, with a single layer beneath this depth that extends to a deeper horizon marking the base of post-rift sediments. The Poisson’s ratio varies from approximately 0.49 close to the seabed to ~0.38 in one of the deepest layers . In the low velocity zone there is an increase of the Poisson’s ratio. We then use full-waveform inversion to resolve finer structure and estimate pore pressures using Kuster-Toksoz theory and Differential Effective Medium theory.

Estimating Overpressure in Deep Sediments of the Eastern Black Sea Basin from Wide-Angle Seismic Tomography

P193 Estimating Overpressure in Deep Sediments of the Eastern Black Sea Basin from Wide-Angle Seismic Tomography C.L. Scott* (National Oceanography Centre Southampton) D.J. Shillington (National Oceanography Centre Southampton) T.A. Minshull (National Oceanography Centre Southampton) R.A. Edwards (National Oceanography Centre Southampton) & N.J. White (University of Cambridge) SUMMARY Data acquired by ocean bottom seismometers (OBS) can give better constraints on the velocity structure of deep basins allowing estimates to be made of pore pressure in deep sediments. The Black Sea is a deep sedimentary basin with a thick infill of ~9 km and is a frontier basin for hydrocarbon exploration. Mud

Crustal structure of the NE Rockall Trough from wide-angle seismic data modeling

Two wide-angle seismic lines located in the northern Rockall Trough were acquired in May 2000. One line (line E) crosses the trough from the continental shelf off Lewis to normal oceanic crust west of Lousy Bank in NW-SE direction. The other line (line D) intersects with line E, crosses the Wyville-Thomson Ridge in a SW-NE direction and ends in the Faeroe-Shetland Basin. Sonobuoy data and expanding spread profiles acquired in the same area have been remodeled. Analysis of the seismic data using travel times and amplitudes reveals an up to 5 km thick sedimentary basin including an up to 1.5 km thick basaltic layer which is present in most of the trough. Further conclusions of this study are that the Rockall Trough is underlain by highly stretched continental crust of ~13 km thickness. The crust thickens to ~24 km beneath Lousy Bank, which is interpreted to be of continental nature. Beneath the Hebrides continental shelf a three-layer continental crust of 26 km is modeled. An up to 12 km thick high-velocity layer is observed underneath the ocean-continent boundary and is interpreted as magmatic underplating resulting from excess volcanism during rifting. No evidence for an underplate layer could be distinguished beneath the trough area. Modeling of the structure of the Wyville-Thomson Ridge revealed no existing igneous core of the ridge confirming existing theories, that it is a compressional structure.