No abstract

The high topography of the Tibetan Plateau was generated by the Cenozoic India‐Eurasia collision. A high‐resolution shear wave velocity model can provide improved understanding of the Tibetan structure and crustal deformation with complicated tectonic evolution. Based on continuous seismic observations at approximately 400 stations, we collected over 10,000 Rayleigh wave phase velocity dispersion curves extracted from ambient noise cross‐correlation functions. A direct surface wave inversion method was applied to obtain an S wave velocity model of the Tibetan crust. A heterogeneous structure including several prominent low‐velocity zones (LVZs) and relatively narrow low‐velocity bands connecting the LVZs is revealed. Our model shows significant crustal low‐velocity structures with lateral variations along the Himalayan front. In the eastern segment of the Lhasa terrane, the LVZ is spatially correlated with Miocene porphyry Cu deposits, which are probably related to strong tearing of the Indian lithosphere, while preexisting weak zones contribute to the LVZs in the western segment. Meanwhile, the LVZ along the Bangong‐Nujiang suture could be considered as a channel for eastward extrusion of ductile material, that is, crustal flow on geological timescales. This “flow” in the middle‐lower crust probably contributed to the formation of the V‐shaped conjugate strike‐slip fault system in central Tibet. The development of conjugate strike‐slip faults, however, ceased near 90°E, since the eastward “flow” was blocked by an intracrustal high‐velocity block in Amdo. This preexisting rigid zone (containing the Amdo microcontinent) hence influences the pattern of material transport inside the Tibetan crust and the deformation of the plateau.

Recent studies have demonstrated that body waves between pairs of stations can be successfully retrieved from ambient noise cross correlation at both regional and global scales, although surface waves are the dominant signals. However, it is still difficult to use these retrieved body wave signals to map lateral depth variations of main structural discontinuities or velocity contrasts because of its low signal‐to‐noise ratio (SNR). In this research, based on a dense seismic array in eastern North China Craton, reflected P wave signals from 410 km and 660 km discontinuities can be successfully recovered from ambient noise cross correlation. To improve SNR, the cross correlations are stacked within each bin with the phase‐weighted stack method. The retrieved P410P and P660P phases from stacked correlations reveal lateral variations of both depths and sharpness of the 410 km and 660 km discontinuities along two profiles, which may be related with hot material upwelling and the effect of stagnant Pacific Plate in the transition zone beneath North China Craton. The imaging results are generally consistent with the results from teleseismic receiver functions, which demonstrate the possibility of mapping high‐resolution topography and sharpness of deep internal discontinuities without earthquake‐station geometric limitations.

Significant left-lateral movement along the Ailao Shan-Red River fault accommodated a substantial amount of the late Eocene to early Miocene India-Asia convergence. However, the activation of this critical strike-slip fault remains poorly understood. Here, we show key seismic evidence for the occurrence of massive lithospheric delamination in southeastern Tibet. Our novel observation of reflected body waves (e.g. P410P and P660P) retrieved from ambient noise interferometry sheds new light on the massive foundered lithosphere currently near the bottom of the mantle transition zone beneath southeastern Tibet. By integrating the novel seismic and pre-existing geochemical observations, we highlight a linkage between massive lithospheric delamination shortly after the onset of hard collision and activation of continental extrusion along the Ailao Shan-Red River fault. This information provides critical insights into the early-stage evolution of the India-Asia collision in southeastern Tibet, which has significant implications for continental collision and its intracontinental response.

We present a 2‐D ambient noise adjoint tomography technique for a linear array with a significant reduction in computational cost and show its application to an array in North China. We first convert the observed data for 3‐D media, i.e., surface‐wave empirical Green's functions (EGFs) to the reconstructed EGFs (REGFs) for 2‐D media using a 3‐D/2‐D transformation scheme. Different from the conventional steps of measuring phase dispersion, this technology refines 2‐D shear wave speeds along the profile directly from REGFs. With an initial model based on traditional ambient noise tomography, adjoint tomography updates the model by minimizing the frequency‐dependent Rayleigh wave traveltime delays between the REGFs and synthetic Green functions calculated by the spectral‐element method. The multitaper traveltime difference measurement is applied in four‐period bands: 20–35 s, 15–30 s, 10–20 s, and 6–15 s. The recovered model shows detailed crustal structures including pronounced low‐velocity anomalies in the lower crust and a gradual crust‐mantle transition zone beneath the northern Trans‐North China Orogen, which suggest the possible intense thermo‐chemical interactions between mantle‐derived upwelling melts and the lower crust, probably associated with the magmatic underplating during the Mesozoic to Cenozoic evolution of this region. To our knowledge, it is the first time that ambient noise adjoint tomography is implemented for a 2‐D medium. Compared with the intensive computational cost and storage requirement of 3‐D adjoint tomography, this method offers a computationally efficient and inexpensive alternative to imaging fine‐scale crustal structures beneath linear arrays.