A. G. Gurbanov1, A. L. Sobisevich2, L. E. Sobisevich2, Yu. N. Nechaev2, V. N. Arbuzkin3, N. I. Prutskii3, E. A. Trofimenko3, I. I. Grekov3
1 Institute of Geology of Ore Deposits, Petrography, Mineralogy,
and Geochemistry, Russian Academy of Sciences, Moscow;
2 Institute of Physics of the Earth, Russian Academy of Sciences, Moscow
3 Federal Caucasus Geological Survey
At the end of the last century volcanologists began to pay more attention to the comprehensive study of the so-called "sleeping'' volcanoes which may become active unexpectedly. As to the Russian territory, an example of these volcanoes is the Akademii Nauk Volcano (Kamchatka) located in the caldera of the same name, where volcanic activity had terminated 28 8 thousand years ago. However, simultaneous volcanic eruptions began unexpectedly on 02.01.1996 at the Karymskii volcano and in the Karymskii Lake filling the caldera of the Akademii Nauk Volcano [Fedotov, 1997]. As to the European part of Russia, Elbrus Volcano is believed to be potentially active [Avdulov, 1962; Avdulov and Koronovskii, 1993; Garetovskaya et al., 1986; Khitarov et al., 1984; Krasnopevtseva, 1984; Masurenkov, 1961, 1971].
The modern morphological classifications of volcanoes are usually based on the view that their shapes are controlled by the composition of magma and by the mechanisms of their volcanic eruptions. The commonly used data are morphological characteristics of the volcano, such as the absolute and relative heights of its cone, or the elevation of its basement. The study of quantitative relationships between these characteristics and the composition of the volcanic rocks are useful for analyzing the rising mechanism of magma and for estimating the depth of its origin. Proceeding from the laws of hydrostatics, the height of a stratovolcano, which had reached its limiting value, can be treated as a value controlled by the depth of the magma chamber. Hence, an important conclusion can be derived that the basic magma generated at a great depth must produce higher volcanoes [Masurenkov, 1972].
At the present time volcanoes are classified into two types. The first type includes active volcanoes of island arcs, most of which are not higher than 1500-2000 m. High volcanoes are not numerous there and can be ranked as basaltic or andesitic volcanoes (with SiO 2 contents of 49-59%) [Fedotov et al., 1981]. The second type includes active volcanoes from young folded areas, which show the proved relationships between their heights and the compositions of the rocks composing them, yet display the general growth of their absolute heights and a great difference in the heights of the volcanoes differing in acidity. There are grounds to believe that the Elbrus Volcano also obeys the above regularity.
Our theoretical studies suggested two mechanisms for the seismoacoustic sounding of the magma chamber: (1) the analysis of a microseismic background in the source area, where the geoacoustic field may include radiation at some typical frequencies, and (2) the analysis of waves reflected from the wave source (some induced wave-type processes) during its active sounding. The use of both methods calls for the preliminary study of the microseismic background in the source region, for the creation of seismic recording channels suitable, in terms of their amplitude and frequency characteristics, to the problem at hand, for their optimum dislocation in place, and for the development of effective methods for seismic data processing and analysis.
The authors of the paper cited above believe that the magma chamber resides at a small depth below the Elbrus volcanic cone, this fact being proved "by the presence of an intensive surface wave in all records.'' They were unable to localize the source zone exactly. Nevertheless, they concluded that "even the data available suggest that with the low epicentral distance (10-15 km) the seismic velocity anomaly suggesting the existence of a zone of high seismic wave attenuation may exists under Elbrus at a depth of 0.5-2 km below the sea level.''
Seismic observations in the Elbrus area were resumed with the onset of the operations of a complex international geological and geophysical expedition which included geoscientists from the Institute of Physics of the Earth, the Institute of Geology of Ore Deposits, Petrography, Mineralogy and Geochemistry, the Institute of Geosphere Dynamics, the Federal Caucasus Geological Survey, and a number of other organizations. In the course of this work the microseismic background was recorded in the area of the Elbrus volcanic edifice.
The first observation site was located in the Baksan R. ravine in the area of the Moscow University Geological Site located at the foot of the volcano. The absolute elevation of the site is 2300 m, its coordinates being 43o16' 09'' N and 42o29' 14'' E. The distance of this recording site from the Elbrus peak is 5.85 km.
The second observation site was located on the Terskol Peak, in the area of the Astrophysical Observatory of the Ukrainian Academy of Science. The coordinates of this site were 43o16' 517'' N and 42o29' 999'' E. The absolute height of the site was 3100 m. The distance from this recording site to the Elbrus Peak was 5.6 km. The choice of the second alternative background recording site was predetermined by the fact that the Azau Glade showed an extremely high microseismic background.
The third observation site, where seismic background was recorded, was located in the tunnel of the Neitrino Observatory with the coordinates of 43o16' 338'' N and 42o40' 878'' E, the altitude of the site being 1740 m, and its distance from the Elbrus Peak being 21.9 km. We begin the description of the study of the seismic environment in the Elbrus volcanic center with the results of recording the microseismic background.
Listed in Table 1 are the mean square values of velocity variations and displacements along all constituents, calculated using 10-minute data samples. The analysis of the resulting data suggests that all of the oscillation constituents are roughly equal (see the data shown in Figure 2 [Khitarov et al., 1984]). This phenomenon is characteristic of the microseismic background at the outcrops of hard rocks.
As follows from Table 1, the microseismic oscillation level in the area of the neutrino measurement site is two orders of magnitude lower than that at the Azau Site and an order of magnitude lower than that at the Terskol Mt. peak. Characteristic of the spectra is the presence of individual peaks. Some peaks might be associated with the industrial noise from various mechanisms, the other seem to be associated with the presence of resonance features in the volcanic cone [Sobisevich, 2001]. The results of our numerous experiments, carried out in the period of time from 1999 to 2002, suggest that the use of modern geophysical instruments, especially, of deformation measurement instruments, allows one to record induced wave processes caused by igneous rock structures and other local heterogeneities [Sobisevich et al., 2001, 2002a].
The fine structure of the local heterogeneities in the area of the Elbrus volcanic cone was studied using a technology developed at the United Institute of the Earth's Physics [Nechaev, 1999; Nechaev and Sobisevich, 2000]. In accordance with the technology offered in these papers, the field of the tectonic breaking of the Central Caucasus lithosphere was plotted using the results of processing the photographs of the ground surface, obtained from the "Resource'' Satellite. The first result of this work was the map of a lineament network for a territory measuring 185 277 km with the Elbrus Volcano located in its central part. The subsequent processing of this map resulted in getting several maps varying from 1 to 50 km in depth, where the values of tectonic shattering were calculated using a network of 1 1 km.
This horizontal section shows the slow growth of tectonic breaking from the southern termination of the epi-Hercynian Scythian Platform toward the region of the Central Caucasus horst-anticlinorium and its subsequent decrease toward the southern Greater Caucasus subsidence. The Laba-Malka zone of the highs is marked by the low values of this field ( < 80 arbitrary units), whereas its western area (gently monoclinal Mesozoic sediments) shows higher values (70-90 arbitrary units), as compared to its eastern margin (involved in the uplifting of the southern segment of the epi-Hercynian Scythian Platform). The protrusions of the Paleozoic basement show the high values of a tectonic breaking field (80-100 arbitrary units). The Tyrnyauz suture zone, composed of Paleozoic and Jurassic rocks, was mapped as a narrow transition zone of elevated horizontal gradients, its eastern segment (east of the Kuban River) being most distinct. The inversion anticlinorium of the Greater Caucasus core, composed of highly folded metamorphic rocks of the Liass-Dogger age (Central Caucasus Horst-Anticlinorium) is distinguished by the maximum values of this field (up to 120 and higher arbitrary units). They occur as individual blocks separated by the areas of lower values ( < 100 arbitrary units).
The more southern region of the synclinoria in the southern Greater Caucasus limb, filled with Malm and Cretaceous isoclinal folded flysch, is represented by the lower values of the field (less than 110 arbitrary units) in the form of individual local anomalies ( < 90 arbitrary units). The Abkhazia-Racha Step, located in the south of the territory concerned and represented by the anticlinoria and steps of the southern limb of the Greater Caucasus (moderately folded nonflysch Mesozoic rocks), showed somewhat lower values ( < 100 arbitrary units).
Considering the behavior of the nearly horizontal lithospheric interfaces in the vicinity of the Elbrus volcanic cone, worthy of mention is their regular rising, this fact suggesting that this volcanic cone is restricted to the region of the anomalous structure of the lithosphere. The upper part of the basaltic crust (about 20 km thick) in its central segment located under the Elbrus Volcano showed anomalous values of the tectonic breaking field. This suggests that:
(1) the Central Caucasus territory had been initially underlain by the basalt crust of the same type, which was close, in terms of its physical and mechanical properties, to its present-day remnants of the pre-Paleozoic basement (blocks and slabs) located west and east of the central anomalous region;
(2) the central block had moved northward and up the section along the nearly meridional crustal faults, and the Transcaucasus transverse uplift was formed during the next stage of the Central Caucasus evolution, when an anomalous region embracing the deep layers of the lithosphere began to form, as indicated by the rise of the Moho discontinuity;
(3) the almost ten-percent decline of the values of the tectonic disintegration in this anomalous region suggests that the physical and mechanical properties of the central block (slab) changed toward their lower values. This suggests the existence of an extension region in the Transcaucasus transverse uplift (in its western periphery) and implies that the local region of abnormally low tectonic fracturing can be interpreted as a potential magma source for the Elbrus Volcano.
The potential existence of a magma source chamber was used as a basis for locating the Elbrus magma chamber in the upper part of the crust. For this purpose an area, 30 30 km in size (with Elbrus Volcano standing in its middle), was chosen for processing space photographs in greater detail. As a result, fields of tectonic breaking were obtained for the horizontal sections corresponding to the average statistical depths of 4, 3, 2, and 1 km.
The position of the Elbrus Volcano in these maps was determined using the lowest values of the tectonic disintegration field in the projection area of the magma chamber.
The magma chamber of the Elbrus Volcano is restricted to the western periphery of the magma source residing at a distance of 10-12 km above it. It appears that magma flows from its source to the magma chamber along tectonically shattered zones.
The above-mentioned igneous rock structures of the Elbrus Volcano are specific formations in terms of geophysics. For instance, magma formation results in the abrupt decline of the viscosity of the rock material in the melting region and, usually, in the decline of its density, as well as in the changes of some other geophysical parameters. These changes, in turn, lead to the rearrangement of the internal structure of the region and to the formation of objects with clearly expressed resonance peculiarities which manifest themselves in the modes of lithospheric deformations and induced seismic fields.
We studied the spectra of the lithospheric deformation modes in the area of the Elbrus Volcano using the analytical results obtained by a group of researchers headed by V. K. Milyukov (Shternberg Institute of Astronomy, Moscow State University), who were engaged for many years in the systematic measurements using the Baksan laser interferometer-deformograph. The objects of these studies were the lithospheric deformations caused by high seismic events (earthquakes) of fairly large magnitudes (Mmax 6.0-7.0 and higher) [Sobisevich et al., 2001, 2002a]. Figure 8 presents a series of relative deformations observed for individual earthquakes.
We found that some modes in the region of extremely low frequencies could be identified with the high-frequency modes of the Earth's natural oscillations. This analysis allowed us to identify the characteristic modes induced during all earthquakes considered, unrelated to the Earth's natural oscillations. To be convinced in this fact the reader is referred to Table 2 which gives the parameters of the modes recorded during at least three of the earthquakes observed.
The experimental results obtained showed that the geoelectric properties of the upper part of the rock sequence were highly variable. The most conductive rocks were recorded under the northern half of the profile, namely, in the Scythian Plate, where the most conductive rocks are the Maikopian deposits. The numerous wells drilled there showed the resistivities of 1-2 and even 0.5-0.6 W m. The total conductivity of the Maikopian and overlying rocks was found to be as high as 1500 S in the Terek-Kuma Trough.
The results of the quantitative interpretation of the frequency sounding and induced polarization data showed that the deposits overlying the Maikopian rocks included 6 or 7 layers of different (mainly low) resistivity. In some areas their correlation was violated by the presence of subvertical shatter zones.
The sedimentary rocks of the mountainous areas are usually poorly conductive. In the south of the North Caucasus Monocline the sedimentary cover consists of Cretaceous terrigenous-carbonate deposits and Upper Jurassic-Early Cretaceous terrigenous rocks. Its total longitudinal conductivity is not higher than 10-15 S. As follows from the quantitative interpretation of the resistivity data, the thickness of the sediments there does not exceed 400-450 m, declining to 150 m in the Podkumok R. Valley. Southward, in the Laba-Malka zone, the Lower Jurassic deposits (sandstone, siltstone, and argillite) are poorly conductive. They have a maximum thickness in the immediate vicinity of the Peredovoi Range in the Mara Basin (up to 500 m), which declines abruptly to 50-150 m in the area of the Malka Uplift, emphasizing the tectonic contact between these and other structural features of the Hercynian basement.
The sedimentary rocks of the Central Caucasus region are also characterized by shatter zones of varying intensity, which are most abundant in the Mineralnye Vody Protrusion. The results of gravity modeling admit the presence of the igneous rocks of the Mara volcanoplutonic rock complex at the base of the sedimentary cover at the northern flank of the Malka Rise. The Hercynian basement along the profile line is heterogeneous in resistivity, though shows less contrasting resistivity values as compared to the sedimentary rocks.
A more complex resistivity distribution pattern was found in the Central Caucasus tectonic block (to an absolute elevation of - 5 km). It emphasizes the synclinal character of the Khasaut tectonically layered zone complicated by subvertical faults and grabens between them.
The southern fault corresponds to the Sredinnyi magmatically active fault which can be traced, outside of the profile, by the centers of the Mara Early to Middle Jurassic volcano-plutonic rocks.
Along the line of the profile this fault is accompanied by a vertical high-resistivity body which is interpreted as a diorite intrusion of Jurassic age.
Also clearly seen is the breaking of the pre-Jurassic basement in the Kislovodsk anticlinal zone, in the area where the sedimentary cover includes the rocks of the Mara Complex. Not less broken are the granitoids of the Malka igneous rock complex in the Bechasy anticlinal zone. The central part of the Malka Massif includes high-conductivity zones with resistivities which are not typical of igneous rocks.
The Shaukamnysyrt fold zone and the Peredovoi Range graben (synclinorium), both located more to the south, occur as a high-resistivity block residing roughly 1 km below the ground surface. The probability of the unity of these tectonic elements has been suggested earlier by G. I. Baranov and I. I. Grekov who inferred the rocks of the Shaukamnysyrt Series under the tectonic zone of the Peredovoi Range. This suggests the greater role of the faults restricting the high-resistivity block. The southern of these faults corresponds to the Pshekish-Tyrnyauz fault zone, the northern, to the not less significant Upper Malka fault. The so-called Northern fault does not continue into the continental crust and is ranked as a near-surface one.
The interpretation of the results of the magnetotelluric sounding, performed at the Elbrus slopes and in its close vicinity revealed the following structure of the consolidated crust. The volcanic rocks showed high resistivity values ( > 1000 W m). The exceptions are the sites located in the water-saturated lake deposits, which lowered the resistivity to 20-30 W m in the upper part of the rock sequence.
The crystalline basement of the Elbrus foundation is composed of the Proterozoic rocks of the Makera and Gondara complexes and Paleozoic granites which show rather high resistivities (hundreds to thousands W m). Resistivivity declines to 40 W m and less in a depth interval of 5-10 km.
This resistivity decline agrees with the universally known fact that as temperature grows to the values of 400o-1000o C, the resistivity of the rocks declines by a few orders of magnitude [Lebedev and Shanets, 1986]. This and the fact that the temperature difference between the walls and the centers of the peripheral sources of andesite-dacite magma of the Kamchatka stratovolcanoes with their radii measuring 3.0-3.5 km is about 1000o C [Fedotov, 1980], suggest that the low-resistivity body discovered at a depth of 5-10 km under the Elbrus Volcano is a magma chamber. The indirect evidence of a low-density body located at a depth of 5-10 km under Elbrus is provided by the interpretation of a negative gravity anomaly under Elbrus. Its magnitude and gradients can be estimated reliably by introducing a body with density of 2.37 g cm-3 with its upper edge at a depth of - 5 km and a thickness of about 10 km. The rise of this low-resistivity body to the north explains the presence of the sources of the Neogene-Quaternary volcanism north of the Peredovoi Range (Tash-Tebe and other sources).
Another low-resistivity anomaly has been found at a greater depth of the crust (25 to 55 km) north of Elbrus. This anomaly, about 15 km wide, plunges steeply (up to 70o ) to the north. Its contour coincides closely with the contour of a low-velocity region, located using the method of reflected earthquake waves, where the velocities of longitudinal waves are abnormally low, as follows from tomographic data. Taking into account that the upper conductivity anomaly is also accompanied by the anomaly of the low velocities of longitudinal waves, this anomalous region can be interpreted as a deep-seated magma source. The high conductivity and low velocity of this magma body suggest its high temperature.
The results of our geological and geophysical investigations, carried out in the Elbrus region, can be summarized as follows:
(1) Our interdisciplinary experiments proved the presence of a magma chamber in the Elbrus volcanic center. The results of the experimental measurements of the wave processes in the vicinity of a magma chamber, evolving under the active external effects, are in good agreement with the theoretical data.
(2) The potential reactivation of the Elbrus Volcano, predicted by some investigators, calls for special investigations aimed to study the structure of the cone of this stratovolcano, including the exact location and monitoring of magma chambers in the Elbrus volcanic area.
(3) The final purpose of studying the Elbrus geological structure is to predict the areas of the potential lava flow by way of locating faults both on the open slopes and on the slopes covered by snow and ice. This can be done using a high-accuracy magnetic survey and magnetotelluric measurements with the subsequent geological study of the located heterogeneities of the physical fields.
(4) The existence of a direct relationship between the conductivity of the rocks and their temperature allows us to recommend the monitoring of the conductivity of the upper and lower conductors using magnetotelluric soundings twice a year at two to four sites (in the north and south of Elbrus) for the purpose of predicting the reactivation of these volcanic centers.
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