Geology and zircon U-Pb geochronology of the Mtkvari pyroclastic flow and evaluation of destructive processes affecting Vardzia rock-cut city, Georgia

摘要

Late Cenozoic subaerial volcanic highland crop out in Eastern Anatolia and the western part of the Lesser Caucasus, part of which, located in Georgia, is known as Samtskhe-Javakheti. The Samtskhe-Javakheti highland (similar to 4500 km(2)) is cut by the Mtkvari river canyon, where, into the thick pyroclastic flow in twelfth-century is hewn the unique city Vardzia. Despite this, characteristics of the pyroclastic flow, such as scale, source, type, genesis, isotopic age and evaluation of destructive geological processes of the Vardzia rock-cut city have not been well studied. Our research has shown that the Mtkvari pyroclastic flow is exposed at a distance of about 35 km from the Karzameti fortress to the Khertvisi fortress, it is inclined northward by 2-4 degrees, and its thickness increases to the north from 40 m up to 80 m. These rocks represent welded, weakly welded and non-welded ignimbrites of the andesitic-dacitic composition. Isotopic parameters of these rocks (epsilon Nd varies between +3 and + 4, and Sr-87/Sr-88 between 0,70341 and 0.70450) indicate that they originated as a result of fractionation of mantle derived melts. As a result of field work, we interpret the Mtkvari ignimbrites as products of megacaldera collapse (modern Niala fields), which nowadays is filled by postcaldera domes of andesitic composition and Quaternary sediments. The result of zircon dating using the U-Pb method with LA-ICP-MS technology is 7.52 +/- 0.21 Ma, which corresponds to the Late Miocene epoch. Geological research of the Vardzia rock-cut city has revealed that it is hewn into weakly welded ignimbrites and for which reason these rocks are under the intensive affecting of weathering and erosion processes. At the same time, the city is completely included in a tectonic block of 900 m length which is detached from the main body of rocks and is gradually subsiding towards the Mtkvari canyon. Nowadays Vardzia block is lowered by 30.6 m in comparison with bordering blocks. In addition, this block is split into several micro-blocks by a joint set, thus reducing its stability. The situation is worsened by the active deep fault running along the Vardzia rock-cut city, which represents a potential earthquake source.