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The Structural and Geodynamic Evolution of the Black Sea Basin
Stuart Egan & David Meredith
1. Regional tectonic setting:
5. Application of modelling of the eastern Black Sea basin:
3. The eastern Black Sea basin:
The Black Sea is a semi-isolated marine basin located north of
Turkey and south of Ukraine and Russia. It covers an area of
approximately 423,000 km2 and has a present maximum
bathymetry of 2200 m. The basin comprises the western and
eastern Black Sea sub-basins, which are separated by the midBlack Sea high. By Neogene times, however, these two sub-basins
had coalesced to form the single basin structure present today. The
evolution of the Black Sea region represents an interference of
tectonic events over geological time in that most of the subsidence
took place within the basin when the immediate surrounding
regions were experiencing compressional deformation. This is
demonstrated by Alpine-Himalayan orogenic belts, including the
Pontides, Greater Caucasus and Crimean Mountains, that surround
the Black Sea.
The Black Sea basin has undergone about 12 - 14
km of subsidence since the early Tertiary. Although
the causal mechanism for this subsidence is still
open to debate, it is commonly considered to have
been initiated by back arc extension related to a
subduction zone lying to the south in Turkey linked
with the closure of the Tethys ocean. There is,
however, a general lack of extensional deformation
within Mesozoic to recent sequences in the Black
Sea, which makes it difficult to attribute the large
amount of subsidence to extensional tectonics
alone.
Models that are based on the magnitude of observed fault controlled deformation do not
generate the thickness of sediment infill in the basin. Similarly, the modelling of
compressional deformation around the edges of the basin structure does little to explain
the large magnitude of subsidence within the central basin.
Uniform lithosphere deformation constrained by thinning of the crust
A modelling approach that quantifies lithosphere deformation according to the amount of
observed crustal thinning/thickening across the basin provides the closest match to
overall subsidence.
Deep lithosphere processes
• The above section has been generated from a combination of regional scale seismic data and published material to focus
on the Late Cretaceous to recent evolution of the eastern Black Sea:
• The earliest deformational event consisted of early Tertiary rifting. Extensional faulting and graben formation is well
developed on the northern and southern continental slopes of the basin. However, the magnitude of extension associated
with this rift phase was not very great (Beta = 1.13).
• Compressional deformation, which probably began in Eocene times, was sufficient to cause flexural subsidence of the
northern and southern continental shelf regions. The effects of this compressional deformation are confined to the basin
margins and there is a gradual change from compression to inversion to extensional tectonics with distance across the
North and South shelf regions.
• The central part of the basin has experienced a large magnitude of subsidence (over 12 km) since the end of the Mesozoic
and shows little evidence of extensional or compressional structures.
2. Structural styles within the western Black Sea:
Section A: Offshore Bulgaria
A combination of regional scale
seismic interpretation, well log
analysis and examination of
published material has been
used to focus on the
Cretaceous to recent evolution
of the Bulgarian, Turkish,
Ukrainian and central Black
Sea regions.
Section B: Offshore Turkey
Uniform lithosphere deformation constrained by crustal faulting
Model results suggest that deep crustal and mantle lithosphere processes, such as
depth dependent stretching (above) or the growth and decay of hot-spots (right) may
have played a significant role during the evolution of the eastern Black Sea. The
influence of such processes may partly account for the origin of the anomalously thin
syn-rift and thick Miocene-Quaternary sequences observed in the basin.
4. Integrated structural and geodynamic modelling:
3D Modelling - variable basin infill and bathymetry
A numerical model has been developed, which integrates crustal
deformation, thermal, isostatic and surface processes (i.e. basin infill and
erosion) in both two- and three-dimensions. It enables the forward modelling
of extensional basin evolution due to rifting followed by subsequent
extensional and compressional events. The adjacent figure shows a typical
starting condition for the modelling, which illustrates a regional cross-section
of undeformed lithosphere. The crustal component of this lithosphere is
assumed to be 35 km thick with a density of 2800 kg.m-3, while the mantle
lithosphere is assumed to be 90 km thick with a density of 3300 kg.m-3. The
modelled lithosphere is thermally conditioned with an equilibrated geotherm.
Section C: Offshore Ukraine
• A new 3-D modelling approach has been used to understand how regional
interactions between geological and geodynamic processes have controlled
subsidence within the Turkish and central regions of the eastern sub-basin.
• The 3-D modelling concentrates upon the effects of bathymetry and quantifying
realistic magnitudes of basin infill over geological time.
• Data constraint for the modelling has been provided by regional sections across
the eastern Black Sea (see yellow, red and blue boxes for location) derived from
depth-converted interpretations of regional seismic data.
• Extension of a 45km thick crust generates syn- and post-rift stratigraphies, which
are comparable in both ratio and magnitude to that observed in the eastern Black
Sea.
The adjacent model shows lithosphere extension due to a
coupled faulting-pure shear process. The model shows a
basement profile with a sequence of closely spaced half
grabens with relative uplift of the footwall. Extension has
also caused heating of the lithosphere temperature field,
which subsequently has cooled to generate subsidence.
This has generated a post-rift stratigraphic sequence that
blankets the underlying fault blocks and syn-rift sequences.
Section D: Central western Black Sea region
Integrated section across the western Black Sea
• Lower
to
middle
Cretaceous rifting mainly
affected the margins of
the basin.
• Late
Cretaceous
to
Eocene
compressional
deformation has caused
the
development
of
inversion structures within
the Offshore Ukraine
region and a thick-skinned
style thrust tectonics over
the Turkish Black Sea
region.
• The central part of the
basin has experienced a
large magnitude of
subsidence (over 14 km)
since
the
middle
Cretaceous. It also
exhibits a "layer-cake"
stratigraphy, with little
evidence of extensional
or
compressional
structures.
In the adjacent figure lithosphere
extension has been modelled by a pure
shear process. This modelling approach is
more suitable when there is little constraint
on the magnitude of fault-controlled
deformation, but where there is
information on the overall thinning or
thickening of the crust. The lithosphere
temperature field can be thermally
conditioned both before deformation and
during deformation (e.g. to represent the
effects of phenomenon such as hot-spots).
6. Summary:
• It is not possible to explain Black Sea subsidence when the magnitude of extension is based solely on the amount of fault
controlled deformation. This is probably due to an underestimate of deformation in the lower crust and mantle lithosphere.
• The large magnitude of Tertiary ("post-rift") subsidence observed in the Black Sea cannot be explained by loading and flexure
caused by adjacent thrust belts.
• Models in which the magnitude of deformation is calculated using crustal thinning/thickening generate amounts of total
subsidence that are comparable with that observed across the eastern Black Sea. However, these models assume the
complete infill of accommodation space and thus simulate overloading of the basin.
• It is suggested that the basin's subsidence history may have relied upon the action of subsurface loading, possibly due to
enhanced mantle extension or transient thermal anomalies, such as hot spot activity.
• Models results show that the magnitude of total subsidence is significantly reduced when accounting for a realistic
bathymetry and a late stage Upper Miocene - Quaternary infill. This suggests that the magnitude of extension may have been
underestimated. In the context of a realistic bathymetry and stratigraphy. It is suggested that extension of a thickened crust
may have accounted for the observed magnitude of overall subsidence.
Acknowledgements
Contact information
Stuart Egan & David Meredith
School of Earth Sciences and Geography,
University of Keele, Keele, Staffs, ST5 5BG, UK
Email:
[email protected];
[email protected]