Multistage tectonic evolution of the Tanlu fault: Insights from upper-crustal anisotropies of the Chao lake segment

Abstract:

Tanlu fault is the largest strike-slip fault in eastern China. It experienced multiple tectonic activities suggested mostly by geological analysis. However, little evidence is from deformations of different depths in the crust. This study presents a high-resolution upper-crustal anisotropic model of shear-wave velocity beneath the Chao lake segment of the Tanlu fault zone (TFZ-CL) using surface wave dispersion data (0.5–12 s) measured from ambient seismic noise. Results show that the anisotropies of fast directions are well consistent with local geological units. (1) The Hefei basin displays NW-SE direction, parallel or subparallel to strikes of the Dabie orogenic belt. (2) The Yinping mountain shows NE-SW fast direction, parallel to the strikes of mountains. (3) The Tanlu fault zone presents complex fast direction at shallow depths and becomes stable at NE-SW direction at great depths. (4) The eastern Chao lake displays a special depth-dependent anisotropic pattern with anisotropy fast directions that vary from NE-SW at shallow depths, NW-SE at intermediate depths, and back to NE-SW at great depths. Combining previous geological researches, we suggest that the newly revealed anisotropic patterns might be associated with fabrics generated by the convergence between the South and North China plates and the multistage tectonic activities of the Tanlu fault. We construct a deformation model at multiple-scale depths, which suggests that the lithosphere deformations are decoupled beneath the TFZ-CL, implying that the entire tectonic background changed dramatically since Mesozoic. We further propose a multistage evolution model of the TFZ-CL. The model well interprets the generation of anisotropies especially the special anisotropic pattern beneath the eastern Chao lake. The anisotropic model obtained in this study could provide new evidence of deformations at deep depths for the multistage tectonic episodes of the Tanlu fault.