Seismic Tomographic Imaging of the Crust and Upper Mantle Beneath the Northeastern Edge of the Qinghai‐Xizang Plateau and the Ordos Area

We investigate the P‐wave velocity structure of the crust and upper mantle down to depth 400km beneath the Ordos region and northeastern Qinghai‐Xizang plateau using the seismic tomographic technique. The data used are taken from the record by the regional network and a passive seismic observation profile across the investigated area, respectively. Our results show clear lateral variations of the crust and upper mantle velocity structure along the profile, which exist not only between different blocks, but also inside blocks. The average P‐wave velocity of the crust and upper mantle beneath the northeastern margin of the Qinghai‐Xizang plateau is lower, and that beneath the Ordos block is higher than the global model. Our observations are coincident with the tectonic activities of these two blocks. Between the northeastern Qinghai‐Xizang and Ordos block exists a transition zone about 200km wide, which on the surface corresponds to the region between Lanzhou and Haiyuan. This transition zone in the upper mantle appears as a stripe with a slope of about 45‐ tilting eastward. Its main feature appears as a mixture of high‐ and low‐velocity media. The strike of this stripe in the upper mantle seems to be controlled by the high‐velocity body beneath the Ordos block. This implies that the stable Ordos block plays a role in resisting the eastward movement of the Qinghai‐Xizang plateau. The upper mantle velocity structure beneath the Qaidam basin in the Qinghai‐Xizang plateau has a low average velocity and that beneath the Qilian block has a high average velocity. The difference between both reaches about 8%. We also find the intrusive low‐velocity bodies in uppermost mantle beneath Zêkog, Lanzhou and Haiyuan areas, respectively. The high seismicity in the north‐south seismic zone should be related closely to the medium heterogeneity and very special deep dynamic environment.