Intracontinental deformation of the Tianshan Orogen in response to India-Asia collision

How the continental lithosphere deforms far away from plate boundaries has been long debated. The Tianshan is a type-example of ongoing lithospheric deformation in an intracontinental setting. It formed during the Paleozoic accretion of the Altaids and was rejuvenated in the Cenozoic, which might be a far-field response to the India-Asia collision. Here we present seismic images of the lithosphere across the central Tianshan, which were constructed from receiver functions and Rayleigh wave dispersions along a N-S-trending linear seismic array. We observe an extensively deformed lithosphere in the Tianshan with inherited, structurally controlled brittle deformation in the shallow crust and plastic deformation near the Moho. We find that earlier multiple accretionary structures were preserved in the crust, which was deformed by pure-shear shortening in the south and thick-skinned tectonics in the north but was limitedly underthrusted by surrounding blocks. A balanced cross-section of Moho discontinuities supports the concept that intracontinental deformation in the Tianshan intensified synchronously with the direct contact between the underthrusting Indian slab and the Tarim Craton in the Late Miocene (~10 Ma). These findings provide a robust and unified seismic model for the Tianshan Orogen, and confirm that effective delivery of the India-Asia collision stress induced the rejuvenation of this intracontinental orogen.

Studying the Depth Structure of the Kyrgyz Tien Shan by Using the Seismic Tomography and Magnetotelluric Sounding Methods

This paper presents new results of detailed seismic tomography (ST) on the deep structure beneath the Middle Tien Shan to a depth of 60 km. For a better understanding of the detected heterogeneities, the obtained velocity models were compared with the results of magnetotelluric sounding (MTS) along the Kekemeren and Naryn profiles, running parallel to the 74 and 76 meridians, respectively. We found that in the study region the velocity characteristics and geoelectric properties correlate with each other. The high-velocity high-resistivity anomalies correspond to the parts of the Tarim and Kazakhstan-Junggar plates submerged under the Tien Shan. We revealed that the structure of the Middle Tien Shan crust is conditioned by the presence of the Central Tien Shan microcontinent. It manifests itself as two anomalies lying one below the other: the lower low-velocity low-resistivity anomaly, and the upper high-velocity high-resistivity anomaly. The fault zones, limiting the Central Tien Shan microcontinent, appear as low-velocity low-resistivity anomalies. The obtained features indicate the fluid saturation of the fault zones. According to the revealed features of the Central Tien Shan geological structure, it is assumed that the lower-crustal low-velocity layer can play a significant role in the delamination of the mantle part of the submerged plates.

The source parameters of earthquakes of Bishkek geodynamic proving ground (Northern Tien Shan)

Based on the method of polarity of signs of P-waves, the focal mechanisms of 1674 earthquakes with M ≥ 1.6, which occurred on the territory of the Bishkek geodynamic proving ground (BGPG) from 1994 to 2020, were determined. Some characteristics of the complete catalogue are presented. Quantitative distributions by the type of mechanisms and diagrams of azimuths of the main stress axes are constructed. A variety of focal mechanism of earthquake is observed, most of them are reverse fault, oblique reverse fault, and horizontal strike-slip fault. The compression axis for most of the events has a north-northwest direction and a sub-horizontal position. For 183, dynamic parameters (DP, source parameters) were obtained: spectral density Ω0, corner frequency f0, scalar seismic moment M0, source radius (Brune radius) r, and stress drop Δσ. The correlations between DP and energy characteristic (magnitude) and scalar seismic moment are investigated. The smallest correlation coefficient was obtained for stress drop.

Volcanology. The Yellowstone magmatic system from the mantle plume to the upper crust

The Yellowstone supervolcano is one of the largest active continental silicic volcanic fields in the world. An understanding of its properties is key to enhancing our knowledge of volcanic mechanisms and corresponding risk. Using a joint local and teleseismic earthquake P-wave seismic inversion, we revealed a basaltic lower-crustal magma body that provides a magmatic link between the Yellowstone mantle plume and the previously imaged upper-crustal magma reservoir. This lower-crustal magma body has a volume of 46,000 cubic kilometers, ~4.5 times that of the upper-crustal magma reservoir, and contains a melt fraction of ~2%. These estimates are critical to understanding the evolution of bimodal basaltic-rhyolitic volcanism, explaining the magnitude of CO2 discharge, and constraining dynamic models of the magmatic system for volcanic hazard assessment.

Recent strike-slip deformation of the northern Tien Shan

The paper presents a geodynamic interpretation of the deep structure and active tectonics of the northern Tien Shan, with particular emphasis on strike-slip motions, which produced a pull-apart in the centre of the Issyk-Kul basin. The study is based on a detailed interpretation of satellite imagery, fault plane solutions of earthquakes, seismic, and geodetic data.

Seismic and magnetotelluric studies show tectonic layering of the Tien Shan lithosphere, with several nearly horizontal viscoelastic layers and the lower layer underthrust northward in the northern Tien Shan. This active process may be responsible for the intricate present-day tectonic framework of the northern Tien Shan.

The recent tectonics of the northern Tien Shan inherits the earlier structure: The lens-shaped Issyk-Kul microcontinent comprising Precambrian-Palaeozoic metamorphic and magmatic rocks is surrounded by thick shear zones which have been involved in the activity over most of the Cenozoic. In the Quaternary the strain propagated as far as the central part of the Issyk-Kul basin.