Home – Home – Jornals – Russian Geology and Geophysics 2001 number 10

In recent two decades, new data on the age of strata and their spatial arrangement and geodynamic formation conditions have appeared, which give new insight into the geologic history of the Tien Shan. This mountain system evolved in several large megastages and stages, from the Archean through the Quaternary, which are recognized according to the predominant oceanic or continental regime of development. The fold-nappe structure of the Tien Shan is the result of crushing of the edge of the ancient supercontinent into a series of small blocks, which were then localized on the periphery of the Paleoasian ocean and underwent accretion, completed in the late Late Paleozoic.
The modern Eurasian plate within Central Asia is separated into several blocks, whose counter and rotational movements determine the recent geodynamics of the region. The Tien Shan lithosphere is layered as a result of the appearance of a migma layer in its lower part, which causes a relatively free transfer and deformation of its upper part.

New data on stratigraphy , tectonics, and magmatism of the Tien Shan ancient rocks suggest the plate-tectonic interpretation of the structure and evolution of the Tien Shan nappe-folded orogen. A new scheme of tectonic regionalization has been proposed for the Kyrgyz Tien Shan. Emphasis is placed on recognition of geodynamic complexes (continents, microcontinents, passive and active margins, open ocean basins, and marginal seas) and structural elements (autochthons, para-autochthons, allochthons, and neoautochthons), governing the spatial arrangement of their units with specific texture and composition. Over the long geologic history of the territory, the continental blocks changed their outlines, and ocean basins appeared and disappeared. The Caledonian and Hercynian orogenies in the Tien Shan underwent a complete Wilson cycle. The Mesozoic history was controlled by within-plate processes of sedimentation and magmatism. Since the Oligocene, the Tien Shan territory has been involved in the recent orogeny.

This paper considers the composition, structure, spatial location, and paleotectonic formation conditions of Precambrian rocks of the Tien Shan and adjacent regions analogous to the Sinian series in China and the Tamian-Vendian strata of the cover of the Russian Platform. A model is proposed for the Precambrian evolution of the Central Asian ocean basin with pre-Sinian accretion and Vendian orogeny. The major events are dated: subduction and continent-marginal volcanism – 830 20 Ma BP, pre-Sinian accretion

The types and subtypes of a density section of the Earth's crust in the zone limited by the Talas-Fergana and East Fergana faults are considered. The zone has an inversion deconsolidated layer in the lower crust (35-50 km). Rock densities are calculated by an empirical formula with the use of seismic tomography data on P- and S-wave velocities. In the geological aspect this territory is remarkable as it involves the South Tien Shan ophiolite belt notching a suture which resulted from the closure of Turkestan paleoocean in the Middle Carboniferous. In the Earth's crust section the suture is traceable to a depth of 15-20 km, is cone-shaped, and is characterized by an increased density of rocks. Analysis of the gravimetric field and density sections suggests that within this cone the Earth's crust is saturated with rocks of ophiolite association.

The paper presents the dip angles of a seismofocal zone, the rate of underthrust, and other kinematic parameters of Middle Carboniferous-Early Permian subduction in northern Fergana, reconstructed from the petrochemical features of coeval magmatites. Comparison of these parameters with those in zones on some modern active continental margins suggests the metallogenic similarity of these regions.

The paper presents new data on several ore fields and mineral deposits of the Tien Shan spatially associated with transregional NW faults, which are easily identified by geophysical methods as they are marked by epicenters of shallow earthquakes and their aftershocks and abnormal heat flux (up to 130 mW/m ²). The faults inherit the strike of older faults and cross structures of the "Tien Shan" orientation.
In a low-seismisity block east of the Talas-Fergana fault (east of 74 ^oE), MT soundings reveal a continuous crustal conductor, and the behavior of P waves indicates the presence of waveguides. The block involves the Barskoon zone of deep faults and a number of rift depressions. A large gold deposit (Kumtor) occurs at the intersection of the Borskoon fault with the Mid Tien Shan back-arc magmatic belt and the "Nikolaev line".

A new conceptual approach has been proposed and substantiated for solving the problem of earthquake prediction and organization of monitoring on the South Baikal geodynamic testing ground. The new concept takes into account first of all general geodynamic factors leading to the appearance of foci of metastable state in the Earth's crust. It has been shown that of great importance are regional variations in the stress state caused by the processes of redistribution of crustal stresses during seismic activity in the region as well as by more distant seismogeodynamic processes on interplate boundaries. It is supposed that with time the physics of a focus should give way to the physics of sufficiently large focal zones. One of the main factors determining the seismic properties of a medium is its block structure. Hence, both experimental and theoretical studies are necessary to clear up the behavior of dynamic processes in such block geosystems.

P-wave delays recorded by two linear arrays of seismographs across the Southern Alps of New Zealand require a high-speed zone in the upper mantle beneath the Alps. The pattern of residuals does not match that predicted for subduction of one plate of mantle lithosphere beneath the other. Instead, the high-speed zone seems to mark a blob of cold lithospheric mantle sinking beneath the eastern part of the Southern Alps, directly beneath the thickest crust, and hence where mantle lithosphere has thickened by pure shear. Calculations of Rayleigh-Taylor instability for a thickening heavy layer buoyed up by a lighter layer show that the form of the sinking blobs of dense material depends strongly on the ratio of effective viscosities of crust and mantle. Where the effective viscosity of the buoyant upper layer (crust) is large, a single blob sinks beneath the area of maximum thickening of the upper layer. In New Zealand, where a single sinking blob is inferred, large-strike-slip shear of the region may have lowered the viscosity of the mantle. Elsewhere, if the viscosity of the upper layer is small, however, two sinking blobs can form adjacent to the area where the more buoyant layer (crust) is shortened horizontally. Between the blobs, the unstable layer (lithospheric mantle) thins. Such a process may occur within the Tien Shan.

On the basis of harmonic analysis of the data for 125 thousand earthquakes from the catalog of the National Earthquake Information Center of the U.S. Geological Survey for the period 1964-90 and of a series of time-dependent changes of the Earth's angular velocity (t) during the same period, significant positive correlation has been established between graphs of |d/dt| and (t), an annual number N(t) of earthquakes for different regions of the Earth averaged over several years depending on their hypocenter depths (so-called T-component of seismic activity). Besides, special attention was paid to studies of spreading and subduction zones. At the same time, in any two adjacent areas along the both sides of the tectonic plate (in subduction zones and in the system of mid-ocean ridges and continental mountain ranges) the correlation coefficient between the functions [N₁(t)

The paper contains a summary of heat flow studies in the Tien Shan since the early 1960s and the "Catalog of heat flow determinations in the Tien Shan and adjacent territories" published for the first time. Heat flow in the region varies in a broad range from 16 to 134 mW/m²) with the average of 50

This paper present results of a comprehensive analysis of the geophysical fields in High Asia, which show the peculiarites of the tectonic structure of the region and geodynamic processes occurring there. Comparison of maps of geophysical fields, namely, geoidal undulations, anomalous magnetic and gravity fields, heat flow, shear-wave attenuation, distributions of group velocities of Rayleigh waves for periods of 10-70 s, and distribution of Pn-wave velocities in the upper mantle of High Asia, shows that they are all caused by an anomalous body which might consist of deconsolidated and hot material and is called by us as the Tibetan plume. We suggest its III-level structure. Comparison of seismicity and geophysical fields shows a particular correlation between the occurrence of earthquakes and the structure of geophysical fields. Gradients of geophysical fields have been found to be indicators of possible zones of strong earthquakes. Dynamic processes in High Asia are governed by combination of collision between the Indostan and Euroasian litospheric plates and the ongoing development of the Tibet plume.

This paper presents the characteristics of the first 3D velocity model of the Tien Shan crust, inferred from the traveltimes of P- and S-waves from local and regional earthquakes, recorded by seismic stations in Kyrgyzstan and adjacent regions. Seismic tomography imaging provided new data on the deep structure of the Tien Shan orogen.
Velocity section along the profile crossing source zones of five strong earthquakes (M 6) shows different structural types of the crust. We describe the peculiarities of the velocity structure of strong-earthquake zones and criteria for their distinguishing: waveguides, subvertical weakened channels, and high-velocity bodies to which the foci of strong events are confined.
We also give examples of geological and geophysical interpretation of the first velocity model and show its weakness. The necessity of new imaging is shown. For this purpose a data set was enlarged and described in detail. The principles of collection of data and calculation of a new model are also considered. Much attention was paid to calculation of a 1D model which is the basis for further computation of the new 3D model. Some preliminary results of 3D inversion for P-wave velocities are presented. We believe that the new model, describing the velocity structure of the Tien Shan in regional scale, will provide a more argued judgment about the geodynamic processes there.

The Tien Shan is the best example of active intracontinental mountain building in the world. In order to determine how strain has accumulated in the range, a 28 station network was deployed in the Tien Shan during 1998

Normalized bimodal interpretation of magnetotelluric sounding in the Kyrgyz Tien Shan mountains provides a reliable geoelectric model for this seismic region. The model contains a conductive layer in the lower crust and subvertical conductive zones in the upper crust. High overall conductivity of crustal zones clearly correlates with low-density zones.

The paper presents results of microseismic and electromagnetic studies of a landslide in the Suusamyr River valley (northern Tien Shan, Kyrgyzstan) and laboratory pulse transmission experiments on ground samples from the landslide. Spectral and polarization patterns of microseisms on the slope show that the main portion of seismic energy in the frequency range of 120 to 210 Hz is related to microfaulting in zones of stress concentration. The high-resolution image of the landslide obtained by the applied techniques of areal TEM soundings and the high sensitivity of seismic parameters to the stress-strain state of the ground demonstrated in the pulse transmission experiments confirm the efficiency of geophysical methods in investigation of landslides and slope stability.

We have determined slip rates on the most active reverse faults, reconstructed an extensive preorogenic erosion surface, constructed local and regional cross sections, and dated syntectonic Tertiary sedimentary rocks by magnetostratigraphy along a north-south transect that spans the Kyrgyz portion of the west-central Tien Shan. The cumulative Late Quaternary shortening rate along this transect is ~12 mm/yr. The trabsect consists of five major fault zones, and the most active faults lie in the interior of the range. Using geometric models developed in other regions of basement-involved determination, we estimate shortening during the Late Cenozoic at 40-80 km. Apparent simultaneous onset of sedimentary basins (at least 3 major basins) about 12 Ma BP is interpreted to mark the onset of the current orogeny. Given the current shortening rate of about 10 mm/yr, measured across active faults and by GPS, we infer that the rate increased with time. We assumed accelerated shortening and have shown that it has always been of similar style, dominated by north-south shortening across east-west trending basement-involved reverse faults. Deformations were localized in five zones, which border the largest and deepest Tertiary basins, show the greatest structural relief, and contain the currently most active faults.

Earlier investigations based on seismological data and results of electromagnetic monitoring of a terrain in one of the most active seismic regions of the North Tien Shan (Bishkek prognostic test ground) revealed that regional and local deformations proceed synchronously. It is important to clear up the mechanism of link of these processes. As known, the seismic process depends on distribution of stresses in a volume of rocks. The pattern of distribution of stresses can be statistically mirrored through allocation of earthquakes in the study terrain. Allocation of seismic events permits us to judge about the mechanism that controls the distribution of stresses.
The obtained maps of allocation of earthquake epicenters show a stable maximum in the Hindu Kush area and a gradual decrease in magnitudes at a distance from it. Preliminary solutions to the model problem on distribution of stresses in an elastic medium demonstrate that the stresses from a central source attenuate much faster than it could be inferred from the observed allocation of earthquakes in the region. It is possible to eliminate this discrepancy by assuming that horizontal forces act from below.
The GPS data received from 1993 to 1999 on the Tien Shan regional networks suggest that some tectonic blocks (especially in the eastern part of the study terrain) move at approximately identical velocities predominantly northward. The impression is created that the blocks move over the surface of a certain ductile horizon in the Earth's crust. Magnetotelluric investigations have revealed an electroconductive horizon in the lower crust throughout South Kazakhstan and the Tien Shan. This geological bed can be just that plastic horizon, on the top of which the Earth's crust moves and through which the horizontal forces are transmitted simultaneously to the entire region at the expense of viscosity of current matter.
It is established that local seismogenic zones coincide with subhorizontal areas of elevated conductivity. These areas are tectonic zones of weakening. Their elevated conductivity is, most likely, caused by inflow of hot solutions from lower-crust horizon through subvertical zones connecting these horizons. The subvertical zones are also channels through which seismogenic zones and lower-crust horizon are interacted by force.

We interpret Global Positioning System measurements of interseismic deformation throughout the western Tien Shan in the context of a block model which accounts for important geologic features (faults) and physical processes (elastic strain accumulation.) Through this analysis we are able to quantify the amount of deformation localized on active structures. In the central part of the belt the Dzhuanaryk fault zone appears to be the most important thrust fault, accommodating nearly five millimeters per year of north-south shortening across it. Conversely, the most widely recognized strike-slip fault in the region, the Talas-Fergana, is found to have very little of the previously estimated right lateral motion.

High density of GPS network in the Tien Shan and long period of observations have allowed us to construct a velocity vector field of movement on the Earth's surface and a strain rate field. A compact uniform area of compressing deformations has been detected. Comparison of the strain rate field with the spatial distribution of weak seismicity for the same period of time showed their considerable correlation. The GPS and seismic data were also compared with data of the magnetotelluric sounding and in particular with geometry of the surface of a crustal conductive layer detected beneath the Tien Shan. The area of maximum compressing deformations coincided with the northern slope of the layer surface, and the strain rate intensity is correlated with the slope angle magnitude of that surface. Nearly all seismic events occur above the surface of the crustal conductive layer. The coincidences suggest that the deformation field and the seismicity distribution are connected by a single tectonic process and depend on the geometry of the crustal layer.

The effect of high-power electromagnetic pulses of MHD-generator on the seismicity of the Bishkek geodynamic test ground has been studied. Local earthquakes became more frequent after the MHD-generator had been started up. High activity of local earthquakes was observed 2 to 4 days after startups. It is shown that electromagnetic pulses cause release of the energy accumulated during tectonic deformation process. The energy releases as a series of relatively weak earthquakes rather than as a catastrophic event.

Measurements performed by means of tiltmeters and expansometers in special-purpose underground observatories are important part of the research of the crustal kinematic parameters in the northern Tien Shan. Long runs of tilts and deformations observed at the Ala-Archa underground station (42.63 ^o N, 74.50 ^o E) have been analyzed. Results of tidal analysis are reported. Slow variations in tilts and deformations reveal seasonal changes and slope effects caused by seismic activity of the region. The annual average rate of meridional compression was 3 10

This paper deals with deformation processes in the Earth's crust recorded by tiltmeters and extensometers in the Northern Tien Shan. The characteristics of instrumentation are reported. The deformation stations located in different geological conditions are described, and the accuracy of measurements of deformations is estimated. Examples of deformation disturbances preceding an earthquake are given. The deformations are of seasonal character. Thus, expansion deformations are recorded in vernal periods, and compressional ones, in the fall. Zones of development of abnormal deformations during preparation of a seismic event are characterized. Abnormal deformations irrelevant to earthquakes are recognized, which are usually due to changes of the groundwater regime.

The 1911, M_s = 8.2 Kemin (Kebin) earthquake in the Northern Tien Shan (Kazakhstan, Kyrgystan) formed a complex system of surface ruptures nearly 190 km long and numerous landslides and rock avalanches up to tens of millions of cubic meters in volume. Judging from their distribution, six fault segments of the Kemin-Chilik and the Aksu fault zones with different strikes, dips, and kinematics have been activated. The Kemin earthquake was one of the strongest events of a sequence of seismic catastrophes that affected the Kungei and Zaili-Alatau mountain ranges between 1887 and 1938. The effects of the Kemin earthquake are well documented in a monograph published soon after the event by K. I. Bogdanovich. In the frame of a European INCO-COPERNICUS program, the surface ruptures, landslides, and rockslides associated to this earthquake have been re-examined in detail. In addition, the large-scale tectonic setting of the Kemin-Chilik and Aksu fault zones has been re-evaluated, and their segments have been identified and described. The whole system forms a sinistral transpressional structure, which controls the formation of the mountain ranges between the Issyk-Kul depression and the Kazakhstan block. The surface ruptures of the 1911 earthquake can presently be observed in the field over a total length of nearly 100 km and generally reactivate longer-term cumulative paleoseismic fault scarps. The presence of well-expressed paleoseismic fault scarps and several tremendous ancient landslides in the Chon-Kemin, Chon-Aksu, and Aksu valleys can be considered as evidence for strong prehistoric earthquakes.

Study was given to a thin structure of pulses of acoustic and electromagnetic emission recorded on axial compression of specimens of rocks at the stage of their dilatantion deformation. Four types of signals have been distinguished in frequency and duration. It is supposed that the type I signals account for the formation of detachment fractures; the type II signals, of shear fractures; and the low-frequency type III signals correspond to vibrations of constructive elements of the loading device. The type IV signals are referred to as pulse electromagnetic emission caused by charged dislocations coming to a grain boundary in minerals with ionic bonds, components of granite.