Abstract The Mid Black Sea High comprises two en echelon basement ridges, the Archangelsky and Andrusov ridges, that separate the western and eastern Black Sea basins. The sediment cover above these ridges has been characterized by extensive seismic reflection data, but the crustal structure beneath is poorly known. We present results from a densely sampled wide-angle seismic profile, coincident with a pre-existing seismic reflection profile, which elucidates the crustal structure. We show that the basement ridges are covered by approximately 1–2 km of pre-rift sedimentary rocks. The Archangelsky Ridge has higher pre-rift sedimentary velocities and higher velocities at the top of basement ( c. 6 km s −1 ). The Andrusov Ridge has lower pre-rift sedimentary velocities and velocities less than 5 km s −1 at the top of the basement. Both ridges are underlain by approximately 20-km-thick crust with velocities reaching around 7.2 km s −1 at their base, interpreted as thinned continental crust. These high velocities are consistent with the geology of the Pontides, which is formed of accreted island arcs, oceanic plateaux and accretionary complexes. The crustal thickness implies crustal thinning factors of approximately 1.5–2. The differences between the ridges reflect different sedimentary and tectonic histories.

Abstract Much of our knowledge on hydrate distribution in the subsurface comes from interpretations of remote seismic measurements. A key step in such interpretations is an effective medium theory that relates the seismic properties of a given sediment to its hydrate content. A variety of such theories have been developed; these theories generally give similar results if the same assumptions are made about the extent to which hydrate contributes to the load-bearing sediment frame. We have further developed and modified one such theory, the self-consistent approximation/differential effective medium approach, to incorporate additional empirical parameters describing the extent to which both the sediment matrix material (clay or quartz) and the hydrate are load-bearing. We find that a single choice of these parameters allows us to match well both P and S wave velocity measurements from both laboratory and in situ datasets, and that the inferred proportion of hydrate that is load-bearing varies approximately linearly with hydrate saturation. This proportion appears to decrease with increasing hydrate saturation for gas-rich laboratory environments, but increases with hydrate saturation when hydrate is formed from solution and for an in situ example.

Abstract The West Iberia margin is a magma-poor rifted margin that resulted from Jurassic to Cretaceous polyphase rifting leading to the opening of the North Atlantic Ocean. The Mesozoic rift structures were overprinted by two compressive tectonic events during Eocene and Miocene times resulting from collision between Iberia, Europe and Africa. The effects of these compressive tectonic events are expressed by faults and folds within the post-rift sedimentary sequence. We mapped and studied these Cenozoic deformation structures throughout the Southern Iberia Abyssal Plain (40°–41°N, 11°–13°W) on the basis of an extensive dataset of time migrated seismic profiles acquired by various academic institutions. Acoustic basement has also been analysed on the basis of its seismic aspect, in order to test potential relationships with the distribution of the post-rift sedimentary deformation. Our observations lead to three major conclusions concerning the deformation affecting the post-rift sediments in the Southern Iberia Abyssal Plain: (1) the deformation occurs within the zone of exhumed continental mantle and not at its transition to continental or oceanic crust; (2) it is localized within a zone overlying basement with well-defined seismic characteristics; and (3) it is closely related to the major topographic features observed in the ocean–continent transition. The localization of the deformation within the zone of exhumed continental mantle and not at its boundaries to the adjacent oceanic and continental crust suggests that the limits between the different types of crust are transitional rather than sharp. Our results show that the zone of exhumed continental mantle represents the weakest zone within the margin that is preferentially deformed during initial convergence. At higher convergence rates, this zone may coincide with the location of a future subduction.

Abstract Although the Iberia–Newfoundland and Alpine Tethys margins are of different age and ultimately had a different fate, they share remarkable similarities. Both pairs of margins show a change from initially distributed and decoupled extension to later localized, coupled and asymmetric extension that results in thinning of the crust and exhumation of subcontinental mantle. The change in the mode of extension together with the localization of deformation reflects an evolution of the bulk rheology of the extending lithosphere. In this paper we summarize the pertinent geological observations for the Iberia–Newfoundland and Alpine Tethys margins. We describe the stratigraphic evolution, the fault geometry, basin architecture, and magmatic and metamophic evolution of the two pairs of margins from initial rifting to final continental breakup. This description forms a basis for understanding the evolution of the bulk rheology and how the various processes interact during progressive lithospheric extension. For the Iberia–Newfoundland and Alpine Tethys margins initial rifting appears to be controlled by inherited heterogeneities and mechanical localization processes, whereas final rifting and lithospheric rupture is controlled by serpentinization, magmatic and thermal weakening. At other margins, these modes may interact in a different way depending on the prerift conditions and the evolution of the rheology during rifting.

Abstract The presence of a well-defined ocean-continent transition (OCT) and the absence of large volumes of extrusive or intrusive rocks on the West Iberia margin make it a good place to investigate how the largely amagmatic rifting and break-up of continental lithosphere evolves into oceanic crust produced by magmatic sea-floor spreading. In the southern Iberia Abyssal Plain there is a broad OCT with a characteristic seismic and magnetic character, distinct from both thinned continental crust and normal oceanic crust, which supports the notion that it consists predominantly of exhumed and serpentinized mantle. Interpretations of magnetic and seismic data indicate that on average only small amounts of syn-rift melt exist within the OCT. Isolated, probably margin-parallel, intrusive melt bodies are scattered within the eastern part of the OCT well beneath the top of acoustic basement. Within the western part of the OCT, closer to unambiguous sea-floor spreading magnetic anomalies, such bodies were later(?) emplaced at higher levels and more closely together in the basement until eventually sea-floor spreading began. The evidence does not support the hypothesis that ultraslow sea-floor spreading can explain the magnetic anomalies observed within the wider parts of the West Iberia OCT, where the OCT evolution is best resolved.

Abstract Recent geophysical work and Ocean Drilling Program drilling in the southern Iberia Abyssal Plain have indicated that, in a transition zone up to 170 km wide between thinned continental crust and oceanic crust, the basement consists of serpentinized peridotite mantle with sparse mafic intrusive or extrusive rocks. There is no evidence for the addition of significant magmatic material to the stretched continental crust landward of this zone during the last phase of rifting, whereas seaward of this zone, where the halfspreading rate is about 10 mm a -1 , the crust rapidly reaches a thickness of c. 6 km, which is normal for Atlantic oceanic crust. Models of melt generation during pure shear, finite-duration continental rifting can successfully reproduce the observed absence of significant syn-rift magmatism on, within and beneath the thinned continental crust if the rifting episode is longer than 10–20 Ma. However, for normal mantle potential temperatures, such models predict significant melt generation in the transition zone seaward of the thinned continental crust even for rift durations longer than 20 Ma. Restricted melting beneath the transition zone might be explained partly by lateral heat loss to the adjacent continental lithosphere, by anomalously low mantle potential temperatures at break-up time, or by depth-dependent stretching such that the observedinfinite stretching factor for the crust is not representative of the lithosphere as a whole. An additional mechanism for restricted melt production involves a transitional state between the end of continental extension and the onset of steady-state sea-floor spreading, during which mantle upwelling is less focused than at normal oceanic spreading centres.