Marie-Françoise Brunet

The South Caspian Basin: a review of its evolution from subsidence modelling

Abstract The basement surface of the South Caspian depression lies at a depth of 20–25 km, making it one of the deepest basins in the world. It occupies the southern, deep-water, part of the Caspian Sea and two adjacent lowlands: the West Turkmenia in the east and the Lower Kura in the west. The basin can be subdivided into several sub-basins with two main depocentres, one in the northern part of the basin, just on the southern flank of the Apsheron Sill, and one, called the Pre-Alborz trough, located in the south-eastern part of the marine basin. The sedimentary fill of the South Caspian Basin has been significantly deformed. Part of it is allochthonous and folded, overlying a ductile detachment zone within the Maikop shale (Oligocene–Early Miocene). The folded succession is unconformably overlain by Upper Pliocene–Quaternary neo-autochthonous sediments. An intense shortening event, related to the NNE–SSW convergence of the Arabian plate with Eurasia, affected the region during the Pliocene–Pleistocene. The thickness of Pliocene–Quaternary sediments alone reaches 10 km. They were deposited in a rapidly subsiding basin and were sourced from the surrounding Caucasus, Alborz, and Kopet-Dagh orogens as well as from the nearby Russian Platform. The thickness of the crust beneath the western central part of the basin is as little as 8 km in the western central part of the basin but exceeds 15 km in the eastern part. Geophysical data and gravimetric modelling provide evidence that the basement of the marine part of the basin comprises a high-velocity, thin complex crust. Subsidence of the basin is in part due to profound thinning of continental crustal or, more likely, to the formation of oceanic crust. This took place in Middle–Late Jurassic times, in the context of back-arc basin development, with possible reactivation during the Cretaceous. However, there remains controversy regarding the timing of oceanic accretion and deep-water deposition in the South Caspian Basin. The present results are based on subsidence analysis complemented by geological data from tectonic units surrounding the South Caspian Basin and on its margins. An additional mechanism, nevertheless, must be invoked to explain the younger, much more rapid Pliocene–Quaternary phase of subsidence that occurred simultaneously with the subsidence of Caucasus-related molasse basins and the uplift and erosion of the Caucasus Orogen. This rapid subsidence phase is probably of compressional origin and a simple elastic model in compression provides comparable amplitudes of subsidence. In addition, the South Caspian Basin is surrounded by orogenically loaded crust that adds to basin downwarping. To the north, the basin is bounded by a subduction zone.

The Black Sea basin: tectonic history and Neogene–Quaternary rapid subsidence modelling

Abstract The Black Sea basin originated as a back-arc basin during Cretaceous times. Continental rifting took place during the Aptian to Albian with large-scale crustal thinning and separation occurring since the Cenomanian, mainly along a former Albian volcanic arc. Both western and eastern Black Sea basins opened almost simultaneously during Cenomanian to Coniacian times. However, during the Santonian to Palaeocene, the Black Sea region was affected by compressional deformation. Apart from a tensional event that took place in eastern part of the region during the Eocene, deepening of the basin has been induced by compressional deformation from latest Eocene to recent times. Kinematic and dynamic modelling of the subsidence history of the Black Sea basin shows that downward bending of the lithosphere beneath the basin due to compressional deformation could be the cause of this rapid additional subsidence.

Permo-Triassic intraplate magmatism and rifting in Eurasia: implications for mantle dynamics.

Abstract At the transition from the Permian to the Triassic, Eurasia was the site of voluminous flood-basalt extrusion and rifting. Major flood-basalt provinces occur in the Tunguska, Taymyr, Kuznetsk, Verkhoyansk–Vilyuy and Pechora areas, as well as in the South Chinese Emeishen area. Contemporaneous rift systems developed in the West Siberian, South Kara Sea and Pyasina–Khatanga areas, on the Scythian platform and in the West European and Arctic–North Atlantic domain. At the Permo–Triassic transition, major extensional stresses affected apparently Eurasia, and possibly also Pangea, as evidenced by the development of new rift systems. Contemporaneous flood-basalt activity, inducing a global environmental crisis, is interpreted as related to the impingement of major mantle plumes on the base of the Eurasian lithosphere. Moreover, the Permo–Triassic transition coincided with a period of regional uplift and erosion and a low-stand in sea level. Permo–Triassic rifting and mantle plume activity occurred together with a major reorganization of plate boundaries and plate kinematics that marked the transition from the assembly of Pangea to its break-up. This plate reorganization was possibly associated with a reorganization of the global mantle convection system. On the base of the geological record, we recognize short-lived and long-lived plumes with a duration of magmatic activity of some 10–20 million years and 100–150 million years, respectively. The Permo–Triassic Siberian and Emeishan flood-basalt provinces are good examples of “short-lived” plumes, which contrast with such “long lived” plumes as those of Iceland and Hawaii. The global record indicates that mantle plume activity occurred episodically. Purely empirical considerations indicate that times of major mantle plume activity are associated with periods of global mantle convection reorganization during which thermally driven mantle convection is not fully able to facilitate the necessary heat transfer from the core of the Earth to its surface. In this respect, we distinguish between two geodynamically different scenarios for major plume activity. The major Permo–Triassic plume event followed the assembly Pangea and the detachment of deep-seated subduction slabs from the lithosphere. The Early–Middle Cretaceous major plume event, as well as the terminal–Cretaceous–Paleocene plume event, followed a sharp acceleration of global sea-floor spreading rates and the insertion of new subduction zone slabs deep into the mantle. We conclude that global plate kinematics, driven by mantle convection, have a bearing on the development of major mantle plumes and, to a degree, also on the pattern of related flood-basalt magmatism.

The Mesozoic-Cenozoic tectonic evolution of the Greater Caucasus

Abstract The Greater Caucasus (GC) fold-and-thrust belt lies on the southern deformed edge of the Scythian Platform (SP) and results from the Cenozoic structural inversion of a deep marine Mesozoic basin in response to the northward displacement of the Transcaucasus (lying south of the GC) subsequent to the Arabia-Eurasia collision. A review of existing and newly acquired data has allowed a reconstruction of the GC history through the Mesozoic and Cenozoic eras. In Permo(?)-Triassic times, rifting developed along at least the northern part of the belt. Structural inversion of the basin occurred during the Late Triassic corresponding to the Eo-Cimmerian orogeny, documented SE of the GC and probably linked to the accretion of what are now Iranian terranes along the continental margin. Renewed development of extensional basin formation in the area of the present-day GC began in Sinemurian-Pliensbachian times with rift activity encompassing the Mid-Jurassic. Rifting led to extreme thinning of the underlying continental crust by the Aale-nian and concomitant extrusion of mid-ocean ridge basalt lavas. A Bathonian unconformity is observed on both sides of the basin and may either correspond to the end of active rifting and the onset of post-rift basin development or be the record of collision further south along the former Mesozoic active margin. The post-rift phase began with deposition of Late Jurassic platform-type sediments onto the margins and a flysch-like unit in its deeper part, which has transgressed the basin during the Cretaceous and Early Cenozoic. An initial phase of shortening occurred in the Late Eocene under a NE-SW compressional stress regime. A second shortening event that began in the Mid-Miocene (Sarmatian), accompanied by significant uplift of the belt, continues at present. It is related to the final collision of Arabia with Eurasia and led to the development of the present-day south-vergent GC fold-and-thrust belt. Some north-vergent retro-thrusts are present in the western GC and a few more in the eastern GC, where a fan-shaped belt can be observed. The mechanisms responsible for the large-scale structure of the belt remain a matter of debate because the deep crustal structure of the GC is not well known. Some (mainly Russian) geoscientists have argued that the GC is an inverted basin squeezed between deep (near)-vertical faults representing the boundaries between the GC and the SP to the north and the GC and the Transcaucasus to the south. Another model, supported in part by the distribution of earthquake hypocentres, proposes the existence of south-vergent thrusts flattening at depth, along which the Transcaucasus plunges beneath the GC and the SP. In this model, a thick-skinned mode of deformation prevailed in the central part of the GC whereas the western and eastern parts display the attributes of thin-skinned fold-and-thrust belts, although, in general, the two styles of deformation coexist along the belt. The present-day high elevation observed only in the central part of the belt would have resulted from the delamination of a lithospheric root.

South Caspian to Central Iran Basins

This book combines interdisciplinary research results using structural geology, geophysics, sedimentology, stratigraphy, palaeontology, palaeomagnetism and subsidence modelling obtained through the MEBE (Middle East Basins Evolution) Programme and other groups in the South Caspian and Northern and Central Iran. A great part of the volume is devoted to Northern Iran (Alborz, Binalud and Koppeh Dagh belts), dealing mainly with the Late Palaeozoic and the Mesozoic Eras. Two papers present subsidence models of the South Caspian Basin since the Jurassic and three papers focus on Central Iran. The data and models in this compilation of papers present a detailed picture and a very comprehensive understanding of the Late Palaeozoic to Cenozoic evolution of the South Caspian and North Iran to Central Iran basins. Geodynamic evolution and sedimentation are mainly controlled by the closure of the Palaeo–Tethys due to collision of Eocimmerian blocks with south Laurasia, opening of the South Caspian Basin, and Neo–Tethys ocean closure associated with Arabia–Eurasia collision.

Northern Caucasus basin: thermal history and synthesis of subsidence models

Abstract Burial histories of the eastern, central and western parts of the Northern Caucasus basin are reconstructed on the basis of well data and seismic sections. Subsidence began in the Early Triassic after the Late Carboniferous–Permian orogeny. Triassic sediments were mainly removed during Late Triassic–Early Jurassic uplift and erosion. Platform cover began to form in the Middle Jurassic and Albian sediments covered the whole territory of the basin. Thermal modelling shows that Jurassic–Eocene subsidence was mainly controlled by Late Triassic–Early Jurassic intrusive warming. This heating event induced thermal uplift of the whole territory followed by exponentially decelerating subsidence due to cooling of the lithosphere. In the southern areas adjacent to Great Caucasus, subsidence was also affected by Caucasian extensional and compressional events. In the Oligocene–Early Miocene, the eastern and the central basins underwent rapid long wavelength subsidence (Maikopian subsidence). The geodynamic cause of this subsidence is probably associated with the mantle flow appearance after cessation of the Tethyan subduction, due to reequilibration of subducted slab. While in the Late Miocene–Quaternary times, the eastern and the western basins underwent foreland-type asymmetrical subsidence due to loading of the Great Caucasus orogen; the central basin was uplifted. According to flexural modelling, the main component of orogen loading was the lithospheric root load; delamination of the latter under the Central Caucasus caused rapid uplift of the orogen and adjacent basin.

The evolution of the southern margin of Eastern Europe (Eastern European and Scythian platforms) from the latest Precambrian-Early Palaeozoic to the Early Cretaceous

Abstract The southern part of the Eastern European continental landmass consists mainly of a thick platform of Vendian and younger sediments overlying Precambrian basement, referred to as the East European and Scythian platforms (EEP and SP). Some specific geological features, such as the Late Devonian Pripyat-Dniepr-Donets rift basin, the Karpinsky Swell, the Permo(?)-Triassic troughs of the SP, and the deformed belt running from Dobrogea to Crimea and the Greater Caucasus, in which rocks as old as Palaeozoic crop out, form a record of the geodynamic processes affecting this part of the European lithosphere. Hard constraints on the Palaeozoic history of the SP are very sparse. The conventional view has been that the SP is a Late Palaeozoic orogenic belt. However, it is shown that the few available data are also consistent with an alternative interpretation in which it is the thinned margin of the Precambrian continent, reworked by Late Palaeozoic-Early Mesozoic rifting events. The geodynamic setting of the margin is classically reported as one of active convergence throughout the Late Palaeozoic and Early Mesozoic, with subduction of the Palaeotethys Ocean beneath Europe. Actually, there are no direct observations constraining the polarity of Palaeotethys subduction in this area although indirect evidence is not inconsistent with the conventional model. In such a case, the sedimentary-tectonic record of the SP suggests that convergence during the Permo-Triassic(?) and certainly during the Early and Mid-Jurassic was oblique. An Eo-Cimmerian (Late Triassic-Early Jurassic) event is widespread and implies a tectonic compressional regime with systematic inversion of most sedimentary basins. There is also a widespread unconformity at the end of the Mid-Jurassic and in the Late Jurassic. These can be interpreted as indicators of compressional tectonics; however, nowhere is there evidence of intense shortening or other orogenic processes. A revised tectonic model is proposed for the area but, given the degree of uncertainty characterizing the geology of this area, it is best considered as a basis for further discussion.

Cenozoic-Recent tectonics and uplift in the Greater Caucasus: a perspective from Azerbaijan

Abstract The Greater Caucasus is Europes highest mountain belt and results from the inversion of the Greater Caucasus back-arc-type basin due to the collision of Arabia and Eurasia. The orogenic processes that led to the present mountain chain started in the Early Cenozoic, accelerated during the Plio-Pleistocene, and are still active as shown from present GPS studies and earthquake distribution. The Greater Caucasus is a doubly verging fold-and-thrust belt, with a pro- and a retro wedge actively propagating into the foreland sedimentary basin of the Kura to the south and the Terek to the north, respectively. Based on tectonic geomorphology – active and abandoned thrust fronts – the mountain range can be subdivided into several zones with different uplift amounts and rates with very heterogeneous strain partitioning. The central part of the mountain range – defined by the Main Caucasus Thrust to the south and backthrusts to the north – forms a triangular-shape zone showing the highest uplift and fastest rates, and is due to thrusting over a steep tectonic ramp system at depth. The meridional orogenic in front of the Greater Caucasus in Azerbaijan lies at the foothills of the Lesser Caucasus, to the south of the Kura foreland basin.

Subsidence and uplift mechanisms within the South Caspian Basin: insights from the onshore and offshore Azerbaijan region

Abstract A combination of fieldwork, basin analysis and modelling techniques has been used to try and understand the role, as well as the timing, of the subsidence–uplift mechanisms that have affected the Azerbaijan region of the South Caspian Basin (SCB) from Mesozoic to Recent. Key outcrops have been studied in the eastern Greater Caucasus, and the region has been divided into several major tectonic zones that are diagnostic of different former sedimentary realms representing a complete traverse from a passive margin setting to slope and distal basin environments. Subsequent deformation has caused folds and thrusts that generally trend from NW–SE to WNW–ESE. Offshore data has been analysed to provide insights into the regional structural and stratigraphic evolution of the SCB to the east of Azerbaijan. Several structural trends and subsidence patterns have been identified within the study area. In addition, burial history modelling suggests that there are at least three main components of subsidence, including a relatively short-lived basin-wide event at 6 Ma that is characterized by a rapid increase in the rate of subsidence. Numerical modelling that includes structural, thermal, isostatic and surface processes has been applied to the SCB. Models that reconcile the observed amount of fault-controlled deformation with the magnitude of overall thinning of the crust generate a comparable amount of subsidence to that observed in the basin. In addition, model results support the tectonic scenario that SCB crust has a density that is compatible with an oceanic composition and is being under-thrust beneath the central Caspian region.

Pre-Mesozoic geodynamics of the Precaspian Basin (Kazakhstan)

Abstract This paper concentrates on the Pre-Mesozoic history of the Precaspian Basin representing more than half of the 20 km of its sedimentary thickness. We present the evolution of sediments deposited in the basin through time by several regional sections of the basin. These image, from the Devonian, the creation and evolution of a deep intercontinental basin which existed from Carboniferous to Permian. Palinspastic reconstructions show also the evolution of the basins margins, their relations with the southern pre-Uralian foredeep in the east and carbonate build-ups in the southeast. We present a reconstruction for the early history of the Precaspian Basin, which became an individual basin only at the end of the Permian as a result of the Uralian orogeny. The basement of the Central Precaspian depression is characterised by a thin crust where a low-velocity layer is absent. A layer of abnormally high velocities exists below the crust, which on the basis of structural and gravitational field data is interpreted as eclogite. We put forward a scenario for the age and emplacement of eclogite lenses at the base of the crust. They are possibly implemented in the hangingwall, from a westward subduction zone existing during the Vendian, before the collision giving birth to the Proto-Urals. Most of the basin subsidence is due to the thinning (mainly during Devonian) and increase of the average density of the crust. Present crustal density has characteristics of intermediate to lower crust interpreted by some authors as oceanic. We propose that the loading effect of the eclogites emplacement may explain also a part of the Precaspian Basin subsidence.