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.     

Methodology of the indirect temperature estimation basing on magnetotelluric data: Northern Tien Shan case study

Electrical conductivity of rocks being closely related to their temperature could serve as a proxy parameter to be used for indirect temperature estimation from the surface electromagnetic (EM) data. Studies are carried out aimed at estimating the feasibility of indirect temperature estimation in the geologically complicated areas of the Earth crust from the electromagnetic data collected at the surface. Basing on the neuronet analysis of magnetotelluric (MT) and temperature data measured at the Bishkek geodynamical testing ground in the northern Tien Shan, optimal methodologies for calibration and application of indirect electromagnetic geothermometer are developed. It is shown that the temperature estimation my means of the EM geothermometer calibrated by 6–8 temperature logs results in 12% average relative error (instead of 30% achieved using only temperature logs). The availability of prior geological information about the region under study and preliminary analysis of the local heterogeneities' indicators determined from the available MT data make it possible rejection of inappropriate site locations that may, in turn, decrease average error to only 11%.The results of the electromagnetic temperature extrapolation in depth indicate that the extrapolation accuracy essentially depends on the ratio between the well length and the extrapolation depth. In particular, in extrapolation to a depth twice as large as the well depth the relative error is 5–6%, and in case of its threefold excess the error is around 20%. This result makes it possible to increase significantly the deepness of indirect temperature estimation in the Earth interior (in particular, for geothermal exploration) based on the available temperature logs.     

Joint three-dimensional inversion of magnetotelluric and magnetovariational data

The problem of quantitative three-dimensional interpretation of the magnetotelluric (MT) data ranks among the most difficult problems in electromagnetic (EM) geophysics. Our paper presents a new rigorous numerical method for MT inversion, based on the integral equations technique. An important feature of the proposed method is the calculation of the Frechet derivative with the aid of a quasi-analytical approximation with an inhomogeneous background. This approach simplifies the algorithm of inversion and requires only a single forward modeling on each iteration. We have also developed a method for a joint inversion of MT and magnetovariational (MV) data. We show in the present paper that the joint inversion of MT impedances and the Wiese-Parkinson vectors can automatically allow for the static shift in the observed data, which is caused by the geoelectric inhomogeneities contained in the near-surface layer.     

Experiments on the dynamic correction of magnetotelluric and magnetovariational data

Pathways of further development of the dynamic correction technique suggested by E. Fainberg are discussed. This technique allows the correction of magnetotelluric (MT) and magnetovariational (MV) data distorted by subsurface irregularities within a broad range of frequencies beyond the static shift interval. This paper addresses new algorithms for the dynamic correction of tensor MT and MV response functions. Moreover, experiments on the dynamic correction of MT and MV data calculated for models containing heterogeneities in sedimentary cover and infinite or half-infinite conductive crustal prism are described. Model experiments show that the dynamic correction can ensure sufficient accuracy of the input data for the reliable solution of the inverse problem of magnetotellurics.     

中天山及邻区S波分裂研究及其动力学意义

本文利用天山及其邻区布设的37个宽频带地震台站记录到的远震波形数据,分别采用最小能量法和旋转相关法对SKS和SKKS波震相进行了偏振分析,计算出了台站下方介质的S波分裂参数：快波的偏振方向（φ）和慢波延迟时间（δt）.本文研究结果表明：中天山内部大多数台站的各向异性快波方向呈NEE-SWW向,与天山走向平行,慢波时间延迟为0.4～1.7 s,这是塔里木、哈萨克斯坦的南北双向俯冲及其导致的天山地区岩石圈地幔南北向缩短变形的直接反映.本文研究发现中天山北部部分台站下方地震各向异性快波方向与慢波延时随方位角呈现规律性的变化,可能暗示该区上地幔各向异性不能仅用单层水平各向异性这一简单模式来解释.75°E以西的天山地区台站下方S波快波方向和延时具有强烈的横向变化,可能与研究区下方存在的小规模对流有关.中天山不同地段地震台站下方各向异性明显不同,进一步证实了天山地区构造变形的复杂性.     

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.     

Method of pseudotopography: A new approach to analysis of magnetovariational and magnetotelluric data

The method of pseudotopography is a new mode of representing magnetotelluric and magnetovariational data based on a 3-D generalization of geoelectric pseudosections widely used at the stage of qualitative interpretation. The paper considers a synthetic model whose layers, imitating the sedimentary cover, crust, and upper mantle, contain a poorly conductive and a few highly conductive prisms differing in strike. The pseudotopography of apparent resistivities demonstrates the static superposition effect: the effects of near-surface chaotic heterogeneities (geoelectric noise) together with the near-surface and crustal prisms are superimposed on the effect of the mantle prism, distorting the information on the deep structure of the Earth. However, the pseudotopographies of the Wiese-Parkinson matrix, horizontal magnetic tensor, phase tensor, and phases of the impedance tensor are free from the superposition effect: arising at definite frequencies, geoelectric noise and the effects of the shallower prisms decay with decreasing frequency, making the effects of deeper prisms recognizable. Thus, it is clearly demonstrated that magnetovariational and phase estimates, being independent of static distortion, can provide reliable information on the deep structure of the Earth and significantly enhance the effectiveness of magnetotellurics.     

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.     

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.     

天山构造带及邻区地壳各向异性

天山构造带位于中国大陆西北部,是典型的岩石圈陆内缩短造山带.本文利用新疆区域数字地震台网2009年1月至2014年12月的近场小地震波形资料,采用剪切波分裂分析对天山构造带及邻区的地壳各向异性特征进行研究,获得了研究区域内39个台站的快剪切波偏振方向和慢剪切波时间延迟.剪切波分裂参数的空间特征显示,研究区上地壳各向异性具有分区性,各向异性特征与局部构造、地壳介质变形和应力分布有关.天山构造带的快剪切波偏振呈现出两个优势方向的特点,第一优势方向大致平行于台站附近断裂和天山构造带的走向,与断裂构造和应力的综合影响有关,另一个优势方向反映了主压应力的直接作用.北天山山前断裂带东段的断裂弯折部位和南天山局部地区的剪切波分裂参数与东、西两侧不同,与准噶尔盆地、塔里木盆地的南北向挤压作用密切相关.快剪切波偏振优势方向的剧烈变化揭示,在准噶尔盆地和塔里木盆地双向挤压隆起的过程中,天山构造带产生了强烈的局部不均匀变形.塔里木盆地西侧快剪切波偏振具有两个优势方向,一个为NNE方向,与帕米尔高原受到印度-欧亚板块碰撞产生的北向挤压作用有关,另一个为NW方向,指出了塔里木盆地区域主压应力方向.准噶尔盆地北部也存在NE和NW两个快波偏振优势方向,主要与断裂的影响有关.天山构造带区域内的慢剪切波时间延迟总体上低于塔里木盆地西侧和准噶尔盆地北部,同时慢剪切波时间延迟的结果也进一步证实了天山构造带的局部强烈变形.

     

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.     

Evidence of a changing size–frequency distribution of landslides in the Kyrgyz Tien Shan,Central Asia

There is a strong possibility that environmental change (whether climate or land use) will be manifest as changes in the size–frequency distribution of landslides. Here, evidence is presented for this from western Kyrgyzstan, Central Asia. Remote sensing and spatial analysis have been applied to map mass movements in the central part of the Maily‐Say Valley and to detect recent landslide activations. The evolution of landslide activity over the past 50 years has been analysed on the basis of pre‐existing landslide maps and new analyses of aerial photographs as well as Quickbird images. Five inventories were produced for the years 1962 (based on the existing map of 1962 and aerial photographs of 1962), 1984 (based on the existing map of 1977 and aerial photographs of 1984), 1996 (based on aerial photographs of 1996), 2002 (based on the existing map of 2003 and Quickbird imagery of 2002) and 2007 (based on Quickbird imagery of 2007). The geomorphologic features contained in the catalogues represent the landslide bodies observed from remote imagery of the corresponding year. Mapped landslides are generally considered as the result of a series of slope failure events. Size–frequency analyses applied to the five landslide inventories show that both the number and size of unstable slopes increased from 1962 (162 objects) to 2007 (208 objects) and the power‐law exponent decreased over time. This changing power‐law exponent may indicate that landslide‐related hazards are increasing. This tendency is documented in more detail for two active landslide zones, one in the main valley and one located to the west of it. Landslide detection methods were used to assist the evolution of slope instabilities. Choosing appropriate thresholds, the image subtraction method based on normalized difference vegetation index (NDVI) allowed accurate detection of new sliding activation in these two zones. This confirmed the results of the more extensive survey that there is a systematic shift in power law exponents and size–frequency distributions for Central Asian landslides. Copyright © 2011 John Wiley & Sons, Ltd.     

中天山地区的Pn波速度结构与各向异性

利用宽频带流动地震台阵GHENGIS和吉尔吉斯地震台网KNET记录的地震波走时数据,反演了中天山地区的Pn波速度结构和各向异性.结果表明,中天山上地幔顶部平均速度偏低,具有构造活动地区的特点和明显的横向非均匀性;中天山南部地幔上涌区的Pn波速度非常低,表明存在较高的热流活动.Pn波速度的变化与地震分布有着密切的对应关系：地震大都发生在中天山北部Pn波高速区上方,而南部的Pn波低速区上方几乎没有地震.这一现象说明地幔上涌引起高温极大地降低了岩石层地幔的强度,并以热传导的方式进入地壳使其失去地震破裂强度而发生韧性变形.中天山北部和南部的各向异性也存在一定的差异,南部各向异性的快波方向为近南北方向,与SKS波的各向异性特征基本一致,反映了地幔物质的迁移方向;北部各向异性的快波方向呈向南凸出的旋转趋势,估计与哈萨克地台南缘楚河盆地地壳块体向天山挤入造成应力场的改变和岩石层变形有关.     

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.     

Occurrence,evolution and ice content of ice-debris complexes in the Ak-Shiirak,Central Tien Shan revealed by geophysical and remotely-sensed investigations

Rock glaciers and large ice-debris complexes are common in many mountain ranges and are especially prominent in semi-arid mountains such as the Andes or the Tien Shan. These features contain a significant amount of ice but their occurrence and evolution are not well known. Here, we present an inventory of the ice-debris complexes for the Ak-Shiirak, Tien Shan's second largest glacierised massif, and a holistic methodology to investigate two characteristic and large ice-debris complexes in detail based on field investigations and remote sensing analysis using Sentinel-1 SAR data, 1964 Corona and recent high resolution stereo images. Overall, we found 74 rock glaciers and ice-debris complexes covering an area of 11.2 km² (3.2% of the glacier coverage) with a mean elevation of about 3950 m asl. Most of the complexes are located south-east of the main ridge of Ak-Shiirak. Ground penetrating radar (GPR) measurements reveal high ice content with the occurrence of massif debris-covered dead-ice bodies in the parts within the Little Ice Age glacier extent. These parts showed significant surface lowering, in some places exceeding 20 m between 1964 and 2015. The periglacial parts are characterised by complex rock glaciers of different ages. These rock glaciers could be remnants of debris-covered ice located in permafrost conditions. They show stable surface elevations with no or only very low surface movement. However, the characteristics of the fronts of most rock glacier parts indicate slight activity and elevation gains at the fronts slight advances. GPR data indicated less ice content and slanting layers which coincide with the ridges and furrows and could mainly be formed by glacier advances under permafrost conditions. Overall, the ice content is decreasing from the upper to the lower part of the ice-debris complexes. Hence, these complexes, and especially the glacier-affected parts, should be considered when assessing the hydrological impacts of climate change. © 2018 John Wiley & Sons, Ltd.     

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.     

Activation of seismicity in Central and South Asia after the Makran earthquakes: Possible acceleration of preparation of large seismic events in the Tien Shan region

Two zones of seismicity (ten events with M _w = 7.0–7.7) stretching from Makran and the Eastern Himalaya to the Central and EasternTien Shan, respectively, formed over 11 years after the great Makran earthquake of 1945 (M _w = 8.1). Two large earthquakes (M _w = 7.7) hit theMakran area in 2013. In addition, two zones of seismicity (M ≥ 5.0) occurred 1–2 years after theMakran earthquake in September 24, 2013, stretching in the north-northeastern and north-northwestern directions. Two large Nepal earthquakes struck the southern extremity of the “eastern” zone (April 25, 2015, M _w = 7.8 and May 12, 2015, M _w = 7.3), and the Pamir earthquake (December 7, 2015, M _w = 7.2) occurred near Sarez Lake eastw of the “western” zone. The available data indicate an increase in subhorizontal stresses in the region under study, which should accelerate the possible preparation of a series of large earthquakes, primarily in the area of the Central Tien Shan, between 70° and 79° E, where no large earthquakes (M _w ≥ 7.0) have occurred since 1992.     

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.     

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.     

Ring structures of seismicity in central Tien Shan and Dzungaria: Possible precursory processes of large earthquakes

It is shown that episodes of comparative seismic quiescence that lasted about 20–25 years in the areas of study alternated with intervals of sharply increased seismicity as series of large (M ≥ 6.9) earthquakes occurred during two to three decades. Since no M ≥ 6.6 earthquake has occurred in the area for as long as 21 years after the 1992 Susamyr event, middle-term prediction would require identification of zones of imminent large earthquakes. More reliable identification of such zones rests on data relating to inhomogeneities in the field of S-wave attenuation in the lithosphere, as well as on the characteristics of ring structures of seismicity. Such structures are formed as zones of seismic quiescence that are bounded by M ? Mth earthquake epicenters, where Mth is the threshold magnitude value. Correlative relationships were previously derived, lgL(Mw) and Mth(Mw), for events with different focal mechanisms (L is the length of the longer axis of a seismicity ring and Mw is the magnitude of the associated large earthquake). These relationships were used to estimate the Mw of large events that can occur in these ring structures. The greatest earthquake with Mw ? 7.5 is probably about to occur in southern Tien Shan, east of the 1949 Khait earthquake rupture. A smaller event (Mw ～ 7.0) can occur in the Kyrgyz Range area. Still smaller earthquakes probably have their precursory areas north and east of Lake Issyk-Kul, as well as in Dzungaria.