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.