A Preliminary Study on Crustal Velocity Structures of Maqin‐Lanzhou‐Jingbian Deep Seismic Sounding Profile

Abstract

[Abstract] The 1 000 km-long Marqin-Lanzhou-Jingbian comprehensive geophysical profile was deployed for the purpose of studying the interaction between the Qinghai-Xizang Plateau block and the Ordos block as well as for studying the deep driving mechanism of tectonic deformation. This paper presents only the preliminary results obtained with deep seismic sounding (DSS) observation data. The crust is clearly stratified. On the whole, it can be divided into two parts, i.e., the upper crust and the lower crust, with interface C as the boundary surface. Each of the two parts contains a number of secondary interfaces. The general trend of lateral variation is that the crust becomes gradually thicker from northeast to southwest, which is mainly due to the variation in the thickness of the lower crust. The average crustal velocity tends to become gradually lower from northeast to southwest-the velocity is obviously low west of Zeku and in Haiyuan region. A number of crustal low velocity layers exist west of Zeku and another low velocity layer exists near Haiyuan. The number of reflecting horizons increases gradually from northeast to southwest. As seen in seismic reflection waveforms, Pc wave is very strong at Haiyuan and it also lasts long. Besides, the complexity coefficients of Pm wave in Haiyuan and west of Zeku are extremely large, much larger than those in other parts. The above results clearly show that anomalous structure exists in the crust and upper mantle west of Zeku and in Haiyuan region, which reflects the interactions between Bayankar block and Chaidamu block, and between Qilian block and Ordos block.

Electrical Conductivity Structure of the Crust and Upper Mantle Along the Profile Maqên-Lanzhou-Jingbian in the Northeastern Margin of the Qinghai-Tibet Plateau

“…But the boundary between the low-resistivity layer and lower part of the layer with increasing resistivity is not clear. At the same position the interface C4 (velocity interface in the lower crust) found in the seismic sounding profile also cannot be well determined [22] . Because of very low resistivity in the surface layer and the existence of low-resistivity layer in the middle-lower crust within the block, the penetration of electromagnetic wave can reach only the depth near the Moho discontinuity, although the period for the measurement data reached 4000s.…”

North China sub-craton lithospheric structure elucidated through coal mine blasting

“…In addition, these waves are relatively stable, without striking changes. Many results show that there is no volcanic activity, no signs of magma intrusion, no strong earthquakes, very few small earthquakes, no low velocity body, a higher P-wave velocity, simple stratification, and no large crustal interface fluctuation [19][20][21] in the interior and ground of the Ordos block, which shows that the deepseated crustal structures within the Ordos block are stable. Continuous, reliable Pm1, Pm2 waves with stable kinematic features may indicate that there is a stable upper-mantle structure within the Ordos block.…”

“…The Pm waves from 154 to 261 km, with clear and continuous phases, simple waveforms, and higher velocities, indicate that the structural inhomogeneity of the Ordos block is not obvious, its structure is simple, and that it is relatively stable. This shows that the Moho is a first-level discontinuity [19][20][21] . The average velocity of this wave is 6.20-6.30 km/s, and the depths of the Moho discontinuity are approximately 41.0-45.0 km.…”

“…The results of deep seismic sounding indicate that, within the crust, there are no low velocity bodies, higher average velocity of the P wave, simpler stratification, and small fluctuation of crustal interfaces. Many results have revealed the stability of the crustal structure within the Ordos block [19][20][21]27] . The continuous, distantly traced, and reliable refracted-wave Pn obtained on the super-range profile as well as the Pm1 and Pm2 waves with strong reflection features and stable kinematic features, tend to suggest that there is a stable deep-seated structure within the Ordos block.…”

Deep seismic sounding data reveal the crustal structures beneath Zoigê basin and its surrounding folded orogenic belts

“…Reflections from interfaces of the crust, P1, P2, P3, P4, and P5, also can be identified on the record sections. The tectonic features in the northeastern margin of Tibetan Plateau, where the structures are characterized by low Pg velocity, weak reflections of the Moho, and strong complicated reflections from mid and lower crust, are quite different from that in stable massifs of the periphery of the plateau, where the structures are characterized by smoothly reflected wave Pg, strong reflections of the Moho, and "transparent" mid and lower crust [8,9]. Obvious variations in travel time and intensity of these seismic phases appear among different tectonic units even inside the northeastern margin of Tibetan Plateau [10].…”