Structural and geological studies in the Naryn and Atbashi depressions (Tien Shan) and geological interpretation of magnetotelluric data

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.

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., 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.

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.

Burbank, McLean, Bullen, Abdrakhmatov, Miller

Well-preserved, actively deforming folds in the Tien Shan of Kyrgyzstan provide a natural laboratory for the study of the evolution of thrust-related folds. The uplifted limbs of these folds comprise weakly indurated Cenozoic strata that mantle well-lithified Palaeozoic bedrock. Their contact is a regionally extensive unconformity that provides a persistent and readily traceable marker horizon. Based on the deformation of this marker, preserved fold geometries support simple geometric models for along-strike gradients in fold amplitude and displacement along the underlying faults, linkage among multiple structures, transfer of displacement among folds and evolution of the folds as geomorphic entities. Subsequent to initial uplift and warping of the unconformity surface, steeply dipping reverse faults cut the forelimbs of many of these folds. Wind gaps, water gaps, recent faulting and progressive stripping of the more readily eroded Cenozoic strata indicate the ongoing lateral propagation and vertical growth of fault-related folds. The defeat of formerly antecedent rivers coincides in several places with marked increases in erosional resistance where their incising channels first encountered Palaeozoic bedrock. Persistent dip angles on the backlimbs of folds indicate strikingly uniform geometries of the underlying faults as they propagate both laterally and vertically through the crust. Deformation switches irregularly forward and backward in both time and space among multiple active faults and folds with no systematic pattern to the migration of deformation. This distributed deformation appears characteristic of the entire Kyrgyz Tien Shan.

Przhiyalgovskii E.S.

AbstractThe detailed geological profile “Karabuk” constructed in recent years, crossing the triad of alpine structures “Naryn Depression - Baibichetoo Uplift - Atbashi Depression [[CHECK_DOUBLEQUOT_ENT]] in the central part of Tien Shan, provides new material for studying the style and parameters of deformations at the late phase of the Indian - Eurasian plate collision. Using original methodological techniques and in accordance with the basic principles of balanced sections method, the horizontal deformation of the rock complex of the Cenozoic sedimentary cover (a meridional reduction in the width of the orogenic belt) was determined, starting from the end of the Neogene. Variations in the thickness of the cover layers determined during the research were taken into account. The presented tectonic reconstruction demonstrates the structure of a single Naryn-Atbashi sedimentary basin before the intense deformations at the late orogenic phase. It is significant, that the magnitude of deformations on individual segments of the profile differs markedly. The cover of the Atbashi depression is more deformed, its width has decreased by 6 km (about 17%) in this section, while the borders of the wider Naryn depression have converged by only 3.7 km (8%). The total South-North contraction of the NBA system measured by us during the Cenozoic is about 10 km (12% of its original width). The values of the contraction from south to north (the horizontal component of the deformation) obtained by us compared with the values given by other researchers.