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Timothy Minshull, University of Southampton (United Kingdom)
Donna Shillington, Lamont-Doherty Earth Observatory (United States)
Caroline Scott, University of Southampton (United Kingdom)
Rosemary Edwards, University of Southampton (United Kingdom)
Nicholas White, University of Cambridge (United Kingdom)
Peter Brown, University of Southampton (United Kingdom)
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The volume of magmatism associated with lithospheric extension and breakup at rifted continental margins varies dramatically worldwide. At some margins, thick sequences of arcuate seaward-dipping reflectors indicate extensive synrift volcanic activity, and wedges of material many kilometres thick with P wave velocities in the range 7.2-7.6 km/s are interpreted as the product of voluminous synrift magmatic intrusion. At other margins, syn-rift magmatism is rare. Such variations in melt production may be attributed to along-strike changes in mantle temperature, extension rate, upwelling velocity, small-scale convection, or mantle composition. In several locations, such as offshore Nova Scotia and the Gulf of California, an along-strike transition between these two types of margin has been observed. However, the nature of such transitions has not been characterised in detail because of a lack of appropriate data. In February-March 2005 we conducted a major seismic experiment across the Eastern Black Sea Basin, a deep rift basin of uncertain age and crustal affinities. We acquired four long wide-angle seismic profiles across different parts of the basin, including a ∼450-km along-strike profile, using a 9-gun, 3140 cu. in. airgun array, tuned to provide a seismic source rich in low frequencies. On each profile, between 14 and 34 four-component ocean bottom seismometers were deployed, and the airguns were fired at 60 or 90 s intervals. Shots were also recorded on land stations up to 50 km from the coast. The lines were approximately coincident with existing industry deep seismic reflection profiles. The experiment resulted in a very high-quality dataset, with reflected and refracted phases recorded from a series of sediment layers, from the crystalline crust, and from the uppermost mantle. Wide-angle seismic data were modelled using both first-arrival traveltime tomography and joint reflection-refraction tomography. Analysis of crustal arrivals revealed substantial along-strike changes in crustal thickness and type in the basin. In the western part of the basin, continental crust thins abruptly from ∼32 km to ∼8 km over ∼30 km distance across the northeastern margin of Archangelsky Ridge, and thinned continental crust appears to be present across the centre of the basin, with no evidence for magmatic addition to the crust during rifting. In contrast, the eastern part of the basin is underlain by 11-13 km crust with a velocity structure typical of thickened oceanic crust, indicating robust magmatism. The transition between these two crustal types takes place over only 20-30 km along-strike and appears to follow an inferred NE-SW-trending transform fault, indicating a close link between magmatism and segmentation of the margin. We explore the possible origins of such an abrupt transition.
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