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Geodynamics & Tectonophysics

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Vol 2, No 3 (2011)

TECTONOPHYSICS

219-243 1447
Abstract

The publication presents stress determinations from geological and kinematical indicators of tectonic stress fields, varying in ranks, for the Kola Peninsula. The objective is to determine possible mechanisms of formation of recent structures in the eastern segment of the Baltic shield and to forecast seismogenic and technogenic hazard of fracturing.

The study is focused on the Kovdor and Khibin Paleozoic alkaline-ultrabasic blocks. Tectonic stresses are reconstructed by M.V. Gzovsky’s method [1954; 1975] based on identification of conjugated shear systems. Neotectonic stresses are studied by the kinematic method [Gushchenko, 1979] on the basis of measurements of tectonic displacment vectors from slickensides (Figure 2). Local stress data processed by the method for determination of general stress fields provide for reconstruction of main normal stresses which are arbitrarily considered as regional stresses [Sim, 1980; 2000]. This study uses the method of bandpattern distribution of fracturing in fault zones [Danilovich, 1961] which determines the main displacement line on the fault plane.

The study of the Zhelezny mining site (Kovdor block, Figures 3 and 4) revealed that elements of fractures of two different ages (centroclinal fractures of the prototectonic genesis and fractures of later tectonic activation) are spatially overlapping due to rock collapse and lacking stability of benches. Numerous inversions and changes of kinematics of relative displacements were reviewed. It was observed that the southeastern wall of the quarry collapsed due to local extension (Fig. 6B
and Photos 1 and 2), and a large fault, i.e. a prototectonic fracture, was dipping towards the quarry. Based on the analyses of local stresses at various points of the quarry (Table 1), two ‘regional’ stress fields can be revealed (Figures 7 and 8). The first paleostess field was associated with reverse faults of the WNW subhorizontal axis of compression and the steeply dipping axis of extension. The second field was related to shear faults; its axis of compression maintained the WNW orientation, while the extension axis was reoriented, and the axis of extension attained submeridional position and a less steep dip. The second field is younger as suggested by traces of two displacements identified on several planes, the youngest of which are shears.

From the analysis of measurements taken at 273 planes with striations, it is evident that striations are distributed in a bandshape pattern. The band of fractures is coincident with the plane of the transition axis of the young ‘regional field (Figure 9); main maximums of density of the planes with striations are symmetrically scattered in a fanlike pattern from the axis of compression and extension of this field. Generally, the striations reflect traces of younger displacements, and their consistency with the axes of the young field supports our conclusions on age relationships between  the two ‘regional’ fields. Four major stages of deformation of the Kovdor block under study are distinguished by analyses of the tectonic stresses (Figure 10).

Within the limits of the Khibin block, 14 local states of stresses are reconstructed for three mining sites (Table 2, Figure 11). At the Central mining site, reindexation of local axis of compression and extension in the fault wings give evidence of the fault activity during the neotectonic stage (Figure 13). The ‘regional’ stress field of the Khibin block is associated with a reserve fault with lowangle NNW orientation of the axis of compression (Figure 14). The tectonophysical studies conducted at the mining sites of the Kola Peninsula give grounds to conclude that activity of faults, which positions are different at the mining sites, is variable, depending on orientations of the faults against the youngest ‘regional’ main stress. From sets of indicators, a relative age of the revealed ‘regional’ fields of stresses is accepted as neotectonic and recent.

For the segments of the Kovdor block under study, four phases of deformation are distinguished, including two early phases revealed by structural indicators, and two last phases revealed from orientations of the axes of the main stresses in the reconstructed ‘regional’ fields. The reverse field of stresses of Deformation Phase 3 (which is a more ancient ‘regional’ field according to reconstructed tectonic stresses) at the Kovdor block and the reversefault field at the mining sites of the Khibin block may reflect a phase of brittle deformation of the rocks after the blocks were exposed to the day surface. Since then the deformation mechanisms might have been determined by two factors which controlled subhorizontal compression: residual gravity stresses in considerably eroded magmatic bodies as ‘recollections’ of being subject to constraint environment at depth [Rebetsky, 2008], and the impact of rifting in the Northern Atlantics. The fact that the neotectonic ‘regional’ stress field of the Kovdor block is fully similar to that of the Central Karelia (Figure 1) give grounds to conclude that the mechanism of deformation of the block under study might have been controlled by both factors. The Khibin block has a lopolithic shape which gradually converts into a centraltype conic structure with depth. It should thus be squeezed upward by the impact of horizontal compression of any genesis, as evidenced by the subvertical extension axis of the general field of the Khibin area and the recent topography as the highest mountains of the Kola Peninsula are located in the block under study.

RECENT GEODYNAMICS

244-265 1658
Abstract
Geological, geophysical and seismogeological studies are now conducted in a more detail and thus provide for determining seismic sources with higher accuracy, from the first meters to first dozens of meters [Waldhauser, Schaff, 2008]. It is now possible to consider uncertainty ellipses of earthquake hypocenters, that are recorded in the updated Earthquake Catalogue, as surfaces of earthquake focus generators. In our article, it is accepted that a maximum horizontal size of an uncertainty ellipse corresponds to an area of a focus generator, and seismic events are thus classified into two groups, earthquakes with nonstiff and stiff foci. Criteria of such a classification are two limits of elastic strain and brittle strain in case of uniaxial (3⋅10–5) or omnidirectional (10–6) compression. The criteria are established from results of analyses of parameters of seismic dislocations and earthquake foci with regard to studies of surface parameters and deformation parameters of fault zones. It is recommendable that the uniaxial compression criterion shall be applied to zones of interaction between tectonic plates, and the unilateral compression criterion shall be applied to low active (interplate) areas. Sample cases demonstrate the use of data sets on nonstiff and stiff foci for separate evaluation of magnitude reoccurrence curves, analyses of structured and dissipated seismicity, review of the physical nature of nonlinearity of recurrence curves and conditions of preparation of strong earthquakes. Changes of parameters of the recurrence curves with changes of data collection square areas are considered. Reviewed are changes of parameters of the recurrence curves during preparation for the Japan major earthquake of 11 March 2011 prior to and after the major shock. It is emphasized that it is important to conduct even more detailed geological and geophysical studies and to improve precision and sensitivity of local seismological monitoring networks in zones of nuclear stations and other hazardous facilities. It is noted that a list of parameters recorded in earthquake catalogues needs to be extended. Based on the above, it will be possible to ensure proper monitoring of stability of the seismic process during construction, operations and decommissioning of nuclear stations.

PALEOGEODYNAMICS

266-288 1172
Abstract
The tectonic and paleogeographic evolution of the Ural-Mongol belt between the cratons of Baltica, Siberia, and Tarim is the key to the formation of the Eurasian supercontinent during Paleozoic time, but the views on this complicated process remain very disparate and sometimes controversial. Three volcanic formations of the Middle Silurian, LowertoMiddle Devonian and Middle Devonian age from the southwestern boundary of the Chingiz Range (NE Kazakhstan) yields what are interpreted as primary paleomagnetic directions that help clarify the evolution of the belt. A singlepolarity characteristic component in midSilurian andesites yields a positive intraformational conglomerate test, whereas dualpolarity prefolding components are isolated from the two Devonian collections. These new data were evaluated together with previously published paleomagnetic results from Paleozoic rocks in the Chingiz Range, and allow us to establish with confidence the hemisphere in which the area was located at a given time. We conclude that NE Kazakhstan was steadily moving northward crossing the equator in Silurian time. These new paleomagnetic data from the Chingiz range also agree with and reinforce the hypothesis that the strongly curved volcanic belts of Kazakhstan underwent oroclinal bending between Middle Devonian and Late Carboniferous time. A comparison of the Chingiz paleolatitudes with those of Siberia shows similarities between the northward motion and rotational history of the Chingiz unit and those of Siberia, which imposes important constraints on the evolving paleogeography of the Ural-Mongol belt.

PERSONALITIES

289-323 1957
Abstract

The publication is devoted to the history of one of the greatest concepts of tectonics of Asia, that has been widely accepted and yet obliterated with time, while the splendors of this concept are doubtful. Numerous citations in the Russian papers to «The Face of the Earth» by Edward Suess and the fact that he was elected a Corresponding Member of the Imperial Saint Petersburg Academy of Sciences clearly demonstrate how highly Suess’s contribution to studies of the structure and geological evolution of Asia was valued by the Russian geological community. Suess’s letters to Vladimir A. Obruchev give evidence how close and productive the relationship between Edward Suess and the Russian researchers was in the late 19th and the early 20th centuries and also illustrate how the great tectonic concept of Asia [Suess, 1908] was born and developed. The idea of centrifugal propagation of tectonic waves of the Altaids from a continental node located somewhere in Siberia was mainly inspired by Suess’s profound scientific intuition. The idea matured after Edward Suess got acquainted with Ivan D. Chersky’s paper [Черский, 1886] that greatly facilitated in shaping and improving this idea. It was mailed to Suess by Vladimir A. Obruchev who translated the paper, attached his own map and provided explanations to Chersky’s ideas. The available historical documents suggest that Vladimir A. Obruchev facilitated communication between the Russian geologists, on the one side, and Edward Suess and other Austrian geologists who conducted geological studies in Asia, on the other side. Being actively involved in exchange of publications and cooperation in field data processing, Edward Suess was aware of all the details of the Russian geological studies.


In addition to the concept of tectonic arcs of the Altaids and descriptions of main geological structures located in Northern Asia and China, Edward Suess adopted a concept of disjunctive dislocations proposed by the Russian geologists. While interpreting the structure of huge territories of Asia that were poorly studied then, he stuck to the orometric geometry principles and thus, unfortunately, missed the Russian conclusions on disagreement between belts and mountain ridges, superimposed folding, mélanges etc., and such features were not incorporated in his concepts.

Papers of Edward Suess, including «The Face of the Earth» which was very positively accepted by the geological scientific community, have never been translated into Russian, unlike other foreign publications. In 1930–1940, the name of Edward Suess gradually vanished from references in the Soviet scientific papers. Such a lapse seems to have resulted from the fact that Suess’s papers were misinterpreted and misunderstood by scientists who adhered to the geosyncline theory. Examples of such errors can be easily discovered by reviewing the history of development of concepts of the Siberian and Russian cratons.

The truly gentlemanly behavior demonstrated by the key researchers of geology of Asia in the late 19th and the early 20th centuries is highly educative and commendable. It is worth studying the scientific reports of Edward Suess and his Russian colleagues with a more detail and comprehensive approach.



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ISSN 2078-502X (Online)