Regional Rift Structure of the Western Black Sea Basin: Map-View Kinematics
Published:December 01, 2015
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Gabor Tari, Mohammad Fallah, Walter Kosi, Zsolt Schleder, Valentin Turi, Csaba Krezsek, 2015. "Regional Rift Structure of the Western Black Sea Basin: Map-View Kinematics", Petroleum Systems in “Rift” Basins, Paul J. Post, James Coleman, Jr., Norman C. Rosen, David E. Brown, Tina Roberts-Ashby, Peter Kahn, Mark Rowan
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The geological understanding of the opening of the Western Black Sea Basin appears to be quite far from being reasonably resolved. The main faults used in the existing map-view reconstruction schemes are either very poorly defined (West Black Sea fault) or simply nonexistent as interpreted earlier (West Crimean fault) and therefore they need be redefined or replaced by other structural elements.
Various kinematic elements and facies boundaries on the conjugate margins of the Western Black Sea (i.e., the Bulgarian, Romanian and Ukrainian margin in the northwest versus the Turkish margin in the southeast) appear to be a key in constraining the opening geometry of the basin. The along-strike changes in the synrift structural pattern of the Bulgarian-Romanian margin, reflecting contrasting crustal rheologies inherited from prerift deformational phases, do appear to have their counterparts in the offshore part of the conjugate Turkish margin including the Pontides. A correlation of regional 2D reflection seismic and well data, and the critical review of the relevant onshore geology did provide some preliminary corresponding tie-points to constrain the kinematics of the basin opening.
If the European margin is fixed in a kinematic reconstruction, the clockwise opening of the rift basin occurred along northwest–southeast trending transform faults around an Euler rotation pole positioned to the southwest of the present Black Sea. The rotational element in the opening of the Western Black Sea Basin, as opposed to the dominantly translational kinematics used in some of the existing kinematic models, is also supported by the broadly triangular shape of oceanic crust imaged in the basin center.
The Black Sea Basin is one of the largest underexplored rift basins (Fig. 1) in the world. During the last two decades many contrasting points of views have been published regarding the kinematics, mechanism, and timing of the opening of both the Western and Eastern Black Sea basins. Whereas academia mostly has focused on the basin margins, the oil and gas industry has produced abundant geological and geophysical data in the shelf and, just recently, in the deep water part of the basin, providing critical insights. The present work is an attempt to summarize the pros and cons of the various models suggested to-date for the Western Black Sea Basin, in light of the deep water reflection seismic and well data acquired in the last few years (Graham et al., 2013; Nikishin et al., 2015a, b). Contrary to previous models (Okay et al., 1994), with a circa 15-20° counterclockwise rotation of the offshore/onshore Pontides and an Euler pole located to the southwest from the Black Sea, we have not found a space problem in the map-view reconstruction of the basin opening.
The overview below focuses on the opening of the Western Black Sea in terms of map-view kinematics, regardless of the timing. A companion paper of this contribution (Schleder et al., this volume) discusses the equally important cross-sectional aspects of the Western Black Sea basin. The stratigraphy of the conjugate margins and the timing and possible driving mechanisms for the opening of the Black Sea basins are discussed separately by Tari (this volume).
Existing Models for the Opening of the Black Sea
Whereas there were earlier attempts to suggest a model for the opening kinematics of the Black Sea basin (Finetti et al., 1988), the first specific map-view model was put forward by Okay et al. (1994). Their model, largely based on geological data from onshore areas surrounding the Black Sea, invoked the separation of a large continental fragment from the Ukrainian and Romanian “Odessa Shelf” by orthogonal rifting during the Albian-Cenomanian (Fig. 2). This continental platelet, which is referred to as the Istanbul Zone of the Western Pontides, drifted southwards at least by 400 km to open up the oceanic Western Black Sea basin. This model assumed the existence of two major strike-slip faults on both ends of the Istanbul Zone continental ribbon. The western fault zone, named West Black Sea Fault by Okay et al. (1994), has been interpreted to lie just west of Istanbul having a north–south trend, separating the Strandja region from the Istanbul Zone (Fig. 1). Unfortunately, the offshore continuation of this speculative right-lateral wrench fault zone has not been identified to-date, as it is assumed to be concealed by the offshore continuation of the Paleogene Balkans folded belt (Doglioni et al., 1996; Stuart et al., 2011; Georgiev, 2012). Note that the almost exactly north–south trend of the Western Black Sea Fault is shown as an almost northwest–southeast trending fault by others (Suc et al.(in press) based on the onshore geology just west of Istanbul.
The existence of the other major postulated strike-slip fault on the eastern end of the Istanbul microplate (Fig. 2) turned out to be equally problematic. The trace of the West Crimean Fault, (Finetti et al., 1988) was extrapolated by Okay et al. (1994) to the south and across the Central Pontides to line up with and explain the prominent reentrant along the southern edge of the Sakarya Zone, shown as suture deflection (Fig. 2). However, using offshore reflection seismic data sets, the West Crimean Fault could not be identified and traced from offshore Crimea across the deepwater basin to the onshore boundary between the Western and Central Pontides in Turkey (Fig. 2) as suggested by Okay et al. (1994). The lack of this postulated fault zone became very clear with the recent acquisition of a superb regional 2D seismic data set across the entire basin (Graham et al., 2013; Nikishin et al., 2015a, b).
An alternative view of the basin opening was offered by Robinson and Kerusov (1997) suggesting that the eastern transform margin of the Western Black Sea Basin runs along the Mid-Black Sea High (a.k.a. Andrusov High) and the Archangelsky Ridge and it continues onshore Turkey between the Central and Eastern Pontides (Fig. 3). Moreover, as a kinematic constraint for the reconstruction of the basin opening, Robinson and Kerusov (1997) suggested that boundary between the Western (Istanbul Series) and Central Pontides (Küre Series) is the continuation of the major Peceneaga-Camena fault on the conjugate margin onshore Romania (Fig. 3). On the western end of the continental fragment used in their reconstruction, Robinson and Kerusov (1997) used the West Black Sea fault as defined by Okay et al. (1994). However, this kinematic assumption leads to a significant overlap of continental crust between the Pontides and the Moesian Platform (Fig. 3).
Okay and Görür (2007) disputed this model based on two arguments. One of them is the perceived space problem associated with the simple restoration of the Istanbul Zone from its present day position to the Ukrainian/Romanian conjugate margin; i.e., it would require substantial extension which has not been observed in the field. The other argument relates to the apparent lack of a major wrench fault observed in the Eastern Pontides.
Nikishin et al. (2015b), in their latest reconstruction, translated and rotated the Istanbul Terrane in a counterclockwise manner along a West Black Sea fault which is very similar to that shown in Figure 2. However, they introduced a new structural element, the Abana fault, which runs along the eastern end of the Central Pontides. Moreover, the western flank of the Andrusov High is shown as a major normal fault (their Figure 11b), instead of a transform fault.
Closing the Western Black Sea Basin Based on Map-View Kinematics
The following discussion is intended to focus on the kinematically important structural elements in the Western Black Sea and its surroundings. These include the Andrusov High, the style and distribution of extension in the basin, the presence and map-view extent of oceanic crust in the basin center, and the correlation of pre-opening kinematic markers on the conjugate margins.
The West Crimean Fault and the Andrusov (Mid-Black Sea) High
Based on regional 2D seismic reflection data, the West Crimean Fault is not running north–south as was postulated by Okay et al (1994). Instead, it has a definite northwest–southeast strike following the southwestern edge of the Andrusov High between Crimea and the eastern Pontides (e.g., Nikishin et al., 2015a). Furthermore, we interpret the overall Andrusov High (a.k.a. Mid-Black Sea High) as a marginal ridge, sensu Mascle and Blarez (1987), associated with the basin-bounding West Crimean transform fault.
As to its continuation onto the onshore basin margin in the Eastern Pontides, like most transform faults, this fault zone should not be expected to dissect the adjacent continental margins. Moreover, the onshore continuation of the West Crimean fault in Turkey should be concealed under the extensive postrift Senonian volcanics of the Eastern Pontides (c.f., Okay and Şahintürk, 1997).
Styles of Extension in the Western Black Sea Basin
The distribution of synrift extension has not been taken into account by the existing reconstructions of the Western Black Sea Basin (Okay et al., 1994; Nikishin et al., 2015a, b) due to the fact that the understanding of the extensional structures is very variable along the conjugate margins and in the basin center.
For example, in a cross-sectional sense, there is a large segment of the Bulgarian margin which does not seem to be affected by Cretaceous synrift faulting in the shelf area (Fig. 5). However, the conjugate Turkish shelf and upper slope are underlain by a system of half-grabens displaying a low-strain distributed wide-rift style extension (Menlikli et al., 2009).
The strikingly similar feature on both margins is the presence of a “hinge-zone” outboard from the present-day shelf, where an abrupt thickening of the basin fill can be observed. This dramatic thickening occurs either across a well-defined large border fault, like on the Bulgarian margin or across a poorly imaged set of faults largely masked by volcanic features, like on the Turkish margin (Fig. 5).
In a map-view sense, the style of extension also displays important changes along-strike due to the preexisting structural fabric. For example, the very large border fault defining the hinge zone in the northeast part of the Bulgarian offshore terminates abruptly to the northeast and is replaced by a series of much smaller faults (Fig. 6). In particular, the Capidava-Ovidiu Fault separating the Moesian Platform sensu lato from Central Dobrogea is interpreted to dissect the Polshkov High trend located in the deep water part of the basin. The overall Polshkov High trend has several structural culminations displaying contrasting extension polarity and fault spacing frequency. We interpret this prominent along-strike change in style of rifting as the signature of the underlying basement fabric.
The Moesian Platform has a markedly different prerift stratigraphic and structural make-up compared to that of Dobrogea (Seghedi, 2012; Georgiev, 2012). The prerift structural architecture of the Bulgarian and Romanian margins appears to control the distribution of extension and the location of transform faults segmenting the Western Black Sea Basin during the Cretaceous rifting. We interpret the prerift Capidava-Ovidiu (Palazu) fault zone (Figs. 1 and 6) being re-used as a transform fault during basin opening.
Presence and Map-View Extent of Oceanic Crust in the Basin Center
The presence or absence of oceanic crust in the Western Black Sea Basin has been debated for a long time as the very thick basin fill, locally thicker than 12 km (Fig. 5), prevented the proper seismic imaging of the bottom of the basin (Finetti, 1988). The recent acquisition of the long-offset (i.e., 10.5 km) and deep penetration seismic data (Graham et al., 2013; Nikishin et al., 2015a, b) is a break-through in this regard. Based on the seismic characteristics of the synrift structure in the basin center, Nikishin et al. (2015b) suggest a roughly triangular shaped distribution for the oceanic crust in the basin (Fig. 7). We believe that even the superb seismic data quality leaves room for a slightly different interpretation; still, we do accept the presence of oceanic crust in the Western Black Sea Basin and incorporate it into our kinematic reconstruction (Figs. 8 and 9).
Correlation of Prerift Kineamtic Markers and Facies Zones on the Conjugate Margins
The along-strike change in the synrift structural pattern of the Bulgarian-Romanian margin described above, reflecting very different crustal rheologies inherited from various prerift deformational episodes, should find its counterpart in the offshore part of the conjugate Turkish margin. We speculate that the abrupt change in the rifting style across the Capidava-Ovidiu (Palazu) fault system shown in Fig. 6 has a counterpart on the Turkish margin. The Eocene thin-skinned thrust fold belt described by Sunal and Tüysüz (2002) in the Pontides between Amasra and Cide (Fig. 7) appears to be very different from the thick-skinned folded belt to the southwest of Amasra, for example in the offshore Eregli embayment (Fig. 5). Admittedly, the along-strike transition cannot be easily tied to a single northwest–southeast trending structural element in the Pontides around Amasra.
As another pair of kinematic tie-points, the boundary between the Western (Istanbul Series) and Central Pontides (Küre Series) is seen as the equivalent of the major Peceneaga-Camena fault on the conjugate margin onshore Romania, following Robinson and Kerusov (1997).
Finally, as an example of correlating prerift facies zones, the Carboniferous coal sequence between Zonguldak and Amasra on the Turkish margin finds its counterpart around Cape Kaliakra in Bulgaria where the same coal strata are well known (Okay et al., 2006).
Closing the Western Black Sea Basin
A combination of the prerift structural and facies elements described above and the synrift structural features such as the hinge zones on the conjugate margins should be used to constrain the kinematic closing of the Western Black Sea Basin. If the European margin is fixed in a kinematic reconstruction, the clockwise opening of the rift basin occurs along northwest–southeast trending transform faults, the most important one being the West Crimean fault as defined in this work (Fig. 4). The Euler rotation pole is positioned to the southwest from the present Black Sea, somewhere within the Aegean Sea. The exact position of the Euler pole requires more work and a more thorough explanation; therefore, it will be published elsewhere.
As to the western edge of the rotated Pontides continental ribbon (including the Istanbul terrane), the remarkable facies change within the Paleozoic and Mesozoic strata between the Istanbul and Zonguldak units of the Istanbul Terrane (Yilmaz et al., 1997; Tüysüz, 1999) is interpreted as the expression of a poorly understood structural feature inherited from the prerift evolution of the region. This northwest–southeast trending feature is used in the rotation of the Pontides as shown in Figures 7 and 8. The rotational element in the opening of the Western Black Sea basin is also supported by the broadly triangular shape of the oceanic crust (Fig. 7, Nikishin et al., 2015b). The overall acute triangle shape of the oceanic crust in the Western Black Sea Basin suggests a rotation angle anywhere between about 10–30°.
We have chosen a circa 12.5° counterclockwise (CCW) rotation of the Pontides in relation to a fixed Eastern European and Moesian Platform margin. The primary constraint on this reconstruction is the match of the synrift “hinge zones” on the conjugate margins (Figs. 7 and 8). Note that the 12.5° CCW rotation angle used is a minimum as the reconstruction is not taking into account the extension associated with several Cretaceous basins; e.g., the Ulus basin, to the south of the hinge zone on the Turkish margin. Cross-sectional reconstruction (Schleder et al., this volume) suggests a slightly larger rotation for the reconstruction.
Regardless, with an Euler pole within a few hundred kilometers to the southwest of the Black Sea and using the relatively well-constrained, slightly curved western edge of the Andrusov High (Fig. 1) as the transform margin of the opening basin, we have found no space problem in the reconstruction (c.f. Okay and Görür, 2007). In addition, the corresponding kinematic markers and facies zones on the conjugate margins line up in a reasonably good match in the reconstruction (Fig. 8).
As to the space problem emphasized by Okay and Görür (2007), the critical issue is the choice of the bounding fault(s) of the rotating Pontides on their western termination (Fig. 7). We consider the trace of the West Black Sea fault, used by all the existing reconstructions (Okay et al., 1994; Robinson and Kerusov, 1997; Nikishin et al., 2015b) as a very poorly constrained fault. Looking at all the other major fault trends segmenting the conjugate margins in the Western Black Sea Basin (Fig. 6), the West Black Sea fault is certainly not north–south trending. We suggest here a northwest–southeast strike for this fault, connecting the area west of Istanbul (where it is undoubtedly constrained in outcrops, Aral Okay, personal communication, 2015) with the Strandzha area in the vicinity of the Bulgarian/Turkish border (Fig. 9) is a much more realistic approximation of this transfer/transform fault. There are elements of a possible northwest–southeast trending structural fabric shown on the existing geologic maps (Fig. 9). For example, the thrust contact between the Grudovo and the Mitchurin/Burgas Groups, close to the Bulgarian coastline (Fig. 9) may be inherited from a pre-existing transfer/transform fault. This speculation could be confirmed or rejected by critical field work in the border zone between Bulgaria and Turkey, which has not been done to-date.
Volcanism in the Jambol-Burgas Segment of the Srednogorie Zone, Bulgaria
The volcanics in the Srednogorie Zone (Fig. 9) have been already considered in the opening record of the Western Black Sea (Görür, 1988). In addition to the existing detailed description of this rifted basin complex onshore (Georgiev et al., 2001, 2012), it is important to note that the volcanics can be followed into the adjacent offshore (Fig. 10).
Therefore, the Senonian volcanics in the Srednogorie Zone to the west of the Burgas embayment represent the onshore continuation of the Western Black Sea rift system (see Fig. 10). Georgiev et al. (2012) interpret the Senonian volcanism in this area as related to rifting in an intra-arc setting.
Taking into account the western termination of the Western Black Sea rift system in the Burgas embayment and its onshore continuation in the Srednogorie Zone of Bulgaria, the axis of the rifting has to be offset by about 200 km to the northwest (Fig. 11). Whether this significant lateral offset can be attributed entirely to one major transform fault or to a few of them, remains to be determined.
Regardless, the kinematic cartoon reconstruction (Fig. 11) illustrates the interpreted westward rift propagation associated with basin opening. The exact timing of the rift propagation remains poorly constrained given the scarcity of well control in the deep water realms of the Black Sea. However, if the model of pulsed rifting (Bosworth, 2015), within which individual segments between the major transform faults were ruptured geologically instantaneously, is applied to the Western Black Sea Basin, the large oceanic segments in the eastern part of the basin should have opened first. This was followed by extension the middle segments with negligible or no oceanic crust, and finally the segments in the Srednogorie developed with the characteristics of an embryonic rift (Fig. 11).
The oroclinal bending of the central Pontides is documented by paleomagnetic declination anomalies (Meijers et al., 2010). The age of this post-opening bending is post-late Cretaceous and pre-late Eocene (Fig. 12). Therefore, it also needs to be retrodeformed for a prerift reconstruction. Note that the oroclinal bending has not been accounted for in the reconstruction shown on Fig. 8. Given the 30–60° differential rotation across the Central Pontides (Fig. 12), their eastern end would line up with Crimea (Fig. 8).
Note that our reconstruction do not address the Aptian/Albian aborted rifting (Schleder et al., 2014) in the Karkinit Basin beneath the Odessa Shelf (Fig. 1). This roughly east–west trending basin is separated from the Western Black Sea basin by the Kalamit High. There are reports of Albian volcanism in the eastern end of the Karkinit Basin and the nearby Crimea area (Nikishin et al., 2013) which suggest that this was failed rift (Schleder et al., 2014).
There is ongoing work addressing the paleogeography of the rifting in the Western Black Sea region by analyzing the detrital zircon spectra of various Cretaceous clastics in the Pontides (Akdogan et al., 2015) and on the conjugate margin in the north (Okay and Nikishin, 2015). We believe that in contrast to the Turkish margin, the Bulgarian, Romanian, and Ukrainian segments of the basin have not been studied with this particular technique to the point that it could help to constrain a kinematic reconstruction.
The map-view kinematic reconstruction of the rifting and opening of the Western Black Sea basin is critically dependent on the definition of the bounding faults of the southern margin of the basin. In particular, the very poorly known West Black Sea fault needs to be redefined as it plays a critical role in the rotational component of the reconstruction. The much better understood West Crimean fault is a major transform fault and the Andrusov High is a marginal ridge associated with it. Contrary to previous models, having a circa 15–20° counterclockwise rotation of the offshore/onshore Pontides and an Euler pole located to the southwest from the Black Sea, there is no space problem in the map-view reconstruction of the basin opening. The prerift kinematic reconstruction used in this work requires additional refinements; e.g. the retro-deformation of the post-opening oroclinal bending of the Pontides of the Turkish margin.
The structural correlation of kinematic and facies marks the conjugate margins of the Western Black Sea basin and underlines the role of prerift structural fabric by segmenting the margins into areas having specific extensional styles. These segments on the conjugate shelves are bound by northwest–southeast trending cross-faults which are interpreted to be the extensions of transform faults associated with the opening of a triangular-shaped oceanic domain. The rifting in the Western Black Sea basin appears to have propagated in an overall westerly direction. The aborted westernmost tip of the rift is located onshore in Bulgaria, in the Jambol-Burgas segment of the Srednogorie intra-arc volcanic sequence.
Reviewers Paul Post and Norman Rosen are thanked for their constructive and helpful comments on the first draft of this paper. Conversations about the various aspects of the geology of the Black Sea with Aral Okay, Dave Roberts, Mike Simmons, Anatoly Nikishin, Zühtü Bati, Özgür Sipahioglu, Gabriel Ionescu, Zamir Bega and David Boote are acknowledged. Peter Pernegr is thanked for drafting most of the figures.
Figures & Tables
Petroleum Systems in “Rift” Basins
- basin analysis
- Black Sea
- continental margin
- East Mediterranean
- extension tectonics
- geophysical methods
- geophysical profiles
- geophysical surveys
- Mediterranean Sea
- ocean basins
- oceanic crust
- plate tectonics
- seismic methods
- seismic profiles
- Southern Europe
- strike-slip faults
- transform faults
- western Black Sea