Multistage tectonic evolution of the Tanlu fault: Insights from upper
crustal azimuthal anisotropy of the Chao Lake segment
Abstract
The Tanlu fault is the largest strike-slip fault in eastern China. It
has experienced multiple episodes of tectonic activity suggested mostly
by near-surface geological analysis. However, little evidence comes from
deformation at different depths in the crust. This study presents a
three-dimensional (3-D) high-resolution upper crustal azimuthally
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) extracted from ambient seismic noise. The results show that
fast directions of azimuthal anisotropy are consistent with local
geological units, hence interpreted as shape-preferred orientations
(SPOs). A special depth-dependent anisotropic pattern is revealed
beneath eastern Chao Lake, where the fast directions vary from nearly
NE-SW at shallow depths, to NNE-SSW or nearly N-S at intermediate depths
and back to NE-SW at great depths. The anisotropic model is combined
with other previously published datasets to construct a deformation
model at multiple depths. The deformation model suggests that
lithospheric deformation is decoupled from the asthenosphere beneath the
TFZ-CL, implying that the deep tectonic setting could have changed
dramatically since the Mesozoic. By connecting the new anisotropic model
and the local tectonic episodes, we further propose a four-stage
tectonic evolution model of the TFZ-CL. The model can well interpret the
generation of anisotropies, especially the specific anisotropic pattern
beneath eastern Chao Lake, providing new evidence of deformation at
great depths to support the multistage tectonic episodes along the Tanlu
fault.