P-wave attenuation variations beneath the central and western Tien Shan from teleseismic waves

Ma X., Zhao L., Xie X., Chang X., Yao Z.

SUMMERY Due to the far-field effect of the India–Eurasia collision, the Tien Shan orogenic belt has been undergoing reactivation and modification. Two end-member models of the geodynamic mechanisms are (1) surface uplift due to crustal shortening caused by lithospheric compression and (2) mountain formation resulting from thermal upwelling of asthenospheric mantle materials generated by lithospheric subduction. However, the topography along the Tien Shan orogenic belt changes significantly, and the deep structure and dynamic process are quite different beneath the Tien Shan orogenic belt from both geological and geophysical observations. Therefore, the reactivation and modification of the Tien Shan orogenic belt are likely influenced by both geodynamic mechanisms, which also generate various thermal anomalies in the crust. Seismic Lg-wave attenuation is very sensitive to crustal material composition and status and can point to the presence of partial melting within the crust resulting from mantle upwelling. In this study, we develop a high-resolution Lg-wave attenuation model between 0.05 and 10.0 Hz in Northwest China and use lateral attenuation variations to infer thermal structures in the crust. The central Tien Shan is characterized by prominent low-QLg anomalies, whereas relatively high-QLg distributions are imaged beneath the eastern and western Tien Shan. The surface uplift and crustal deformation are mostly related to the upwelling of hot mantle materials in the central Tien Shan and are likely induced by lithospheric compression in the eastern and western Tien Shan. However, low-Q anomalies are observed in the junction between the Pamir Plateau and western Tien Shan, indicating that the uplift in the south of the western Tien Shan is related to thermal subduction-induced upwelling and intrusion into the crust due to the collision between the Indian and Eurasian plates. The Kazakh Shield, characterized by pronounced high-QLg values, is likely a cold and hard terrane, and hence blocks the far-field effect of the India–Eurasia collision.

Cui R., Cui Q., Li G., Huangfu P., Zhou Y.

As one of the most important active intracontinental orogenic belts in the world, the Tianshan has experienced the closure of the Paleo-Asian Ocean, rejuvenation, and intracontinental orogeny caused by the far-field effect of the India-Eurasia collision. Here, based on the delayed seismic waveforms recorded by the China National Seismic Network (CNSN) from local and teleseismic earthquakes, we find an obvious low-velocity anomaly (LVA) of 6% at depths of 35–110 km, which is just beneath the eastern Central Tianshan, near the highest peak of Yilianhabierga Mountain. We constrain the lateral distribution of the LVA with arrival delays of teleseismic P waves, and the vertical distribution and velocity contrast of the LVA with waveform fitting of local seismic P waves. Combined with previous results, the LVA could be related to the rejuvenation of the Tianshan caused by the India-Eurasia plate collision. After the bidirectional subduction of the Tarim lithosphere and Junggar lithosphere into the Tianshan upper mantle, the hot upwelling asthenosphere filled the space in which the delaminated lithosphere was previously occupied. Then, the hot upwelling asthenosphere heated the lithosphere and resulted in the partial melting of the accretionary wedge, which is normally believed to have high water content. Our results can provide important insight into the lithospheric deformation processes and tectonic evolution in the Tianshan.

Liu D., Zhao L., Yuan H., Sun W., Xiao W.

Yang Y., Zeng Z., King S.D., Shuang X.

• Deep and intermediate-depth earthquakes in the Hindu Kush are controlled by thrust faulting. • The Pamir region is dominated by strike-slip stress regime with shallow earthquakes. • Two seismic zones represent a double-sided subduction system beneath the Pamir-Hindu Kush. The Pamir-Hindu Kush region at the western end of the Himalayan-Tibet orogen is one of the most active regions on the globe with strong seismicity and deformation and provides a window to evaluate continental collision linked to two intra-continental subduction zones with different polarities. The seismicity and seismic tomography data show a steep northward subducting slab beneath the Hindu Kush and southward subducting slab under the Pamir. Here, we collect seismic catalogue with 3988 earthquake events to compute seismicity images and waveform data from 926 earthquake events to invert focal mechanism solutions and stress field with a view to characterize the subducting slabs under the Pamir-Hindu Kush region. Our results define two distinct seismic zones: a steep one beneath the Hindu Kush and a broad one beneath the Pamir. Deep and intermediate-depth earthquakes are mainly distributed in the Hindu Kush region which is controlled by thrust faulting, whereas the Pamir is dominated by strike-slip stress regime with shallow and intermediate-depth earthquakes. The area where the maximum principal stress axis is vertical in the southern Pamir corresponds to the location of a high-conductivity low-velocity region that contributes to the seismogenic processes in this region. We interpret the two distinct seismic zones to represent a double-sided subduction system where the Hindu Kush zone represents the northward subduction of the Indian plate, and the Pamir zone shows southward subduction of the Eurasian plate. A transition fault is inferred in the region between the Hindu Kush and the Pamir which regulates the opposing directions of motion of the Indian and Eurasian plates.

Li W., Chen Y., Yuan X., Xiao W., Windley B.F.

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. This study presents seismic images across the central Tianshan. The results show that Tianshan’s crust was extensively deformed according to its inherited properties, but was limitedly underthrusted by surrounding blocks

Zhang B., Bao X., Xu Y.

Abstract The initiation and evolution of compressional intracontinental orogens are favored by rheologically weak lithosphere underneath; however, how this weakened lithosphere responds to the regional stress regime remains vigorously debated. The Tien Shan mountains in central Asia provide the best example to illustrate the deep deformational responses to intracontinental orogenesis. We present new constraints on the nature of seismic anisotropy in the crust and upper mantle of the central Tien Shan through shear-wave splitting analyses. Our results reveal a sharp change in the orientations of crustal anisotropic fabrics on two sides of the mountains. The convergence-parallel fast orientations in the northern segment are closely related to the lower-crustal simple-shear deformation caused by the underthrusting of the Kazakh Shield, whereas the depth-independent orogen-parallel fast orientations in the southern segment suggest vertically coherent pure-shear thickening of the Tien Shan lithosphere in response to the northward indentation of the Tarim Basin. The thickened lithosphere has partly foundered into the deep mantle, contributing to the accelerated shortening deformation in the late Cenozoic. Our observations demonstrate the complex tectonic processes in the Tien Shan and suggest that the rheological properties of bounding blocks can play a significant role in shaping the lithospheric structures of intracontinental orogens.

Hapaer T., Tang Q., Sun W., Ao S., Zhao L., Hu J., Jiang M., Xiao W.

The Tien Shan orogenic belt is located in the western region of central Asia, which has been previously considered to have formed in the Paleozoic and reactivated in the Cenozoic. However, the dynamic processes and deep detailed structure of the Tien Shan orogen remain elusive. Seismic studies are expected to provide profound clues on the deep structure and dynamic mechanism of the mountain-building process. Regional Pn traveltime tomography provides opportunities for constructing a more detailed P-wave velocity model of the uppermost mantle linking the shallow crust and the upper mantle. In this work, a 3-D P-wave high-resolution velocity model in the uppermost mantle of the western and central Tien Shan is constructed by inverting the 20,940 Pn arrival time of the 1108 local and regional earthquakes retrieved from the ISC-EHB catalog. The 3-D fast-marching tomography (FMTOMO) package is used to invert the 3-D uppermost mantle velocity model with a resolution of 1° × 1°. We clearly observe descending high-velocity anomalies, extending from the lower crust down to 85 km, suggesting double subduction of the Kazakh lithosphere in the north and Tarim lithosphere in the south beneath central Tien Shan. Furthermore, a prominent low-velocity feature in the upper mantle of central Tien Shan might result from asthenospheric upwelling. The high-velocity anomaly in the upper mantle of the western Tien Shan may suggest the southward subduction of the Kazakh plate to the western Tien Shan and Fergana basins. A significant low-velocity anomaly in the uppermost mantle beneath the southwestern Tarim Basin might result from plume intrusion with evidence of mantle xenolith or plume-lithosphere interaction and its consequent upwelling.

Cui Q., Zhou Y., Li J., Song X., Gao Y., Cui R.

As an active intracontinental orogenic belt within the far field influence of the India-Eurasia collision, the Tien Shan is a natural laboratory to study the mechanisms of the large-scale mountain building. In this study, we image crustal seismic structures beneath the central Tien Shan using the H - κ - c method with the harmonic corrections on Ps and its crustal multiples ( PpPs , PpSs + PsPs ) in P -wave receiver functions (RFs). The RFs are calculated from three-component seismograms recorded by 43 broadband stations located in central Tien Shan and surrounding areas. The robust and high-resolution images of the crustal thickness ( H ) and average Vp / Vs ratio ( κ ) are finally obtained. From our observations, the crustal thickness beneath the central Tien Shan is larger (50– 65 km) than those beneath the Kazakh Shield (~45 km) and Tarim Basin (~42 km). The κ image denotes a general distribution of κ which is higher than 1.73 in most parts of the central Tien Shan, as well as the margins of the Kazakh Shield and Tarim Basin, while lower than 1.73 only in some sporadic areas of this orogenic belt. Our seismic results suggest the complex intracontinental collision with the inhomogeneous crustal thickening in the mountain building. Moreover, the κ image implies the partial melting within the crust beneath the central Tien Shan. Combining with previous seismic studies, we infer that the magmatic intrusion of the hot upwelling mantle materials induces the partial melts within the crust, and also contributes to the rejuvenation of the Tien Shan orogenic belt. • Crustal structures beneath the central Tien Shan are effectively constrained with the H-κ - c method. • The nonuniformly thickened crust suggests that there is a complex intracontinental collision in the mountain building. • The generally high κ (> 1.73) imply the partial melting associated with the magmatic intrusion of hot upwelling mantle.

Lü Z., Lei J., Zhao L., Fu X., Chen J., Li G., Kong Q.

The Tien Shan is one of the highest, youngest, and most active orogenic belts at the southwestern margin of Central Asia. The topography of the Central Tien Shan is characterized by nearly parallel orogenic belts with interposing intermontane basins, providing an ideal site to investigate the formation and modification of the intermontane basins. Here we construct a high-resolution crustal velocity model of the Central Tien Shan using full-wave ambient noise tomography based on high-quality Rayleigh wave empirical Green's functions at periods of 5–50 s extracted from the dense seismic array during 1997–2000. Our new model reveals two high-velocity crustal anomalies beneath the Naryn Basin and Issyk-Kul Basin, respectively, and widespread low-velocity anomalies in the middle to lower crust beneath the mountain belts. We propose that the high-velocity crustal features of the Naryn Basin and Issyk-Kul Basin may be the remnant structures from the Palaeozoic collision. Widespread low-velocity anomalies reflect mantle upwelling and partial melting due to the Kazakh and Tarim lithosphere underthrusting, which have been consequently modifying the overlying crustal structures. High-resolution tomographic results reveal significant crustal heterogeneities beneath the Central Tien Shan, which shed new light on the continental growth mechanism and the crustal deformation of intermontane basins. • A high-resolution crustal velocity model is constructed beneath the Central Tien Shan. • Two crustal high-velocity anomalies are imaged beneath the Naryn Basin and Issyk-Kul Basin. • Widespread crustal low-velocity anomalies associated with the overlying crustal structures.

Sycheva N.A.

This study is concerned with Western and Central Tien-Shan. It is an area of intracontinental collision and is of great interest for the study of the geodynamic processes that are occurring in the crust. The area of study was investigated using the method of seismotectonic deformation (STD). The STD was calculated on the basis of the approaches proposed by Yu.V. Riznichenko and S.L. Yunga. We used the ISC (International Seismological Centre, London) catalog for estimating the seismicity distribution, for calculating the mean annual rate of STD (STD intensity) IΣ, and the parameter of concentration of earthquake-generating ruptures KСР. The catalog includes over 84 000 earthquakes for the period 1902–2019. The distribution of the parameters mentioned above was calculated for three depth ranges: 0–5, 5–25, and over 25 km. We identified areas of intensive seismotectonic deformation, seismic activity, and high concentration of earthquake-generating faults. The study of the entire earthquake-generating layer gave the result that the maximum STD intensity IΣ = ~9 × 10–8 yr–1 was obtained for the junction zone between Southern Tien-Shan and Northern Pamirs. As to the north part of the area of study, high values of STD intensity were obtained for the western Terskey Alatau: IΣ = ~2 × 10–9 yr–1. At all the depths studied here, the maximum rate of earthquakes occurs in the Gissar-Kokshaal earthquake-generating zone. In the northern part, high seismicity is characteristic for the mountain ranges that encircle the Issyk Kul Basin (Terskey Alatau, Kungei Alatau, and Zailiisky Range). The area of study typically exhibits a high level of the concentration of earthquake-generating ruptures, with most of these lying at depths of 0–5 km. Our study of STD directivity is based on data for focal mechanisms of 11 376 earthquakes occurring in 1949–2020. We plotted diagrams showing the distribution of azimuthal directions for the principal stress axes. The azimuth of the compression axis for most events falls within the sector 300°–360°. The resulting STD maps were inspected to determine the directions of shortening and lengthening axes, and to note that the deformation settings show a great diversity in the area of study. The STD tensors obtained for the depths 5–25 km (the earthquake-generating layer) were used to find the distributions of the Lode–Nadai coefficient με, of the sum of the horizontal components (ХХ + YY), and of the vertical component (ZZ). We determined areas that show simple compression and maximum shortening. The models of crustal deformation derived by the STD method and from GPS data were compared to find a fairly good consistency.

Cai Y., Wu J., Wang W.

The structure and physical properties of the crust in the Tianshan region are important for studying the tectonic evolution process within the Tianshan crust. We extracted the receiver functions by collecting the teleseismic waveform from 43 permanent and temporary stations in the Tianshan region (40–46° N, 78–92° E), and obtained the crustal thickness and Poisson’s ratio by using the H–κ–c stacking method to eliminate the effects of the anisotropy and dipping interface in the crust. The results from the H–κ–c stacking method are more robust than those obtained from traditional H–κ stacking. The crustal thickness in the study area ranges from 38 to 62 km with an average of 51 km. The overall crustal thickness is found to be thicker at the Tianshan Mountains but thinner in the basins to the north and south. It is assumed that these features are related to the subduction of Tarim Basin and Junggar Basin beneath the Tianshan Mountains, where insertion and subduction occurred, resulting in crustal shortening and thickening. The Poisson’s ratio in most areas of the Tianshan Mountains ranges from 0.23 to 0.27, close to the average for continental crust. In the contact zone between the southern margin of Junggar Basin and Tianshan Mountains and from the northern margin of Tarim Basin to the western part of the Tianshan Mountains, the Poisson’s ratio reaches as high as 0.3. Based on the velocity structure, electrical conductivity, density, and heat flow study, the higher Poisson’s ratio near the contact zones is probably related to partial melting of the mass with low melting point from the basins due to the subduction. The Tianshan Mountains and Tarim Basin show a clear unbalanced isostatic gravitational state.

Huangfu P., Li Z., Zhang K., Fan W., Zhao J., Shi Y.

The ongoing India-Asia collision principally regulates the Cenozoic tectonic deformation of the Asian interior, and builds a far-away but active spectacular intraplate orogen—Tian Shan. However, the deep processes and dynamics of far-field deformation propagation and the resultant Tian Shan building remain ambiguous. Here, we construct systematic numerical models with variable thermo-rheological properties of the orogen-featured blocks and convergence rates, which reveal that the far-field effect of India-Asia collision on the Tian Shan building is strongly controlled by the direct collision of Indian lithospheric mantle with the rigid Tarim block beneath western Tibet. The model results, together with the well-established geological and geophysical constraints, not only reconcile the first-order crustal and lithospheric structures of the western Tibetan plateau and Tian Shan, but also confirm a >30 Myr time lag between the initial India-Asia collision and the far-field Tian Shan building.

Soto Castaneda R.A., Abers G.A., Eilon Z.C., Christensen D.H.

Seismic deployments in the Alaska subduction zone provide dense sampling of the seismic wavefield that constrains thermal structure and subduction geometry. We measure P and S attenuation from pairwise amplitude and phase spectral ratios for teleseismic body waves at 206 stations from regional and short-term arrays. Parallel teleseismic travel-time measurements provide information on seismic velocities at the same scale. These data show consistently low attenuation over the forearc of subduction systems and high attenuation over the arc and backarc, similar to local-earthquake attenuation studies but at 10× lower frequencies. The pattern is seen both across the area of normal Pacific subduction in Cook Inlet, and across the Wrangell Volcanic Field where subduction has been debated. These observations confirm subduction-dominated thermal regime beneath the latter. Travel times show evidence for subducting lithosphere much deeper than seismicity, while attenuation measurements appear mostly reflective of mantle temperature less than 150 km deep, depths where the mantle is closest to its solidus and where subduction-related melting may take place. Travel times show strong delays over thick sedimentary basins. Attenuation signals show no evidence of absorption by basins, although some basins show signals anomalously rich in high-frequency energy, with consequent negative apparent attenuation. Outside of basins, these data are consistent with mantle attenuation in the upper 220 km that is quantitatively similar to observations from surface waves and local-earthquake body waves. Differences between P and S attenuation suggest primarily shear-modulus relaxation. Overall the attenuation measurements show consistent, coherent subduction-related structure, complementary to travel times.

Shrivastava A., Liu K.H., Gao S.S.

Seismic attenuation is an important parameter for characterizing subsurface morphology and thermal structure. In this study, we use P-wave amplitude spectra from 588 teleseismic events recorded by 477 broadband seismic stations in the southeastern United States to examine the spatial variations of seismic attenuation in the crust and upper mantle. The resulting seismic attenuation parameter (∆t*) measurements obtained using the spectral ratio technique reveal a zone of relatively low attenuation in the Gulf of Mexico Coastal Plain and the southwestern terminus of the Piedmont province. Spatial coherency analysis of the ∆t* observations suggests that the center of the low attenuation layer is located within the uppermost mantle at about 70 km depth. This low attenuation anomaly lies along the suture zone between Laurentia and Gondwana and approximately coincides with the east-west trending Brunswick magnetic anomaly. The origin of this low attenuation anomaly can be attributed to low attenuation bodies in the form of remnant lithospheric fragments in the deep crust and the uppermost mantle. The contribution of scattering to the observed ∆t* is estimated by calculating the ratio of amplitude on the transverse and vertical components in the P-wave window. Relative to the rest of the study area, the Gulf of Mexico Coastal Plain demonstrates weaker scattering which is suggestive of a more homogenous crustal and uppermost mantle structure.

Rybin A.K., Bataleva E.A., Matiukov V.E., Morozov Y.A., Nepeina K.S.

Abstract New results of a detailed study of the deep structure of the Central Tien Shan along the Son-Kul magnetotelluric (MT) profile crossing the Son-Kul Lake are reported. Based on the results of magnetotelluric data modeling, the regional and local geoelectric anomalies in the lithosphere are studied and their quantitative characteristics are given. Geological interpretation of the geoelectric cross-section was carried out, which supported the existing ideas about the block–hierarchical structure of the upper part of the Earth’s crust. This corresponds to the tectonophysical concepts of the sequential inserted subordination of large and smaller elements of the zone–block structure consisting of stable blocks and limiting mobile zones, which are distinguished by the high dislocation of the geological substrate. The integral pattern of the distribution and morphology of zones of high electrical conductivity in this segment of the Central Tien Shan crust may reflect discretely localized palm tree–type structures associated with the evolution of transgressive suture zones of localized deformation during the Hercynian and Alpine tectogenesis.