Azimuthally Anisotropic Structure in the Crust and Uppermost Mantle in Central East China and Its Significance to Regional Deformation Around the Tan‐Lu Fault Zone. Issue 3 (1st March 2022)

• Record Type: Journal Article
• Authors: Bem, Terhemba Shadrach; Liu, Chuanming; Yao, Huajian; Luo, Song; Yang, Yuan; Liu, Bin
• Is Part Of: Journal of geophysical research. Volume 127:Issue 3(2022)
• Page Start: n/a
• Page End: n/a
• Publication Date: 2022-03-01
• Subjects: ambient noise tomography -- azimuthal anisotropy -- crust and uppermost mantle -- deformation -- central‐southern Tan‐Lu Fault -- central East China; Geomagnetism -- Periodicals; Geochemistry -- Periodicals; Geophysics -- Periodicals; Earth sciences -- Periodicals; 551.1
• DOI: 10.1029/2021JB023532
• Languages: English
• ISSNs: 2169-9313

Abstract

Abstract: Seismic anisotropy can be linked to different mechanisms, structures, and sources. We constrained the 3‐D depth‐dependent shear wave speed isotropic and azimuthally anisotropic structures with Rayleigh wave phase velocity from ambient noise in central Eastern China. We compared the results with other models and found that in the upper crust, the maximum horizontal stress direction differs mainly from the anisotropy that coincides with the strikes of major faults and geologic structures. We inferred that the anisotropy within proximity of faults might be related to fault fabrics, including fractures that strike parallel to the fast axes. The Tan‐Lu Fault controls the anisotropy in the crust to the uppermost mantle. The direction of fast axes in the fault zone is NNE‐SSW to N‐S, coinciding with the fault strike and trend that matches the fault shape on the surface. Southeastern North China craton has a weak anisotropy in the crust and uppermost mantle. The sources are possibly connected to the thrusting, tight trending folds, and dipping thrusts observed to be sub‐parallel to the Dabie Orogenic Belt (DOB). We propose that the NNE‐SSW Zhangbaling uplift group comes from the extension and ductile shearing in the Tan‐Lu Fault zone and the dragging of the northeastern South China Craton in the crust and the uppermost mantle. The N‐S polarized fast axes in the lower crust and uppermost mantle across the Sulu orogen and Xuzhou thrust‐and‐fold belt are the products of the resultant effect of subduction and deep Tan‐Lu faulting. Plain Language Summary: Seismic anisotropy is the dependence of seismic velocity of the medium on wave propagation direction. When observed, it gives information about the nature of material deformation. We used surface waves recovered from background noise to determine the anisotropy and its connection to the Tan‐Lu fault system, the largest fault system in Eastern China. Our results show that seismic anisotropy in the upper crust is likely caused by fault zone fabrics and the regional stress field. In the middle crust to the uppermost mantle, the anisotropy is parallel to the strike of the Tan‐Lu fault, indicating that the fault penetrates to the uppermost mantle and controls the anisotropy. The southeastern North China craton has weak anisotropy in the crust and uppermost mantle. The cause of anisotropy is likely the thrust faults resulting from compression that strike similar to the trend of the Dabie Orogenic Belt. The extension and shearing in the Tan‐Lu Fault zone is likely what caused the anisotropy at Zhangbaling uplift. The direction of anisotropy changes to N‐S in the north of the study region in the lower crust and uppermost mantle. The cause might be related to the resultant effect of subduction and deep Tan‐Lu faulting. Key Points: Azimuthal anisotropy in the crust and uppermost mantle deviates from stress, shear‐wave splitting, and absolute plate motion direction Within proximity of regional faults, anisotropy might be related to fault fabrics including fractures that strike parallel to the fast axes The direction of anisotropy coincides with the strike of the Tan‐Lu Fault signifying structural sources in the crust and uppermost mantle