The geological and geodynamic evolution of the eastern Black Sea basin: insights from 2-D and 3-D tectonic modelling

School of Earth Sciences and Geography, University of Keele, Keele, Staffs, ST5 5BG, UK
Tectonophysics 01/2002; DOI: 10.1016/S0040-1951(02)00121-X

ABSTRACT Subsidence mechanisms that may have controlled the evolution of the eastern Black Sea have been studied and simulated using a numerical model that integrates structural, thermal, isostatic and surface processes in both two- (2-D) and three-dimensions (3-D). The model enables the forward modelling of extensional basin evolution followed by deformation due to subsequent extensional and compressional events. Seismic data show that the eastern Black Sea has evolved via a sequence of interrelated tectonic events that began with early Tertiary rifting followed by several phases of compression, mainly confined to the edges of the basin. A large magnitude (approximately 12 km) of regional subsidence also occurred in the central basin throughout the Tertiary. Models that simulate the magnitude of observed fault controlled extension (β=1.13) do not reproduce the total depth of the basin. Similarly, the modelling of compressional deformation around the edges of the basin does little to enhance subsidence in the central basin. A modelling approach that quantifies lithosphere extension according to the amount of observed crustal thinning and thickening across the basin provides the closest match to overall subsidence. The modelling also shows that deep crustal and mantle–lithosphere processes can significantly influence the rate and magnitude of syn- to post-rift subsidence and shows that such mechanisms may have played an important role in forming the anomalously thin syn-rift and thick Miocene–Quaternary sequences observed in the basin. It is also suggested that extension of a 40–45 km thick pre-rift crust is required to generate the observed magnitude of total subsidence when considering a realistic bathymetry.

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    ABSTRACT: The numerical results of thermal modeling studies indicate that the lithosphere is cold and strong beneath the Black Sea basin. The thermal lithospheric thickness increases southward from the eastern Pontides orogenic belt (49.4 km) to Black Sea basin (152.2 km). The Moho temperature increases from 367 °C in the trench to 978 °C in the arc region. The heat flow values for the Moho surface change between 16.4 mW m−2 in the Black Sea basin and 56.9 mW m−2 in the eastern Pontides orogenic belt. Along the southern Black Sea coast, the trench region has a relatively low geothermal potential with respect to the arc and back-arc region. The numerical studies support the existence of southward subduction beneath the Pontides during the late Mesozoic–Cenozoic.
    Geoscience Frontiers. 07/2013; 4(4):389–398.
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    ABSTRACT: We investigated a high-flux seepage site in the Eastern Black Sea in order to quantify the content of shallow gas hydrates and to elucidate their physico-chemical behavior. Pressure and non-pressure sediment cores, as well as venting gas were collected at the Batumi seep area (BSA) in about 845 m water depth. Sediments represented late glacial to Holocene deposits. In gravity cores, hydrates were absent in top sediments (lithological Unit 1) but abundant below ca. 0.9 mbsf. Here, hydrates occurred as massive aggregates in deeper sections of the Unit 2 and as disseminated pieces in the underlying Unit 3. Gas from the degassing of pressure cores and from hydrates as well as vent gas were dominated by CH 4 (>99.9 mol-% of light hydrocarbons, LHC). Enrichments in CH 4 and C 2 H 6 accompanied by depletions in C 3 H 8 and C 4 -isomers in hydrate-associated gas relative to vent gas resulted from molecular fractionation during hydrate precipitation. Volumetric gas/bulk sediment ratios determined by pressure core degassing approached 20.3. CH 4 concentrations reflected hydrate saturations of 5.2% in Unit 2 and 21% of pore volume in Unit 3. It is calculated that over the entire BSA covering 0.5 km 2 about 11.3 kt of hydrate-bound CH 4 exist in shallow sediments. X-ray diffraction showed structure I hydrate to prevail. Stable O isotope ratios of authigenic carbonates signify that hydrate decomposition along with gas discharge into overlying sediments occurs episodically. From the rough seafloor topography and carbonate data we conclude that in situ dissociation and/or upfloating of shallow-buried hydrates are a typical feature of the BSA. INTRODUCTION The Black Sea basin comprises the world's largest reservoir of dissolved methane (9.6 × 10 4 kt [1]), which is primarily supplied from seeps and de-composing hydrates [2]. It is estimated to contain ca. 10–50*10 3 km 3 of hydrate-bound methane [3]. So far, numerous hydrocarbon seepage sites fuel-led from reservoirs in the deeper subsurface were discovered mainly above the upper boundary of the gas hydrate stability zone (GHSZ), which for pure methane hydrates (structure I, sI) is located at about 710 to 720 m water depth [4-5]. In addition, distinct areas of intense hydrocarbon seepage within the GHSZ and associated with shallow hydrates were recognized in recent years (e.g. [6] for refs.). Especially shallow-buried hydrates are sensitive to changes in the environmental conditions control-ling hydrate stability (i.e. temperature, pore water salinity, hydrocarbon availability, hydrostatic pressure; e.g. [7]) compared to their deeply buried counterparts. In the case one or more of these factors change, submarine hydrates might disso-ciate and release significant amounts of light hydrocarbons (LHC) to the hydrosphere with con-sequences for the seafloor topography, biogeo-chemical carbon cycling, and global climate. However, information on total methane amounts trapped in hydrates at individual seepage sites in the Black Sea is sparse. This is mainly due to the technical effort required to determine true gas and hydrate concentrations (i.e. pressure sampling techniques [5, 8]) in deep sea sediments. In sediments overlying hydrates the anaerobic oxidation of methane (AOM) typically takes place and affects the vertical hydrate distribution. The AOM is mediated by a consortium of methano-trophic archaea and sulfate-reducing bacteria in a transition zone where methane ascending towards the seabed and seawater-derived sulfate meet. Authigenic methane-derived carbonates, formed as by-products of the AOM, might be used as archives of biogeochemical processes since they preserve the geochemical signature of the inter-stitial water and therefore reflect varying methane seepage activity and hydrate decomposition (e.g. [9-11]). However, authigenic carbonates from deep sea hydrocarbon seep site in the Black Sea have barely used to evaluate the long-term stability of associated hydrates, so far [12]. An aspect of parti-cular interest is whether hydrate-derived methane is released constantly over time or mostly in form of huge bursts rapidly exhausting the hydrate re-servoir. An improved understanding of processes affecting hydrate formation and dissociation in the past will contribute to a refined prediction of similar processes in the future if global warming leads to an increase of deep water temperatures. In this study we present total amounts of hydrate-bound methane contained in surface sediments of a highly active hydrocarbon seepage area, the Batumi seep area, in the Eastern Black Sea. More-over, we discuss the stability of hydrates associated to this seepage site in the past. For the study pre-sented here, we analyzed pressurized and non-pressurized sediment cores as well as authigenic methane-derived carbonates using several state-of-the art techniques.
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    ABSTRACT: Detailed acoustic investigation of bubble streams rising from the seafloor were conducted during R/V Meteor cruise M72/3a at a deep submarine hydrocarbon seep environment. The area is located offshore Georgia (eastern part of the Black Sea) at a water depth between 840 m and 870 m. The sediment echosounder Parasound DS-3/P70 was used for detecting bubbles in the water column that causes strong backscatter in the echographs ("flares"). Employing the swath echsounder Kongsberg EM710 flares in the water column were mapped along the entire swath width of approximately 1000 m at high spatial resolution. The exact location of the flares could be extracted manually. Subsequently, the horizontally looking sonar Kongsberg digital telemetry MS1000 mounted on a remotely operated vehicle (ROV) was utilized to quantify the flux of bubbles. A model was developed that is based on the principle of finding the "acoustic mass" in order to quantify the bubble flux at various seeps. The acoustic approach from the backscatter data of the ROV sonar resulted in bubble fluxes in the range of 0.01 to 5.5 L/min (corresponding to 0.037 to 20.5 mol CH4/min) at in situ conditions (˜850 m water depth, ˜9°C). Independent flux estimations using a funnel-shaped device showed that the acoustic model consistently produced lower values but the offset is less than 12%. Furthermore, the deviation decreased with increasing flux rates. A field of bubble streams was scanned three times from different directions in order to reveal the reproducibility of the method. Flux estimations yielded consistent fluxes of about 2 l/min (7.4 mol CH4/min) with variations of less than 10%. Although gas emissions have been found at many sites at the seafloor in a range of geological settings, the amount of escaping gas is still largely unknown. With this study presenting a novel method of quantifying bubble fluxes employing a horizontally looking sonar system, it is intended to contribute to the global effort of better constraining bubble fluxes at deep-sea settings.
    Geochemistry Geophysics Geosystems 10/2008; 9(10):10010-. · 2.94 Impact Factor