Geoelectric section of the Central Tien Shan: Sequential inversion of the magnetovariational and magnetotelluric data along the Naryn Line

The paper presents the results of 2D inversion of deep magnetotelluric (MT) and magnetovariational (MV) soundings along the Naryn Line. The method of partial (sequential) inversions is used. According to this method, at the first stage, magnetovariation responses are used for the localization of deep anomalies of electrical conductivity, and then the magnetotelluric sounding data are invoked to refine the structure of the host medium and the structural details in the upper part of the section. It is shown that this approach enables one to estimate the informativeness of separate components of the electromagnetic field, to reduce the distorting influence of the near-surface geoelectric inhomogeneities, and to increase the stability of the final solution of the inverse problem.     

Geoelectric section of the Central Tien Shan: Analysis of magnetotelluric and magnetovariational responses along the Naryn geotraverse

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.     

The pattern of the paleo- and present-day stresses of Northern Tien Shan

The study of neotectonic stresses of the North Tien Shan is carried out within the northern slopes of the Kyrgyz Ridge bordering the Chyua submontane depression. The purpose of the study was to reconstruct the neotectonic stresses from the geological indicators and to compare the obtained results with the data on the present-day stress state of this region. The work is based on the analysis of the field in situ data acquired in the expeditions in 2009 and 2011. For the first time for the Northern Tien Shan region, the general (averaged) neotectonic stresses, which are distinct between the uplifts and depressions, are reconstructed. The deformation of the positive topographic features at the recent tectonic stage takes place in the conditions of thrust faulting with the meriodional horizontal compression axis and subvertical tension axis, while deformation of the negative landforms is dominated by normal faulting with the vertical compression axis and northnortheast trending subhorizontal tension axis. Based on the field data, separate sites with different types of local stress tensors, which are determined by the Lode-Nadai parameter, are revealed. The neotectonic geodynamical regime within the depressions and uplifts and the variations in the type of the stress tensor agree with the characteristics of the present-day stress state reconstructed from seismological data.     

The state of the lithosphere in the junction zone of Tarim and Tien Shan according to the petrological interpretation of the magnetotelluric data

The geological-geophysical and petrological-geochemical studies of the Earth’s crust and upper mantle are combined to estimate the state of the lithosphere at the junction zone of Tarim and Tien Shan. The laboratory measurements of electric conductivity in the rocks sampled from the upper mantle and lower crust considered against the geoelectrical and thermal models revealed lherzolite, granulite, and eclogite massifs in the deep section of the Tarim and Tien Shan junction zone. The experimental results suggest that the crustal thickness in the southern Tien Shan attained 35–40 km 70 Ma ago.     

Titanomagnetites and ilmenites from the Early Cenozoic basalts and limburgites of the Northern Tien Shan

Based on X-ray spectrometry and thermomagnetic analysis, the chemical composition is studied and the Curie points are determined for the Early Cenozoic basalts and limburgites of the Northern Tien Shan. The microprobe analysis used in combination with the thermomagnetic analysis unambiguously identified a series of homogeneous primarily magmatic titanomagnetites in the studied samples against the broad variety of grains of titanomagnetite that underwent single-phase heterophase oxidation, which are often undetectable by a microprobe alone. The titanium content or TiO₂/FeO ratio in titanomagnetites and, correspondingly, the Curie points reflect the depth of the most recent equilibrium state of the magma, i.e., the depth of the magma sources. According to the dependence of the content of primarily magmatic titanomagnetite on the source depth, the latest equilibrium state of the magmatic melt of the Northern Tien Shan occurred at a depth of 40 ± 5 km. The obtained results agree with the reduction in the seismic velocities by a few percent below the Moho revealed by seismic tomography.     

Directional secular variation of the geomagnetic field in Late Devonian volcanics of the Northern Tien Shan

We have paleomagnetically studied sixty andesite-basalt lava flows of the Late Devonian (Frasnian) in the Northern Tien Shan. Stepwise thermal demagnetization revealed a high-temperature dualpolarity magnetization in most of the samples. The primary origin of this component was established by the positive reversal and conglomerate tests. We found a clear difference in angular dispersion of this component between the lower and upper portions of the section. In the former, the dispersion is close to the predictions of various models of geomagnetic secular variation, while the scatter in the top part of the sequence is smaller by a factor of four. We conclude that this distinction is most likely due to the geomagnetic field behavior and hypothesize that the magnitude of secular variation could vary severalfold over time intervals of 10⁵−10⁶ years. This is in sharp contrast with other models of secular variation, where this magnitude has been assumed to be rather time-independent (for a given latitude). We discuss probable implications and alternative interpretations of the observed data.     

Crust and mantle of the Tien Shan from data of the receiver function tomography

A 3-D velocity model of the Tien Shan crust and upper mantle is constructed through the inversion of the receiver functions of P and S waves together with teleseismic traveltime anomalies at nearly 40 local seismic stations. It is found that in the vast central region, where no strong earthquakes have been known over the past century, the S wave velocity at depths of 10–35 km is lower than in adjacent regions by up to 10%. These data are evidence for mechanical weakness of the crust preventing the accumulation of elastic energy. Apparently, the lower velocity and the weakness of the crust are due to the presence of water. The weakness of the crust is one of the possible reasons for the strain localization responsible for the formation of the present Tien Shan but can also be due in part to the young orogenesis. The crustal thickness is largest (about 60 km) in the Tarim-Tien Shan junction zone. The crust-mantle boundary in this region descends by a jump as a result of an increase in the lower crust thickness. This is probably due to the underthrusting of the Tien Shan by the Tarim lithosphere. This causes the mechanically weak lower crust of the Tarim to delaminate and accumulate in nearly the same way as an accretionary prism during the subduction of oceanic lithosphere. In the upper mantle, the analysis has revealed a low velocity anomaly, apparently related to basaltic outflows of the Upper Cretaceous-Early Paleogene. The Cenozoic Bachu uplift in the northern Tarim depression is also associated with the low velocity anomaly. The Naryn depression is characterized by a high velocity in the upper mantle and can be interpreted as a fragment of an ancient platform.     

Radon monitoring in the Northern Tien Shan with application to the process of tectonic earthquake nucleation

Preliminary results of space-time radon monitoring in the Northern Tien Shan are presented. The study area was chosen on the basis of the known seismic events. A basic model of the observed effects is determined. The radon exhalation is shown to depend nonlinearly on variations in the stress state of the medium and the recording mode of the radon field. A geodynamic model of earthquake nucleation is presented and the expected effects in the radon field are examined. Results of measuring radon field variations before earthquakes are presented. It is shown that noticeable and characteristic changes in the radon field are observed, respectively, 90–100 and 3–5 days before a seismic event. Constraints are gained on the response of the radon field within a sedimentary rock mass to the passage of waves generated by distant earthquakes. The data presented in the paper show that radon monitoring can be successfully used for studying the process of earthquake nucleation. The model of tectonic earthquake nucleation proposed in the paper uses data of multidisciplinary geophysical monitoring (including radon monitoring) to obtain a deterministic indicator of earthquake nucleation.     

Palinspastic restorations for the south Tien Shan mountains and Tajik depression based on paleomagnetic data

In terms of plate tectonic concepts, horizontal movement are responsible for the formation of fold belts, and modern structures are interpreted by the position of the belt relative to the zones of plate collision. Paleomagnetic data allow the reconstruction of the former positions of these zones relative to a geographic grid, their pre-deformation patterns being important to the solution of problems in fold belt tectonics (Van der Voo et al., 1980; Khramov et al., 1982).Fold structures within the south-western spurs of the Tien Shan Mountains and the intermontane basins separating them were the first to be studied (Rzhevsky, 1966). Paleomagnetic studies aimed at understanding the tectonic setting of the region have been undertaken during the ensuing years and also cover adjacent areas of Central Asia (Khramov et al., 1982).In this paper an attempt is made to summarize some paleomagnetic results, produced palinspastic reconstructions of the region and produce a framework to help understand the formation of one of the crucial parts of the Alpine-Himalayan belt.     

Seismotectonic deformations and recent tectonics of the Tien Shan

Seismotectonic strains (STSs) are calculated on the basis of the catalog of focal mechanisms of earthquakes including more than 5000 events compiled at the Institute of Seismology, National Academy of Sciences of the Kyrgyz Republic, in order to investigate deformation processes in the Tien Shan (38°–44°N, 68°–80°E). A modern approach to STS classification based on the identification of 11 deformation settings (including four main, two extreme, and five transitional settings) is applied for constructing STS maps. Areal distributions of the Lode-Nadai coefficient, vertical component, and angle of the stress-state type are obtained. The results of the calculations are verified by comparing them with STS calculations using focal mechanisms of 116 strong earthquakes from the centroid moment tensor catalog of Harvard University that occurred in the studied region in 1976–2003.     

Geodynamic processes in seismically active areas of the Tien Shan from monitoring data with the use of nuclear explosions

Recent geodynamic processes in the Tien Shan region are studied by the analysis of time series of effective velocities and traveltime delays relative to the IASPEI-91 traveltime curve of the weakly refracted wave P _n from nuclear explosions at the Semipalatinsk test site over the period of 1968–1989. The time series were constructed for 10 seismic stations located at distances of 800–1200 km from the test site in the regions of the Northern, Central, and Southern Tien Shan. The twenty-year period of observations at stations in the North Tien Shan showed a significant decrease in traveltime delays by 0.20–0.76 s, which corresponds to a 0.2–0.7% increase in seismic velocities. An opposite pattern is observed at stations of the Central and Southern Tien Shan: traveltime delays increased by 0.2–0.5 s and, accordingly, seismic velocities dropped by 0.2–0.5%. These results suggest the predominance of compression processes in the crust and upper mantle during the period of observations in Northern Tien Shan and extension processes in the Central and Southern Tien Shan. The series of velocities and traveltime residuals are characterized by the presence of rhythmic oscillations of various amplitudes and periods against a linear trend. A correlation between variations in kinematic parameters and yearly numbers of earthquakes is observed at all stations. Diagrams of the spectral time analysis reveal rhythms with periods of 2–3 and 5–7 yr. The data obtained in this study are consistent with results of studying the stress-strain state of the Tien Shan crust from focal mechanisms of earthquakes and the velocities of recent crustal movements from GPS data. It is found that the amplitude of variations in kinematic parameters of the P wave at stations located in seismically active regions (the Tien Shan, Kopet Dagh, the Caucasus, Altai, and Sayany) is two to five times higher compared to aseismic regions (the Russian and Kazakh plains).     

Strong Seismic Activity of the Terskey Ala-Too Range Adyrs,Northern Tien Shan,in the Holocene from Radiocarbon Analysis Data

Izvestiya, Physics of the Solid Earth - Abstract—We studied historical and paleoseismic deformations in the southern Issyk-Kul depression—on the territory of adyrs (piedmonts) of the...     

Stress drop in the sources of intermediate-magnitude earthquakes in northern Tien Shan

The paper is devoted to estimating the dynamical parameters of 14 earthquakes with intermediate magnitudes (energy class 11 to 14), which occurred in the Northern Tien Shan. For obtaining the estimates of these parameters, including the stress drop, which could be then applied in crustal stress reconstruction by the technique suggested by Yu.L. Rebetsky (Schmidt Institute of Physics of the Earth, Russian Academy of Sciences), we have improved the algorithms and programs for calculating the spectra of the seismograms. The updated products allow for the site responses and spectral transformations during the propagation of seismic waves through the medium (the effect of finite Q-factor). By applying the new approach to the analysis of seismograms recorded by the seismic KNET network, we calculated the radii of the sources (Brune radius), scalar seismic moment, and stress drop (release) for the studied 14 earthquakes. The analysis revealed a scatter in the source radii and stress drop even among the earthquakes that have almost identical energy classes. The stress drop by different earthquakes ranges from one to 75 bar. We have also determined the focal mechanisms and stress regime of the Earth’s crust. It is worth noting that during the considered period, strong seismic events with energy class above 14 were absent within the segment covered by the KNET stations.     

Density Structure,Isostatic Balance and Tectonic Models of the Central Tien Shan

A new combined satellite-terrestrial model of the gravity field is used together with seismic data for construction of a density model of the lithosphere of the Central Tien Shan and for estimation of its isostatic balance. The Tien Shan is one of the most active intraplate orogens in the world, located about 1,500 km north of the convergence between Indian and Eurasian plate, and surrounded by stable Kazakh platform to the north and the Tarim block to the south. Although this area was extensively studied during recent decades, several principal problems, related to its structure and tectonics, remain unsolved up to now: (1) various geodynamic scenarios have been discussed so far to explain tectonic evolution, such as direct “crustal shortening,” intracontinental subduction and some others, but no definite evidence for any of them has been found. (2) Still, it is not clear why Tien Shan grows so far from the plate boundary at the Himalayan collision zone. Gravity modeling can provide valuable constraints to resolve these questions. The results of this study show that: (1) there exists a very strong deflection of the Tien Shan lithosphere from isostatic equilibrium. At the same time, the patterns of the isostatic anomalies are very different in the Western and Central Tien Shan. The latter one is characterized by much stronger variations. The best fit of the modeling results is found for the model according to which the Tarim plate partially underthrusts the Central Tien Shan; (2) negative density anomalies in the upper mantle under the central block possibly relate to magmatic underplating during the initial stage of the tectonic evolution. Therefore, the weak lithosphere could be the factor that initiates mountain building far away from the collision zone. Alternatively, this might be a gap after detachment of the eclogised lower crust and lithospheric lid, which is filled with the hot asthenospheric material.     

A study of static shift removal from magnetotelluric data

Near-surface inhomogeneities can distort magnetotelluric (MT) data, thereby limiting interpretational accuracy. In the specific case of near-surface galvanic distortion the resulting frequency independent shift in apparent resistivity sounding curves can be most readily identified in data which are predominantly one-dimensional (1 D). We here consider data from a detailed grid survey which are thoroughly contaminated by parallel offsets. The data were collected over a limited portion of the basalt-covered Paraná basin in Brazil. The data appear largely 1 D over a three decade bandwidth down to a low frequency limit of about 0.1 Hz. A realistic conceptual model consists of a variable thin-sheet over a basin in which the layer parameters vary sufficiently slowly (laterally) that a 1 D interpretation of the static corrected data is valid down to some maximum depth (in this case basement). We consider the performance of three methods which attempt to remove static offsets and provide assessments of the configuration of the geoelectric units of the basin. The three methods consist of curve shifting, statistical/spatial averaging, and the application of parameter constraints. The unambiguous removal of static offsets undoubtedly requires one or more independent constraints at each measurement location. In the absence of such control the three methods are necessarily statistical and each must supply a constraint that has least conflict with the data characteristics. Simple spatial averaging of our data proves effective but the degree of success is generally not known. Parametric constraints, applied as uniform layer resistivities, allow phase-only (non-distorted) data inversion. Inverse models obtained from this approach are found to provide accurate estimates of the depth to the main resistivity interface, the base of the basalt cover, in the vicinity of a control well.     

Strong Paleoearthquakes along the Aksuu Border Fault according to the Results of Dating the Offset Terrace Complex of the Chon-Aksuu River,Northern Tien Shan

The study and radiocarbon dating of the low alluvial terraces of the Chon-Aksuu River, in the Northern Issyk-Kul region, which were broken by the Kebin (Kemin) earthquake of 1911 (Ms = 8.2, Io = 10 to 11), are carried out. The obtained radiocarbon dated ages refer to the second half of the Holocene. Since that time, at least eight strong earthquakes took place along this (Chon-Aksuu) segment of the Aksuu border fault. Three seismic events, including the earthquake of 1911 occurred in the second millennium A.D. This outburst of seismic energy was preceded by two millennia of seismic quiescence, which set in after another pulse of seismic activation. The latter lasted for 1.5 millennia and included five strong earthquakes. The recurrence period of seismic events during the activations is 300–600 years. Hence, the seismic regime along the Chon–Aksuu segment of the Aksuu border fault in the second half of the Holocene was a succession of two seismic activations, each with a duration of 1.0–1.5 ka, which were separated by a 2-ka interval of seismic quiescence. Therefore, the absolute datings of the river terraces of different ages which have been broken by a seismogenic rupture can serve as a reliable source of information about the age of the strong earthquakes that occurred along the seismogenic fault.     

Solar forcing of the Northern hemisphere land air temperature: New data

It has previously been demonstrated that the mean land air temperature of the Northern hemisphere could adequately be associated with a long-term variation of solar activity as given by the length of the approximately 11-year solar cycle. Adding new temperature data for the 1990s and expected values for the next sunspot extrema we test whether the solar cycle length model is still adequate. We find that the residuals are now inconsistent with the pure solar model. We conclude that since around 1990 the type of Solar forcing that is described by the solar cycle length model no longer dominates the long-term variation of the Northern hemisphere land air temperature.     

Three-dimensional inversion of the single-profile magnetotelluric data

It is preferable to use the three-dimensional (3D) magnetotelluric inversion, which provides volumetric geoelectric models, to handle the array input data. However, the soundings are frequently conducted on the single profiles or on the profiles that are considerably spaced apart from each other. We explore the possibilities of the 3D inversion of such data by the example of a three-layer model containing three local inhomogeneities. We previously showed that the simple processing of the data and their 1D or 2D inversion enable reconstructing the background cross section and locating all the three inhomogeneities. In the present paper, we use this information for constructing several versions of the starting model and carrying out the smoothing 3D inversion of the data. The experiments show that if the background cross section is incorporated into the starting model, the final model provided by the inversion closely reproduces the real distribution of all geoelectric parameters. At the same time, if the starting model that hosts the inhomogeneities has the form of a homogeneous half-space, the inversion is not able to reconstruct an adequate final model.

     

Three-dimensional inversion of the single-profile magnetotelluric data

It is preferable to use the three-dimensional (3D) magnetotelluric inversion, which provides volumetric geoelectric models, to handle the array input data. However, the soundings are frequently conducted on the single profiles or on the profiles that are considerably spaced apart from each other. We explore the possibilities of the 3D inversion of such data by the example of a three-layer model containing three local inhomogeneities. We previously showed that the simple processing of the data and their 1D or 2D inversion enable reconstructing the background cross section and locating all the three inhomogeneities. In the present paper, we use this information for constructing several versions of the starting model and carrying out the smoothing 3D inversion of the data. The experiments show that if the background cross section is incorporated into the starting model, the final model provided by the inversion closely reproduces the real distribution of all geoelectric parameters. At the same time, if the starting model that hosts the inhomogeneities has the form of a homogeneous half-space, the inversion is not able to reconstruct an adequate final model.