科研之友
微信
新浪微博
Facebook
分享链接
摘要
The north-south tectonic belt (NSTB) is a north-south meridional tectonic boundary between the eastern and western Chinese mainland with a very complex structure, showing significant changes in geology, geomorphology, and geophysical field characteristics on both sides. Meanwhile, the NSTB is seismically active zone, so also named the well-known north south seismic belt (NSSB). Thus, the NSTB is regarded as a unique natural laboratory for understanding continental interiors and lithospheric deformation. The study region of this paper is located in the northern NSTB, including the Songpan-Garze and Kunlun-Qilian fold belts, which are terranes of the northeastern margin of the Tibetan Plateau, and the Alxa and Ordos blocks which are west portions of the North China craton (NCC). This work uses knowledge of seismic anisotropy to provide important constraints on deformation patterns of the crust and lithosphere mantle during an orgency process. It is based on 695 new shear-wave splitting observations from a dense temporary seismic array and 122 published results from permanent seismic stations to map variations in the deformation of the northern segment of the NSTB. The new XKS (SKS, SKKS, and PKS) shear wave splitting observations include 674 measurements from portable deployments in the NSTB (2013-2015, the ChinArray Phase II) and 21 measurements from temporary stations deployed in the Ordos block (2010-2011, the Ordos Array). We determine the XKS fast wave polarization directions and delay times between fast and slow shear waves for 695 new seismic stations in the northern segment of the NSTB using both the grid searching method of minimum transverse energy and stacking analysis method. To obtain a reliable estimate of splitting parameters, the following criteria are taken as diagnostics for successful splitting parameter estimations: (1) Clear XKS arrivals and distinct tangential component. (2) The horizontal particle motion is elliptical when anisotropy is present. (3) The two horizontal fast- and slow-component waveforms are coherent. (4) The particle motion becomes linear following correction for anisotropy. And (5) successful removal of tangential energy in the case of core phases. The results at most stations are good, the error of azimuth is less than 100, and the error of delay time is less than 0. 2 s. The fast polarization directions and delay times do not depend on back azimuths, thus a single layer of anisotropic fabric is able to sufficiently explain the data without the need for additional layer. Based on 817 observations, we develop an anisotropic image of upper mantle in the northern segment of the NSTB. In the study region, the fast-waves trend in NW SE in the northeastern margin of the Tibetan Plateau, Alxa block, and western and northern margins of the Ordos block. The fast-wave directions are in nearly E-W in the Qinling orogen. Within the Ordos block, the fast-wave directions trend in nearly N-S in the north, but switch to nearly E-W in the south. The value of delay time in the Ordos block is not only less than that in its margins, but also less than that in other tectonic units. Especially, the value of delay time in the conjunction of the northeastern margin of the Tibetan Plateau, Alxa block and Ordos block is the largest and considerably larger than other areas. This implies the value of delay time in the stable units is less than that of the active units. Analysis of the fit between the fast-wave direction of shear-wave splitting and predicted fast axis orientation calculated from the surface deformation field indicates the coherence between surface deformation and mantle deformation in the northeastern margin of the Tibetan Plateau, Alxa block, western and northern margins of the Ordos block, and the crust is coupled with the mantle. These results suggest the vertical coherent deformation of the lithosphere plays a major role in the observed seismic anisotropy. In the Qinling orogen, both the lithosphere mantle and eastward asthenospheric mantle flow contribute to the observed anisotropy. Within the Ordos block, there exist weak anisotropy and thick lithosphere, and the shallow deformation is inconsistent with the deep deformation, suggesting the anisotropy of the stable Ordos block is possibly caused by "fossil" anisotropy frozen in the ancient NCC.
- 出版日期2016-11
- 单位中国地震局
