The geodynamic evolution of the Precaspian Basin (Kazakhstan) along a north-south section

Several hypotheses exist for the origin and evolution of the Precaspian Bas in. There are more than 20 km of sediments deposited, yet there is little c onsensus on the causes of the subsidence. Except for the presence of a thic k salt layer (Lower Permian), the main problem is the chronostratigraphic i nterpretation of the sediments in the centre of the basin, where the calibr ation of seismic data with well data from the basin margins is problematic at deep levels. The age of the deepest sediments could be either Riphean or Devonian. A generalised cross-section of the Precaspian Basin, roughly per pendicular to the central elongated E-W depocentre, is used to represent th e basin's evolution. Several evolutionary steps from the end of the Riphean until the Present are demonstrated. Tectonic subsidence analysis indicates that there are six main phases of evolution: (1) subsidence during an acti ve rifting phase in Riphean times; (2) rifting during the Vendian-Ordovicia n (poorly dated); (3) significant subsidence during the Late Devonian in an extensional context possibly due to back-are rifting; (4) acceleration of subsidence during the Late Carboniferous-Permian, synchronous with or just following the closure of the Uralian Ocean, a major subsidence phase in the Dniepr-Donets Basin and possible subduction to the south of the basin; (5) renewed rifting during the Triassic coincident with a general phase of ext ension in Eurasia and the opening of the Neo-Tethys; and (6) neotectonic su bsidence resulting from crustal down-bending in a generally compressional s etting. The nature of the crust underlying the basin is not well known. It could be Riphean-early Palaeozoic or Devonian oceanic crust or continental crust attenuated during the several episodes of supposed rifting between th e Riphean and the Triassic. Estimations of crustal thickness in the basin v ary and they depend on the interpretation of a high-velocity layer situated at the base of the crust. The presence and general distribution of this la yer is confirmed by gravity data. It may be considered as the uppermost man tle, as oceanic crust metamorphosed into eclogite at depth during collision and then exhumed and emplaced at the base of the crust, or as lower crust transformed in situ (into eclogites?). The crustal thinning factor leading to the observed present crustal thickness - assuming an initial thickness o f 40 km - is 3.3 in the two first cases and 2 in the latter, if the crust i s considered to be continental. The geophysical and subsidence data are dis cussed in terms of basin-forming mechanisms such as: (1) intracontinental r ifting of early Palaeozoic, Devonian or Permo-Triassic ages; (2) oceanisati on of continental crust during Riphean or Devonian rime; (3) eclogititisati on at the base of the crust in the upper mantle or by metamorphism of subdu cted oceanic crust; and (4) down-bending due to compressional forces or man tle flow induced by subduction-collision processes in Carboniferous and Rec ent times. (C) 1999 Elsevier Science B.V. All rights reserved.