Geochemical, Isotopic and Petrological Constraints on the Origin and Evolution of the Recent Silicic Magmatism of the Greater Caucasus

Significant volumes of rhyolites and granites of the Pliocene-Pleistocene age are exposed in the collision zone of the Greater Caucasus, Russia. The volcanic history of the region includes ignimbrites and lavas associated with the Chegem caldera (2.9 Ma) and Elbrus volcano (1.98 and 0.7 Ma) and rhyolitic necks and granites in Tyrnyauz (1.98 Ma). They are characterized by a similar bulk and mineral composition and close ratios of incompatible elements, which indicates their related origin. The 1.98 Ma Elbrus ignimbrites, compared to the 2.9 Ma Chegem ignimbrites, have elevated concentrations of both compatible (Cr, Sr, Ca, Ni) and incompatible elements (Cs, Rb, U). We argue that the Elbrus ignimbrites were produced from magma geochemically similar to Chegem rhyolites through fractionation crystallization coupled with the assimilation of crustal material. The 1.98 Ma Eldjuta granites of Tyrnyauz and early ignimbrites of the Elbrus region (1.98 Ma) are temporally coeval, similar mineralogically, and have comparable major and trace element composition, which indicates that the Elbrus ignimbrites probably erupted from the area of modern Tyrnyauz; the Eldjurta granite could represent a plutonic reservoir that fed this eruption. Late ignimbrites of Elbrus (0.7 Ma) and subsequent lavas demonstrate progressively more mafic mineral assemblage and bulk rock composition in comparison with rhyolites. This indicates their origin in response to the mixing of rhyolites with magmas of a more basic composition at the late stage of magma system development. The composition of these basic magmas may be close to the basaltic trachyandesite, the flows exposed along the periphery of the Elbrus volcano. All studied young volcanic rocks of the Greater Caucasus are characterized by depletion in HSFE and enrichment in LILE, Li, and Pb, which emphasizes the close relationship of young silicic magmatism with magmas of suprasubduction geochemical affinity. An important geochemical feature is the enrichment of U up to 8 ppm and Th up to 35 ppm. The trace element composition of the rocks indicates that the original rhyolitic magma of Chegem ignimbrites caldera was formed at >80%–90% fractionation of calc-alkaline arc basalts with increased alkalinity. This observation, in addition to published data for isotopic composition (O-Hf-Sr) of the same units, shows that the crustal isotopic signatures of silicic volcanics may arise due to the subduction-induced fertilization of peridotites producing parental basaltic magmas before a delamination episode reactivated the melting of the former mantle and the lower crust.

Tectonostratigraphy and major structures of the Georgian Greater Caucasus: Implications for structural architecture, along-strike continuity, and orogen evolution

Although the Greater Caucasus Mountains have played a central role in absorbing late Cenozoic convergence between the Arabian and Eurasian plates, the orogenic architecture and the ways in which it accommodates modern shortening remain debated. Here, we addressed this problem using geologic mapping along two transects across the southern half of the western Greater Caucasus to reveal a suite of regionally coherent stratigraphic packages that are juxtaposed across a series of thrust faults, which we call the North Georgia fault system. From south to north within this system, stratigraphically repeated ~5–10-km-thick thrust sheets show systematically increasing bedding dip angles (<30° in the south to subvertical in the core of the range). Likewise, exhumation depth increases toward the core of the range, based on low-temperature thermochronologic data and metamorphic grade of exposed rocks. In contrast, active shortening in the modern system is accommodated, at least in part, by thrust faults along the southern margin of the orogen. Facilitated by the North Georgia fault system, the western Greater Caucasus Mountains broadly behave as an in-sequence, southward-propagating imbricate thrust fan, with older faults within the range progressively abandoned and new structures forming to accommodate shortening as the thrust propagates southward. We suggest that the single-fault-centric “Main Caucasus thrust” paradigm is no longer appropriate, as it is a system of faults, the North Georgia fault system, that dominates the architecture of the western Greater Caucasus Mountains.

SEISMICITY of the NORTHERN CAUCASUS in 2015

Seismic monitoring in the region in 2015 was carried out by a seismic network consisting of 59 stations. Digital equipment was installed at all stations in the second half of the year. The network capability was assessed by the level of seismic noise at the stations: in most of the region, the network provided registration of an earthquake from КR7.0, in the central (including the Greater Sochi region) and eastern parts of the region – КR6.0, and in some local zones with КR5.5. 2,276 earthquakes were registered, 17 earthquakes were felt in the settlements of the Caucasus. The maximum intensity VII at MSK-64 (SSI-17) scale was noted from the earthquake in the territory of Azerbaijan. The earthquake on November 3, which occurred on the platform territory within the Stavropol arch, felt IV at MSK-64. The strongest earthquakes were recorded in the Terek-Caspian and Kura troughs and in the eastern part of the Greater Caucasus. The seismicity of the North Caucasus in 2015 in accordance with the seismicity scale "SOUS-09" was set as the "background average" for the observation period from 1962 to 2015.

Supplemental Material: Tectonostratigraphy and major structures of the Georgian Greater Caucasus: Implications for structural architecture, along-strike continuity, and orogen evolution

Structural orientation data and raw data from geochemical and thermochronological analyses.

Supplemental Material: Tectonostratigraphy and major structures of the Georgian Greater Caucasus: Implications for structural architecture, along-strike continuity, and orogen evolution

Structural orientation data and raw data from geochemical and thermochronological analyses.

Strong acceleration of glacier area loss in the Greater Caucasus over the past two decades

An updated glacier inventory is important for understanding glacier behavior given the accelerating glacier retreat observed around the world. Here, we present data from new glacier inventory at two time periods (2000, 2020) covering the entire Greater Caucasus (Georgia, Russia, and Azerbaijan). Satellite imagery (Landsat, Sentinel, SPOT) was used to conduct a remote-sensing survey of glacier change. The 30 m resolution Advanced Spaceborne Thermal Emission and Reflection Radiometer Global Digital Elevation Model (ASTER GDEM; 17 November 2011) was used to determine aspect, slope and elevations, for all glaciers. Glacier margins were mapped manually and reveal that in 2000 the mountain range contained 2186 glaciers with a total glacier surface area of 1381.5 ± 58.2 km2. By 2020, glacier surface area had decreased to 1060.9 ± 33.6 km2. Of the 2223 glaciers, fourteen have an area > 10 km2 resulting the 221.9 km2 or 20.9 % of total glacier area in 2020. The Bezingi Glacier with an area of 39.4 ± 0.9 km2 was the largest glacier mapped in 2020 database. Our result represents a 23.2 ± 3.8 % (320.6 ± 45.9 km2) or −1.16 % yr−1 reduction in total glacier surface area over the last twenty years in the Greater Caucasus. Glaciers between 1.0 km2 and 5.0 km2 account for 478.1 km2 or 34.6 % in total area in 2000, while it account for 354.0 km2 or 33.4 % in total area in 2020. The rates of area shrinkage and mean elevation vary between the northern and southern and between the western, central, and eastern Greater Caucasus. Area shrinkage is significantly stronger in the eastern Greater Caucasus (−1.82 % yr−1), where most glaciers are very small. The observed increased summer temperatures and decreased winter precipitation along with increased Saharan dust deposition might be responsible for the predominantly negative mass balances of two glaciers with long-term measurements. Both glacier inventories are available from the Global Land Ice Measurements from Space (GLIMS) database and can be used for future studies.

A New Structural and Tectonic Model for a Mature Field in the Greater Caucasus

Executive Summary This article deals with the necessity the re-interpret seismic data at mature fields and is based on the field data located in the territory of the Greater Caucasus. The field was discovered back in the Soviet Union (1935).

REGULARITIES OF THE MICROELEMENT COMPOSITION FORMATION OF SOILS UNDER THE MOUNTAIN-MEADOW VEGETATION OF THE GREATER CAUCASUS

Report. The purpose of the work is to identify the features of the formation of the microelement composition of soils under the mountain meadow vegetation of the Greater Caucasus based on the analysis of literary materials and the results of our own field research. Methods. The study of the microelement composition of soils under subalpine and alpine vegetation was carried out on the territory of the Teberdinsky State Biosphere Reserve. Traditional methods of soil-geochemical studies were used with the laying of soil sections, the selection of soil samples and their analysis for the content of four trace elements (Zn, Cu, Pb and Cd). Determination of trace elements was carried out by voltammetric and atomic absorption methods. The humus content was determined by the Tyurin method with wet salting, the pH of the water extract was determined potentiometrically. Statistical processing of the obtained data was performed in the Statistica 10 program. The microelement composition of soil-forming rocks was compared with the clarks of chemical elements in the upper part of the continental crust; the microelement composition of mountain-meadow soils was compared with the clarks of the soils of the world. The radial distribution of trace elements in the soil profile was analyzed. The qualitative trace element composition of soils was characterized as a sequence of decreasing the content of trace elements in the humus horizon. Results. It is established that the microelement composition of soils under the mountain-meadow vegetation of the Western Caucasus is formed under specific conditions that affect the course of soil processes. High solar insolation, low temperatures, intensive humidification throughout the year affects the features of the processes of humification, the formation of clay minerals in the soil and other products of intra-soil weathering. The predominance of acid hydrolysis processes leads to the predominant accumulation of aluminosilicates, Fe hydroxides, chelated organomineral complexes in the soil profile, which play a leading role in the binding of trace elements. The microelement composition of mountain-meadow soils under subalpine vegetation is formed with more intensive processes of humus formation and oglinivaniya. These soils are characterized by a more pronounced biogenic accumulation of Cu and Zn in the humus horizon, the illuvial nature of the Cd distribution is more pronounced. The microelement composicomposition of mountain-meadow soils under alpine and rock-scree vegetation is formed against the background of relatively weakened processes of humus formation, humus accumulation and oglinivaniya. This affects the lower intensity of biogenic accumulation of trace elements, their leaching into the lower part of the profile. Conclusions. The main regularities of the formation of the microelement composition of mountain-meadow soils are determined by the special conditions in which these soils develop. The fixation of trace elements in mountain-meadow soils occurs mainly on aluminosilicates, Fe, Mn hydroxides and chelated organomineral complexes, which largely form the silty fraction. The movement of silty particles along the soil profile leads to the redistribution of trace elements associated with them. The granulometric composition, which is an indicator of the content of the silty fraction and its distribution along the soil profile, is of great importance when characterizing the microelement composition of mountain meadow soils. The established regularities of the formation of the microelement composition of mountain-meadow soils allow us to determine the main directions of economic activity that will contribute to the preservation of their ecological state. This is, first of all, the rational use of pasture resources of mountain meadows with the introduction of a system of alternating mowing, changing the main pastures with spare ones during the year for their restoration. An important component should be monitoring changes in the trace element composition of mountain meadow soils, which will allow timely response to changes and make adjustments to the structure of the use of these soils.