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 π/2. This variation can be best explained by a two-layer anisotropic model (φ lower = 132°, δt lower = 0.8 s, φ upper = 83°, δt 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 the crust and upper mantle possibly have vertically coherent deformation in the margins of the Ordos Block, whereas the internal deformation characteristics of the Ordos Block are complex and require further study.