Tectonic paleostress fields and structural evolution of the NW-Caucasus fold-and-thrust belt from Late Cretaceous to Quaternary

Abstract

The NW-Caucasus fold-and thrust belt essentially corresponds to the inverted western Flysch Zone of the Great Caucasus Mountains, a deep basin that developed from Late Jurassic to Eocene times between the Scythian Plate to the north and the Transcaucasian terranes to the south (the Shatsky Ridge, SW of the NW-Caucasus zone). The Flysch Basin was strongly affected by compression in Late Eocene times, when the characteristic WNW trending folds and thrusts of the NW-Caucasus belt developed (some authors regard the main compressive deformation as Miocene in age). By means of remote sensing analysis, we elucidate the geometry of major structures in the belt: WNW trending south-vergent thrusts and folds, and major vertical and transverse NNE–SSW to NE–SW deep fault zones. The later structures are interpreted as ancient faults that were active during the development of the Flysch Basin. Paleostress investigations reveal seven main tectonic episodes in the evolution of the NW-Caucasus since Late Cretaceous. Combining structural interpretation, remote sensing analysis and paleostress field reconstruction, we propose a model for the structural evolution of the belt. During the Late Cretaceous–Paleocene, the western Caucasus zone was under transtensional regime with an E–W to NE–SW trending σ 3 that generated oblique normal movements along NNE–SSW transverse faults and WNW–ESE margins of the Flysch Basin. This tectonism could correspond to rifting related to the formation of the Eastern Black Sea Basin. At the Paleocene–Eocene boundary, a transpressional event with an E–W to NW–SE trending σ 1 developed and the NNE–SSW to NE–SW trending faults could have been inverted. This event could correspond to an attenuation in the Eastern Black Sea Basin formation or to the incipient accretion of the Transcaucasian terranes. During the Eocene, another E–W to NW–SE oblique extension (-transtensional event) affected the Flysch Basin that could be related to a known rifting phase in the Eastern Black Sea Basin. Strong NNE–SSW to NE–SW compression characterises Late Eocene tectonism. The fold-and-thrust belt developed at this time as a result of the direct collision of the Shatsky Ridge with the Scythian Plate. A NE–SW extension followed the Late Eocene event, related to basin development around the newly formed fold belt. A WNW–ESE oblique contraction affected the belt during the early Miocene as the result of Arabian Plate convergence with the Caucasian system. The latest inferred event is a compressional regime, with NNW–SSE trending σ 1 that is affecting the NW-Caucasus belt from Sarmatian times until the present. Under this oblique compression, the belt has deformed as in a dextral shear zone and the thrust surfaces have acquired lenticular shapes. This study highlights the occurrence of oblique movements in the NW-Caucasus area prior to and after the dominant Late Eocene compression. From the Late Cretaceous until the Eocene, the structural development of the NW-Caucasus was closely related to the evolution of the Eastern Black Sea Basin. From the Late Eocene until Quaternary times, it was rather related to the Arabia–Eurasia plate convergence.