Research Station of the Russian Academy of Sciences in Bishkek

Measurements at 400 campaign-style GPS points and another 14 continuously recording stations in central Asia define variations in their velocities both along and across the Kyrgyz and neighboring parts of Tien Shan. They show that at the longitude of Kyrgyzstan the Tarim Basin converges with Eurasia at 20 2 mm/yr, nearly two thirds of the total convergence rate between India and Eurasia at this longitude. This high rate suggests that the Tien Shan has grown into a major mountain range only late in the evolution of the India-Eurasia collision. Most of the convergence between Tarim and Eurasia within the upper crust of the Tien Shan presumably occurs by slip on faults on the edges of and within the belt, but 1-3 mm/yr of convergence is absorbed farther north, at the Dzungarian Alatau and at a lower rate with the Kazakh platform to the west. The Tarim Basin is thrust beneath the Tien Shan at 4-7 mm/yr. With respect to Eurasia, the Ferghana Valley rotates counterclockwise at 0.7Myr -1 about an axis at the southwest end of the valley. Thus, GPS data place a bound of 4 mm/yr on the rate of crustal shortening across the Chatkal and neighboring ranges on the northwest margin of the Ferghana Valley, and they limit the present-day slip rate on the right-lateral Talas-Ferghana fault to less than 2 mm/yr. GPS measurements corroborate geologic evidence indicating that the northern margin of the Pamir overthrusts the Alay Valley and require a rate of at least 10 and possibly 15 mm/yr.

Convergence of 29 ± 1 mm/yr between the NW corner of the Indian plate and Asia is accommodated by a combination of thrust and strike‐slip faulting on prominent faults and apparent distributed deformation within the Hindu Kush, Pamir, South Tien Shan and Kohistan Ranges. An upper bound to the slip rate of known faults is obtained by ignoring distributed strain and rotation: convergence occurs on thrust faults north of the Peshawar Basin (13 ± 1 mm/yr) and in the Alai‐South Tien Shan (12 ± 2 mm/yr), and shear on the northeast‐trending northern Chaman‐Gardiz‐Konar system (18 ± 1mm/yr) and the Darvaz‐Karakul fault zone (11 ± 2 mm/yr). Slip rates on the Herat and Talas‐Ferghana faults are small (<2 mm/yr). Shortening not attributable to known active faults occurs within the Hindu Kush and central Pamir (16 ± 2 mm/yr) with concomitant east‐west extension in the latter of 9 ± 2 mm/yr. This diversity of strain styles confirms the importance of mechanical heterogeneity to continental tectonics and shows that the Pamir, although less than half the size, behaves more like Tibet than like a linear belt of localized deformation.

Abstract GPS velocities measured in the Pamir and surrounding regions show a total of ~30 mm/yr of northward relative motion between stable Pakistan and Eurasia. The convergence budget is partitioned into 10–15 mm/yr of localized shortening across the Trans‐Alai Thrust, which bounds the Pamir on the north, consistent with southward subduction of intact lithosphere. Another 10–15 mm/yr of shortening is distributed across the Chitral Himalaya and Hindu Kush, suggesting that Hindu Kush seismicity might be related to northward subduction of Indian lithosphere. Modest shortening at <5 mm/yr occurs north of the Trans‐Alai Thrust, across the South Tien Shan and between the Ferghana Valley and Eurasia. Negligible north‐south shortening occurs within the high Pamir, but as much as 5 mm/yr, and perhaps 10 mm/yr, of east‐west extension occurs within this region. This extension is matched by a comparable amount of east‐west shortening in the Tajik Depression. The localization of shortening to the margins of the Pamir combined with observations of distributed internal extension implies that the east‐west vertically averaged, horizontal compressive normal stress is smaller than the north‐south compressive stress.

The results of reflection CMP seismic profiling of the Central Tien Shan in the meridional tract 75–76° E from Lake Song-Köl in Kyrgyzstan to the town of Kashgar in China are considered. The seismic section demonstrating complex heterogeneous structure of the Earth’s crust and reflecting its near-horizontal delamination with vertical and inclined zones of compositional and structural differentiation was constructed from processing of initial data of reflection CMP seismic profiling, earthquake converted-wave method (ECWM), and seismic tomography. The most important is the large zone of underthrusting of the Tarim Massif beneath the Tien Shan.

We analyze retrospectively/prospectively the transient variations of six different physical parameters in the atmosphere/ionosphere during the M7.8 and M7.3 earthquakes in Nepal, namely: 1) outgoing longwave radiation (OLR) at the top of the atmosphere (TOA); 2) GPS/TEC; 3) the very-low-frequency (VLF/LF) signals at the receiving stations in Bishkek (Kyrgyzstan) and Varanasi (India); 4) Radon observations; 5) Atmospheric chemical potential from assimilation models; and; 6) Air Temperature from NOAA ground stations. We found that in mid-March 2015, there was a rapid increase in the radiation from the atmosphere observed by satellites. This anomaly was located close to the future M7.8 epicenter and reached a maximum on April 21–22. The GPS/TEC data analysis indicated an increase and variation in electron density, reaching a maximum value during April 22–24. A strong negative TEC anomaly in the crest of EIA (Equatorial Ionospheric Anomaly) occurred on April 21, and a strong positive anomaly was recorded on April 24, 2015. The behavior of VLF-LF waves along NWC-Bishkek and JJY-Varanasi paths has shown abnormal behavior during April 21–23, several days before the first, stronger earthquake. Our continuous satellite OLR analysis revealed this new strong anomaly on May 3, which was why we anticipated another major event in the area. On May 12, 2015, an M7.3 earthquake occurred. Our results show coherence between the appearance of these pre-earthquake transient’s effects in the atmosphere and ionosphere (with a short time-lag, from hours up to a few days) and the occurrence of the 2015 M7.8 and M7.3 events. The spatial characteristics of the pre-earthquake anomalies were associated with a large area but inside the preparation region estimated by Dobrovolsky-Bowman. The pre-earthquake nature of the signals in the atmosphere and ionosphere was revealed by simultaneous analysis of satellite, GPS/TEC, and VLF/LF and suggest that they follow a general temporal-spatial evolution pattern that has been seen in other large earthquakes worldwide.

A detailed study of the harmonic technique, which exploits the generation of harmonics resulting from excitation of the nonlinearity of the single Langmuir probe characteristic, is presented. The technique is used to measure electron temperature and its fluctuations in tokamak plasmas and the technical issues relevant to extending the technique to high bandwidth (200 kHz) are discussed. The technique has been implemented in a fast reciprocating probe in the TEXTOR tokamak, gaining the ability to study denser and hotter plasmas than previously possible. A corrected analytical expression is derived for the harmonic currents. Measurement of the probe current by inductive pickup is introduced to improve electrical isolation and bandwidth. The temperature profiles in the boundary plasma of TEXTOR have been measured with high spatial (∼2 mm) and temporal (200 kHz) resolution and compared to those obtained with a double probe. The exact expansion of the probe characteristic in terms of Bessel functions is compared to a computationally efficient power series. Various aspects of the interpretation of the measurement are discussed such as the influence of plasma potential and density fluctuations. The technique is well suited to study fast phenomena such as transient plasma discharges or turbulence and turbulent transport in plasmas.

We present the crustal resistivity structure of the Pamir and Southern Tian Shan orogenic belts at the northwestern promontory of the India-Asia collision zone. The magnetotelluric (MT) data were recorded along a roughly north-south trending, 350 km long corridor from the Pamir Plateau in southern Tajikistan across the Pamir frontal ranges, the Alai Valley and the southwestern Tian Shan to Osh in the Kyrgyz part of the Fergana Basin. In total, we measured at 178 sites, whereof 26 combine broad band and long period recordings. One of the most intriguing features of the 2-D and 3-D inversion results is a laterally extended zone of high electrical conductivity below the Pamir Plateau, with resistivities below 1 m, starting at a depth of 10-15 km. The high conductivity can be explained with the presence of partially molten rocks at middle to lower crustal levels, possibly related to ongoing migmatization and/or middle/lower crustal flow underneath the Southern Pamir. This interpretation is consistent with a low velocity zone found from local earthquake tomography, relatively high v p /v s ratios, elevated surface heat flow, and thermomechanical modelling suggesting that melting temperatures are reached in the felsic middle crust. In the upper crust of the Pamir and Tian Shan, the Palaeozoic-Mesozoic suture zones appear as electrically conductive, whereas the compact metamorphic rocks of the Muskol-Shatput Dome of the Central Pamir are highly resistive. The intra-montane basin of the Alai Valley-sandwiched between the Pamir and Tian Shan-exhibits a generally conductive upper crust that bifurcates into two conductors at depth. One of them connects to the active Main Pamir Thrust, which is absorbing most of today's convergence between the Pamir and the Tian Shan. Several deeper zones of high conductivity in the middle and lower crust of Central and Northern Pamir likely record fluid release due to metamorphism associated with active continental subduction/delamination.

Abstract The implications of recent seismological and resistivity data for the geometry and orientations of neotectonic faults are generally consistent with the morphotectonic model of Gorny Altai as an area of crustal failure at the junction of two relatively stable blocks. The model predicts motions under general NW compression mainly on right-lateral strike-slip faults accompanied by systems of pinnate reverse and extensional faults. The locations and mechanisms of aftershocks that followed the 2003 Chuya earthquake (Gorny Altai) indicate long seismic activity generated by a neotectonic NW right-lateral strike-slip fault which separates the North Chuya and South Chuya ranges from the Kurai-Chuya system of intermontane basins. The plane of the northwestern termination of the active fault zone dips in the SE direction, beneath the ranges, at about 70°. MT data show two types of conductors that reach the surface, namely, nearly vertical zones along neotectonic faults between the blocks not involved into vertical motion, according to morphotectonic evidence, and inclined zones between the uplifted (subsided) blocks. We interpret the former as strike-slip faults and the latter as reverse or reverse oblique faults, which always dip beneath the uplifted blocks and record the general compressional setting.

The structure of the vector field of present-day horizontal velocities on the Earth’s surface is investigated with the use of the territory covered by the Central-Asian GPS network as an example. A method of identification of groups of GPS points (statistically rigid clusters), for which the rate of change in distances between them is virtually zero, is proposed and realized. Sites of the Earth’s surface (regions) containing such groups of GPS points, within the required measurement accuracy, move in the horizontal plane as two-dimensional rigid bodies. The clustering algorithm, which is based on the Student t statistics in determining the so-called statistical “sample cost,” is developed and carefully tested. The results of identification of regions take into account possible random errors in velocity measurements and do not depend on the chosen frame of reference. The method of identification of regions is sufficiently stable with respect to variations in the number of GPS sites used for clustering. Of all the sites of the Central-Asian GPS network, 323 points were selected for clustering. These sites were measured from 3 to 11 times over an 11-year interval of observations (1995–2005). The estimates of errors of velocity measurement for these sites must not exceed 1.0 mm/yr. As a result, 29 statistically rigid clusters, containing from 3 to 17 GPS sites, were identified, and the kinematic regimes of motion of regions corresponding to these clusters were determined with respect to the stable part of Eurasia. With the general direction of the translatory motion of regions toward the north, the majority of them rotate counterclockwise. Nearly one third of GPS sites do not participate in the formation of clusters; these points fall into the interregional space (IRS), which is characterized by increased strain rates. The IRS structure is partitioned into zones with four directions, of which two directions virtually coincide with directions of the principal axes of the regional strain rate tensor, and the two other directions are oriented diagonally to the principal axes. The axis of the maximum rate of the regional shortening has a north-northwestern orientation. It is in this direction (mainly along the IRS) that the crust’s contraction takes place. There is no spatial correlation between IRS zones and geological faults; however, their angular distributions and the directions of strike-slips on them are interrelated. The resulting patterns of regional motions and IRS deformations consistently reflect the dynamic action of the Indian plate on the territory under investigation.

Abstract Based on seismic data from existing seismic networks in Kyrgyzstan, we have constructed new crustal models of seismic velocity and attenuation for P and S wave s beneath the Kyrgyz Tien Shan. With data from more than 6,000 events recorded by the international KNET network, the most detailed structures were detected in the central‐northern part of the study region, where the Kazakh Shield collides with the northern Tien Shan. The independently computed 3‐D distributions of P and S wave attenuation show features that are consistent with the main structural elements. The high‐attenuation areas correspond to folded areas of the northern Tien Shan, whereas the partitions of the stable Kazakh Shield and the Issyk Kul block match with the low‐attenuation areas. The velocity model reveals some structures that help to determine the details of the collision processes in the northern Tien Shan. In the upper crust, we observe the alternation of several higher‐ and lower‐velocity anomalies that likely represent the interaction of brittle and ductile crustal layers of the collided Kazakh and Tien Shan plates. In deeper sections, both P and S wave velocity models show a prominent low‐velocity anomaly just beneath the northern boundary of Tien Shan. We propose that this anomaly represents an anomalous crustal thickening at the point of underthrusting of the Kazakh Plate beneath Tien Shan.

Abstract High-density array MT soundings of the crust in the seismically active northern Tien Shan were performed using Phoenix MTU-5 stations in the Bishkek Geodynamic Polygon, at the junction of the Chu basin and the Kyrgyz Range. The MT transfer functions were determined to an accuracy of 1–2% (amplitude) and about 0.5–0.8 deg (phase) in most of 145 soundings. Preliminary analysis of the collected data aimed at estimating the geoelectrical dimensionality. The Bahr decomposition analysis indicated the presence of local 3D structures in the crust of the area superposed on the regional 2D structure.

Abstract Magnetotelluric soundings (MTS) in the Kyrgyz Tien Shan along 74° and 76° E profiles reveal conductors in the crust which delineate the boundaries of the At-Bashi accretionary-collisional zone and the Issyk-Kul microcontinent. Correlated to earthquake converted-wave patterns (vP) along the MANAS profile collected in 2007, the geoelectric model for the At-Bashi zone lends support to the hypothesis that the position and dip of large thrust sheets, as well as the way and direction of exhumation of eclogites in this zone, are similar to those in Northwest China. Petrological analysis, geothermobarometry, and elastic P-wave velocities measured in laboratory on lower-crust and upper-mantle xenoliths indicate that at the time when the xenoliths were dragged to the surface about 70 Ma ago, the Moho was 20 km shallower than now (35 km against 55 km) and the heat flux was 20 mW/m2 higher (80 against 60 mW/m2).

Abstract This study continues the work by Mikhail Gzovsky on geological (tectonophysical) criteria for seismic risk. It is suggested to perform seismic-risk zoning according to parameters of normal and shear stresses on fault planes converted from results of tectonophysical stress reconstructions. The approach requires the knowledge of both dip and strike of the respective fault segments. Slip geometry is estimated from stress tensor, assuming that it is directed along shear stress. The suggested approach is applied to faults in the northern Tien Shan, and the current stress parameters are reconstructed using source mechanisms of catalogued earthquakes recorded by the KNET seismological network of the RAS Science Station in Bishkek. Stress modeling is performed by the method of cataclastic analysis providing constraints on stress ellipsoids, as well as on relations between the spherical and deviatoric components of the stress tensor. Plotted on the Mohr diagram, the fault stress points allow estimating whether the respective fault segments are close to the critical state (brittle failure). The suggested seismic-risk zoning of faults in the northern Tien Shan reveals up to 25 km long hazardous fault segments.

The results of longstanding GPS measurements in the northwestern part of Central Asia are discussed. These results impose certain constraints for modeling of intraplate tectonic processes. In the territory covered by observations, the velocity vectors of recent motions of the Earth’s surface relative to the stable portion of Eurasia decrease northward. The plane field of velocities, which rules out the development of extension zones, indicates the impossibility of the mountain building driven by ascending mantle flows beneath the lithosphere of these regions. The nonuniform spatial distribution of the motions is suggestive of the discrete character of the Earth’s crust and its deformation. The crust is brittle, at least in its upper part, and capable of breaking into blocks. The blocks, which move at different velocities, interact with one another and change their original orientation and position, while experiencing independent deformations. This phenomenon has been exemplified in the Tarim Block and the Tien Shan. Within the limits of the constraints imposed by the GPS measurements, the mechanism of intracontinental mountain building related to the lateral flow of asthenospheric material and to the drag of the overlying lithospheric layers is discussed. This mechanism springs from Argand’s ideas [2, 29] and the plate tectonic concept [10, 23]. The upper-mantle convective flow in the direction of the Indian Plate’s motion was the main cause of the crustal deformation. The detachment of the lithospheric mantle from the Indian Plate approximately 25 Ma ago and its subduction beneath the Himalayas and Tibet, along with simultaneous ascent of the remaining crust and uplift of the Tibetan Plateau, allowed the mantle flow to spread far northward beneath the Asian continent. This process is accompanied by consecutive separation and sinking of the cooling asthenospheric material over the entire area from the Himalayas to Siberia as the subcrustal material cools. As a result, the flow velocity decreases, the roof of the active flow plunges, and the lithosphere becomes thicker. The motion and deformation of the lithospheric layers dragged by deep flow cannot follow the asthenospheric flow strictly, owing to the rigidity of the layers. Therefore, a difference of tangential velocities originates between the flow and the lithosphere, thus giving rise to horizontal shear stresses. These stresses affect the overlying lithospheric layers, including the crustal ones, and bring about their drag and tectonic delamination. Simultaneously, the decreasing velocity in the direction of the mantle flow results in bending of the lithospheric layers that is accompanied by local warping of the crust and its stacking and fragmentation into blocks. The different velocities of block motions lead to their mechanical interactions. This scenario of intracontinental mountain building allows an explanation of the many specific features of tectonic processes and orogeny in within-plate mountainous regions.

The Talas Fergana/Karatau Fault, is a major tectonic boundary separating the Kazakh-Turan domain to the west from the Tian Shan domain to the east. During the Jurassic, movements along the fault led to the opening of several basins. Still, the Mesozoic kinematics of the fault and the geodynamic mechanism that led to the opening of these basins are largely unconstrained. Located at its southwestern termination, the Yarkand-Fergana Basin is certainly the best exposed and however still poorly understood. In this study, we provide new sedimentological description of the Jurassic series from the northern part of the Yarkand-Fergana Basin as well as new palynological data. Following a Middle–Late Triassic period dominated by regional erosion, the onset of sedimentation in the Yarkand-Fergana Basin occurred during the Sinemurian(?)–Pliensbachian. The basin opened as a half graben controlled by the Talas Fergana/Karatau Fault and separated from the Fergana Basin by basement highs. Extension persisted during the late Pliensbachian–Middle Jurassic, leading to a general widening of the Yarkand-Fergana Basin. Finally, Late Jurassic–Early Cretaceous renewed tectonic activity in the area led to the inversion of the north Yarkand-Fergana Basin. The Early to Middle Jurassic timing of development of the Yarkand-Fergana Basin suggests that the coeval movements along the Talas Fergana/Karatau Fault are not associated to the collision of the Qiangtang block along the southern margin of Eurasia. We favor the hypothesis of an opening controlled by transtension related to far field effects of back-arc extension along the Neo-Tethys subduction zone to the west.

Abstract In this study we present a detailed analysis of natural stresses in the Northern Tien Shan crust averaged in a window of 10–15 km obtained from seismological data of the local KNET network. The transformation of focal mechanism data into the parameters of the stress tensor was based on the method of cataclastic analysis of rupture displacement elaborated by Yu.L. Rebetsky (Institute of Physic of the Earth, Moscow). The results, including the orientation of the principal stress axes and the reduced stresses, are presented for four depth layers. It was shown that the central part of the study area is dominated by horizontal compression, while multiple domains characterized by horizontal shear and superimposed compression or pure horizontal shear are also present (uppermost layers in the eastern part of the Chuya depression, Suusamyr depression and adjoining regions, in the central part of the Kyrgyz Range). There are also several large domains of high and low effective confining pressure, which defines the corresponding deviator stress, according to the Coulomb–Mohr law. It was shown that relatively strong earthquakes are correlated with zones with low levels of effective pressure where the ruptures are characterized by lower resistance to brittle fracturing, i.e., Coulomb friction stresses. It was also shown that a distinct segment of the ~ 60 km E–W striking fault on the northern slope of the Kyrgyz Range generates a uniform distribution of stresses, corresponding to a dextral slip along of its edges.

The behavior of the variations in the crustal electrical conductivity in a wide range of periods is studied from the data of magnetotelluric soundings (MTS) during the Kambarata experiment (a strong industrial explosion to construct the blast-fill dam on the Naryn river), as well as at Aksu, a stationary geophysical monitoring point. The concept of the interrelation between the stress-strain state of the medium and the change in the apparent electrical resistivity, which is based on the idea of the redistribution of mineralized solutions between the crack networks, is confirmed experimentally. A procedure of azimuthal monitoring is developed, which allowed us not only to identify the anomalous changes in the module and phase of apparent resistivity but also to establish the directions of their maximum increases and decreases (the axes of compression and tension). For 34 points of deep MTS in the territory of Central Tien Shan, the depth intervals in the upper crust that are most sensitive to the changes in the stress-strain state of the medium are established. The variations in the electrical conductivity are compared with the solar-lunar tidal impacts. It is shown that by analyzing the recorded time series, it is possible to recognize the characteristic signs of the changes in the stress-strain state of the medium that are caused by seismic events.

During the past two decades, at the Research station (Bishkek) more than a hundred magnetotelluric and magnetovariational soundings were carried out on the Naryn geotraverse that intersects the Tien Shan region from Lake Balkhash to the Tarim Basin along the 76° E meridian. Integration and complex interpretation of the data of these soundings with improved resolution and reliability of the geoelectric model of the Central Tien Shan section became an urgent challenge. Our paper presents a complex of methods for processing and invariant analysis of the electromagnetic data developed for the solution of this problem. Its application allowed us to validate the choice of the 2D interpretation model for the Naryn Line and to form the adequate ensemble of the data to be inverted. The developed approaches will also be useful in similar studies in the other mountain regions.

Applications of features of pseudonoise signals (PNS) in systems of active geoelectric exploration are investigated by mathematical simulation and experimentally under actual field conditions using a special measuring complex. A significant gain in the output signal/noise ratio (of about 100 times) is obtained with a measuring system using PNS compared with conventional electric exploration systems because of the use of the correlation processing of special PNS. It ensures a high accuracy of the transient signal recording, especially for large time periods. As a result, it becomes possible to measure electrical parameters of the Earth’s crust in a wider range of depths at a lower power of the sounding device.

Abstract The basement surface and sediments of the Kochkor basin have been studied by structural geological and geophysical methods. The work included high-resolution mapping of the southern basin margin, with a focus on Cenozoic basin stratigraphy, structural unconformities, basement/sediment contacts, and latest deformation (folds, fractures, and faults). Magnetotelluric (MT) soundings provided insights into basin and basement structure, especially important in the poorly exposed eastern flank of the basin margin. The sections across the southern margin of the basin based on geological and geophysical data reveal deformation in both the basement and the sediments. Deformation in sediments shows up as folding, conformal peneplain surfaces, large faults, low-angle detachment faults, and related thrust-fold belts. Thrusting in sediments is inferred to result from cataclastic flow and mass redistribution in disintegrated basement granites. With this mechanism, the total amount of thrusting in the central part of the basin can exceed the convergence of the basin sides.