STRUCTURE AND EVOLUTION OF THE TEREK-CASPIAN FOLD-AND-THRUST BELT: NEW INSIGHTS FROM REGIONAL SEISMIC DATA

The Terek-Caspian fold-and-thrust belt along the northern flank of the Greater Caucasus Mountain Range together with the adjacent foreland basin is one of the oldest oil-producing regions in Russia. Despite the long history of exploration, recently acquired seismic data has provided new insights about the structural architecture and evolution of this area. Its structural development during the Neogene was constrained by a syn-extensional tectonic fabric inherited from Jurassic rifting and extension of the Great Caucasus Basin. The structural framework of this basin controlled the distribution of syn-extensional deposits, and the Cenozoic reactivation of lateral ramps resulted in along-strike variations in structural style. Thus western, central and SE segments of the Terek-Caspian foldbelt are recognised and are referred to here as the Terek-Sunzha fold zone, the Dagestan Promontory, and the Maritime Zone in southern Dagestan.

Three principal episodes of Cenozoic compression in the Terek-Caspian fold-and-thrust belt took place. The first episode in the Oligocene resulted in the inversion of pre-existing normal faults with the coeval development of a foreland basin to the north of the thrust belt. The dominance of sediments of northerly provenance in the foreland basin suggests there was only moderate uplift of the Greater Caucasus at this time. However, significant uplift of the orogenic belt took place during later phases of Sarmatian (Late Miocene) and Akchagylian (Late Pliocene) compression. Erosion of the uplifting Greater Caucasus gave rise to the development of large-scale, northerly prograding clinoforms which are clearly observed on seismic profiles in the foreland basin. Shortening was largely accommodated by wedge-shaped thrusting facilitated by the presence of mechanically weak stratigraphic units.

Structural development of the Terek-Sunzha fold zone in the west of the Terek-Caspian fold-and-thrust belt was largely controlled by a Tithonian salt layer which provided an efficient basal detachment surface and which also supplied material to squeezed diapirs in front of the belt. To the east, the plan-view shape and internal architecture of the Dagestan Promontory were influenced by the areal extent of the Lower-Middle Jurassic depocentre of the palaeo-Volga delta which is up to 10 km thick. In the Maritime Zone, the style of shortening was mostly controlled by the presence of a pre-existing structural high composed of folded Palaeozoic-Triassic strata in front of the fold-thrust belt.