Formation of the superdeep South Caspian basin: subsidence driven by phase change in continental crust

Russian Geology and Geophysics 48 (2007) 1002–1014 www.elsevier.com/locate/rgg

Formation of the superdeep South Caspian basin: subsidence driven by phase change in continental crust E.V. Artyushkov * Institute of the Physics of the Earth, Russian Academy of Sciences, 10 ul. Bol’shaya Gruzinskaya, Moscow, 123810, Russia Received 22 December 2006

Abstract The large hydrocarbon basin of South Caspian is filled with sediments reaching a thickness of 20–25 km. The sediments overlie a 10–18 km thick high-velocity basement which is often interpreted as oceanic crust. This interpretation is, however, inconsistent with rapid major subsidence in Pliocene-Pleistocene time and deposition of 10 km of sediments because the subsidence of crust produced in spreading ridges normally occurs at decreasing rates. Furthermore, filling a basin upon a 10–18 km thick oceanic crust would require twice less sediments. Subsidence as in the South Caspian, of ≥20 km, can be provided by phase change of gabbro to dense eclogite in a 25–30 km thick lower crust. Eclogites which are denser than the mantle and have nearly mantle P velocities but a chemistry of continental crust may occur beneath the Moho in the South Caspian where consolidated crust totals a thickness of 40–50 km. The high subsidence rates in the Pliocene-Pleistocene may be attributed to the effect of active fluids infiltrated from the asthenosphere to catalyze the gabbro-eclogite transition. Subsidence of this kind is typical of large petroleum provinces. According to some interpretations, historic seismicity with 30–70 km focal depths in a 100 km wide zone (beneath the Apsheron-Balkhan sill and north of it) has been associated with the initiation of subduction under the Middle Caspian. The consolidated lithosphere of deep continental sedimentary basins being denser than the asthenosphere, can, in principle, subduct into the latter, while the overlying sediments can be delaminated and folded. Yet, subduction in the South Caspian basin is incompatible with the only 5–10 km shortening of sediments in the Apsheron-Balkhan sill and south of it and with the patterns of earthquake foci that show no alignment like in a Benioff zone and have mostly extension mechanisms. © 2007, IGM, Siberian Branch of the RAS. Published by Elsevier B.V. All rights reserved. Keywords: Crustal structure; crustal subsidence; phase change; lithospheric rheology; seismicity; subduction; South Caspian basin

Introduction Some basins in the continent interior and along the margins have sediment accumulations as thick as 15–20 km or more. They are, namely, the South Caspian, Black Sea, and Barents basins where the sedimentary cover overlies high-velocity consolidated crust thinned to 10–15 km in the deepest parts. Because of the low thickness and high P velocity near that of the basalt layer, the basement in these basins is often interpreted as oceanic crust (e.g., Jackson et al., 2002; Verba et al., 2001). It was recently discovered, however, that the deposition centers of the Barents and Caspian basins lied over continental crust that experienced major subsidence caused by phase change with consolidation of mafic rocks (Artyushkov, 2005; Artyushkov and Yegorkin, 2005). The inference stemmed from the fact that the sedimentary accumulation of

* Corresponding author.

18–22 km was much thicker than required to fill a basin upon an oceanic crust of that thickness (12–15 km). Furthermore, subsidence in the Barents and Caspian basins lasted for hundreds of million years, or times longer than the characteristic subsidence time of oceanic crust (80 Ma). The Moho was found out to delineate a thick layer of eclogites with their P velocities about the same as in mantle peridotites. The South Caspian sedimentary basin (Fig. 1) is one of world largest petroleum provinces. Deep seismic soundings (Baranova et al., 1990; Neprochnov, 1968) and reflection profiling (Glumov et al., 2004; Knapp et al., 2004) indicated sediment thicknesses of 15–25 km or more and a consolidated crust attenuated to at least 10–12 km. The VP velocities reaching as high as 6.6–6.9 km/s (Baranova et al., 1990) or even 7.1 km/s (Jackson et al., 2002) in the crust were taken for an indication of its oceanic origin. The controversy was mainly about the time and tectonic setting of crust formation. The most broadly invoked idea was that the South Caspian was either a remnant of the Tethys (Dercourt et al., 1986; Nadirov et al., 1997) or a Mesozoic or an Early Cenozoic

E-mail address: [email protected] (P.I.Chekhovich) 1068-7971/$ - see front matter D 2007, IGM, Siberian Branch of the RAS. Published by Elsevier B.V. All rights reserved. doi:10.1016/j.rgg.200 7. 11. 0078

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Fig. 1. Main tectonic units of South Caspian basin and its surroundings, modified after (Udintsev, 2003). 1 — Alpine orogens, 2 — sedimentary basins of Middle Caspian and adjacent part of Turan platform, 3 — deep sedimentary basins of South Caspian and its surroundings.

back-arc basin (e.g., Zonenshain and Le Pichon, 1986). Sengör (1990) interpreted it as an oceanic pull-apart basin that followed a Late Mesozoic shear zone parallel to the Caucasus, Alborz, and Kopet Dagh ranges. An alternative hypothesis (Artyushkov, 1993) attributed the origin of the South Caspian basin to consolidation of mafic rocks in the lower continental crust as a result of gabbro-eclogite phase change whereby the P velocity increased to typically mantle values. Then, the Moho should trace the top of lower crust high-grade dense rocks rather than the top of the mantle. The historic seismicity with hypocentral depths to ∼70 km and the free-air gravity pattern prompted the idea that the oceanic lithosphere in the northern basin margin was in the initial stages of subduction under the Middle Caspian region (Allen et al, 2002; Jackson et al., 2002; Knapp et al., 2004). Axen et al. (2001) hypothesized subduction of the South Caspian under the Alborz Range in the south. The aim of this study is to investigate geological and geophysical evidence of the structure and history of the South Caspian basin to see whether its basement is oceanic or continental crust and whether it undergoes subduction.

Crustal structure The >20 km deep sedimentary basin of South Caspian (Figs. 1 and 2) is located in the center of the Alpine-Himalayan orogen. Its northern boundary is along the Apsheron-Balkhan sill that links the orogenic structures of the Greater Caucasus and the Kopet Dagh. The basin borders the Middle Caspian sedimentary basin filled with 5–7 km of sediments in the north and the 15 km deep Kura basin in the west. In the east, the

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South Caspian grades into the West Turkmenian basin which has a sedimentary thickness of 10–12 km or, possibly, 20–30 km in the northeast (Udintsev, 2003) and is bounded by the Kopet Dagh Range further in the east. The southern and southwestern borders of the South Caspian are formed by the Alborz and Talysh mountains. The northern half of the South and Middle Caspian basins has been densely covered with seismic profiles (Figs. 12, 13 in Glumov et al., 2004). Figure 2 shows the respective map of basement topography based on reflection profiling data. The map for the southern, Iranian, sector is, however, tentative being compiled mainly from gravity data. DSS data from the South and Middle Caspian basins collected in 1956 (Aksenovich et al., 1962; Neprochnov, 1968) were reprocessed recently to obtain velocity models for profiles I and II (Baranova et al., 1990). See Fig. 3 for the profile locations and Fig. 4 for the velocity models. According to these models, the thickness of sediments (hsd) along profile I (Fig. 4, A) is 25–28 km, the consolidated crust is as thin as 5–6 km (hcc), and the Moho is as shallow as 30–35 km. The sediment thickness decreases rapidly, the crust becomes thicker, and the Moho deepens to 50 km in the NE direction. The corresponding estimates for profile II (Fig. 4, B) are hsd = 22 km, hcc = 12 km, and a Moho depth of 30–35 km. The sharp transition to the platform that lies northeast of the basin is shown as a steep fault border. P velocities remain low (VP ≤ 4.8 km/s) as far as the basement, to 20 km or deeper, in both profiles which never occurs in other sedimentary basins. The low seismic velocities are often explained by low consolidation and high porosity of sediments (Baranova et al., 1990). The DSS-derived Moho and basement depths being accurate to ±2–3 km, the respective hcc estimates may bear a large error and the actual crust thickness may range from 5 to 15 km. The estimates of P velocities in the thin consolidated crust appear to be not very accurate either. Therefore, the VP velocities in Fig. 4 are rather tentative. Reflection profiling along two short profiles in the northern part of the basin showed a sediment accumulation of hsd = 26–28 km overlying a basement of hcc = 10 km which was interpreted as oceanic crust (Knapp et al., 2004). A model of the composite W–E profile (Fig. 5) based on the synthesized earlier DSS data and some land profiles of converted and surface waves (Jackson et al., 2002) images the profile western (Kura basin) and eastern (Turkmenia) flanks lying over a typical continental crust with a granite layer. In this model, the consolidated crust beneath the South Caspian with VP = 7.1 km/s is only 12 km in the west and 18 km thick in the east.

Deposition history The South Caspian sedimentary basin is a large petroleum province with hydrocarbon resources of 50 BBOE or more (Glumov et al., 2004). The sedimentary cover comprises two main lithologic units with their rocks brought to the surface

Fig. 2. Basement topography in South Caspian basin and its surroundings (sediment depths in km), modified after (Glumov et al., 2004).

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the Late Pliocene, the whole Caspian basin became again filled with water. Late Pliocene and Pleistocene deposition added 3–5 km of sediments. The modern sea depth in the South Caspian (1 km) is close to that in the earliest Pliocene but the Pliocene sea level was 800–1000 m lower than now. To fill a dry basin of the depth ∆h, sediments of the density ρsd are required to have the thickness hsd = [ρm / (ρm − ρsd)] ∆h,

Fig. 3. Locations of DSS profiles (I, II) in South and Middle Caspian basin reprocessed in 1990, modified after (Baranova et al., 1990), and seismic reflection profiles III, IV (line F–F′) of sediments in South Caspian basin (Glumov et al., 2004). 1 — shot points along profiles I, II; 2 — bottom seismographs; 3 — land stations.

in numerous mud volcanoes (Grigoriants, 2001). The lower succession (10 km thick) consists mainly of marine mudrocks deposited from the Paleocene to the latest Miocene. Hydrocarbon source rocks are Oligocene highly bituminous anoxic mudrocks of the Maikop Formation varying in thickness from hundreds of meters to 2–3 km (Glumov et al., 2004; Jones and Simmons, 1997). Organic contents in Middle Miocene sediments reach 10% and more. According to Mamedov (1992), the base of the succession in the western basin part may be composed of Jurassic volcanics extending from the Kura basin and the basement in the east may be overstepped by Cretaceous sediments. The deposition of the lower sedimentary unit completed in the Late Miocene when the Caspian basin became separated from the Black Sea soon after the Messinian salinity crisis at about 5 Ma. That is, the other half of the sediment has accumulated since that time. Judging by the 600–700 m incision of the Paleo-Volga channel in the Middle Caspian basin (Gadzhiev and Popkov, 1988), the Early Pliocene water level in the South Caspian can have been about 800–1000 m lower than the global sealevel. Nevertheless, the western part of the area was apparently still occupied by a sea (Glumov et al., 2004) with about 1 km of water, according to the elevation of deposits carried by tributary rivers. In the Early Pliocene, the South Caspian basin rapidly accumulated fluvial-deltaic sands, sandstones, and mudrocks of the Balakhan Group, a productive hydrocarbon reservoir. The group is also called the Redbed Group in the east of the basin where it contains abundant redbeds. The thickness of the Early Pliocene productive series is 3–4 km over a great part of the area but reaches 7–8 km in the west, in the South Apsheron basin. In

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taking into account the isostatic crustal subsidence under sediment load, where ρm = 3330 kg/m3 is the mantle density. The South Caspian sediments are not much consolidated (Narimanov, 1993) and their mean density (ρsd) is mainly within 2500 kg/m3 (Brunet et al., 2003). Substituting (ρsd) = 2500 kg/m3 and ∆h = 1 km into (1) gives hsd 3−4 km, or 6–7 km less than the upper succession (10 km). Therefore, up to 6–7 km of sediments were deposited for the past 5 Ma at the account of crustal subsidence. The deposition of the 10 km thick lower succession began in the earliest Cenozoic (Zonenshain and Le Pichon, 1986) or, most likely, in the Mesozoic (Dercourt et al., 1986; Nadirov et al., 1997; Sengör, 1990), judging by the presence of Jurassic and Cretaceous limestones among products of mud volcanism in the Baku Archipelago (Glumov et al., 2004; Grigoriants, 2001). Therefore, it took at least 50 Myr or rather 100 to 150 Myr and occurred in the conditions of poor sediment transport from the surrounding areas where there were no high mountains at that time. The rapid deposition of the upper succession for the past 5 Myr was due to voluminous riverine transport maintained by active erosion in the rapidly growing ranges of Greater and Lesser Caucasus, Talysh, Alborz, and Kopet Dagh. Thus, subsidence was mainly kept up by deposition.

Minor contribution to subsidence from lithospheric flexure If the South Caspian basement were oceanic crust, the deposition of the ∼10 km thick lower sedimentary succession would be associated with filling an oceanic basin. Subsidence of oceanic crust produced in a spreading ridge normally takes about 80 Myr and occurs at decreasing rates (Watts, 2001). Subsidence in the South Caspian began from at least 50 to 150 Ma and would have been fully or nearly completed by the Pliocene. On the contrary, it accelerated in the Pliocene and Pleistocene to let accumulation of another ∼10 km thick sedimentary succession. The Apsheron-Balkhan sill along the northern basin edge (Fig. 1) bridges the Greater Caucasus with the Kopet Dagh range, where strong compression acted in the Neogene. That was the reason why the South Caspian oceanic crust was suggested to undergo northward subduction under the Middle Caspian region along the sill (Allen et al., 2002; Jackson et al., 2002), proceeding mainly from seismicity data of numer-

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Fig. 4. Crustal structure along DSS profiles I (A) and II (B), modified after (Baranova et al., 1990). For locations of profiles see Fig. 3. 1 — reflectors; 2 — basement surface; 3 — Moho; 4 — regions of velocity inversion. See text for explanation of points a, b, c.

Fig. 5. Crustal structure in western Kura basin, Caspian Sea and its eastern surroundings from DSS and converted-wave data (Jackson et al., 2002). 1 — sediments; 2 — granite layer (VP = 5.8–6.5 km/s), 3 — basalt layer (VP = 6.5–7.8 km/s); 4 — upper mantle; 5 — reflection lines; 6 — locations of seismographs.

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Fig. 6. Earthquake mechanisms and depths (km) of large events originated at >30 km in northern South Caspian and southern Middle Caspian, modified after (Jackson et al., 2002). 1 — shallow marine basins, 2 — deep marine basins; 3 — earthquake mechanisms. Left panel enlarges box that frames earthquakes in right panel (map).

ous earthquakes with centroid depths of 30 to 70 km (Fig. 6). Allen et al. (2002) explained the rapid increase in subsidence rates by the development of a flexural basin in front of the subduction zone, as in the case of oceanic trenches on active margins. Another hypothesis associated subsidence with loading from the Alborz range (Axen et al., 2001) because the major and rapid Pliocene-Pleistocene subsidence in the South Caspian was coeval with uplift of the Alborz mountains on its southern border (Alavi, 1994). Downward elastic flexure should increase toward a convergent plate boundary. The model of Fig. 4, A images a convergent boundary beneath the Apsheron-Balkhan sill in the basin deepest part at the point c. Note that instead of dipping, the basement flattens out rapidly in the NE direction from the point a toward c, and its slope in the segment bc is about five times as low as that in ab. In the model of Fig. 4, B, the basement in the basin deep part even rises slightly toward the northern edge. DSS data cannot resolve crustal structures less than 10 km wide, and a basement flexure close to the hypothetical convergent boundary in the north of the South Caspian (Fig. 4) might have been missed. Knapp et al. (2004) distinguished a 5 km deep basement flexure at 25 km far from the ApsheronBalkhan sill. However, their inference appears ambiguous because continuous bedding in the lower sedimentary succession is strongly disturbed by mud diapirs and is poorly detectable. Even though there is a local flexure near the sill, it cannot influence the subsidence in the ∼250 km wide area in the south. Oceanic trenches on active margins commonly produce large gravity lows. A 100 km wide linear low that fringes the southern edge of the Apsheron-Balkhan sill in the free-air gravity field over the South Caspian, with a mean northward anomaly increase of 60 mGal, was interpreted as another line of evidence to support the subduction to the north (Allen et al., 2002). The free-air gravity anomalies, however, poorly represent the departure from isostasy in regions where the crust thickness and topography are very uneven. To estimate isostatic gravity anomalies in these regions, one has to cover

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an area at least 200 km around. According to these estimates (Kaban, 2002; Kaban et al., 1998), large isostatic gravity lows do exist over the Apsheron-Balkhan sill, as well as south and north of it. They reach –120 mGal in the northwestern South Caspian and in the southwestern Middle Caspian giving way to a ∼250–300 km wide elongate zone of gravity highs farther in the south. The gravity highs fluctuate smoothly over the area between a few tens and 50–70 mGal. Therefore, even though lithospheric flexure may account for subsidence in the basin north, it does not involve the remaining part of the basin. A downward basement flexure to the north contradicts reflection profiling data. See, for instance, the rise of the N22pr1 base to 7 km near the Apsheron-Balkhan sill (Fig. 7) in the seismic cross section along profile IV in the central and northern South Caspian (Glumov et al., 2004). The cross section provides quite reliable ages of reflectors to a depth of 10–12 km though the deeper strata, imaged quite faithfully, are timed less reliably. There are few published data available for the southern, Iranian sector of the basin, and its basement topography (Fig. 2) is inferred mainly from gravity. The absence of large isostatic gravity lows virtually rules out any considerable lithospheric flexure toward the Alborz range. Furthermore, the variation in sediment thickness from as thin as 1–2 km near the range to ∼20 km at ∼100 km away (Fig. 2) is contrary to that expected in the case of flexure under the load from the Alborz mountains hypothesized by Axen et al. (2001). Seismic data (Jackson et al., 2002) either give no support for subduction under Alborz. Earthquake mechanisms in the Alborz region indicate crustal compression and left-lateral strike-slip faulting along the range northern edge. The origin depths of reliably located earthquakes in the area do not exceed 15 km.

Sediment thickness required to fill a basin upon oceanic crust The thickness of basaltic oceanic crust (h0c ) is mainly uniform and averages about 7 km (White et al., 1992); the mean ocean depth outside spreading ridges is h0w ≈ 5.5 km (Cloos, 1993). The profiles of Figs. 4 and 5 show abrupt large variations in crustal thickness (hcc) from 5–10 to 12–18 km which are hardly found in oceans. Thick crust of hcc = 12–18 km may occur over hotspots and in oceanic plateaus which occupy a minor area of today’s oceans (Udintsev, 1975, 1989–1990, 2003). Therefore, the existence of oceanic crust appears unlikely in the South Caspian basin. Isostasy requires that subsidence of an oceanic basin with the crust thickness hc be fully compensated by deposition of sediments of the thickness hsd = [(ρm − ρw)/(ρm − ρsd)]h0w − [(ρm − ρoc)/(ρm − ρsd)] (hc − h0c ),

(2)

where ρoc = 2900 kg/m3 is the density of oceanic crust. At

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Fig. 7. Cross section of South Caspian sediments obtained by reflection profiling along line F–F′ (profile IV in Fig. 3), modified after (Glumov et al., 2004). 1 — age limits; 2 — reflectors; 3 — basement; 4 — productive bed and its number. Vertical scale exaggerated to ten times horizontal scale.

hc = 12–18 km in the central South Caspian, the sediment thickness would be hsd ≈ 9.5 − 12.7 km.

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Yet, the sediment thickness reaches at least 20 km in two areas of the basin (Fig. 2). Replacement of the 1 km deep water of the South Caspian with sediments and the related isostatic crustal subsidence under the sediment load would require another ∼3 km thick succession. The resulting thickness of ∼23 km is about twice the value according to (3). The same estimates can be obtained for the profiles of Fig. 4, A, B. The great sediment thickness in the South Caspian basin is either inconsistent with suboceanic crust which, being lighter than oceanic crust, would be covered with a lesser amount of sediments. Still less sediments would be needed to fill a basin on a lower-density continental crust attenuated by extension. Therefore, the South Caspian basin cannot have been produced by extension, more so that the area was subject to compression during the recent subsidence.

Thick continental crust beneath the South Caspian Maintaining a consolidated crust at a depth of ∼20 km in the South Caspian basin requires a thick layer of rocks denser than mantle peridotites to lie beneath the Moho. Of widespread lithospheric rocks, only mafic eclogites and garnet granulites

possess this density (Christensen and Mooney, 1995; Sobolev and Babeiko, 1994) and have P velocities about VP in mantle peridotites. The available seismic models of the South Caspian basin interpreted these rocks as mantle material and placed them below the Moho. However, their average major-element compositions are of crustal affinity. Therefore, the basement of the South Caspian basin is consolidated continental crust, much thicker than the 10–18 km inferred from seismic data (see below). Earlier we (Artyushkov, 1993, 2005; Artyushkov and Baer, 1987; Artyushkov and Yegorkin, 2005; Artyushkov et al., 2000) discovered that much of subsidence of continental crust could be provided by phase change of gabbro into denser granulite and/or eclogite in the lower crust. This transition is very slow in anhydrous crust and can maintain subsidence kept up by deposition at a rate of ∼10 to 100 m per million years, which commonly occurs on shelves. But it can become orders of magnitude faster in the presence of minor amounts of fluid (Austrheim, 1998). Active fluids infiltrated into the lower crust from small plumes at certain evolution stages in many sedimentary basins. They catalyzed eclogitization or granulitization of gabbro and, hence, accelerated the respective density increase and subsidence (Artyushkov, 1993) to form deep marine basins in 1–10 Myr. That was, namely, the mechanism responsible for the origin of a relatively deep basin of the Bazhenov Sea in West Siberia (Artyushkov, 1993; Artyushkov and Baer, 1987). Rapid crustal subsidence driven

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by gabbro-to-eclogite (granulite) transition acted also in other hydrocarbon basins, such as the Persian Gulf, the Caspian basin, and the Barents Sea, being a typical feature of petroleum provinces (Artyushkov, 1993; Artyushkov and Baer, 1987). Gabbro-to-eclogite (granulite) transition catalyzed by active fluids from the asthenosphere can have been the key mechanism of the major and rapid Pliocene-Quaternary subsidence in the South Caspian. The finds of Pliocene to latest Miocene marine sediments brought to the surface in mud volcanoes indicate that the area may have experienced other episodes of rapid subsidence prior to that of Pliocene-Quaternary. In Pliocene-Miocene time, the South Caspian was a deep sea, where up to ∼10 km of sediments had accumulated. The earliest subsidence at high rates occurred apparently in the beginning of the Cenozoic or in the end of the Cretaceous. Rapid subsidence at the Eocene-Oligocene boundary produced a series of deep marine basins in the northern Crimea, in the peri-Caucasus and Middle Caspian regions, and in the Turan platform (e.g., Artyushkov et al., 2000; Gadzhiev and Popkov, 1988), and acted in the Kura basin as well (Udintsev, 2003). Hence, the South Caspian likewise can have undergone subsidence at that time and thus became deeper. Rapid subsidence having relation neither to extension of lithosphere nor to its flexure in front of a subduction zone provide more evidence that the South Caspian lies on continental crust. The crust above the Moho in the deepest basin parts is no thicker than 10–12 km. It had been overlain by ∼10 km of sediments in the latest Miocene before the recent subsidence episode. At that time, the respective crustal layer was at a depth of 10–22 km at a pressure under 600 MPa (6 kbar). Phase change of mafic rocks at this pressure can account for only a small density increase of ≤100 kg/m3 (Bousquet et al., 1997; Cloos, 1993; Dobretsov et al., 2001; Korikovskii, 2002). Assume that basalts (gabbro) with the density ρgb in a layer of the thickness hgb transformed into garnet granulites of the density ρgl. In the conditions of isostasy, this phase change would cause crustal subsidence and deposition of sediments of the thickness hsd = (ρm / ρgl) [(ρgl − ρgb )/ (ρm − ρsd)]hgb.

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It follows from (4) that at hgb = 10–12 km and ρgl − ρgb ≤ 100 kg/m3, hsd ≤ 1.3−1.6 km, which is much less than the amount of recent subsidence in the South Caspian basin. Therefore, the subsidence would be impossible upon oceanic crust. The basalt layer in the continental crust occurs at depths where density increase by phase change can be considerable. See, for instance, Fig. 6.49 in (Dobretsov et al., 2001) and Fig. 1 in (Doin and Henry, 2001). Furthermore, the continental basalt layer is normally times thicker than oceanic crust. Therefore, rapid major subsidence without large lithospheric extension or flexure is possible only in continental crust, by eclogitization (granulitization) of mafic rocks in its lower part. Substituting the original density of gabbro in the lower crust assumed to be ρgb = 2930 kg/m3 into (4), along with hsd = 23 km, ρec = ρgl = 3500−3600, and ρsd = 2500 kg/m3, gives a 31–35 km thick layer of gabbro required to transform into

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eclogites to maintain the South Caspian subsidence. The consolidated crust is now 12 km thick above the Moho, and, together with that below the Moho, its initial thickness must have been 43–47 km. The crust in the neighboring Kura basin is 47 km and the basalt layer is 40 km thick (Fig. 5). Eclogites that underlie the South Caspian basin are as thick as hec = (ρgb/ρec)hgb = 25−29 km, and the crust base below is thus at a depth of ∼60 km, which is only ∼5 km deeper than beneath the Kura basin. The South Caspian is part of a larger sedimentary basin (Fig. 1) which comprises also the Kura, West Turkmenian, and Middle Caspian basins. The crust in all these areas includes a granite layer (Figs. 4, 5), and is obviously continental. These areas likewise experienced rapid subsidence in the Pliocene and Pleistocene and the ensuing deposition of 7–8, 3–5 and 1.5–2 km of sediments, respectively. Substituting the 800 m sea depth in the Middle Caspian would add about 2 km more sediments. The fact that the basin depth decreases toward the flanking mountains (toward the Greater and Lesser Caucasus in the Kura (Fig. 2) and Kopet Dagh in West Turkmenian basins) is inconsistent with lithospheric flexure but agrees with the phase change of gabbro into eclogites or granulites as the principal driving mechanism of the PliocenePleistocene subsidence. The synchronicity of rapid subsidence in the South Caspian and in its surroundings is another line of evidence that its basement is continental crust. Therefore, the crust above the Moho can be expected to have felsic and intermediate compositions. Hydrous minerals in these rocks dissociate to produce garnet, a high-density phase, when heated to ≥400 °C (Bousquet et al., 1997; Korikovskii, 1979), and lower crust temperatures at 20–30 km may reach 600–800 °C (Glumov et al., 2004). The formation of garnet enhances P velocities in felsic, as well as in mafic rocks. This may account for relatively high P velocities (∼ 7 km/s) in the granite layer beneath the South Caspian (Jackson et al., 2002). High-velocity thin consolidated crust above the Moho exists in other deep sedimentary basins upon continental crust, e.g., in the Barents and cis-Caspian basins. High P velocities in the granite layer associated with hightemperature metamorphism appears to be quite a common phenomenon. Shikhalibeily and Grigoriants (1980) hypothesized that rocks above the Moho in the South Caspian were the basalt layer of continental crust in which the granite layer was eliminated by erosion. Then, the crust would have stayed long above the sea level over a thick hot low-density mantle. Cooling of crust and mantle takes 50 to 100 Myr (McKenzie, 1978) and its rates decay rapidly with time. Therefore, this mechanism appears to be unlikely to allow a major PlioceneQuaternary subsidence in the South Caspian. Softening of lithosphere in the South Caspian The effective elastic thickness of lithosphere (Te) is related to the characteristic flexure width L as (Artyushkov, 2003) (Te)km ∼ 5.3⋅10−2[(L)km]4/3.

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Lithosphere in stable continental regions can be subject to flexure under surface or deep-seated loads over distances (L) from one to several hundred kilometers, and Te can be from a few tens to about 100 km (Burov and Diament, 1998). Rapid major crustal subsidence in some sedimentary basins produced a downward flexure of 10–12 km in ∼20 to 50 km wide zones, i.e., lithosphere softened whereby its elastic component became 5–10 km thinner (Artyushkov, 2003; Artyushkov et al., 2000). Lithospheric softening is caused by active fluids that infiltrate from small plumes and wet mineral crystals. The adsorption surface layer can lower strength and viscosity of polycrystalline rocks, which is known as Rehbinder’s effect (Rehbinder and Wengstrom, 1937; Salnikov and Traskin, 1987). Thus, fluid infiltration catalyzes phase transitions and, simultaneously, causes strong softening of lithosphere. The South Caspian basin is surrounded by steep basement flexures of tens of kilometers wide and up 10–12 km deep (Fig. 2). A 50 km wide and 11 km deep flexure is imaged also in Fig. 7, b in the basin northern part. At L = 20–50 km, Te ∼ 3−10 km according to (5), i.e., lithosphere has been subject to recent softening. Softening can account for the absence of large earthquakes over the main part of the basin.

possibility is most often neglected and basin formation is believed to result from lithospheric extension (e.g., Kenzie, 1978; Wernicke, 1985) or elastic flexure at continental convergent boundaries (e.g., Royden, 1993). The model below simulates a sedimentary basin upon continental crust filled with sediments of the mean density ρsd and the thickness hsd (Fig. 8, a). The mean density of consolidated continental lithosphere of the thickness d beneath the sediments (ρcn cs ) is compared with the mean density of oceanic lithosphere of the thickness D (ρoc) under a water layer of the thickness hw (Fig. 8, b). Both the oceanic and continental plates are assumed to be in isostatic equilibrium, and the asthenospheric pressure at the lithospheric base beneath the continental basin is assumed to be equal to the pressure at the same depth beneath the ocean. Assume that the depth difference between these levels is ha (Fig. 8). Then, hsd + d = hw + D + ha.

(6)

ρsdhsd + ρcn cs d = ρwhw + ρocD + ρaha.

(7)

Excluding ha from (7) and using (6) gives

ρcn cs − ρa = (ρoc − ρa)D/d + [(ρa − ρsd)hsd − (ρa − ρw)hw]/d. (8) A possibility of subduction of continental lithosphere in deep sedimentary basins As it was already noted, there was a hypothesis that the South Caspian basement was oceanic crust in the initial stage of northward subduction (Allen et al., 2002; Jackson et al., 2002; Knapp et al., 2004). It turns out, however, that the sedimentary basin lies upon a basement of consolidated continental crust. The other question is whether it undergoes subduction. Lithosphere (either oceanic or continental) can subduct into the mantle if its mean density exceeds the density of asthenosphere (e.g., Cloos, 1993; Doin and Henry, 2002), which presumably is ρa = 3200 kg/m3 (Artyushkov, 1993). Continental lithosphere includes a granite layer and thus has a mean density commonly lower than the asthenosphere. Yet, continental upper crust in many orogens contains coesite, diamond, and other minerals that formed at depths of 100– 150 km (e.g., Chopin, 1984; Dobretsov et al., 2001; Sobolev and Shatsky, 1990; Wang et al., 1989). The presence of high-pressure rocks in continental crust was explained in terms of its subduction into the mantle either by slab pull from subducting oceanic lithosphere (e.g., Chopin, 1984) or by push from a strong compressive force (e.g. Chemenda et al., 2000). Both models treat the gabbro-to-eclogite transition as a common phenomenon that accompanies and facilitates subduction rather than being its principal driving mechanism (e.g., Doin and Henry, 2002). Lower continental crust beneath many sedimentary basins is composed of heavy garnet granulites and/or eclogites (Artyushkov, 1993, 2005; Artyushkov and Yegorkin, 2005), and it is eclogitization (granulitization) that appears to be responsible for the greatest part of the subsidence. This

The mean density of oceanic lithosphere (ρoc) increases with its age. The youngest age at which oceanic lithosphere remains slightly heavier than the asthenosphere and can be involved in subduction is 10 Myr (Cloos, 1993). Assume that this is the age of oceanic lithosphere in the model of Fig. 8, b, and its density is roughly equal to the asthenospheric density. The first term in the right-hand side of (8) approaches zero and can be neglected. Then, it follows from (8) that the continental lithosphere is heavier than the asthenosphere (ρcn cs − ρ) > 0 at hsd > [(ρa − ρw) / (ρa − ρsd) hw.

(9)

The mean water depth upon a 10 Myr oceanic lithosphere is hw ≈ 3.5 km (Cloos, 1993). Then, (9) reduces to the condition hsd > 3.5[(ρa − ρw)/(ρa − ρsd)] km.

(10)

Fig. 8. A deep sedimentary basin upon continental (a) and oceanic (b) crust in isostasy conditions.

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The available estimates of crust density derived from seismic data are very different (Chriestensen and Mooney, 1995), and the density of lithospheric mantle on continents shows considerable lateral variations (Jordan, 1997; Kaban et al., 2003). It is thus important that (10) is free from parameters of the consolidated lithosphere but includes only the thickness (hsd) and mean density (ρsd) of sediments which are usually quite exactly known. At ρsd = 2500 kg/m3 in the South Caspian basin, (10) fulfills at hsd > 10.9 km, or approximately twice as low as the actual sediment thickness. Therefore, the basin basement is much heavier than the underlying asthenosphere. It follows from (10) that hsd > 11.7–12.7 km in basins with ρsd = 2550−2600 kg/m3. The Barents Sea, the cis-Caspian basin, and the Vilyui basin have much greater sediment thicknesses. A great sediment thickness is required for full compensation in many marine basins, no matter whether they lie over continental or oceanic crust, such as the Western Black Sea, the Gulf of Mexico, and the Eastern Mediterranean basins. Therefore, the basement under those basins must be heavier than the asthenosphere. Sediments in continental sedimentary basins can delaminate from the basement under strong compression and stay as thrusts and buckled folds in orogens. The underlying consolidated lithosphere, if it is heavier than the asthenosphere, can subduct into the mantle. Equation (10) holds for a lithosphere with tightly coupled granite and basalt layers but the latter can decouple in a soft lithosphere with a strongly metamorphosed lower crust and a heavy lithospheric mantle. Thus the lower part of the lithosphere becomes involved in subduction while the overlying lighter granite layer delaminates to make part of an orogen, together with buckled sediments. This crustal structure occurs in the Pamirs, the Urals, and the Alps (Artyushkov, 1993, Figs. 6.2 and 6.4). Much of the Earth’s oceanic lithosphere is older than 10 Myr and is denser than the asthenosphere. Then, one might expect widespread subduction in oceanic interior as well as on active margins, which is actually not the case. Equation (10) fulfills in deep continental sedimentary basins over thick eclogitic lower crust, but there is commonly no subduction in these basins. No subduction occurs either in the lower continental slopes on passive margins where the crust is thin and the thick lithosphere has a high mean density. Compression acts on a great part of continental lithosphere (Zoback, 1992) and would induce at least the onset of subduction, but this is a very rare case, if any. The reason is most likely that the high strength of the lower lithosphere prevents its steep flexure before subduction and formation of a fault between the subducting and overriding plates.

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compression. Therefore, there are all necessary conditions for subduction. Most of earthquakes in the southern, western, and eastern basin surroundings originate in the crust at ≤30 km or most often 15 km (Jackson et al., 2002) but M = 5–6 events near the Apsheron-Balkhan sill, especially at distances within 100 km to the north, have depths below 30 km (Figs. 6 and 9). If deep earthquakes are related to subduction, the subducting plate should be at least 100 km long, taking into account its slope. Sediments south of the sill reach a thickness of 20–25 km and are poorly consolidated, with plastic mudrocks abundant in the lower section. Strong compression in these conditions can be expected to cause delamination and buckling of sediments, possibly, together with the granite layer. Compression in the South Caspian basin began in the Late Pliocene 3.4 Ma (Devlin et al., 1999). The Early Pliocene hydrocarbon source rocks became delaminated from the underlying plastic sediments but no large thrusts formed though folding was rather intense. Late Pliocene and Pleistocene deposits are much thinner in anticlines than in synclines (Fig. 10), whereas the sediment thickness in the underlying older productive series is nearly uniform laterally. The amount of lateral compression (shortening) is proportional to fold dip. Dips of folds in the profiles of Figs. 7 and 10 are actually not very steep being most often within a few degrees, i.e., shortening is quite low. To estimate its order of magnitude, consider, for example, a harmonic fold ζ = ζ0 + (a / 2)[1 − cos (2πx / l)], where x is the horizontal coordinate, a is the fold amplitude, and l is the fold width. At a shallow dip (a2π2/l2 << 1), the difference between the length of a fold and its modern width l is

∆l = a2π2/4l.

(11)

For instance, the amplitude of a fold at the boundary of productive beds N22pr1 and N22pr2 under the point f2 in Fig. 7 is a = 1.5 km and its width is l = 21 km. The length-width difference according to (11) is ∆l ≈ 260 m. The fold corresponding to the reflector SG-A at the Vezirov rise in the

Absense of subduction in the South Caspian The South Caspian basin overlies a basement which is of a very low strength and is heavier than the asthenosphere, judging by the great sediment thickness. Lithosphere in the central part of the Alpine-Himalayan orogen is subject to N-S

Fig. 9. Projection of earthquakes originated at ≥30 km onto line orthogonal to Apsheron-Balkhan sill. After data from (Jackson et at, 2002; Balakina et al., 1996). Zero at x axis corresponds to sill southern edge. Shown are earthquakes and their magnitudes in Apsheron-Balkhan sill (1), Apsheron Peninsula west of sill (2), and in Bolshoi Balkhan region east of sill (3).

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E.V. Artyushkov / Russian Geology and Geophysics 48 (2007) 1002–1014

Fig. 10. Seismic cross section of Pliocene-Quaternary sediments in South Caspian basin along profile 844705, modified after (Glumov et al., 2004). SG-1 is top of productive series (base of Akchagyl). Section covers greatest part of profile III (Fig. 3). Line 844705, sea bottom rises, named after Fioletov (39°26′ N, 49°57′ E), Vezirov (39°37′ N, 50°24′ E, 36 km), Abikh (39°06′ N, 51°02′ E, 60 km), Gamburtsev (38°52′ N, 51°12′ E, 32 km), Avakyan (38°45′ N, 51°27′ E, 22 km), Khanlar (38°42′ N, 51°46′ E, 33 km).

profile of Fig. 10 has its amplitude a = 4 km and the width l = 25 km. The initial length of the reflector is longer than its modern width for ∆l ≈ 1.6 km. Inasmuch as the 183 km long profile of Fig. 10 comprises only three large folds, the total excess of the length of SG-A over the modern length of the profile is far less than 10 km. The upper sedimentary succession (productive series) in Fig. 7 appears much more strongly deformed than the lower succession, because the initial height of folds has been increased by mud diapirism (Glumov et al., 2004). Mud diapirs are well pronounced (Fig. 10), and shortening in the basin estimated from folding of the lower succession is quite small. For instance, it is no greater than a few kilometers at the base of N1–2 along the profile in Fig. 7. Furthermore, folds in different parts of the basin are chaotically oriented (Glumov et al., 2004) while folds parallel to the Apsheron-Balkhan sill are relatively few. Thus, the reflection profiling evidence is inconsistent with recent subduction. Earthquakes near the Apsheron-Balkhan sill, projected onto the sill-orthogonal plane in Fig. 9, show no alignment along the sill (Fig. 6). They cluster mostly around its center in an isolated field about 100 km across the sill strike (see circles in Fig. 9) and obviously do not align in a seismic zone dipping beneath the Middle Caspian. Only one moderate event (M = 5.0) located the farthest in the north has a centroid depth of 73 km according to (Engdahl et al., 1998) but 33 km according to the Harvard CMT solution (Harvard, 2000), i.e., the hypocenter location is of low accuracy. Therefore, the event hardly is useful to characterize deformation north of the sill. An earthquake at a depth of 61 km occurred south of the sill center where an ∼40 km thick consolidated continental crust lies under 20 km of sediments, i.e., it originated in the lower crust. Other earthquakes beneath the sill and north of it have depths of 30–60 km. The crustal base may be at a depth of ∼50 km because subsidence of crust overlain by 5 to 15 km of sediments would require high-grade metamorphism of rocks in the thick lower crust, with high-velocity heavy garnet

granulites and eclogites under the Moho. Thus, most earthquakes fall in the lower crust, especially four largest events (M ≥ 6.0) with depths about ∼30 km. Earthquakes show mainly extension mechanisms corresponding to normal faulting (Jackson et al., 2002), whereas slopes of oceanic trenches on active margins are normally dominated by compression. Normal faulting may be attributed to ongoing uneven consolidation of mafic rocks, as it was hypothesized for the Vranch zone in the southern Carpathians (Artyushkov et al., 1996). Then, large earthquakes may be associated with consolidation of heavy mafic rocks in subducting blocks delaminated from the crust. Some events with depths at ≥30 km occurred also west of the Apsheron-Balkhan sill, near the Apsheron Peninsula and north of it, as well as east of the sill, in the Bolshoi Balkhan region (triangles and squares, respectively, in Fig. 9). Together with earthquakes in the sill central part (circles in Fig. 9), they form a pattern that shows nothing like a Benioff zone. Thus, seismicity data provide no support to the idea of subduction under the Apsheron-Balkhan sill.

Conclusoins The South Caspian lying over a high-velocity (VP = 7 km/s) thin (10–18 km) basement has been often interpreted as an oceanic basin filled with ∼20 km of sediments (e.g., Allen et al., 2002; Zonenshain and Le Pichon, 1986). However, the sedimentary thickness is about twice that needed to fill a basin upon oceanic crust as thick as in the South Caspian. Maintaining consolidated crust at a depth of 20 km requires 20–25 km of eclogites, denser than mantle peridotites, to occur under the Moho. By their chemistry, eclogites belong to the crust but have typically mantle seismic velocities, and are thus placed beneath the Moho in many seismic models. The Moho in the South Caspian basin is overlain by high-grade felsic and intermediate rocks with P velocities up

E.V. Artyushkov / Russian Geology and Geophysics 48 (2007) 1002–1014

to 7 km/s. Their high density is due to metamorphism with formation of garnet at T ≥ 400 °C. Together with eclogites lying under the Moho, the basin basement totals a thickness of 40–50 km, which corresponds to a continental crust. Crustal subsidence in the South Caspian was induced by phase change of gabbro into denser eclogites. Subsidence occurred at rapidly increased rates at least twice, at the Eocene/Oligocene boundary and in Pliocene-Pleistocene time. The first episode of rapid subsidence produced (or deepened) a marine basin and the other episode was associated with deposition of a 10 km thick sedimentary succession in 5 Myr. The increased subsidence rates may have been due to the effect of active fluids that catalyzed the gabbro-eclogite transition. Rapid subsidence as in the South Caspian is impossible upon oceanic crust. Rapid and major crustal subsidence was found out to be typical of hydrocarbon basins (Artyushkov, 1993, 1995; Artyushkov and Baer, 1987; Artyushkov and Yegorkin, 2005). The reliability of this criterion as diagnostic for discovery of hydrocarbon reservoirs elsewhere was confirmed by the evidence of rapid subsidence in the large petroleum province of South Caspian. The rapid Pliocene-Pleistocene subsidence in the South Caspian with deposition of 10 km thick sediments was explained in terms of a flexural basin developing in front of a subduction zone in the south or in the north (Allen et al., 2002; Axen et al., 2001; Knapp et al., 2004). However, even though it existed, the flexure could be located within a few kilometers wide zone along the Apsheron-Balkhan sill and caused no influence on subsidence outside that strip. The South Caspian lithosphere apparently experienced strong softening as a result of fluid infiltration during the recent rapid subsidence, judging by the presence of steep basement flexures up to 10–12 km deep. Relatively large historic earthquakes (M = 5–6) located at depths ≥ 30 km beneath the Apsheron-Balkhan sill and north of it within a ∼ 100 km wide zone were interpreted as indication of northward subduction (Allen et al., 2002; Knapp et al., 2004). The consolidated lithosphere as in the South Caspian basin, with its density higher than the asthenosphere due to phase change, could in principle be involved in subduction with delamination of the overlying lighter sediments on condition of strong lateral compression and softening. Had subduction occurred in the South Caspian, the subducting plate would have been ∼ 100 km long, and the delaminated sediments would have experienced shortening of the same magnitude. Yet, judging by very shallow fold dips in the area, shortening was no more than 5–10 km. Therefore, hardly there has been any subduction. Furthermore, slopes of oceanic trenches on active margins are normally dominated by compression but most of earthquakes in the northern South Caspian show extension mechanisms. The earthquakes originate mostly in the crust (at 30 to 50 km) and show no alignment like a Benioff zone. Therefore, the basin with its thick sedimentary fill lies over continental crust, and the extension focal mechanisms may record normal faulting associated with ongoing consolidation of mafic rocks by eclogitization.

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The study was supported by grant 03-06-64166 from the Russian Foundation for Basic Research and was carried out as part of projects ESD-1b and ESD-6 of the RAS Earth Science Department.

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