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.     

Array magnetotelluric soundings in the active seismic area of Northern Tien Shan

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.     

The thermo-tectonic history of the Song-Kul plateau, Kyrgyz Tien Shan: Constraints by apatite and titanite thermochronometry and zircon U/Pb dating

The Song-Kul Basin sits on a plateau at the Northern and Middle Kyrgyz Tien Shan junction. It is a lacustrine basin, occupied by Lake Song-Kul and predominantly developed on igneous basement. This basement was targeted for a multi-method chronological study to identify the different magmatic episodes responsible for basement formation and to constrain the timing of the development of its present-day morphology. Zircon U/Pb dating by LA-ICP-MS revealed four different magmatic episodes: a Late Cambrian (~ 500 Ma) island arc system, a Late Ordovician (~ 450 Ma) subduction related intrusion, an Early Permian (~ 290 Ma) collisional stage, and a Middle to Late Permian (~ 260 Ma) post-collisional magmatic pulse. Middle to Late Triassic (~ 200–230 Ma) titanite fission-track ages and Late Triassic – Early Jurassic (~ 180–210 Ma) apatite fission-track ages and thermal history modeling indicate the Song-Kul basement was already emplaced in the shallow crust at that time. An exhumed fossil apatite fission-track partial annealing zone is recognized in the bordering Song-Kul mountain ranges. The area experienced only minor post-Early Mesozoic denudation. The igneous basement was slowly brought to apatite (U–Th)/He retention temperatures in the Late Cretaceous–Palaeogene. Miocene to present reactivation of the Tien Shan does not manifestly affect this part of the orogen.     

Petrological interpretation of the magnetotelluric data of the depth zone of the Tarim and Tien Shan junction

Electrical conductance of lower crust and mantle rocks samples from the Tarim and Tien Shan junction zone were measured in a laboratory under P-T conditions determined on the basis of thermobarometry data. The geothermic curve for the moment of xenolith delivery (K - \rlap- P)(K - \rlap{--} P) corresponding to a thermal flux of 80 mW/m² was determined. Its comparison with the present-day geothermic curve corresponding to 60 mW/m² demonstrates general cooling of the lithosphere. The Moho surface position also changed from 35–40 km (as the boundary between lherzolites and granulites) to 55–60 km, as a proper geophysical Moho discontinuity. Results of laboratory measurements of electrical conductance of upper mantle and lower crust rocks samples in correlation to geoelectric and thermal models make it possible to define lherzolite, granulite, and eclogite massifs in the depth zone of the Tarim and Tien Shan junction.     

Thermo-tectonic history of the Issyk-Kul basement (Kyrgyz Northern Tien Shan,Central Asia)

Lake Issyk-Kul occupies a large Late Mesozoic–Cenozoic intramontane basin between the mountain ranges of the Northern Kyrgyz Tien Shan. These ranges are often composed of granitoid basement that forms part of a complex mosaic assemblage of microcontinents and volcanic arcs. Several granites from the Terskey, Kungey, Trans-Ili and Zhetyzhol Ranges were dated with the zircon U/Pb method (SHRIMP, LA-ICP-MS) and yield concordant Late Ordovician–Silurian (~ 456–420 Ma) emplacement ages. These constrain the “Caledonian” accretion history of the Northern Kyrgyz Tien Shan in the amalgamated Palaeo-Kazakhstan continent. The ancestral Tien Shan orogen assembled in the Early Permian when final closure of the Turkestan Ocean ensued collision of Palaeo-Kazakhstan and Tarim. A Late Palaeozoic structural basement fabric formed and Middle–Late Permian post-collisional magmatism added to crustal growth of the Tien Shan. Permo‐Triassic cooling (~ 300–220 Ma) of the ancestral Tien Shan was unraveled using ⁴⁰Ar/³⁹Ar K-feldspar and titanite fission-track (FT) thermochronology on the Issyk-Kul granitoids. Apatite thermochronology (FT and U–Th–Sm/He) applied to the broader Issyk-Kul region elucidates the Meso-Cenozoic thermo-tectonic evolution and constrains several tectonic reactivation episodes in the Jurassic, Cretaceous and Cenozoic. Exhumation of the studied units occurred during a protracted period of intracontinental orogenesis, linked to far-field effects of Late Jurassic–Cretaceous accretion of peri-Gondwanan blocks from the Tethyan realm to Eurasian. Following a subsequent period of stability and peneplanation, incipient building of the modern Tien Shan orogen in Northern Kyrgyzstan started in the Oligocene according to our data. Intense basement cooling in distinct reactivated and fault-controlled sections of the Trans-Ili and Terskey Ranges finally pinpoint important Miocene–Pliocene (~ 22–5 Ma) exhumation of the Issyk-Kul basement. Late Cenozoic formation of the Tien Shan is associated with ongoing indentation of India into Eurasia and is a quintessential driving force for the reactivation of the entire Central Asian Orogenic Belt.     

Structure of the basement surface and sediments in the Kochkor basin (Tien Shan): geological and geophysical evidence

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.     

Underthrusting of Tarim beneath the Tien Shan and deep structure of their junction zone: Main results of seismic experiment along MANAS Profile Kashgar-Song-Köl

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.     

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.