Deep Structure of the Lithosphere in the Central Tien Shan along the Son-Kul Magnetotelluric Sounding Profile

Leonov M.G., Morozov Y.A., Przhiyalgovskii E.S., Rybin A.K., Bakeev R.A., Lavrushina E.V., Stefanov Y.P.

The article provides geological data on the morphostructural differentiation of sedimentary basins and the results of tectonophysical and digital modeling reflecting the shape and possible mechanisms of this process. It is shown that morphostructural differentiation is the fundamental property of sedimentary basins, associated with interaction of rock masses in the basement–sedimentary cover system. The specific features of rock mass deformation in the basement and cover of sedimentary basins, as well as the kinematic conditions characterizing the morphostructural differentiation of sedimentary basins, are considered.

Leonov M.G., Morozov Y.A., Stephanov Y.P., Bakeev R.A.

Our study was focused on narrow linear zones that penetrate to different depths the crust and have complex infrastructure. Rocks in such zones are more intensively tectonically altered in comparison with the background. ‘Flower structures’ and ‘zones of concentrated deformation’ (ZCD) are the terms to describe these zones. The field study results combined with the data of tectonophysical and computational modeling data and supplemented by the literature analysis gave grounds for the following conclusions. In the experiments, as well as in nature, ZCDs show similar and, in some cases, identical morphological and infrastructural features and have similar stages of their evolution. A ZCD is mainly a reflection of the transpression setting. Its formation is accompanied by 3D plastic shear flow of matter and dilatancy of the deformed volume. A ZCD may be associated with the development of the ‘basement – cover’ system. It may also occur due to the intra-cover tectogenesis that does not influence the basement. Locations of ZCDs are spatially regular and predetermine the tectonic divisibility of the crust and lithosphere.

Przhiyalgovskii E.S., Lavrushina E.V., Batalev V.Y., Bataleva E.A., Leonov M.G., Rybin A.K.

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.

Rybin A.K., Bataleva E.A., Morozov Y.A., Leonov M.G., Batalev V.Y., Matyukov V.E., Zabinyakova O.B., Nelin V.O.

New magnetotelluric data were obtained for the Karabuk profile crossing the Naryn basin–Baibichetoo Ridge–Atbashi basin geodynamic system (Central Tien-Shan). The complex geological–geophysical cross section along the profile provides a good agreement between the surface tectonic structures and the deep geoelectric model. The electric conductivity anomalies revealed as subvertical conductors striking along the flanks of basins may be explained by the zones of dynamic influence of faults and cataclasis of granite.

Morozov Y.A., Smul’skaya A.I., Kulakovskiy A.L., Matveev M.A.

The results of an instrumental and analytical investigation of the products of mineral and textural transformations in the tectonic slickensides and fault gouge in the near-surface terrigenous sedimentary rocks (clays, arkose sandstones, shungites) which have undergone localized deformations in fault zones of presumably seismogenic origin are presented. Based on this, several peculiarities in the behavior of a dynamic slip in the upper transition horizon from aseismic to seismogenic mode of faulting in the Earth’s crust are elucidated. The changes in the mineral phase compositions of the fault facies against the protolith composition are estimated; the parameters of the temperature regime and thermal energy balance of deformational metamorphic reactions are determined. The probable causes of instability in the faults, the mechanisms of the loss of strength, the weakening and strengthening during seismogenic dynamic slip are considered. The role of tribochemical phenomena in the course of a rock’s transformation into a fault gouge and the related energy effects are discussed. An inventory of the possible energy costs on the processes of transforming material in dynamic slip zones is compiled.

Bataleva E.A., Przhiyalgovskii E.S., Batalev V.Y., Lavrushina E.V., Leonov M.G., Matyukov V.E., Rybin A.K.

Based on a complex study of the upper crust structure in the southern margin of Kochkor basin (Northern Tien Shan), including study of the structure of the Cenozoic sedimentary cover, the deep geoelectrical structure, the structural unconformities, and occurrences of recent deformations in the basement rocks, new geological–geophysical cross sections are constructed. The cross sections show both fault structures that penetrate the cover from the basement and flat interplate detachments with related fold-overthrust structures. The comparison of the cross sections has established the absence of common planes of fault extensions along the entire margin of the hollow, except for the zone where the margin and the hollow adjoin, which can be caused by the zones of dynamic influence of secondary faults, the zones of fracturing, and the zones of cataclasis of blockwise disintegrated granite massifs.

Rybin A.K., Leonov M.G., Przhiyalgovskii E.S., Batalev V.Y., Bataleva E.A., Bragin V.D., Morozov Y.A., Schelochkov G.G.

We studied the infrastructure of granite massifs of the Central Tien Shan and its correlation with the electric conductive layer of the upper crust, which made possible to reveal new peculiarities of the structure of the granite layer in the region and to clarify the nature of low resistivity layers.

Morozov Y.A., Leonov M.G., Alekseev D.V.

The geodynamic settings of the evolution of Cenozoic basins in the North and Middle Tien Shan and their Paleozoic framework have been estimated from a kinematic analysis and detailed structural geological mapping of key sites. Two stages of their development in different geomechanical settings are distinguished. It is suggested that in the late Oligocene, Miocene, and early Pliocene, the pull-apart basins developed under the setting of simple left-lateral shear or transtension. Presumably in the late Pliocene and Quaternary, this setting gave way to right-lateral transpression; extension in the basins was changed by compression with the formation of local fold-thrust structural elements. The reconstruction of geomechanical sections was tested by analog tectonophysical simulation. The natural and experimental structural assemblies and patterns reveal satisfactory convergence.

Batalev V.Y., Bataleva E.A., Egorova V.V., Matyukov V.E., Rybin A.K.

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).

Makarov V.I., Alekseev D.V., Batalev V.Y., Bataleva E.A., Belyaev I.V., Bragin V.D., Dergunov N.T., Efimova N.N., Leonov M.G., Munirova L.M., Pavlenkin A.D., Roecker S., Roslov Y.V., Rybin A.K., Shchelochkov G.G.

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.

Berdichevsky M.N., Dmitriev V.I.

Buslov M.M., De Grave J., Bataleva E.A., Batalev V.Y.

The complex crustal structure of the Tien Shan has a strong impact on the distribution of strain induced by the India–Eurasia collision, with intracontinental deformation in Eurasia’s interior as a distant effect. The northward propagation of the India–Eurasia deformation front is suggested by the rejuvenation of mountain ranges and intermittent intramontane basins. The Tien Shan basement is formed by the rigid, heterogeneous Precambrian blocks (microcontinents) of Tarim, Issyk-Kul (or Central Tien Shan) and Aktyuz-Boordin, surrounded by a ‘soft’ matrix of Paleozoic accretion–collision belts. The Kyrgyz Tien Shan Mountains are situated between the active structures of the Tarim Plate and the Pamir indenter (south), and the stable Kazakhstan Shield (north). Underplating by the Tarim Plate and thrusting by the Pamirs are responsible for the building of the Cenozoic Tien Shan, the reactivation of its inherited structural fabric and the tectonic layering of the upper lithosphere underlying the area. Large earthquakes ( M  > 6) delineate the northern and southern margins of the Issyk-Kul microcontinent, indicating that crustal heterogeneity influenced the location of active structures in the northern Kyrgyz Tien Shan.

Rodi W., Mackie R.L.

We investigate a new algorithm for computing regularized solutions of the 2-D magnetotelluric inverse problem. The algorithm employs a nonlinear conjugate gradients (NLCG) scheme to minimize an objective function that penalizes data residuals and second spatial derivatives of resistivity. We compare this algorithm theoretically and numerically to two previous algorithms for constructing such “minimum‐structure” models: the Gauss‐Newton method, which solves a sequence of linearized inverse problems and has been the standard approach to nonlinear inversion in geophysics, and an algorithm due to Mackie and Madden, which solves a sequence of linearized inverse problems incompletely using a (linear) conjugate gradients technique. Numerical experiments involving synthetic and field data indicate that the two algorithms based on conjugate gradients (NLCG and Mackie‐Madden) are more efficient than the Gauss‐Newton algorithm in terms of both computer memory requirements and CPU time needed to find accurate solutions to problems of realistic size. This owes largely to the fact that the conjugate gradients‐based algorithms avoid two computationally intensive tasks that are performed at each step of a Gauss‐Newton iteration: calculation of the full Jacobian matrix of the forward modeling operator, and complete solution of a linear system on the model space. The numerical tests also show that the Mackie‐Madden algorithm reduces the objective function more quickly than our new NLCG algorithm in the early stages of minimization, but NLCG is more effective in the later computations. To help understand these results, we describe the Mackie‐Madden and new NLCG algorithms in detail and couch each as a special case of a more general conjugate gradients scheme for nonlinear inversion.

Ma X., Huang Z.

Zabinyakova O., Bataleva E., Medved I.

The paper is dedicated to consider the approach of longitudinal electrical conductivity maps construction for Central Tien Shan region and to complete the comparison analysis of these maps with related seismic tomography data. The results of joint analysis of longitudinal electrical conductivity distribution and seismic tomography velocity models with respect to geological and geophysical senses allow to confirm the already known and to establish the new patterns of structure and geodynamics of the Tien Shan segment of Central Asia such as the manifestation of characteristic structural ensembles, reflection of the longitudinal and transverse segmentation of the Tien Shan, the existence within the framework of a “single” orogen of segments with different near-surface and deep structure and morphological expression. As well, the correlation between the velocity characteristics and geoelectric properties was found. This correlation allows to identify signs of reflection of fault structures in geophysical fields and to clarify the territorial and structural position of both visible and hidden faults. Additionally, in the paper, the geological interpretation of geophysical anomalies is considered.

Bataleva E.A., Matiukov V.E.

The paper presents the results of a comprehensive analysis of geological studies with data on the deep structure obtained by various geophysical methods for the Tien Shan territory. The areas of junction of different tectonic segments are characterized by anomalies of increased electrical conductivity, are clearly manifested in the magnetic and gravity fields and correspond to the zones of deep faults. Studying of these objects having high permeability and providing heat and mass flow as well as transport of fluids, is the main object of this work. In order to identify the boundaries of the largest tectonic fragments of the Tien Shan the geoelectric and velocity sections along the profiles intersecting the main tectonic elements of the Tien Shan were analyzed. Comparison of velocity anomalies in seismotomographic sections and objects of abnormally low electrical resistivity in geoelectric sections of the Earth’s crust indicates the existence of zones of increased fracturing or plasticity in the indicated intervals of the Earth’s crust, as well as the coincidence of waveguides and zones of increased conductivity. The distribution of seismic events clearly shows the zones of the Northern, Middle and Southern Tien Shan and the segmentation of the Western and Central Tien Shan.

Polyak B.G., Bataleva E.A., Rybin A.K.

Abstract The research shows new results of isotope-helium studies carried out in the Tien Shan - Naryn and Atbashi basins. Sampling of thermal mineral springs in the Eastern Tien Shan was carried out in order to identify traces of mantle emanations in fluids and to assess the degree of permeability of the Earth’s crust for the introduction of deep masses, which would make it possible to clarify the nature of the interaction of the crust and mantle in the zone of recent orogeny. Concentrations of helium isotopes were measured in gas and water samples from 6 thermal mineral springs in the Central Tien Shan. It was found that fluids from three sources contain an abnormally high amount of mantle helium. In the gases of the Narzan source, the ratio 3He/4He in the gas phase (597÷600) × 10−8 reaches the Central Asian maximum found earlier on the Fergana ridge in the Kyzyl-Beles source (510÷630) × 10−8. The obtained results support the ideas on the existence of two sublatitudinal positive helium isotope anomalies on the northern and southern flanks of the Eastern Tien Shan - in the zones of junction of the epiplatform neoorogene with more ancient tectonically stable structures.

Bataleva E.A., Nepeina K.S.

Abstract Based on the analysis of deep geophysical (geoelectric and seismic) models of the Central Tien Shan, structures with the morphology resembling the crown of palm trees or the shape of a flower were identified. Geoelectric models are considered along a series of regional profiles (75º, 76º, 76º 30’). The length of the profiles intersecting all the main tectonic structures of the Tien Shan ranges from 75 to 250 km. Particular attention was paid to those zones of concentrated deformation, where the tectonic regime combines the conditions of shear and lateral compression (transpression zones). The structure of the collisional - accretionary wedge of the Atbashi zone in the distribution of electrical and velocity characteristics of the geological section is considered. Geoelectric models plotted along a series of regional profiles identify areas of increased electrical conductivity and show “flower structures”. The integral picture of the distribution and morphology of zones of increased electrical conductivity in the segments of the Earth’s crust of the Central Tien Shan may reflect a discretely localized manifestation of palm tree structures due to the evolution of transpressive suture zones during the Hercynian and Alpine tectogenesis.