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An Interpretation of the Basement Structure of the Black Sea Based on SPAN Seismic Data and Gravity Modelling

Abstract

It is widely accepted that the Black Sea consists of two separate back arc basins which opened at different times during the Cretaceous. Most authors accept that, at least in part, both these basins are floored by oceanic crust, even though there a no magnetic stripes. Long offset SPAN seismic data (imaging to more than 30 km) and geophysical modelling using published gravity data support this view and allow the distribution of oceanic crust vs. stretched continental crust to be mapped in both the West and East Black Sea. The nature of the transition between the two different types of seems to be somewhat different from that visible on other passive margins imaged by SPAN data. There do not seem to be any SDR's, yet there is only very limited exposure of sub continental mantle. We explore the implications of this transition, and argue that it may reflect changes in the strain rate in different parts of the basins. There is an impact on the nature of Black Sea opening, and it may be that existing opening models for the Western Black Sea basin are in need of modification. The Black Sea is surrounded by fold and thrust belts. We argue that the beginnings of subduction of the oceanic crust can be seen beneath part of the mid Black Sea high, and that the original oblique outer marginal detachment on which the ocean opened is beginning to be inverted as a subduction zone with contractional deformation of the volcano-sedimentary pile in the outer marginal trough. Deep crustal structure is an important element in hydrocarbon maturation – the heat generated in the crystalline basement contributes significantly to the maturation of source rocks in sedimentary cover. The quantity of radiogenic heat generated by the upper continental crust depends on its thickness, and is therefore a direct function of the amount of stretching (ß). Oceanic crust, born at 1300 C at mid ocean ridges cools with time and distance away from spreading centres, while the quantity of heat derived from the volcanic edifices which give rise to SDR's is probably rather small, and that from the serpentinised sub-continental mantle exposed to the sea bed is probably negligible. Maps of heat flow and radiogenic heat production like those presented here provide direct and important input into petroleum systems models.