The geological and geodynamic evolution of the eastern Black Sea basin: insights from 2-D and 3-D tectonic modelling
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 backarc Black Sea (BS) basin was formed in the Late Cretaceous-Palaeocene at the hinterland of the Pontide magmatic arc due to subduction of the Neotethys ocean below the southern Eurasian continental margin. At present the BS consists of two large depressions-the West- and East-Black Sea basins (WBS and EBS) filled with thick (up to 12 km) Cretaceous and younger sediments and underlain by a crust of oceanic/suboceanic type. The sediments mask poorly investigated crystalline crust that is thought to comprise an accretional collage of microplates and terranes of different affinities. To investigate the lithospheric structure of the BS we performed a 3-D gravity analysis and local seismic tomography study. 3-D gravity backstripping analysis allowed us to separate the gravity signal from different parts of the crustal model and then, by subtracting the crustal effect from the observed field, to obtain gravity anomalies of presumed mantle origin only. The broad positive long wavelength component of this might be indicative of good isostatic equilibrium of the deep structure of the Black Sea, that is, that the negative gravity effect of sediments is almost totally compensated by the strong positive gravity impact of Moho shallowing. Velocity structure of the BS lithosphere has been studied by P-wave local seismic tomography. It uses the traveltimes of the earthquakes occurring inside the study region and recorded by permanent seismic stations around the BS. Initial data were corrected for the effect of the crust. The resulting model shows the BS lithosphere as being rather heterogeneous with two domains of increased velocity in its western and eastern parts. The gravity analysis and seismic tomography approaches were integrated by calculating the upper-mantle gravity effect of the tomography model and comparing this to the mantle gravity signature inferred from the gravity analysis itself. The integrated results suggest the presence of rheologically strong and cold continental lithosphere below the BS, similar to Precambrian lithosphere of the East European Platform.Geophysical Journal International 04/2013; 193(1):287-303. · 2.72 Impact Factor
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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 ), which is primarily supplied from seeps and de-composing hydrates . It is estimated to contain ca. 10–50*10 3 km 3 of hydrate-bound methane . 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.  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. ) 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 . 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: Pore pressure above the hydrostatic (overpressure) is common in deep basins. It plays an important role in pore fluid migration, represent a significant drilling hazard, and is one of the factors controlling slope stability and deformation in seismically active areas. Here, we present an inverse model to calculate overpressure due to disequilibrium compaction and aquathermal pressuring. We minimize a function that contains the misfits between estimates from our forward model and observed values using a non-linear least squares approach. The inverse model allows the introduction of observed seismic and geological constraints such as P-wave velocity (Vp) and density data, and depth of the layer boundaries, for a better pore-pressure prediction. The model output also provides estimates of: (1) surface porosity, (2) compaction factor, (3) intrinsic permeability at surface conditions, (4) a parameter controlling the evolution of the intrinsic permeability with porosity, (5) the ratio between horizontal and vertical permeability and (6) uncompacted thickness (so sedimentation rate assuming known time intervals), for each sedimentary layer. We apply our inverse approach to the centre of the Eastern Black Sea Basin (EBSB) where the Vp structure has been inferred from wide-angle seismic data. First, we present results from a 1-D inverse model and an uncertainty analysis based on the Monte Carlo error propagation technique. To represent the observed rapid change from low Vp to normal Vp below the Maikop formation, we impose a zero overpressure bottom boundary, and subdivide the layer below the Maikop formation into two sublayers: an upper layer where the rapid change is located and a lower layer where the Vp is normal. Secondly, we present the results from a 2-D inverse model for the same layers using two alternative bottom boundary conditions, zero overpressure and zero flow. We are able to simulate the observed Vp, suggesting that the low velocity zone (LVZ) at ˜3500-6500 m depth below the seabed (mbsf) can be explained by overpressure generated due to disequilibrium compaction (>90 per cent) and to aquathermal pressuring (<10 per cent). Our results suggest that the upper sublayer, below the Maikop formation, behaves as a seal due to its low permeability ˜0.3-2 × 10-14 m s-1. This seal layer does not allow the fluids to escape downwards, and hence overpressure develops in the Maikop formation and not in the layers below. This overpressure was mainly generated by the relatively high sedimentation rate of ˜0.29 m ka-1 of the Maikop formation at 33.9-20.5 Ma and an even higher sedimentation rate of ˜0.93 m ka-1 at 13-11 Ma. We estimate a maximum ratio of overpressure to vertical effective stress in hydrostatic conditions (λ*) of ˜0.62 at ˜5200 mbsf associated with an overpressure of ˜42 MPa.Geophysical Journal International 08/2013; 194(2):814-833. · 2.72 Impact Factor