Anisotropic regime across northeastern Tibet and its geodynamic implications
Graphical abstract
Introduction
The continental collision of the northward-drifting Indian plate and the relatively stationary Asian plate ~ 55 million years ago caused formation of the Tibetan plateau and continued to propel the growth and expansion of this plateau through on-going underthrusting of the Indian plate. As the leading edge of the plateau's northeastward expansion, the northeastern margin of Tibetan plateau (NE Tibet) are currently undergoing shortening/thickening and topographic uplift as being incorporated into the plateau (Meyer et al., 1998). To clarify the mode of lithospheric deformation in this boundary area is thus a key issue for understanding the mechanism of plateau growth and expansion. In addition to imaging the lithospheric structure in a direct way by using seismological methods such as receiver function and tomography, seismic anisotropy is another important facet that serves to examine deformation in the lithosphere and upper asthenosphere.
Shear wave splitting provides us with an effective method to identify the seismic anisotropy in the upper mantle. The method gives the measurements of the delay time between the two orthogonally polarized quasi-shear waves and the polarization direction of the fast quasi-shear wave (Silver, 1996, Savage, 1999). Quite a few shear wave splitting measurements have been carried out in NE Tibet, most of which are base on dataset of regional seismic networks (e.g., Li et al., 2011a, Li et al., 2011b, Zhang et al., 2012b, León Soto et al., 2012). These previous studies generally tended to support the vertically coherent deformation of the lithosphere (England and Houseman, 1986, Holt, 2000, Flesch et al., 2005, Wang et al., 2008) while Li et al. (2011b) argued for significant decoupling between the crust and mantle in favor of crustal flow (Clark and Royden, 2000, Royden et al., 2008).
The published results based on regional seismic networks show us an overview of the regional anisotropy. Herein we add new data from a dense linear array to the dataset of observations to show the detailed knowledge of lateral variations of seismic anisotropy across NE Tibet. This is of great significance for more thorough understanding of deformation in this boundary area. Results of shear wave splitting based on a 550 km long and dense broadband seismic profile are present here. The profile traverses the entirety of NE Tibet and extends northward to the southern Alxa block (the westernmost NCC) (Fig. 1). We inspected the lateral variations of seismic anisotropy along the profile in detail and attempted to gain a new insight into the lithospheric deformation in NE Tibet, by integrating our analysis on anisotropy with other correlated geological/geophysical information.
Section snippets
Data and methods
We deployed 38 three-component broadband seismograph stations in NE Tibet in October 2011 and kept them in good running for 17 months till March 2013. Each of the stations was equipped with a Guralp 3T/3ESP sensor (32 Guralp 3T and 6 Guralp 3ESP) and a Reftek-130 data acquisition system. This observation profile is oriented NNE with an average station interval of ~ 15 km (Fig. 1).
Shear wave splitting analysis was applied on the teleseismic seismograms of earthquakes with magnitudes Ms ≥ 5.3, using
Lateral variations of splitting parameters across NE Tibet
We finally obtained a total of 218 pairs of well-defined XKS splitting parameters (Supplementary Table S2) from high-quality measurements on the seismograms of 78 teleseismic events (Fig. 1C, Supplementary Table S1). The epicenters show a relatively good azimuthal coverage based on which we divided the events into three back-azimuthal (BAZ) ranges: BAZ-1 (90–180°), BAZ-2 (− 45–45°) and BAZ-3 (180–315°), as shown in Fig. 1C. We then stacked the splitting observations of the events in each BAZ
Depth localization of anisotropy
The seismic anisotropy detected by XKS wave splitting is a composite result along the ray path from the core–mantle boundary to the receiver-side surface. Fresnel zone analysis (e.g., Alsina and Snieder, 1995) can help us estimate the depth of the anisotropy. This method is based on the idea that the different splitting observations at two closely located stations are mainly contributed by the non-overlapped part of the Fresnel zones. Here we also observed that significant variations of
Geodynamic implications and conclusions
XKS wave splitting measurements along our seismic array reveals significant lateral variations of seismic anisotropy across NE Tibet. Fresnel zone analysis indicates the anisotropy should be mainly confined to lithospheric scale. The shear wave velocity contrast between the Qilian–Alxa blocks and the KL-WQL–SPGZ blocks was revealed by our observation of large decrease of SKS travel-time residuals from south to north across the WQLF zone as well as previous tomography studies. It supports the
Acknowledgment
We are grateful to Earthquake Administration of Gansu Province for their generous assistance with field work. Data used was authorized and provided by Sinoprobe, a Chinese government-funded earth science program (http://sinoprobe.cags.ac.cn/index.html). This work was supported by Sinoprobe02 with grants 201011042 and 201311156, the National Natural Science Foundation of China (grants 41430213, 41590863, 41174081 and 41274096) and China Geological Survey 12120115027101.
References (49)
- et al.
Crust and upper mantle structure of the North China craton and the NE Tibetan plateau and its tectonic implications
Earth Planet. Sci. Lett.
(2013) - et al.
Tearing of the Indian lithospheric slab beneath southern Tibet revealed by SKS-wave splitting measurements
Earth Planet. Sci. Lett.
(2015) - et al.
Constraining the extent of crust–mantle coupling in central Asia using GPS, geologic, and shear wave splitting data
Earth Planet. Sci. Lett.
(2005) - et al.
Seismic anisotropy and implications for mantle deformation beneath the NE margin of the Tibet plateau and Ordos plateau
Phys. Earth Planet. Inter.
(2011) - et al.
Seismic anisotropy of the Northeastern Tibetan Plateau from shear wave splitting analysis
Earth Planet. Sci. Lett.
(2011) - et al.
Mantle flow and lithosphere–asthenosphere coupling beneath the southwestern edge of the North American craton: constraints from shear-wave splitting measurements
Earth Planet. Sci. Lett.
(2014) - et al.
Amount of Asian lithospheric mantle subducted during the India/Asia collision
Gondwana Res.
(2013) - et al.
Active thrusting and folding in the Qilian Shan, and decoupling between upper crust and mantle in northeastern Tibet
Earth Planet. Sci. Lett.
(1990) - et al.
Amphibole and lower crustal seismic properties
Earth Planet. Sci. Lett.
(2008) - et al.
Crustal structure and Moho geometry of the Northeastern Tibetan plateau as revealed by SinoProbe-02 deep seismic-reflection profiling
Tectonophysics
(2014)
SplitLab: a shear-wave splitting environment in Matlab
Comput. Geosci.
Identifying global seismic anisotropy patterns by correlating shear-wave splitting and surface waves data
Phys. Earth Planet. Inter.
Seismic evidence for the North China plate underthrusting beneath northeastern Tibet and its implications for plateau growth
Earth Planet. Sci. Lett.
Crustal structure across northeastern Tibet from wide-angle seismic profiling: constraints on the Caledonian Qilian orogeny and its reactivation
Tectonophysics
Upper mantle deformation beneath central–southern Tibet revealed by shear wave splitting measurements
Tectonophysics
Small-scale sublithospheric continental mantle deformation: constraints from SKS splitting observations
Geophys. J. Int.
Development of lattice preferred orientation in clinoamphiboles deformed under low-pressure metamorphic conditions. ASEM/EBSD study of metabasites from the Aracena metamorphic belt (SW Spain)
J. Struct. Geol.
Observations of frequency-dependent Sn propagation in northern Tibet
Geophys. J. Int.
Shear-wave splitting in the upper-mantle wedge above the Tonga subduction zone
Geophys. J. R. Astron. Soc.
Topographic ooze: building the eastern margin of Tibet by lower crustal flow
Geology
Finite strain calculations of continental deformation; 2, comparison with the India–Asia collision zone
J. Geophys. Res.
Structure of the crust and mantle down to 700 km depth beneath the East Qaidam basin and Qilian Shan from P and S receiver functions
Geophys. J. Int.
Present-day crustal motion within the Tibetan plateau inferred from GPS measurements
J. Geophys. Res.
Lithospheric structure and geodynamic model of the Golmud–Ejin transect in northern Tibet
Cited by (36)
Northeastward expansion of the Tibetan Plateau: Seismic anisotropy evidence from shear-wave splitting measurements
2022, Journal of Asian Earth SciencesCitation Excerpt :The XKS splitting measurement is a composite result along the ray path with poor depth resolution, and the origin of anisotropy can root at any depth between the receiver stations and the core-mantle boundary. Although the depth resolution is relatively poor, the depth localization of anisotropy obtained by XKS splitting measurements has been constrained by some previous researches (e.g., McNamara et al., 1994; Herquel et al., 1995; Li et al., 2011b; Huang et al., 2013; Ye et al., 2016). Thus, the origin of seismic anisotropy observed in the present study region should be firstly determined.
A distinct contrast in the lithospheric structure and limited crustal flow across the northeastern Tibetan Plateau: Evidence from Vs and Vp/Vs imaging
2022, TectonophysicsCitation Excerpt :While the Kunlun-West Qinling block (~3.5 km/s average Vs in the crust and ~ 4.35 km/s average Vs in the uppermost mantle) represents a transition to the hotter and weaker lithosphere characteristic of the Songpan-Ganzi block (~3.4 km/s average Vs in the crust and ~4.25 km/s average Vs in the uppermost mantle). SKS travel-time residuals are significantly decreased from the Songpan-Ganzi and Kunlun-West Qinling blocks to the eastern Qilian and Alxa blocks across the West Qinling fault (blue dots in Fig. 4, Ye et al., 2016). In addition, Rayleigh surface wave imaging results reveal that the Qilian block and the Alxa block display higher shear-wave velocities, compared with those of the Songpan-Ganzi and Kunlun-West Qinling blocks (e.g. Li et al., 2013).