Postcollisional tectonics and seismicity of Georgia

Authors:  Sh. Adamia,V. Alania, N. Tsereteli, O. Varazanashvili, N. Sadradze, N. Lursmanashvili, A. Gventsadze

During the Oligocene, marine Tethyan basins were replaced by euxinic basins, which are considered to represent the beginning of syncollisional development between the Arabian and Eurasian plates in Georgia. Ongoing collision during Miocene– Pleistocene times caused inversion of topography such that fold-and-thrust moun- tain belts of the Great and Lesser Caucasus, and the intermontane foreland basins in between the two mountain belts, were formed where intra-arc and back-arc basins had been. Analysis of seismic sections showing growth strata in intermountain fore- land basins indicates that the thrust system in Georgia was active ca. 4 to 3.5 Ma.

Beginning in the late Miocene, coeval with molasse deposition in the foreland basins, subaerial volcanic eruptions occurred, characterized by intensively frac- tionated magma of suprasubduction-type calc-alkaline series from basalts to rhyo- lites. Outcrops of the magmatic rocks are exposed along the boundaries of the main tectonic units of the region. Pyroclastic rocks of the first volcanic stage (Goderdzi Formation) contain Upper Miocene–Lower Pliocene petrified subtropical wood and other floral remnants. Marine deposits of the Goderdzi Formation are represented by sandy diatomite, which hosts Upper Miocene nanoplankton.

In addition to volcanism, earthquakes indicate active tectonics in Georgia. Some of the major earthquakes have proven to be devastating; i.e., the Racha earth- quake of 29 April 1991, with Ms = 6.9, was the strongest ever recorded in Georgia. The fault plane solution data for 130 earthquakes show that the territory of Georgia is currently under latitudinal compression, longitudinal extension, and an overall crustal thickening.

A complex network of faults divides the region into a number of separate blocks. The boundary zones between these terrains represent maximum geodynamic activity.