Crustal deformation and its dynamic mechanisms beneath the different tectonic blocks of the central Tibetan Plateau from P-wave Receiver Functions

SUMMARY Receiver function (RF) analysis is an indispensable method that is widely utilized to image the deep structure of the crust and upper mantle. Although the P-wave receiver function (PRF) has the advantages of a higher signal-to-noise ratio and higher resolution than S-wave receiver function, the information about P-to-S converted (Ps) phases from velocity discontinuities of the crust and upper mantle is usually obstructed by multiples from shallower crustal interfaces. Sometimes it is difficult to identify useful information about lithospheric discontinuities. In this paper, we propose a method for suppressing multiples and isolating the Ps phases of RFs. This task is accomplished by exploiting the differences in arrival times resulting from the slowness between the conversion and the multiples that occur at the same interface in the PRFs and by separating them in the Radon domain, which enables removal of crustal and sedimentary multiples. This method can effectively remove multiples and isolate the useful signals of Ps phases contained in the PRFs. We test the method on synthetic PRFs and demonstrate that crustal multiples can be effectively eliminated or suppressed and that the Ps phase of lithospheric discontinuities can be coherently traced. Next, we apply the method to real PRF data collected from the seismic station of the China Array seismic experiment (Phase II and Phase III) in the Ordos block and its adjacent area, and successfully obtain an improved common conversion point stacking image of lithospheric discontinuity structures in the depth domain, particularly for detecting the lithosphere–asthenosphere boundary.

Crustal thickness (H) and Vp/Vs ratio (κ) images beneath the central Tien Shan revealed by the H-κ-c method

Crustal structure beneath the central and western North China from receiver function analysis

The southeast margin of Tibet is the region in clockwise rotation around the Eastern Himalayan Syntaxis due to the India–Eurasia collision and the resistance of the stable Sichuan Basin and South China block. However, the dynamic processes involved in the evolution and deformation of the region remain poorly understood due to a lack of reliable geophysical observations. We collected abundant seismic data recorded by 108 permanent broadband stations deployed in the SE margin of Tibet since 2000, and obtained 4536 pairs of P-wave receiver functions (PRFs) with high signal-to-noise ratio. In this study, we have implemented a novel two-step data inversion procedure that can reduce the dependence of the inversion results on the initial model. We first use low-frequency PRFs obtained by iterative deconvolution in the time domain, and then an initial model consisting of a series of 2-km-thick isotropic layers to fit velocity models, and thus determine an overall statistical solution by means of the bootstrap resampling technique. This statistical solution is then regarded as a new initial model to adjust high-frequency PRFs. Hence, the same resampling process is executed again to estimate the optimal S-wave velocity structure below each station. The results provide an accurate 3D image of the crust and uppermost mantle in the SE margin of Tibet. We infer a wide intra-crustal low-velocity zone that varies laterally and in depth, which is thinner or even absent in the most southern part of Yunnan. Our hypothesis is that this low-velocity zone is the result of the accumulation of lower crustal flow coming from central Tibet. Furthermore, we show that this lower crustal flow extends largely through the Sichuan–Yunnan diamond-shaped block, and that there are significant variations in both crustal velocity structure and deformation mechanism across the great strike-slip faults of the Jinshajiang–Red River and Xiaojiang fault systems.

The lithospheric structure beneath southeast Tibet revealed by P and S receiver functions

Crustal structure and deformation beneath eastern and northeastern Tibet revealed by P-wave receiver functions

The P- and S-wave receiver functions and dispersion curves of the fundamental Rayleigh wave are used to study the lithosphere within the Central Anatolian Plateau. The results for eight broadband seismic stations are presented. It is established that within the plateau, the crust with a thickness of about 35 km is underlain by the mantle lid with its bottom at a depth of about 60 km. The velocities of longitudinal (Vp) and shear (Vs) waves in this layer are at most 7.6 and 4.5 km/s, respectively, and the Vp/Vs ratio is close to 1.7 (i.e., by 6% lower than in the standard IASP91 and PREM models). Such a low velocity ratio is characteristic of rocks having high orthopyroxene content. Beneath the high-velocity mantle lid, the S-wave velocity decreases to 4.0–4.2 km/s and the Vp/Vs ratio is close to its standard value (1.8). At most stations, the P-wave receiver functions do not contain seismic phase P410s, which is formed at the global seismic boundary at a depth of 410 km. The seismic boundary at a depth of 410 km is related to the olivine-spinel phase transformation, and its absence can indicate the anomalously low olivine content and high basalt content. This anomaly is probably associated with the subduction of a large amount of oceanic crust during the closure of the Tethys. The results of the study overall indicate the high informativity of the used method.

In this study, we used the P-wave receiver functions (PRFs) to investigate the crustal structure of northern Morocco, located at the westernmost edge of the Mediterranean, near to the boundary between the African and Eurasian tectonic plates. This region is an integral part of the complex crustal deformation and tectonic system associated with the Alpine orogeny, characterized by concurrent compressional and extensional processes. These dynamics have led to the development of various structural and tectonic models aimed at explaining the area‘s geological evolution. The significant tectonic activity, evident in frequent seismic events, and complex lithospheric deformation, makes it an ideal location for studying crustal variations, lithospheric interactions, and mineralogical contrasts.To achieve these objectives, we utilized high-quality seismic broadband data from the TopoIberia and Picasso seismic experiments, provided by the Scientific Institute, as well as from the broadband seismic stations operated by the National Center for Scientific and Technical Research (CNRST). The PRFs were extracted by decomposing teleseismic P-waves to isolate the effects of the local crustal structure. The dataset covers a wide range of regional stations, and the RFs provide detailed insights into crustal thickness, density and velocity contrasts, as well as deep discontinuities. Our preliminary results reveal significant variations in Moho depth, ranging from approximately 22.7 km in the eastern part of the region to 51.7 km in the western part. These variations correlate with changes in Vp/Vs and Poisson’s ratios, indicating mineralogical heterogeneity, with compositions spanning from mafic to felsic. These findings provide new constraints for tectonic models and enhance our understanding of the geodynamic processes involved, particularly the interactions between the crust and the upper mantle. This study not only improves our understanding of active tectonics and crustal composition in northern Morocco but also offers valuable insights for refining evolutionary models of the Western Mediterranean within its complex geodynamic context.Keywords: Teleseismic event, P-wave, Receiver functions, Seismic Network, Vp/Vs ratio, Poisson ratio, Crustal structure, Mineralogical composition, Seismotectonics, Northern Morocco.

We model the depth and Vs structure of the Hales discontinuity (H‐D) beneath Eastern Dharwar Craton (EDC) and Southern Granulite Terrain (SGT) using P wave receiver function (P‐RF) analysis and joint inversion with Rayleigh wave phase velocity dispersion. We calculate P‐RFs at higher frequency (fmax = 0.46 Hz), compared to previous studies, to show that the H‐D P‐to‐S converted phase (Phs) is distinct from crustal reverberations. Phs at stations Hyderabad (HYB) and Gauribidanur (GBA), in the EDC, arrive at ∼11.5 s and ∼12 s, respectively. From joint inversion, the H‐D is modeled at 107 ± 5 km and 113 ± 4 km depth, with ∼3% and ∼4% Vs increase, beneath HYB and GBA, respectively. For station Kodaikanal (KOD), in SGT, the Phs destructively interferes with the negative midcrustal reverberation at most ray‐parameters, which explains its apparent absence in previous studies. We isolated P‐RFs where Phs is distinct at ∼11 s and model it at 102 ± 3 km depth. Common conversion point stack profiles constructed by depth migrating P‐RFs through the Vs model show an undulatory nature of the H‐D. From data of mantle xenoliths (Wajrakarur kimberlite field), we calculate Vs of mantle peridotite and eclogite, using published bulk rock compositions. At the H‐D depth and temperature derived from Indian shield geotherm, we observed a good match to the Vs structure of the H‐D. Our results support the geodynamic model of the H‐D being an interface of paleosubducted eclogitic oceanic crust embedded within the upper mantle peridotite. Global observations of mantle reflectors within the continental lithosphere, at depths similar to H‐D, have been related to relict subduction and independently support our model.

SUMMARY Crustal structure across the India–Asia collision zone (Tidding Suture) in the northeast Himalaya bounded by the Eastern Himalayan Syntaxis (EHS) is investigated using the P-wave receiver function (RF) method. The analysed data included three-component waveforms of teleseismic earthquakes recorded by a linear array of 11 broad-band seismic stations. The RFs and inverted shear wave velocity models reveal azimuthally varying crustal structure. The RFs for earthquakes from the northeast back azimuths are conspicuous by absence of P-to-S converted phase at the Moho discontinuity. Inverted velocity model ascribe this to absence of the typical step velocity jump at the Moho in a narrow section of the EHS bordering the indenting Indian Plate and pierced by travelling waves. In contrast, teleseismic waves arriving from southeast back azimuth sample different litho-tectonic blocks of the Himalayan collision zone and inverted models show northeast dipping Moho across the Tidding Suture. Compared to an overall thickness of >70 km in the northwest and central Himalaya, the crust across the Tidding Suture is only about 55 km thick. This is attributed to a slower rate of convergence in this part of the collision zone. The Moho structure beneath Indian Plate and southeastern Tibetan Plateau reveal opposite dip directions with their colliding margin placed just east of the Walong Thrust. The inverted shear wave velocity models show evidence of intracrustal low-velocity layer whose strength varies across the Tidding Suture. The magnitude of velocity reduction beneath Lohit Plutonic Complex (Trans Himalaya) favour partial melt as a possible mechanism whereas south of the Tidding Suture, where velocity reduction is comparatively less, fluids generated by dehydration reactions appear to be the source for velocity reduction.

The vertical component P -wave receiver function is an important source of data in studies of the crust/mantle transfer function for determining Earth structure under isolated receivers or under receiver arrays. This waveform illuminates a missing aspect of the wave propagation in receiver function studies that employ only the horizontal components of motion, and yields complementary constraints on near-receiver heterogeneity and P -wave propagation. The vertical component P -wave receiver function is formed using an array estimate for the effective teleseismic source function that is then deconvolved from all the vertical and horizontal components of ground motion at each station in the array. One-dimensional, three-dimensional, and stochastic wave-propagation models are used to test the robustness of the technique. Breakdown of single-station receiver function deconvolution occurs because of high levels of noncorrelated noise between the ground-motion components. Receiver functions for stations of the southern California TERRAscope array are investigated using the array technique. Vertical receiver functions for stations in the Los Angeles Basin and Long Valley Caldera show high-amplitude secondary arrivals that cannot be explained by simple 1D structures but probably reflect wave propagation in 3D basin structures. Three-component receiver functions from the station at Mammoth Lakes, California, show pathological behavior where the horizontal components of ground motion exceed the amplitude of the vertical components, suggesting extreme topographic and 3D velocity heterogeneity. Use of all three components of the receiver function in modern passive array experiments is encouraged to reduce the problems of nonuniqueness in determining Earth models.

A new technique for P‐wave receiver function (PRF) inversion, within the framework of wave equation‐based adjoint tomography and referred to as receiver function adjoint tomography (RFAT), has been developed to obtain models of Vp, Vs, and density. This innovative technique fits the synthetic PRFs with observed PRFs and can better image the lateral variations of Vs from the crust to the uppermost mantle than traditional 1‐D PRF inversion. We utilized RFAT to perform high‐resolution imaging beneath a dense seismic array in Southeastern Tibet, revealing low‐velocity zones extending from the uppermost mantle to the crust, as well as an eastward dipping Moho under the Red River Fault (RRF). Our inversion results provide direct evidence for the existence of a distinct asthenospheric upwelling channel beneath the RRF, and further highlight the effectiveness of RFAT for accurately imaging subsurface structures.

The scattering properties of terrestrial planetary bodies can provide valuable insights into their shallow seismic structure, meteoritic impact history, and geological activity. Scattering properties of the shallow crusts of Earth, Mars, and the Moon are investigated by constructing P‐wave receiver functions (PRFs) from teleseismic waveforms with high signal‐to‐noise ratios. The authors’ analysis reveals that strong coda waves lead to significant variations in the PRF waveforms calculated using different time windows, and the stability of the PRF is primarily influenced by the fractional velocity fluctuation. Synthetic PRFs for various scattering media confirm these observations. Comparing the observed and synthetic PRFs, it is found that the fractional velocity fluctuation in the shallow crust is greater than ∼0.2 for the Moon but less than ∼0.2 for Earth and Mars. The authors further discuss possible mechanisms that could have affected the fractional velocity fluctuation and suggest that the distinct fractional velocity fluctuation between the Moon and Earth/Mars is mainly due to differences in the water content of the crustal rocks of the three planetary bodies.

The northeastern Tibetan Plateau is one of the key regions to explore the geodynamics of the Tibetan Plateau. In 1958, a research team led by Prof. Zeng Rongsheng did low-frequency seismic exploration in the Qaidam Basin, which was a prelude to deep geophysical research in the northeastern Tibetan Plateau. Since 1960s, a series of scientific projects have been carried out, including 27 deep seismic sounding profiles, which provided good coverage and fundamental constraints on velocity structure of the crust and upper mantle of tectonic units in the region. Among them, a number of deep seismic reflection profilings were used to reveal fine crustal structures of key tectonic areas. Moreover, crustal electrical structure and density structure were inferred from magnetotelluric sounding profiling and Bouguer gravity anomaly data, respectively. 3-D P-wave and S-wave velocity structures were determined using body-wave travel time tomography, surface-wave group velocity and phase velocity tomography based on seismic data. Since 2000, with the dramatically increased number of broadband seismic stations, application of modern seismology methods, such as teleseismic receiver function inversion, ambient noise imaging, as well as shear-wave splitting and seismic anisotropy, led to further understanding of structure and deformation of crust and upper mantle in the northeastern Tibetan Plateau. Among published research papers, most results on the deep structure are compatible. For example, the ambient noise imaging, surface wave tomography and receiver function inversion jointly show a wide distribution of low shear wave velocity in the middle and lower crust. Deep geophysical explorations show coexistence of crustal thickening, low P-wave velocities, low resistivity and high heat flow values. H- κ stacking analysis of receiver functions shows low-to- moderate crustal Poisson’s ratios in the Qilian fold system, the northern Songpan-Garze block and the west Qinling orogenic belt. The crustal Poisson’s ratios in the northeastern Tibetan Plateau are obviously lower than those in the central plateau. These compatibilities are crucial to understanding basic features of the deep structure. However, several issues related to the dynamics of the region remain in debate: (1) low velocity-high conductivity layer in the upper-middle crust; (2) crustal thickening mode; (3) crustal and mantle anisotropy; (4) lower crustal channel flow; (5) southward subduction of the Eurasian lithosphere. No consensus has been reached for these issues yet at present. Although several deep seismic sounding profiles and magnetotelluric sounding profiles collectively displayed evidence of existence of a low velocity-high conductive layer in the upper-middle crust, results of some other profiles did not show such layer. Mechanism of crustal thickening in the northeastern Tibetan Plateau can be summarized in the following three end-member hypotheses: (1) uniform crustal thickening; (2) lower crustal thickening; (3) upper crustal thickening. A prevailing view is that the crustal shortening generates folding and deformation of the upper crust, and fragment stacking is the main mode of crustal thickening. However, this is inconsistent with the crustal model from deep seismic sounding profiles. Different deep tectonic models and interpretations are causes of the on-going debate on issues such as the “lower crustal channel flow” and “southward subduction of Eurasian continent”. One of the reasons for lack of consensus might be that the resolution power of the existing seismic data is still not high enough to identify the details in deep crust and upper mantle. The national and regional seismic networks, and large-scale temporary seismic array observation, which is currently being implemented, will greatly improve the reliability and resolution of the target model. This is an effective way to enhance the knowledge of crustal and upper mantle structures and geodynamics in the northeastern Tibetan Plateau.

This dissertation investigates the lithospheric structure of the Eastern Anatolia and Caucasus region through an integrated approach employing Two-Plane Wave Tomography (TPWT), Ambient Noise Tomography (ANT), P-wave Receiver Functions, and joint inversion of Receiver Functions with Surface Waves, utilizing new data from the CNET (Caucasus Seismic Network) seismic experiment. The tectonic framework of the Greater Caucasus, Lesser Caucasus, and Eastern Anatolia is predominantly shaped by the continental collision between the northward- moving Arabian Plate and the Eurasian Plate. This ongoing convergence has resulted in the formation of the 1,500-meter-high Eastern Anatolian Plateau, a diffuse deformation zone along the plate boundary, and Mount Elbrus (5,642 meters), the highest mountain in Europe (Philip et al., 1989; Reilinger et al., 2006). Comparisons are drawn to earlier stages of continent--continent collision, such as between the Indian and Eurasian Plates, to enhance our understanding of mountain-building processes in similar tectonic settings (Şengör et al., 1979). The extended TPWT results span from Eastern Anatolia to the Caucasus, providing an updated and refined model compared to Skobeltsyn et al., (2014). Integration of ANT enhances depth resolution at crustal levels, while P-wave receiver functions are utilized to analyze discontinuity structures. The joint inversion of receiver functions with combined ANT and TPWT data yields a high-resolution S-velocity structure from the surface to 100 km depth. Key findings include the identification of a low-velocity zone in Eastern Anatolia across all depths, suggesting asthenospheric upwelling and a thin mantle lid at approximately 100 km depth near the Lithosphere-Asthenosphere Boundary (LAB). Crustal thickness increases from south to north in Eastern Anatolia, with possible crustal thickening observed in the Lesser Caucasus. In the eastern Greater Caucasus, evidence points to a northward-dipping flat ongoing subduction, and a potential new subduction zone may be forming in northern Iran, also oriented northward. The weak, low-velocity crust beneath the eastern Greater Caucasus indicates strain weakening due to flexural loading, which facilitates the formation of thick sedimentary rock layers reaching up to 15-18 km in eastern Georgia, central Azerbaijan and potentially deeper in South Caspian. Crustal thickening in Lesser Caucasus is also evident from joint inversion results.