Upper mantle anisotropy in the Ordos Block and its margins

Abstract

Based on the polarization analysis of teleseismic data, SKS (SKKS) fast-wave directions and delay times between fast and slow shear waves were determined for each of the 111 seismic stations from both permanent and temporary broadband seismograph networks deployed in the Ordos Block and its margins. Both the Silver and Chan and stacking analysis methods were used. In this way, an image of upper mantle anisotropy in the Ordos Block and its margins was acquired. In the western and northern margins of the Ordos Block, the fast-wave directions are consistently NW-SE. The fast-wave directions are mainly NWW-SEE and EW in the southern margin of the Ordos Block. In the eastern margin of the Ordos Block, the fast-wave directions are generally EW, although some run NEE-SWW or NWW-SEE. In the Ordos Block, the fast-wave directions trend near N-S in the north, but switch to near EW in the south. The delay time between fast and slow waves falls into the interval 0.48-1.50 s, and the average delay time at the stations in the Ordos Block is less than that in its margins. We suggest that the anisotropy of the stable Ordos Block is mainly caused by "fossil" anisotropy frozen in the ancient North China Craton. The NE-trending push of the northeastern margin of the Tibetan Plateau has caused NW-SE-trending lithospheric extension in the western and northern margins of the Ordos Block, and made the upper mantle flow southeastwards. This in turn has resulted in the alignment of the upper mantle peridotite lattice with the direction of material deformation. In the southern margin of the Ordos Block, the collision between the North China and Yangtze blocks resulted in the fast-wave direction running parallel to the collision boundary and the Qinling Orogen. Combining this with the APM and velocity structure of the Qinling Orogen, we propose that eastward-directed asthenospheric-mantle channel flow may have occurred beneath the Qinling Orogen. In the eastern margin of the Ordos Block, the complex anisotropic characteristics of the Fenhe Graben and Taihang Orogen may be caused by the interaction of western Pacific Plate subduction, regional extensional tectonics, and the orogeny. For station YCI, the apparent splitting parameters (the fast-wave directions range from 45° to 106° and the delay times range from 0.6 to 1.5 s) exhibit systematic variations as a function of incoming polarization with a periodicity of /2. This variation can be best explained by a two-layer anisotropic model ( lower = 132°, t lower = 0.8 s,  upper = 83°, t upper = 0.5 s). The upper layer anisotropy beneath station YCI can again be attributed to "fossil" anisotropy frozen in the ancient North China Craton. The lower layer anisotropy is affected by the tectonic activity of the western Ordos Block. The NW-SE trending extension caused by the NE trending push of the northeastern margin of the Tibetan Plateau affected the deformation of the lower anisotropic layer beneath station YCI. By comparing the fast-wave directions with GPS velocity directions, we see that ...

Distinct upper mantle deformation of cratons in response to subduction: Constraints from SKS wave splitting measurements in eastern China

“…Zhao et al, 2007;L. Zhao et al, 2007;Liu et al, 2008;Wang et al, 2008;Chang et al, 2009;Bai et al, 2010;Li and Niu, 2010;Chang et al, 2011;Huang et al, 2011;Li et al, 2011;Zhao et al, 2011). We note however that some of these studies obtained fast directions that varied by more than 30°for a given station.…”

“…This report showed that the uniform fast directions of the anisotropy in eastern China (WNW-ESE) are generally consistent with the absolute plate motion direction of the Eurasian plate, suggesting that the anisotropy is mainly located in the asthenosphere. Regional studies using better seismic station coverage (e.g., Zhao and Zheng, 2005;Liu et al, 2008;Bai et al, 2010;Li and Niu, 2010;Zhao and Xue, 2010;Chang et al, 2011;Li et al, 2011;Zhao et al, 2011) show greater spatial variations in splitting parameters however.…”

“…Previous regional studies have focused on the North China Craton, Northeast China or Tibet (Huang et al, 2000;Zhao and Zheng, 2005;Lev et al, 2006;Zhao and Zheng, 2007;Liu et al, 2008;Wang et al, 2008;Zhao et al, 2008;Bai et al, 2010;Li and Niu, 2010;Zhao and Xue, 2010;Chang et al, 2011;Li et al, 2011;Zhao et al, 2011). For example, a series of shear wave splitting measurements performed by Zhao andZheng (2005, 2007), D. Zhao et al (2007), L. Zhao et al (2007), Zhao et al (2008Zhao et al ( , 2011, and Zhao and Xue (2010) using data from 571 broadband stations, revealed relatively complicated upper mantle patterns beneath the North China Craton and adjacent areas.…”

Three‐dimensional lithospheric S wave velocity model of the NE Tibetan Plateau and western North China Craton

“…The Alxa block and the Ordos block, which are traditionally considered to be parts of the western North China Craton (NCC), have resisted the northeastward expansion of the Tibetan Plateau [ Chang et al ., ; Shen et al ., ]. Whether or not these two blocks have been modified is a subject of debate.…”

Phase Velocity Tomography of Rayleigh Wave in Qinling‐dabie and Its Adjacent Areas Using Ambient Seismic Noise

“…Large areas of high-pressure metamorphic rocks (HP) and ultrahigh pressure metamorphic rocks (UHP) have been exposed in the Hong'an and Dabie areas, resulting in a higher degree of research but a lower level in other areas. In addition, many scholars also have great differences and disputes on the existence of the Tibetan Plateau eastward flow channel under the Qinling Mountains orogenic belt (Enkelmann et al, 2006;Chang et al, 2011;He et al, 2015;Zhang et al, 2015;Wang et al, 2016). The main advantage of noise imaging is that it can obtain more short period dispersion data without relying on the location of the source, and the resolution of the shallow crust is greatly improved (Wang and Gao, 2014).…”