Subsidence History and Basin-Fill Evolution in the South Caspian

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Subsidence history and basin-ﬁll evolution in the South Caspian Basin from geophysical mapping, ﬂexural backstripping, forward lithospheric modelling and gravity modelling

N. A. ABDULLAYEV1,2*, F. KADIROV1 & I. S. GULIYEV1 1Institute of Geology, Azerbaijan National Academy of Sciences, H. Javid Avenue, 29A, Baku AZ1143, Azerbaijan 2BP Caspian Ltd, Xazar Centre, Port Baku, 153 Neftchilyar Avenue, Baku AZ1010 *Corresponding author (e-mail: [email protected])

Abstract: This study summarizes the subsidence history and aspects of the geodynamic evolution of the South Caspian Basin based on the integration of geophysical observations, and subsidence and gravity modelling on selected two-dimensional (2D) profiles. This analysis implies the presence of an attenuated ‘oceanic-type’ crust in the northern portion of the South Caspian Basin, demonstrates characteristics of basin subsidence on variable crustal types and describes sediment-fill evolution in several different parts of the basin. Modelling conducted in this study shows that the observed pattern of subsidence and sedimentation in the South Caspian Basin can be explained by a process of thermal subsidence following Jurassic rifting and further enhanced subsidence that resulted from sediment-induced loading in the Late Tertiary, especially after a large-scale base-level fall after 6 Ma.Variation in crustal type is reflected in differences observed in the degree of subsidence and sediment fill in the overlying stratigraphy. The western part of the South Caspian Basin has subsided differently to the eastern part because of this difference in crustal type. This is also confirmed by gravity modelling, which shows that the South Caspian Basin crustal density is compatible with an oceanic composition in the western part of the South Caspian Basin: the crust in the eastern part of the basin, however, is thicker.

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21 The South Caspian Basin (SCB) is located between layer (Vp , 4.8 km s ) and a ,10–18 km-thick 21 the mountain ranges of the Great Caucasus, Talesh, high-velocity (Vp between 6.4 and 7.4 km s ) Alborz and Kopet Dagh (Fig. 1). The SCB is a large ‘basaltic-type’ layer (Jackson et al. 2002), sug- intermountain basin situated within the Alpine– gesting that the SCB could be underlain by an Himalayan collision zone (Jackson et al. 2002; ‘oceanic-type’ (oceanic affinity) crust. Recent rein- Reilinger et al. 2006; Kadirov et al. 2008), the terpretation of the seismic refraction data of the unique characteristics of which are anomalously SCB shows that the sedimentary layer is underlain thick sediment cover, extensive compressional by a thinned 10 km-thick crust, that the Moho dis- folding, mud volcanism and relatively thin crust. continuity beneath the basin is at depths of between More than 25 km of sediment thickness has accu- 35 and 40 km, and that velocities below the Moho mulated in the basin since its inception, mostly discontinuity increase from 8.0 to 9.0 km s21 in the Tertiary (Allen et al. 2002; Artyushkov (Piip et al. 2012). The SCB region with the thin, 2007). The northern boundary of the SCB is a sub- high-velocity crust is characterized along its north- merged line of structural highs that forms the so- ern edge by deep-focus earthquakes (Jackson et al. called Absheron–Pribalkhan Ridge or, simply, the 2002; Khain 2005; Artyushkov 2007). This crust of Absheron Ridge (Fig. 1). oceanic affinity is also believed to be undergoing According to deep seismic refraction studies subduction beneath the Absheron Ridge (Golonka conducted in the 1960s (Mangino & Priestley 2007). This contrasts with the western Turkmeni- 1998; Piip et al. 2012) and seismic reflection pro- stan area, east of the Caspian Sea, where teleseis- filing (Glumov et al. 2004; Knapp et al. 2004) mic data suggest the crust is composed of 15– the thickness of sediments in the SCB is esti- 20 km of ‘granitic’ upper-crustal material and mated to be between 25 and 30 km. The results 20 km of ‘basaltic’ lower crust (Mangino & Priest- of refraction studies and teleseismic receiver ley 1998; Piip et al. 2012). function analysis show that the region around Previous crustal studies of the SCB also included SCB contains a 15–20 km-thick, low-velocity subsidence and gravity modelling by Brunet et al.

From:Brunet, M.-F., McCann,T.&Sobel, E. R. (eds) Geological Evolution of Central Asian Basins and the Western Tien Shan Range. Geological Society, London, Special Publications, 427, http://doi.org/10.1144/SP427.5 # 2015 The Author(s). Published by The Geological Society of London. Publishing disclaimer: www.geolsoc.org.uk/pub_ethics Downloaded from http://sp.lyellcollection.org/ by guest on September 26, 2021

N. A. ABDULLAYEV ET AL.

Fig. 1. Shaded relief map showing the location of the South Caspian Basin and the surrounding mountain ranges. The black dashed-line polygon shows a region covered by the teleseismic receiver function study from Mangino & Priestley (1998); the red solid line shows an east–west crustal cross-section after Mammadov (1992) used for subsidence and gravity modelling by Brunet et al. (2003); the black solid lines show two 2D lithospheric-scale gravity models constructed by Granath et al. (2007); the yellow solid line shows the geological cross-section used in the 2D gravity modelling by Kadirov (2000) and Kadirov & Gadirov (2014), originally sourced from USSR Ministry of Geology (1990).

(2003), and gravity modelling by Granath et al. interpreted earthquake databases, sparse deep seis- (2007) and Kadirov & Gadirov (2014) (Fig. 1). mic refraction and reflection data coverage, as well The modelling results in these studies were con- as poorly constrained subsidence modelling, cannot strained by a number of Soviet-era deep reflection definitely confirm or deny the presence of oceanic lines and refraction data. More recent ultra-deep crust or explain the limited extent of such crust reflection profiles acquired over the last decade in the SCB. Therefore, in this study we describe were used by Knapp et al. (2004), Mammadov ‘oceanic-type’ crust, implying crustal thicknesses (2008), Egan et al. (2009) and Green et al. (2009). and properties similar to oceanic crust, without They have provided new insights into basin struc- inferring definite crustal type. ture and evolution by means of their subsidence This study synthesizes previous studies with modelling and structural restoration. geophysical observations available to the Geologi- Despite numerous studies, many details of the cal Institute of Azerbaijan (GIA) and BP, including internal crustal structure and evolution of the basin a number of deep two-dimensional (2D) seismic are unclear. Uncertainty over the crustal type and reflection profiles, some of which have been pub- composition of the crust underlying the SCB is dis- lished in Knapp et al. (2004). These seismic profiles cussed in a variety of studies, such as Artyushkov have record lengths of up to 20 s two-way time (2007), Knapp et al. (2004), Glumov et al. (2004) (TWT) and reveal deep basin structure. Interpret- and Mammadov (2006, 2008). The majority of ation of these seismic profiles, and other offshore authors assume that there is a subduction of South and onshore seismic and well data, was accumu- Caspian lithosphere underneath continental litho- lated into an integrated set of depth and isopach sphere of the Central Caspian Basin (Khalilov maps across the entirety of the SCB. The three et al. 1987; Granath et al. 2000; Allen et al. 2002; structural ‘geoseismic profiles’ presented in this Knapp et al. 2004; Golonka 2004, 2007; Egan et al. paper were combined from the various 2D seismic 2009; Green et al. 2009). However, insufficiently reflection profiles and also include integrated Downloaded from http://sp.lyellcollection.org/ by guest on September 26, 2021

SUBSIDENCE IN THE SOUTH CASPIAN BASIN published information on the deep crustal structure age of basin opening in the Paleocene–Eocene of the SCB. (Adamia et al. 1977; Allen et al. 2002; Kazmin & Verzhbitskii 2011), or a pull-apart mechanism Tectonic and stratigraphic framework for the basin formation (Sengo¨r 1990), we assume Jurassic back-arc rifting as a cause of the SCB The tectonostratigraphic framework of the SCB is opening in this paper. summarized in Figure 2, being a modified strati- Jurassic-age opening is associated with volcanic graphic column from Abdullayev et al. (2012). Plate sediments penetrated in the Saatly superdeep well tectonic reconstruction for the SCB and palaeo- (onshore Azerbaijan), where more than 4400 m of environment interpretations for key tectonostrati- Jurassic basalts, andesites, diorites and tuffs were graphic units shown in this stratigraphic column encountered (Shikhalibeyli et al. 1988). No known can be found in Jones & Simmons (1996), Geologi- Jurassic-age volcanic rocks have been penetrated cal Institute of Azerbaijan (2003), Golonka (2007), offshore and not much is known of their offshore Hudson et al. (2008) and Van Baak (2010). extent. Knapp et al. (2004) described, from 2D A Jurassic-age origin of the SCB has been pro- reflection data, a very prominent north-dipping posed by a number of researchers (Zonenshain & reflector reaching maximum depths of 26–28 km, Le Pichon 1986; Otto 1997; Granath et al. 2000; which they interpreted as a basement–cover bound- Brunet et al. 2003). Basin opening is believed to ary of Jurassic age. This reflector has also been have been caused by a back-arc-rifting episode identified on 2D seismic profiles that we have used behind subduction of the Neotethyan Ocean in in this study and is onlapped by what is assumed the Mid-Jurassic–Early Cretaceous (Berberian to be a post-Jurassic succession. No obvious down- 1983; Zonenshain & Le Pichon 1986; Golonka to-basin faulting of this level can be discerned from 2007). While some authors propose an alternative 2D seismic profiles.

Fig. 2. Stratigraphic column for the South Caspian Basin, Azerbaijan with major geological events and local stratigraphic stages (suites) indicated. Modiﬁed from Abdullayev et al. (2012) and showing the average sediment thicknesses for each key stratigraphic unit (yellow brackets). Stratigraphic ages are from Popov et al. (2006) and Hudson et al. (2008). Formations: PS, Productive Series; NKG, Post-Kirmaky Shaly Suite; NKP, Post-Kirmaky Sandy Suite; KS, Kirmaky Suite; PK, Under-Kirmaky Sandy Suite; KAS, Kalin (or Qala) Suite; Meot, Meotian; Pont, Pontian; Serr, Serravallian; Burd., Burdigalian; Lang., Langhian; Aq, Aquitanian. Downloaded from http://sp.lyellcollection.org/ by guest on September 26, 2021

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The stratigraphic column in Figure 2 shows four total organic carbon (TOC) occur within the key post-Jurassic megastratigraphic complexes that Maykop and Diatom formations, and constitute can be described in terms of the sediment fill: the main source rocks for the hydrocarbon accumulations of the SCB. The age of the bound- † A Cretaceous–Early Palaeogene sequence ary between the Maykop Series and the over- (145–33.9 Ma) denotes a shelf to basin tran- lying Middle–Late Miocene Tarkhanian and sition of carbonate and clastic rocks on the Diatom Series is believed to be located close to margins of the back-arc rift (Golonka 2007). the base of Burdigalian age at about 15.9 Ma The Great Caucasus Proto-Caspian back-arc (Hudson et al. 2008). For onshore Azerbaijan, basin was undergoing rifting that finished some all of the Oligo-Miocene formations vary in time before Santonian, as evidenced by volcanic thickness between about 0.5 and 3 km from activity in Georgia (Golonka 2004). Egan et al. north to south across the Absheron Peninsula; (2009) interpreted the Mesozoic rocks of the however, much of the thickness variation could Great Caucasus as representative of deposition be tectonic in origin as this unit is an important in a passive margin/rift setting, with a transition detachment zone in the SCB structures (Devlin from the shallow-marine carbonates on the et al. 1999). From offshore seismic mapping, southern edge of Scythian–Turan Platform to we estimate Oligo-Miocene thickness to be deep-water mudstones. The Paleocene–Eocene between about 1 km in the central parts of the sediments are exposed in the easternmost Great SCB and 3 km in the north, in areas adjacent to Caucasus area as the ‘Koun’ Formation. These the Absheron Ridge. The isopach map for the are predominantly mudstone units deposited in Maykop, published in Green et al. (2009), shows slope to deep-water conditions throughout the thicknesses under the Absheron Ridge, the Scythian Platform (Golonka 2007). Onshore Absheron Peninsula and the Eastern Great Cau- outcrop and well data suggest thicknesses of casus thrust belt, where there has been tectonic 2–5 km for the Cretaceous interval, and up to thickening and structural repetition. 2 km of Paleocene and Eocene rocks (Geological † The Late Miocene–Early Pliocene ‘Productive Institute of Azerbaijan 2003). For offshore SCB, Series’ (6–2.6 Ma) is an important stratigraphic we observe between 5 and 11 km total thick- episode that contains the main clastic reser- ness for Cretaceous, Paleocene and Eocene voirs of the SCB in oil and gas fields such combined, with reflective packages being sig- as Azeri-Chirag-Guneshli and Shah-Deniz. The nificantly below well control. Unit thickness is Productive Series (PS) is well known from greatest in the northern portion of the SCB south wells and exposures onshore, offshore wells of the Absheron Ridge, as shown in isopach and seismic data. The PS consists of regionally maps published by Green et al. (2009). Seismic extensive fluvio/deltaic sandstones, interbedded mapping and depth conversion of the north- with regionally extensive lacustrine mudstones dipping basal reflector shows that depth to this acting as seals. Both Green et al. (2009) and sequence varies from 14 km in the southern Abdullayev et al. (2012) related the formation part of the SCB to between 20 and 26 km in of PS to large-scale, possibly Messinian-age the area south of the Absheron Ridge (Green (6–5.5 Ma) base-level fall that isolated the SCB et al. 2009). Nothing is known about the distri- from the global oceans. The magnitude of this bution, provenance and exact age of these event was estimated to be in the order of 1.5– rocks offshore, but this interval is thought to 2 km of vertical base-level fall and more than contain mostly deep-water clastics and carbon- 900 km of basinwards shift of onlap between ates by analogy with the basin margins. the Pontian-age highstand deltaic margin in the † A Oligocene–Late Miocene sequence (33.9– Central Caspian and first PS age onlap in the 6 Ma) is exposed in the Great Caucasus and con- SCB (Abdullayev et al. 2012). This base-level sists of largely marine clastics, known to occur fall integrated the continental river drainage of within a wide region across the Caspian and the Russian Platform and Central Asia into the Black Sea (Golonka 2007). Oligocene and SCB, thus creating a sediment sink that accumu- Miocene rocks of the SCB have been deposited lated about 6 km of sediment thickness over an during a compressional episode, related to the extremely short timespan (2–3 Ma). The PS collision of the Arabian continent with southern across the offshore SCB is relatively unstruc- Eurasia (Allen et al. 2002; Hudson et al. 2008). tured and progressively onlaps pre-existing stra- The Oligo-Miocene Maykop, Diatom, Sarma- tigraphy above the base-level fall unconformity tian, Meotian and Pontian formations (Fig. 2) (Abdullayev et al. 2012). are of wide geographical extent, and are exposed † Late Pliocene and Pleistocene (2.6 Ma–pre- in western parts of the Absheron Peninsula and sent). Finally, the PS is capped by an Akchagyl in the Great Caucasus. Intervals of locally high marine transgression episode that reconnects Downloaded from http://sp.lyellcollection.org/ by guest on September 26, 2021

SUBSIDENCE IN THE SOUTH CASPIAN BASIN

the SCB to the global oceans. Following this duration. Our modelling assumes the same age for transgression, a deeper-water lacustrine environ- the start of the rifting. ment was established in response to a continued Initial rift subsidence caused by fault defor- compressional episode related to the Arabia– mation and crustal attenuation is followed by long- Eurasia collision (Allen et al. 2002). Pleistocene term gradual subsidence caused by lithospheric shelf-margin complexes of the Absheron For- cooling and is termed thermal subsidence (Parsons mation develop in this lacustrine environment. & Sclater 1977; McKenzie 1978; Sclater & Christie These shelf-margin complexes prograded into a 1980). Basin burial curves for the SCB in Brunet deep-water lake developing slope and basin- et al. (2003) and Egan et al. (2009), using typi- floor turbidites (Abdullayev 2000). The com- cal compaction trends for the basin, demonstrate pressional episode responsible for the formation exponentially decreasing thermal subsidence after of the major offshore buckle folds and structures an initial rifting episode. A study by Egan et al. is mainly a Late Pliocene–Pleistocene event (2009) showed that thermal subsidence in the SCB (Devlin et al. 1999; Egan et al. 2009). Structural rapidly decreased between 100 and 90 myr ago, growth and rapid progradation of shelf margins and the remaining total subsidence rate was rela- during this time also created conditions for sub- tively low up to the Oligocene. However, in the marine slope failures, which were sourced along Miocene, the total subsidence process intensified, the basin margin and from adjacent anticlines and the basin bathymetry decreased from between (Richardson et al. 2011). 2 and 3 km to less than 1 km due to a rapid infill by clastic sediments of Pliocene PS (Mammadov 2008). The total subsidence and sedimentation rate Subsidence modelling during that period was anomalous for basins of Subsidence in the SCB comparable history; the SCB is truly unique in this respect. Allen et al. (2002) and Green et al. (2009) In this study we continue and expand upon subsi- showed that total water-loaded tectonic subsidence dence modelling conducted by Green et al. (2009), of the SCB is between 5.5 and 6.5 km on average, which describes the influence of rapid sediment and that around 30% (2 km) of this tectonic subsi- loading on the total subsidence of Mesozoic-age dence has been created since 6 Ma (Allen et al. ‘oceanic-type’ extended crust during the Late Ter- 2002). It is also known that the SCB is out of iso- tiary. The age of rifting in Green et al. (2009) was static equilibrium (Kadirov & Gadirov 2014) and assigned to a notional Late Jurassic date (145 Ma) negative Bouguer anomalies reach up to 2120 and was assumed to be instantaneous or limited in mGal (Fig. 3a). It is estimated that up to 2 km of

Fig. 3. (a) Depth to basement map integrating reflection seismic and published data from Glumov et al. (2004). (b) Bouguer gravity anomaly (mGal) modified from Kadirov & Gadirov (2014). Modelled profiles are shown in red and are numbered. Downloaded from http://sp.lyellcollection.org/ by guest on September 26, 2021

N. A. ABDULLAYEV ET AL. tectonic subsidence is uncompensated and, presum- horizons available from BP and GIA, or from pub- ably, results from compressive forces acting on the lished maps where the former were not available. SCB plate since the Pliocene (Zonenshain & Le Published sources include maps of Top Base- Pichon 1986). ment, depth maps and key isopachs by Guliyev et al. (2003), Glumov et al. (2004) and Green et al. Workflow summary (2009), which extend onshore around the SCB. The interpreted seismic horizons (with ages assigned) To understand the subsidence evolution of the were: Top Basement (145 Ma); Base Oligocene SCB and variations in subsidence across the basin ‘Maykop’ (33.9 Ma); Lower Miocene ‘Top May- rather than along a single profile, integrated and kop’ (15.9 Ma); Base Lower PS (6 Ma); Intra- iterative subsidence modelling was conducted on Lower PS (5.6 Ma); Base Pliocene or Base of three regional cross-sections. Modelling included Middle PS (5.3 Ma); Base of Upper PS (4 Ma), five key steps: Top PS (2.6 Ma); and Seabed (0 Ma). † Flexural backstripping including simple assump- Figure 3a shows the most basal of the maps, tions for crustal stretching and thermal sub- interpreted as Top Basement, which was generated sidence (Green et al. 2009) performed in by merging basement surface shown by Green ‘FlexDecomp’ software provided by Badley et al. (2009), with onshore maps from Guliyev Geoscience Ltd. et al. (2003) and Glumov et al. (2004). For the † Whole-crust (and lithosphere) extensional for- gravity input to the modelling, we used the Bouguer ward modelling with thermal subsidence perfor- gravity field of the SCB, as shown in Kadirov (2000) med in ‘Stretch’ software provided by Badley and Kadirov & Gadirov (2014), which is a publicly Geoscience Ltd. available map set. This Bouguer gravity map is † Two-dimensional flexural backstripping using shown in Figure 3b. the constraints generated by the forward model- Each profile was flexurally backstripped to ling. This provides a better consistency between restore to the post-rift palaeobathymetry, as in the the two modelling procedures (Kusznir et al. method shown in Roberts et al. (1998). Flexurally 1995). A comparison here can be made between backstripping a profile sequentially ‘strips off’ suc- the total sediment thickness of the model and the cessively decompacted and unloaded sedimentary present-day sediment thickness to see whether a layers from the total stratigraphy. For each layer in match can be achieved. the stratigraphic column, the backstripping process † One-dimensional subsidence modelling to con- unloads and then decompacts the underlying sedi- strain the values of 2D crustal stretching mod- ments (restoring palaeo-sediment thickness) and, els using 1D backstripping and the strain-rate finally, calculates the equivalent water load. The inversion method (White 1994). This has been sections were backstripped with the thermal sub- modelled using ‘backstrip1d’ script, written by sidence incorporated into the modelling using first Dr Alistair Crosby (BP, formerly from the Cam- an assumed stretch factor, (b), value of b ¼ 1 bridge Basin Research Group). implying no crustal deformation and then b ¼ 100 † Gravity profile modelling conducted using to approximate the unlimited extension of ‘oceanic- ‘GMsys’ software by Geosoft to constrain the type’ crust. Constraints of the forward model were subsidence modelling results. then used to vary the b-factor between these two The modelling workflow has a number of limit- extremes. ations and assumptions. The results of the modelling Reverse modelling of compaction in the ‘Flex- have been used only qualitatively and in conjunction Decomp’ software follows the relationships with mapped observations to interpret crustal struc- defined by Sclater & Christie (1980). The program ture and basin evolution of the SCB. default values for the compaction parameters are set for a 50:50 shale:sand mix. From regional under- Modelling assumptions standing, the lithology for the geoseismic sections was assumed to be largely mudstone with some Three modelled profiles across the SCB are shown sandstone input into the PS (up to 50%) from by numbers 1–3 in Figure 3, where they overlie which average compaction parameters were calcu- depth to basement and Bouguer anomaly maps, lated. The modelling presented here uses a principal respectively. These profiles were iteratively mod- value for effective elastic thickness (Te)of3km elled using the workflow outlined above. The that is generally considered typical for a standard assumptions and parameters used in the modelling rift basin (White 1994; Roberts et al. 1998), which are described below. the SCB could have been following its forma- For the surfaces used in the modelling exercise, tion. Sensitivities with higher Te of 10 km have we have used nine seismic horizons taken from also been modelled in the SCB and it was either an interpreted set of depth-converted seismic shown (Green et al. 2009) that, while local-scale Downloaded from http://sp.lyellcollection.org/ by guest on September 26, 2021

SUBSIDENCE IN THE SOUTH CASPIAN BASIN geometries within the SCB changed, this did not The final step in estimating the tectonic subsi- impact the overall subsidence history along the dence and benchmark b-factors from forward mod- modelled cross-section. elling is a 1D backstripping and strain-rate inversion The sea-level curve used for palaeobathymetry of a number of pseudo-wells posted along the geo- corrections until the Pliocene is the eustatic curve seismic sections and the application of the method of Haq et al. (1987), which is a default in ‘FlexDe- described in White (1994). The modelling takes comp’ software but can be customized. During the observations of horizon depth, age and palaeo-water backstripping process at 6 Ma, a sea-level change depth from a well, and calculates the tectonic of 21.5 km was introduced to correct for the substi- (i.e. water-loaded) subsidence history to determine tution of water-filled by air-filled subsidence, as a the variation of lithospheric strain rate with time result of the regional base-level fall at the base of that is required to match the tectonic subsidence the PS. observations. Essentially, the 1D inverse method Crustal parameters were further refined in the calculates the rifting history that best fits the back- second stage of modelling, which involved forward stripped well data. A 1D backstripping code also modelling of the lithosphere, capable of generating makes predictions of stretching factors and heat sufficient subsidence to accommodate the observed flux through time. This method is 1D and also stratigraphic thickness of the SCB. The ‘Stretch’ assumes that all subsidence is due to rifting. There- software we used for modelling utilizes a modifi- fore, an assumption of no further crustal stretching cation (simplification) of the flexural-cantilever following Late Jurassic rifting was made in the 1D model (Kusznir & Egan 1989; Kusznir et al. 1991; backstripping, and subsidence post-36 Ma was not 1995; Green et al. 2009) that models lithosphere fitted to the model. This allowed the determina- extension as pure shear only without reference to tion of pure tectonic subsidence from initial crustal specific basement fault geometries. This model stretching and thermal subsidence, without the influ- differs from the 1D model of McKenzie (1978) in ence from other causes, which is not possible in that the flexural response to loading is incorporated. ‘FlexDecomp’ software. The solutions shown are A uniform pure-shear rift subsidence incorporating the smoothest fit solutions that fit the data and which flexural isostasy was modelled. The forward- produce the most realistic extension and strain rate modelling workflow was conducted by first assum- history, not simply the best attainable fit (White ing 35 km of unstretched crust, instantaneously 1994). Limited numbers of data points that can be stretching it using a ductile pure-shear model and reliably dated in the early part of basin history is a then allowing the crust to thermally subside for limitation to this method. 145 Ma. Lithospheric thickness in the model was Finally, crustal structure was validated using assumed to be 125 km, and the density of the crust gravity modelling along the same profiles and cross- and mantle were assumed to be 2800 and 3300 checked with ultra-deep seismic reflection profiles kg m23 respectively. Modelled basin stratigraphy where available. The gravity model was applied was then compared to the observed stratigraphic using forward modelling, including best fit of the overlay to see whether a match could be achieved. initial model to the observed gravity profile, recal- The modelling was then repeatedly optimized to culation of the anomaly, and comparison of final produce a best match, with the output being a 2D observed and modelled anomalies. An update of profile of the b-factor that is the best match to a geological and layer density model to match modern-day stratigraphy. observed and modelled gravity profiles was made, The ‘Stretch’ software does not model the flex- enabling a better-informed decision on crustal ural subsidence related to compression and, for structure. It is with this fit that a potential for a this reason, stretching was overestimated, exceed- thin ‘oceanic-type’ crust beneath the SCB, with an ing b ¼ 20 in some areas near the Absheron Ridge. increased density of 3000 kg m23, can be ascer- In areas away from the Absheron Ridge model- tained. Using this view of crustal structure and sedi- led b-factors were much lower. The modelling ment fill, a model was constructed incorporating allowed a qualitative analysis of crustal stretching, both seismic observations and the results of model- but needed further calibration to provide realistic ling to describe differences in crustal structure and basin evolution scenarios. evolution of basin fill inside the SCB. Following forward modelling, 2D flexural backstripping was re-run using b-factors derived from Geoseismic profile 1. This profile runs approxi- the forward-modelling process, and post-rift palaeo- mately north–south and is built up of available bathymetry was restored as in Green et al. (2009). surface sets of various 2D reflection seismic pro- Despite using modelled b-factors that achieve files, some of which are ultra-deep 2D TWT pro- model match with stratigraphy, some of the exces- files. It is broadly similar to the profile modelled sive ‘stretching’ required to do so can be explained in Green et al. (2009) but differs by being con- by other factors. structed to avoid structurally complex parts of the Downloaded from http://sp.lyellcollection.org/ by guest on September 26, 2021

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Fig. 4. Geoseismic Profile 1 through the South Caspian Basin and the Absheron Ridge showing the variable thicknesses across the profile. The profile is similar in orientation to the profile used in Green et al. (2009).

Absheron Ridge. The model consists of nine lay- surface to palaeobathymetry following rifting (Fig. ers, corresponding to seismically mappable inter- 5d). Results restore bathymetry depths to about vals covering the Jurassic to the present (Fig. 4). A 2–2.5 km, similar to values in Green et al. (2009), portion of the profile, north of the Absheron Ridge, corresponding to present-day sediment thickness of represents continental crust of the Central Caspian up 20 km. Although there is a good match over the Basin, while the southern portion is located over majority of the area, this is achieved with b-factors the SCB crust. The profile is characterized by an that may be too high in some places. This suggests increase in thickness, from south to north, in the that other subsidence mechanisms are required to SCB portion of Mesozoic–Lower Palaeogene age. keep b-factors below values of 10, which are more The full profile was subjected to a restoration with acceptable for highly stretched crust. ‘FlexDecomp’ software initially with constant b- Along the profile, there is a zone of highly fluc- factors, based on the assumption of changing crustal tuating reconstructed palaeobathymetry that is prob- types across the profile. The northern portion of the ably not realistic (Fig. 5d). This zone is located over profile includes significant crustal shortening under implied underthrusting of ‘oceanic-type’ crust under the Absheron Ridge and cannot be confidently and to the north of the Absheron Ridge. This may restored using this methodology. have resulted in significant local tectonic thickening The flexural backstripping was complemented in the South Caspian sedimentary sequence imme- by forward lithospheric modelling along the pro- diately to the south of the Absheron Ridge. The file. The results of forward modelling are shown observed anomalous depth and thickness can be in Figure 5a, where the model restores the pro- explained by compressional shortening of the Ter- file to a maximum sediment thickness of about tiary stratigraphy. A more realistic modelling of 25 km, which corresponds to a stretching factor, b, this zone requires a precise geological interpretation of c. 20 in the thickest portion of the Mesozoic– of the basin margins. Palaeogene wedge, decreasing to average values of One-dimensional backstripping using strain-rate b ¼ 2 in the south (Fig. 5b) that are typical of atte- inversion flow (White 1994) on the pseudo-well nuated continental crust. The northern portion of from the central part of the profile just to the south the profile is located on fairly thick Scythian Plate of the Absheron Ridge was undertaken to compare continental crust and cannot be confidently model- subsidence analysis with 2D forward modelling led with high stretch factors. Values above b ¼ 20 and strip away potentially dramatic flexural effects for oceanic crust are fairly unrealistic and are (Fig. 6). An attempt has been made to ignore enhanced by flexural effects of the load over the recent subsidence events, which are not extensional Absheron Ridge. However, these crustal parameters in nature, by not fitting data younger than 36 Ma show a good match between forward modelled and and using an increased smoothing factor. This observed stratigraphy across the SCB portion of achieves a b-factor of 7 and water-loaded subsi- the profile (Fig. 5c). dence of 6–6.5 km, which still implies a highly atte- The resultant laterally variable b-factors were nuated crust at this location. However, these values then applied to the profile, restoring Top Basement of stretching factor are much smaller than values Downloaded from http://sp.lyellcollection.org/ by guest on September 26, 2021

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Fig. 5. Results of the forward lithospheric modelling of Profile 1. (a) 2D modelling results with crustal extension to match the stratigraphy, which indicates attenuated crust. (b) Resultant value of the initial b-factor, reaching maximum values of b ¼ 20. (c) Modelling results with stratigraphy overlay, with the red dashed line representing the depth to basement and the black solid line representing the approximate Base Tertiary reflector. (d) Restored palaeobathymetry at the Top Jurassic (145 Ma) using the crustal b-factor derived from the modelling, and showing average water depths of between 2.5 and 3 km. from 2D forward modelling that best fit the stratigra- thicknesses have been assigned to layers of the phy. This suggests that the substantial portion of tec- crust and the upper mantle similar to those of tonic subsidence at this location was not generated Kadirov & Gadirov (2014). These were then modi- by extension. fied in order to achieve the best fit between the In addition to flexural backstripping and for- observed gravity and the calculated gravity: man- ward lithospheric modelling, gravity modelling of tle densities were kept at 3300 kg m23; ‘oceanic- the profile was conducted to support the modell- type’ crustal densities were kept at 2950 kg m23; ing results as outlined above. Density values and ‘continental-type’ crustal densities were varied Downloaded from http://sp.lyellcollection.org/ by guest on September 26, 2021

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Fig. 6. (a) Geoseismic Profile 1 showing the location of the pseudo-well where the 1D subsidence model applied (shown by a red dot on the map); (b) water-loaded subsidence history using a constrained model; (c) initial b-factors derived from the constrained model. The black dashed line shows 36 Ma, after which no subsidence fit was applied. between 2750 and 2800 kg m23; Mesozoic-age (we used lower continental crust densities of sequence was assigned values of 2600 kg m23; 2800 kg m23). The values of the Bouguer gravity and the Tertiary-age sequence was averaged at field slowly increase towards positive values, south 2400 kg m23. These modified values of density of the SCB. and layer thickness were kept within bounds provided by the 2D forward modelling and 1D back- Geoseismic profile 2. A west–east geoseismic pro- stripping, and by available knowledge of elevated file extracted from available map sets (Fig. 3) was Moho depths and sediment thicknesses (Glumov similarly modelled to describe crustal type and et al. 2004). The gravity model matches the subsidence to the south of the Absheron Ridge. observed gravity field of Profile 1 (Fig. 7), which Profile 2 (Fig. 8) begins onshore at the Absheron supports the presence of denser, ‘oceanic-type’ Peninsula near Baku, traverses the western SCB to crust (5–7 km) underneath the northern portion the SE for 290 km, reaching the modern-day of the SCB, below and to the north of the Absheron Turkmen Shelf, where it turns and changes direc- Ridge. This gravity observation is also well sup- tion to the NE and runs for 220 km to the Balkhan ported by reflection seismic and earthquake obser- Mountains of Western Turkmenistan. The profile vations (Kadirov et al. 2008; Kadirov & Gadirov is oblique to some of the ultra-deep seismic lines. 2014). This area corresponds to large negative The model consists of nine layers correspond- Bouguer anomaly (Fig. 3b) and is best modelled ing to seismically mappable intervals covering as a root of South Caspian crust that has displaced the Jurassic to the present (Fig. 8). The profile is the normal lithospheric mantle, creating a large characterized by significant changes in the Meso- isostatic anomaly, as shown in Granath et al. zoic–Palaeogene thickness along the profile, being (2000). The southern part of the profile has a very 10–5 km thick in the western sector and 4–5 km small negative Bouguer anomaly, and can be mod- thick in the eastern sector. Deformation style elled with thicker (+10 km) and less dense crust also changes along the profile, with Tertiary-age Downloaded from http://sp.lyellcollection.org/ by guest on September 26, 2021

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Fig. 7. Gravity modelling results of Profile 1 showing the observed (dotted line) and modelled (solid line) Bouguer gravity anomaly. The density model shows: sediment layers in yellow and green; the upper crust in light purple; the lower crust in deeper purple; the oceanic crust in red-brown. compressional folding in the western sector and ‘oceanic-type’ highly attenuated crust (5–7 km largely undeformed parallel-lying stratigraphy in thick), while the eastern part with its low b-factor the eastern sector. values (10–15 km thick) shows the presence of Similar to Profile 1, after initial flexural back- less attenuated crust. The boundary of these two stripping, without thermal input, we conducted implied crustal types lies on the modern shelf lithospheric forward modelling and calculated the margin of the palaeo-Amudarya Delta in the Turk- amount of crustal stretching required to accom- menistan sector of the Caspian Basin and represents modate the available stratigraphy. Figure 9a shows a significant tectonic step. the results of forward modelling within both Figure 9c depicts the forward-modelling result syn-rift (yellow) and post-rift (blue) successions. with the stratigraphic overlay, and shows a rea- The amount of stretching required along this pro- sonable match between observed and modelled file is significant and the calculated total b-factors stratigraphy, allowing for differential subsidence are quite high. Figure 9b shows the calculated between western and eastern parts of the profile. b-factor, where values increase from approximately Figure 9d shows the Top Basement surface restored b ¼ 3 in the east to more than b ¼ 20 in the west. to palaeobathymetry just after rifting, using later- The western part of the model corresponds to ally variable b-factors derived from forward

Fig. 8. Geoseismic Proﬁle 2 through the South Caspian Basin showing variable structural styles and sediment thickness differences between the western and eastern parts of the basin, especially in the Mesozoic–Palaeogene section. Productive Series thicknesses are generally consistent around 5–6 km. Downloaded from http://sp.lyellcollection.org/ by guest on September 26, 2021

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Fig. 9. Results of forward lithospheric modelling of Profile 2. (a) 2D modelling results with crustal extension to match the stratigraphy. (b) Resultant value of the initial b-factor, exceeding values of b ¼ 20. (c) Modelling results with stratigraphy overlay with the red dashed line representing the depth to basement, and the black solid line representing the approximate Base Tertiary reflector. (d) Flexural backstripping results with restored palaeobathymetry at the Top Jurassic (145 Ma) using the crustal b-factor derived from the modelling, and showing average water depths of between 2.5 and 3 km. modelling. The section shows a palaeobathymetry One-dimensional subsidence and strain-rate of approximately 3 km in the western portion of inversion modelling was conducted on two pseudo- the profile and approximately 1.0–1.5 km in the wells (Fig. 10). Location 1 is in the middle of eastern portion. the Mesozoic–Palaeogene depocentre (Fig. 10a), Downloaded from http://sp.lyellcollection.org/ by guest on September 26, 2021

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Fig. 10. (a) Geoseismic Profile 2 showing the location of two pseudo-wells where a 1D subsidence model was applied (shown by red dots on the map with pseudo-wells labelled as locations 1 and 2). (b) Tectonic subsidence history using the constrained model for Location 1, where the black dashed line shows the 36 Ma cut-off after which no subsidence fit was applied. (c) Initial b-factors derived from the constrained model for Location 1. (d) Tectonic subsidence history using the constrained model at Location 2. (e) Initial b-factors derived from the constrained model at Location 2. about 20 km south of the Absheron Ridge. Ignor- Mesozoic–Palaeogene depocentre thins towards ing any subsidence beyond 36 Ma, the 1D model the Turkmenistan coast. One-dimensional modelling predicts initial b-factor values of approximately for this location predicts initial b values of approxi- 4.5 (Fig. 10c), with water-loaded subsidence of mately 2.5 (Fig. 10e), with water-loaded subsidence 6 km (Fig. 10b). This is significantly lower than of 4.5 km (Fig. 10d). The b-factors derived from b-factors derived from the forward modelling 2D forward modelling required to match stratigra- required to match stratigraphy, which would imply phy (Fig. 10a) are similar to the values from 1D mod- that a large component of the tectonic subsidence elling. This implies that at this location crustal is not of extensional origin even this far south. extension and thermal subsidence could explain Location 2 (Fig. 10a) used in 1D subsidence most of the history of the tectonic subsidence. and strain-rate inversion modelling is located on There is an observed difference in gravity field the eastern portion of the profile, where the signature and crustal structure between the two Downloaded from http://sp.lyellcollection.org/ by guest on September 26, 2021

N. A. ABDULLAYEV ET AL.

Fig. 11. Gravity modelling results of Profile 2 showing the observed (dotted line) and modelled (solid line) Bouguer gravity anomaly. The density model shows: sediment layers in yellow and green; the upper crust in light purple; the lower crust in deeper purple; the oceanic crust in red-brown. pseudo-well locations on the profile. The eastern sediments in the northern and middle part of the portion of the profile crosses the so-called ‘Godin profile thinning towards the south, with the Tertiary Massif’ feature (Mammadov 2008), which is char- wedge also thinning slightly from north to south. acterized by a rapid change of gravity towards a The change in thickness of the Mesozoic wedge in positive Bouguer anomaly (+10 to +20 mGal), this profile is more gradual and not as dramatic as and a thicker crust between 12 and 15 km (Granath in Profile 1. The northern portion of the profile is et al. 2000, 2007; Glumov et al. 2004; Mammadov involved in the compression and uplift of the 2008; Kadirov & Gadirov 2014). The profile trails Absheron Peninsula, and is, therefore, not a proper the edge of the gravity anomaly with values close reconstruction. This portion of the profile is in- to +0to+10 mGal. The gravity modelling calcu- volved in compressional deformation and uplift lation matches the observed gravity field of Profile related to collisional tectonics, therefore modelling 2 (Fig. 11), which supports the presence of denser, for this part is inaccurate, as it does not reflect long- ‘oceanic-type’ crust (5–6 km thick) in the western wavelength flexural effects. portion of the profile and thicker ‘continental-type’ Figure 13a, b demonstrates the results of forward crust (10–15 km thick) to the east, with a change modelling with syn-rift and post-rift sedimentary observed across the boundary. The western portion fill. The middle and northern part of the profile of the profile (represented by Location 1) corre- south of the Absheron Ridge shows a resulting sponds to thicker sediment cover and the eastern crustal thickness of roughly 5–6 km. In the south- portion of the profile (represented by Location 2) ern part of the profile, the crust thickens rapidly corresponds to thinner sediment cover, with the to between 10 and 15 km. The resulting crustal- change occurring across a gravity-observed, deep- type variation can be seen in Figure 13a, where seated tectonic block. Mammadov (2008) interprets the b-factor value varies from around b ¼ 3 in the this tectonic feature as a fragment of the continen- south to approximately b ¼ 13 in the north, where tal crust or ‘microcontinent’ on the border of the the crust is modelled as being thinnest. These values ‘oceanic’ SCB. are probably too high to be realistic for oceanic crust as the software is not adapted to the problem, but Geoseismic profile 3. To continue to reconstruct the they are required to accommodate the deposited initial basin shape of the ‘oceanic-type’ crust in the post-rift stratigraphy during modelling and show western part of the SCB, modelling along a third a good match with stratigraphy (Fig. 13c). These profile was conducted using the same workflow as results indicate that the total thickness of sedi- described for the previous two profiles. The profile ments, interpreted along Profile 3, can be accommo- starts at the Absheron Peninsula near Baku and dated by subsidence of the sediment succession ends close to the Iranian shore of the Caspian Sea overlying an ‘oceanic-type’ crust on the northern (Fig. 3). This profile was chosen to represent a con- part of the profile, slowly transitioning to an attenu- tinuous north–south cross-section across the west- ated continental crust in the south, but lacking the ern portion of basin. sharp transition observed in Profile 2. The geological cross-section of this profile One-dimensional subsidence modelling was (Fig. 12) illustrates the thicker Mesozoic-aged conducted on the pseudo-well extracted from the Downloaded from http://sp.lyellcollection.org/ by guest on September 26, 2021

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Fig. 12. Geoseismic Profile 3 through the northern and southern parts of the South Caspian Basin showing variable structural styles and sediment thicknesses. middle portion of Profile 3 (Fig. 14) to compare with One-dimensional backstripping and strain-rate results of 2D modelling on the same profile, as well inversion modelling was undertaken in four loca- as similar 1D modelling results shown on profiles 1 tions, shown by red circles in Figure 16, for (Fig. 6) and 2 (Fig. 10). Tectonic subsidence in this comparison with 2D models. Shown on the map pseudo-well was calculated to be about 4.5 km and are values of the b-factor derived from both the the initial b-factor on this profile was calculated at 2D ‘Stretch’ forward model and from the 1D sub- b ¼ 3.5, which is only slightly smaller than the sidence model (the latter shown in brackets), b ¼ 5 from the 2D forward modelling at the which excludes the last 36 Ma from tectonic sub- same location. sidence. An excessive b-factor has been calcula- Gravity modelling along this profile was also ted for the northern part of the SCB from the 2D carried out (Fig. 15). The best match between the modelling, and relatively lower values have been observed and modelled gravity signature can be calculated elsewhere for both 1D and 2D modell- achieved by the density model using a 15 km-thick ing, being quite similar for the eastern part of the ‘oceanic-type’ crust layer in the south of the pro- SCB (Fig. 16). For the 2D forward model, the file, thinning to between 7 and 8 km in the north of b-factor maximum exceeds b ¼ 20 but, for the con- the profile, where it is potentially consumed under strained model, maximum values are around b ¼ 7 the Absheron Ridge. The thickness of the sediment (Profile 1). layer in the profile gradually increases from 15 km Subsidence modelling of the SCB shows that in the southern part of the profile to 20 km in the excessive sediment thickness of between 24 and northern part. 25 km can be explained by significant crustal extension in the Late Jurassic followed by thermal subsidence, and sediment loading and compaction, Discussion with sediment loading and compression increas- ing dramatically in the Late Tertiary. The amount The iterative modelling applied to the SCB was of crustal stretching is high with values of up to used as a tool to confirm a number of key obser- b ¼ 7, indicative of what could be called highly vations about the crustal structure and stratigraphic attenuated or even ‘oceanic-type’ crust (Fig. 16). evolution of the SCB. It also allowed for the syn- Gravity modelling also confirms an ‘oceanic-type’ thesis of modelling and observation, subdividing thin and dense crustal model. the geodynamic history of the SCB into several tec- Variations in crustal structure controlled the tonostratigraphic stages, from basin formation to magnitude of subsidence following rift extension, present day. with the modelling showing that strongly marked Downloaded from http://sp.lyellcollection.org/ by guest on September 26, 2021

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Fig. 13. Results of the forward lithospheric modelling of Proﬁle 3. (a) 2D modelling results with crustal extension to match the stratigraphy. (b) Resultant value of the initial b-factor. (c) Modelling results with stratigraphy overlay, with the red dashed line representing the depth to basement and the black solid line representing the approximate Base Tertiary reﬂector; data outside the black square is interpolated. (d) Flexural backstripping results with restored palaeobathymetry at the Top Jurassic (145 Ma) using crustal b-factor derived from the modelling, and showing average water depths around 3 km. thickening of the Mesozoic–Palaeogene-age sedi- and eastern portions of the basin through a structural ment wedge in the NW portion of the SCB is step also observed on gravity data (Fig. 15). located in an area of transition from a thicker attenu- The conceptual model, compatible with these ated crust (15 km) to very thin ‘oceanic-type’ crust. observations, shows that the current SCB geometry This marked change occurred between the western could be interpreted as an inherited rift basin Downloaded from http://sp.lyellcollection.org/ by guest on September 26, 2021

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Fig. 14. (a) Geoseismic Proﬁle 3 showing the location of the pseudo-well where the 1D subsidence model was applied (shown by a red dot on the map). (b) Water-loaded subsidence history using the constrained model. (c) Initial b-factors derived from the constrained model. The black dashed line shows 36 myr ago, after which no subsidence ﬁt was applied.

Fig. 15. Gravity modelling results of Proﬁle 3 showing the observed (dotted line) and modelled (solid line) Bouguer gravity anomaly. The density model shows: sediment layers in yellow and green; the upper crust in light purple; the lower crust in deeper purple; the oceanic crust in red-brown. Downloaded from http://sp.lyellcollection.org/ by guest on September 26, 2021

N. A. ABDULLAYEV ET AL.

Fig. 16. Interpretation of the crustal structure shown over the depth to basement map. This map shows a comparison of b-factors derived from the modelling of 2D lines and 1D strain-rate inversion. fabric. The largest of the modelled b-factors (b . 5) a block of attenuated continental crust named the can be placed within an elongated zone of the ‘Godin micro-continent’ by Mammadov (2008). rifted margin, orthogonal to Jurassic-age SW–NE This deep-seated basement feature could be inter- extension (outlined by a green dashed polygon in preted as a transform fault and underlies a major Fig. 16). The northern boundary of this depocentre Pliocene-age structural ridge named ‘Abikh Uplift’ is the Absheron Ridge and the western boundary or ‘Alov’ (Guliyev et al. 2003). The southern of this depocentre is the West Caspian Fault as boundary of the depocentre is not clear but may described in Kadirov et al. (2008). This proposed coincide with a gravity maximum near the Seﬁdrud rift depocentre is ﬂanked to the east by a deep-seated Delta. These key crustal boundaries have a major NE–SW basement feature (shown as a black dashed impact on the tectonic framework of the basin line on Fig. 16) separating the depocentre from and related depositional styles. There are also Downloaded from http://sp.lyellcollection.org/ by guest on September 26, 2021

SUBSIDENCE IN THE SOUTH CASPIAN BASIN indications that another potential rift depocentre enhanced as a result of a 1.5–2 km Messinian-age might have been present on the SE margin of the base-level fall at 6 Ma with a basinward shift of SCB in the so-called Pre-Alborz (Mazandaran) over 1000 km, followed by rapid deposition of ter- deep, where the depth to the base of the Pliocene rigenous and lacustrine sediments of the PS. The exceeds 8 km (Guliyev et al. 2003; Glumov et al. importance of this large-scale base-level fall event 2004) and basement features have been identified for sedimentation and subsidence is covered in on seismic data (Babayev & Gadzhiev 1998). detail in Green et al. (2009) and Abdullayev et al. Within the area of the proposed NW rift depo- (2012). centre, total subsidence following rifting was sig- The total subsidence that has produced the nificantly enhanced through thermal subsidence current basin shape may result from the interplay and sediment loading, which is reflected in the of all these factors. Modelling shows that extremely calculated b-factors and observed sediment thick- attenuated ‘oceanic-type’ crust, with high calcula- nesses. Outside of this depocentre, Mesozoic– ted b-factors over the NW portion of the basin, is Palaeogene age thickness is relatively limited. In necessary in order to generate the observed sedi- addition, the Mesozoic–Palaeogene-age sequence ment thickness. It is not possible to accommodate probably also contains some (probably limited) this sediment thickness with a single average crus- syn-rift stratigraphy. The original extent and orien- tal thickness and a single b-factor as an unrealistic tation of the rift depocentre and the mechanism for initial water depth will result. This, again, implies the generation of oceanic crust within this basin are the complicated nature of the crustal structure impossible to explain from any type of subsidence across the basin. modelling attempted, but the highly attenuated nature of the crust over part of the basin hints at complicated rift fabric geometries along the north- Conclusion ern margin of the Mesozoic Neotethyan Ocean. The Late Tertiary evolution of the SCB was a Iterative subsidence modelling of the SCB pre- complex interplay of continuing sediment loading sented in this study shows that the observed charac- and far-field flexural effects related to the uplift of ter of subsidence and sedimentation in the SCB the Caucasus and the potential subduction of the can be explained by a process of thermal subsidence SCB plate under the Absheron Ridge. Late Tertiary and sedimentary loading over ‘oceanic-type’ or atte- compression and subduction has a large, but loca- nuated continental crust, and is compatible with lized, impact on tectonic (water-loaded) subsidence, similar studies by Egan et al. (2009) and Green especially in the region south of the Absheron et al. (2009). Forward modelling of lithospheric Ridge, and accounts for the discrepancy between extension and gravity modelling confirms the pres- the 2D forward modelling and the 1D subsidence ence of variable crustal types, and infers a tentative modelling results outlined above. It can be esti- Jurassic-age boundary of the rifted margin and its mated from the discrepancy that about 1.5–2 km basin: delineated by the Absheron Ridge in the of additional tectonic subsidence has been gener- north, the West Caspian Fault in the west and the ated by Late Tertiary compression after about ‘Godin Massif’ in the east. In addition, gravity mod- 34 Ma. Alternative causes for the excessive tectonic elling shows that negative anomalies north of the subsidence in the western portion of SCB could Absheron Ridge correspond to dense ‘oceanic-type’ be related to ‘dynamic’ or residual topography in crust, possibly undergoing underthrusting or sub- response to convective downward flow (downwel- duction under the continental crust of the Scythian ling) within the mantle (Winterbourne et al. 2009). Plate, and the negative anomaly south of the Studies by Crosby & McKenzie (2009) and Winter- Absheron Ridge corresponds to an approximately bourne et al. (2009) demonstrate that positive and 10 km-thick Mesozoic–Palaeogene wedge over- negative deviations from the established global lying same attenuated ‘oceanic-type’ crust. Positive age–depth trend for oceanic crust are common gravity anomalies in eastern and southern portions and can be as large as +1 km. They mostly correlate of the SCB correspond to thinner sediment succes- with long-wavelength gravity anomalies over oce- sions, overlying relatively less attenuated crust. anic crust, such as the large negative anomaly under Late Tertiary compression in the western SCB the SCB. Negative residual topography could also has had a more localized impact on subsidence, explain the isostatically uncompensated nature of especially in the region south of the Absheron the western part of the SCB. Ridge. Modelling presented in this study indicates In addition, simple subsidence modelling cannot that an additional compression mechanism is prob- account for the marked increase in the rate of depo- ably necessary to reduce crustal stretching to more sition observed during the Pliocene PS, not related realistic stretching factors, but the amount of that to specific tectonic compressional events. Total additional subsidence varies between western and subsidence during the Miocene was significantly eastern parts of the SCB. Downloaded from http://sp.lyellcollection.org/ by guest on September 26, 2021

N. A. ABDULLAYEV ET AL.

Further investigation of the modelling methods Abdullayev, N. R., Riley,G.W.&Bowman, A. 2012. is recommended to address underlying deficiencies Regional controls on lacustrine sandstone reser- of extensional models, and more constraints from voirs: the Pliocene of the South Caspian Basin. In: Baganz, O. W., Bartov, Y., Bohacs,K.&Numme- new reflectivity refraction or gravity data, as well dal as better depth constraints, are required for the vali- , D. (eds) Lacustrine Sandstone Reservoirs and Hydrocarbon Systems. American Association of Pet- dation of these models. roleum Geologists, Memoirs, 95, 71–98. The largest volume of sediments was accu- Adamia, S. A., Buadze,V.I.&Shavishvili, I. D. 1977. mulated in the basin from the Oligocene onwards. The great caucasus in the phanerozoic; A geodynamic Average sedimentation rates were 500–600 m model. In: Jankovic, S. (ed.) Metallogeny and Plate Ma21, reaching a maximum during the Lower Plio- Tectonics in the Northeastern Mediterranean. Faculty cene PS. The sediment input and, therefore, total of Mining Geology, Belgrade University, Belgrade, subsidence dramatically accelerated following 215–229. Allen, M. B., Jones, S., Ismail-Zadeh, A., Simmons, Messinian-age base-level fall at 6 Ma. More than Anderson 6 km of terrestrial and lacustrine sediments were M. & , L. 2002. Onset of subduction as the cause of rapid Pliocene-Quaternary subsidence in the deposited during the PS, which represents a large- South Caspian Basin. Geology, 30, 775–778. scale lowstand systems tract capped by Akchagyl Artyushkov, E. V. 2007. Formation of the superdeep marine deposits. The impact of PS deposition on South Caspian basin: subsidence driven by phase subsidence was a rapid acceleration of subsidence change in continental crust. Russian Geology and Geo- rate as a result of sediment loading, not a tectonic physics, 48, 1002–1014. event (Green et al. 2009). The effect of this sedi- Babayev, D. Kh. & Gadzhiev, A. N. 1998. About exist- ment loading is most pronounced in the western ence of new massif in the south of the South Caspian portion of the SCB, which contains thicker PS. basin on complex geophysical data. In: Reports of the The process of basin evolution and subsidence 2nd Azerbaijan International Geophysical Conference, Baku, Azerbaijan, 30 September–2 October 1998, in the SCB modelled in the study can be subdivided Azerbaijan National Geophysics Committee, 240 (in into five main tectonostratigraphic stages: (1) Late Russian). Jurassic-age rifting, assumed to be of limited dur- Berberian, M. 1983. The southern Caspian: a compres- ation; (2) Mesozoic–Lower Palaeogene thermal sional depression floored by a trapped, modified subsidence; (3) Oligocene–Lower Miocene-age oceanic crust. Canadian Journal of Earth Sciences, increase in subsidence rates, probably related to 20, 163–183. the onset of compression and Caucasus uplift; (4) Brunet, M.-F., Korotaev, M. V., Ershov,A.V.& Nikishin rapid sedimentation and sediment loading during , A. M. 2003. The South Caspian Basin: a the PS following a dramatic base-level fall; and (5) review of its evolution from subsidence modelling. Sedimentary Geology, 156, 119–148. renewed Pleistocene-age compression, folding and Crosby,A.G.&McKenzie, D. 2009. An analysisof young hydrocarbon entrapment with continuing rapid sub- ocean depth, gravity and global residual topography. sidence. The hydrocarbon systems of the SCB are Geophysical Journal International, 178, 1198–1219. quite unique because of such rapid sedimentation. Devlin, W., Cogswell,J.et al. 1999. South Caspian Basin: young, cool, and full of promise. GSA Today, This work mostly reflects doctoral research conducted by 9,1–9. the corresponding author at the Geological Institute of Egan, S., Mosar, J., Brunet, M.-F., Bochud,M.&Kan- Azerbaijan. The authors express their gratitude to acade- garli, T. 2009. Subsidence and uplift mechanisms mician Professor P. Z. Mammadov (Azerbaijan State Oil within the South Caspian Basin: insights from the Academy) and Dr G. Riley (BP) for their advice and onshore and offshore Azerbaijan Region. In: Brunet, comments, and also to Dr A. Crosby for advice of the 1D M.-F., Wilmsen,M.&Granath, J. W. (eds) South subsidence workflow. The authors are also grateful to BP Caspian to Central Iran Basins. Geological Society, Exploration Ltd for permission to show map data, and to London, Special Publications, 312, 219–240, http:// Badley Geoscience Ltd (in particular to Dr A. Roberts) doi.org/10.1144/SP312.11 for permission to use the ‘FlexDecomp’ and ‘Stretch’ GEOLOGICAL INSTITUTE OF AZERBAIJAN 2003. Atlas of modelling software, as well as providing helpful com- Lithological–Paleogeographical Maps of Azerbaijan. ments. The authors would also like to thank Dr M. F. Geology Institute of the Azerbaijan (GIA), National Brunet, S. Egan and L. 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