Density inhomogeneities, isostasy and flexural rigidity of the lithosphere in the Transcaspian region

The 3-D lithospheric-density model for the southeastern part of the Caspian Sea and the Transcaspian area, practically coinciding with the territory of the Turkmen Republic, has been constructed based on geophysical data and in accordance with the principle of isostasy. From the model selected the anomalous density of the subcrustal layer between the Moho discontinuity and the 100-km depth level is found to be — 100 kg/m³ under the Tien-Shan, ? 50 kg/m³ under the Kopet-Dag mountain area, + 80 kg/m³ under the central region of the South Caspian basin, ?50 kg/m³ under the eastern part of the basin, known as the West Turkmenian depression, and + 45 kg/m³ under the Murgab depression.

Significant disturbances of the local isostasy are determined both in the northern and central areas of the South Caspian basin and also in the area of the Kara-Bogaz swell of the Turan platform and for the Kopet-Dag foredeep. indicating a high level of stresses in the lithosphere. The shape of the Turan plate determined by the seismic profiling is accounted for by elastic deformation resulting from the forces acting on the southern edge of the plate in the area of the Turan plate-Kopet-Dag collision. The elastic thickness of the Turan plate is estimated as 25 ± 5 km. The results obtained seem to confirm the idea that the decomposition of the Turan plate has taken place in the zone of the plates interaction and the decomposed material is situated under the Kopet-Dag ridge.

We propose that the Kara-Bogaz swell is supported by the mantle material upwelling whereas the subsidence of the adjacent part of the South Caspian basin may be due to the downgoing mantle flow i.e., a small convection cell is suggested in that area.