Radiolarians of the upper Cenomanian and lower Turonian from deposits of the Ananuri Formation, the western Caucasus (Lazarevskoe area)

Diverse radiolarians (over 70 species) are detected in cherty rocks above the bituminous shale horizon, the marker of anoxic event OAE-2 recorded across the Cenomanian-Turonian boundary in the upper part of the Ananuri Formation of flyschoid deposits, the Lazarevskoe area of the western Caucasus. The radiolarian assemblages studied are comparable in composition with radiolarians from concurrent Cenomanian-Turonian boundary strata in other Mediterranean regions (e.g., in the Crimea and Turkey). The lower radiolarian assemblage includes index species Dactyliosphaera silviae of synonymous Cenomanian zone. Alievium superbum present in the upper assemblage is index species of the relevant Turonian zone. Within the studied flyschoid sequence, sediments indicative of the above event (bituminous shales and cherts) are confined to upper elements of flysch rhythms.     

Blueschists of the Amassia-Stepanavan Suture Zone (Armenia): linking Tethys subduction history from E-Turkey to W-Iran

The Amassia–Stepanavan blueschist-ophiolite complex of the Lesser Caucasus in NW Armenia is part of an Upper Cretaceous-Cenozoic belt, which presents similar metamorphic features as other suture zones from Turkey to Iran. The blueschists include calcschists, metaconglomerates, quartzites, gneisses and metabasites, suggesting a tectonic mélange within an accretionary prism. This blueschist mélange is tectonically overlain by a low-metamorphic grade ophiolite sequence composed of serpentinites, gabbro-norite pods, plagiogranites, basalts and radiolarites. The metabasites include high-P assemblages (glaucophane–aegirine–clinozoisite–phengite), which indicate maximal burial pressure of ∼1.2 GPa at ∼550°C. Most blueschists show evidence of greenschist retrogression (chlorite—epidote, actinolite), but locally epidote-amphibolite conditions were attained (garnet—epidote, Ca/Na amphibole) at a pressure of ∼0.6 GPa and a temperature of ∼500°C. This LP–MT retrogression is coeval with exhumation and nappe-stacking of lower grade units over higher grade ones. ⁴⁰Ar/³⁹Ar phengite ages obtained on the high-P assemblages range between 95 and 90 Ma, while ages obtained for epidote-amphibolite retrogression assemblages range within 73.5–71 Ma. These two metamorphic phases are significant of (1) HP metamorphism during a phase of subduction in the Cenomanian–Turonian times followed by (2) exhumation in the greenschist to epidote-amphibolite facies conditions during the Upper Campanian/Maastrichtian due to the onset of continental subduction of the South Armenian block below Eurasia.     

Glacier variations in the Caucasus Mountains from 1960 to 2020北大核心CSCD

Glacial meltwater is an important freshwater resource in the Caucasus and is important for regional irrigation and hydroelectricity generation. This paper analyses the spatial and temporal patterns of glacier change in the Caucasus Mountains from 1960 to 2020 based on Landsat images, coherence images from Sentinel-1 image pairs, GLIMS glacier inventory and WGMS glacier mass balance data. The results of the study show that in 2020 there were 1912 glaciers in the Caucasus Mountains, with a total area of（1 087. 36±66. 44）km2. The total glacier area shrank by（587. 36±98. 66）km2（35. 07±5. 89%）between 1960 and 2020, with an average annual shrinkage rate of（0. 58±0. 10%）·a-1. The area shrinkage rates of Caucasian glaciers for 1960-1986, 1986-2000 and 2000-2020 are（0. 44±0. 20%）·a-1, （0. 66±0. 77%）·a-1 and（0. 96±0. 31%）·a-1, respectively, indicating that Caucasian glaciers in a state of accelerated retreat over the last 60 years. Analysis of mass balance information shows that both the Djankuat and Garabashi glaciers in the Caucasus have been in a strong negative equilibrium for nearly 60 years, with a significant acceleration of mass deficit after 2000. Analysis of climate data suggests that the strong warming is the main reason for the accelerated retreat of glaciers in the Caucasus mountains in recent decades. © 2022 The authors.     

Linking the NE Anatolian and Lesser Caucasus ophiolites: evidence for large-scale obduction of oceanic crust and implications for the formation of the Lesser Caucasus-Pontides Arc

In the Lesser Caucasus and NE Anatolia, three domains are distinguished from south to north: (1) Gondwanian-derived continental terranes represented by the South Armenian Block (SAB) and the Tauride–Anatolide Platform (TAP), (2) scattered outcrops of Mesozoic ophiolites, obducted during the Upper Cretaceous times, marking the northern Neotethys suture, and (3) the Eurasian plate, represented by the Eastern Pontides and the Somkheto-Karabagh Arc. At several locations along the northern Neotethyan suture, slivers of preserved unmetamorphozed relics of now-disappeared Northern Neotethys oceanic domain (ophiolite bodies) are obducted over the northern edge of the passive SAB and TAP margins to the south. There is evidence for thrusting of the suture zone ophiolites towards the north; however, we ascribe this to retro-thrusting and accretion onto the active Eurasian margin during the latter stages of obduction. Geodynamic reconstructions of the Lesser Caucasus feature two north dipping subduction zones: (1) one under the Eurasian margin and (2) farther south, an intra-oceanic subduction leading to ophiolite emplacement above the northern margin of SAB. We extend our model for the Lesser Caucasus to NE Anatolia by proposing that the ophiolites of these zones originate from the same oceanic domain, emplaced during a common obduction event. This would correspond to the obduction of non-metamorphic oceanic domain along a lateral distance of more than 500?km and overthrust up to 80?km of passive continental margin. We infer that the missing volcanic arc, formed above the intra-oceanic subduction, was dragged under the obducting ophiolite through scaling by faulting and tectonic erosion. In this scenario part of the blueschists of Stepanavan, the garnet amphibolites of Amasia and the metamorphic arc complex of Erzincan correspond to this missing volcanic arc. Distal outcrops of this exceptional object were preserved from latter collision, concentrated along the suture zones.     

Isotopic evidence (He,B, C) for deep fluid and mud mobilization from mud volcanoes in the Caucasus continental collision zone

The Caucasian orogenic wedge formed as a consequence of the closure of the Tethyan Ocean, and numerous fields of active mud volcanoes pepper the area adjacent to the Black and Caspian Seas. Stable isotope ratios of boron, helium, and carbon have been measured for gas, fluid and sediment samples from active mud volcanoes of Taman Peninsula and Georgia to estimate the sources and mobilization depths of the fluid phase and mud. Boron concentrations in mud volcano fluids were found to be 5–35× higher than seawater. Fluid isotope ratios vary between ¹¹B=22 and 39, while isotope ratios of the smectite- and illite-rich extruded mud are considerably depleted in heavy ¹¹B (¹¹B=–8 to +7). B contents of these muds are ~8× higher than modern marine sediments. This suggests that liquefaction prior to mud volcanism was accompanied by both B enrichment and isotope fractionation, most likely at an intermediate depth mud reservoir at 2–4 km.The hydrocarbon-generating source beds to the mud volcanoes are located at 7 to >10 km depth in the folded Maikop Formation and are of proposed Oligocene–Miocene age. The most likely mechanism is re-hydration of these shales by both hydrocarbons and a geochemically mature fluid from greater depth within the orogenic wedge. Such a deep fluid source is supported by our results from gas analyses, which imply an admixture of minor amounts (less than 1%vol) of ³He (Georgia), thermogenic ¹³C in methane as well as "ultraheavy" ¹³C in CO₂ (both Taman and Georgia). The overall results attest active local flow of geochemically different fluids along deep-seated faults penetrating the two study areas in the Caucasian orogenic wedge, with the waters as well as the gases coming from below the Maikop Formation.     

Activité du gaz radon dans l'air du sol et sismicité locale : exemple du bassin de l'Arax (Arménie)Radon soil-gas concentrations and local seismicity: case of the Arax basin (Armenia)

A survey of soil gas radon concentrations has been carried out at three sites, in the seismic area of Armenia, from 1996 to 1999. The seismicity generates opposed behaviours at the different sites. This heterogeneity is related to the sites locations in the tectonic frame. An increase of radon concentration occurs inside the tectonic micro-blocks during local seismic activity. On the contrary, an abrupt co-seismic decrease is recorded close to the fault area. These variations can be due to ‘pore-collapse’ phenomena which expulse the pore-gas and increases Rn concentration in soil at intra-blocs sites, but also to pore-elastic deformations and microfracturing modifications in the fault area. To cite this article: K. Kharatian et al., C. R. Geoscience 334 (2002) 179–185.     

Maximum Expected Magnitude Assessment in a Geo Computer Environment: Case Study

Different approaches to seismic hazard assessment are compared. Each of them could be applied more or less successfully for territories which are stable in time and have a high level of seismic activity. A long-term seismic catalogue, not only including historical but also paleoearthquake data, is an essential requirement. But, in practice, such an ideal situation is very rare. Initial data is usually poor and short-term. Seismic hazard assessment could be more complicated for regions which are transient between relatively stable platforms and active mountain massifs. A new step in geoinformation technology for seismic hazard assessment based on a GEO computer environment is presented, its application is illustrated by the real case hazard evaluation for the territory of the Stavropol region, which is situated between the Russian platform and the Great Caucasus. The regional catalogue covers a time period of about 150 years. Though the region under consideration is not large, seismic activity is variable in space, from almost aseismic zones to rather active areas. GEO allows us to incorporate different techniques and all available information in the analysis, including those which are very difficult to formalize. The space distribution of the maximum expected earthquake magnitude is determined as a function of geological and geophysical data. An important feature of GEO is that it makes it possible to control the result of complicated algorithms through some relatively simple physical reasons.     

欧洲大高加索厄尔布鲁斯山的形成与演化①

厄尔布鲁斯山位于土耳其板块俯冲至西徐亚板块之下而形成 的大高加索弧形构造带的弧顶部位,本身为晚更新世和全新世两个火山口组成的复式火山锥 。厄尔布鲁斯山隆升过程和机制可划分为4个阶段中生代板块俯冲形成大高加索相对隆起 区;新生代早期板块碰撞使大高加索成为强烈隆起区;新生代晚期发育了厄尔布鲁斯复式火 山锥,并使之成为欧洲最高峰;许多迹象表明,厄尔布鲁斯火山现代并未熄灭,厄尔布鲁斯 山以10 mm/a的速度继续隆升。     

大高加索山脉冰川反照率时空分布及与物质平衡的关系

利用MOD10A1和MYD10A1冰雪反照率数据, 分析了大高加索山脉地区加拉巴西冰川和德扬库特冰川的反照率时空分布特征, 并使用五种反照率聚合方法, 对这两条冰川2002—2019年夏季反照率与物质平衡关系开展相关性研究.结果表明, 加拉巴西冰川和德扬库特冰川反照率随年份增加而明显减小, 两条冰川在夏季都出现了最低反照率, 加拉巴西冰川在冬季出现最高反照率, 而德扬库特冰川则出现在春季; 加拉巴西冰川夏季高海拔和低海拔区域的反照率差异较德扬库特冰川更为明显; 两条冰川夏季反照率均与夏季物质平衡表现出一定的正相关性, 其中加拉巴西冰川的相关性更加显著, 使用平均最小反照率方法时两者相关系数最大可达0.874(p＜0.05), 德扬库特冰川使用加权平均反照率方法时两者相关系数最大为0.765(p＜0.05).德扬库特冰川反照率与物质平衡相关性较低的原因可能是坡向和低海拔导致了其更易受暖湿气流影响, 冰川消融中短波辐射能占比较低, 反照率对其消融的影响小.