The Structural and Geodynamic Evolution of the Black Sea Basin

1
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 mid-Black 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.
Uniform lithosphere deformation constrained by
crustal faulting
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.
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.
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
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.
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.
4. Integrated structural and geodynamic modelling

• 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.

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 B Offshore Turkey
Section C Offshore Ukraine
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
6. Summary

• 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.

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).
Acknowledgements

• 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.

• 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.

Contact information
Stuart Egan David Meredith School of Earth
Sciences and Geography, University of Keele,
Keele, Staffs, ST5 5BG, UK Email
s.s.egan_at_esci.keele.ac.uk d.j.meredith_at_esci.keel
e.ac.uk