2D Tomographic imaging of the P-wave velocity structure in the upper crust beneath the southern Beishan tectonic belt

doi:10.13745/j.esf.sf.2022.2.2

Abstract

Abstract:

The Huahai Basin-Beishan tectonic belt is located north of the northeastern margin of the Qinghai-Tibetan Plateau and is the junction zone connecting the Tethys and Paleo-Asian Ocean domains. Since the Late-Neoproterozoic it has undergone multi-era, multi-stage evolution involving multiple plate splitting, subduction, collision and merging events. The subsequent overthrusting and strike-slip faulting since the Mesozoic, in particular the northward expansion of the northeastern margin of the Tibetan Plateau caused by the far-field effect of the collision between the Indian and Eurasian plates in the Cenozoic, formed the present complex geological/geomorphological structure. The crustal structure records the overprinting of the tectonics, whereas the upper crust is a natural notebook valuable for understanding the outward growth of the NE Tibet and its role in the transformation of the adjacent tectonic units. In this paper, based on the first arrive seismic wave (Pg phase) data from the 180 km-long deep seismic reflection profile completed by the Chinese Academy of Geological Sciences in 2018, we applied the tomographic inversion method to determine the P-wave velocity structure in the upper crust beneath the Huahai Basin-southern Beishan tectonic belt, 0-4 km deep underground. It was found that the three basins, Huahai, Zongkouzi and Zhagehao Basins, have a relatively low P-wave velocity and small vertical velocity gradient; the Late-Paleozoic granite outcrop shows obvious high-velocity anomalies and large vertical velocity gradient; and the left strike-slip Altyn fault zone across the southern margin of the Huahai Basin is a north-dipping high-angle strike-slip fault as deep as cutting through the basin basement at the least. In addition, many low-velocity anomalies revealed the extend of fault development in the southern Beishan.

Key words: southern Beishan, Huahai Basin, tomography, upper crustal structure

CLC Number:

P631.4
P542

WU Guowei, XIONG Xiaosong, GAO Rui, CHEN Xuanhua, LI Yingkang, WANG Guan, WANG Xiaocheng, REN Haidong. 2D Tomographic imaging of the P-wave velocity structure in the upper crust beneath the southern Beishan tectonic belt[J]. Earth Science Frontiers, 2022, 29(2): 402-415.

Figures/Tables 11

Fig.1 Geological map of the study area showing the location of the deep seismic reflection profile (survey line in red line)and the geological structure of the area. Modified after [1].

Fig.2 Simplified geological map of the Huahai Basin-Beishan tectonic belt. Blue line is the survey line of the deep reflection profile, where the numbers indicate the distance between the sampling points and the south end of the profile.

Fig.3 Flowchart of seismic wave travel tomography data processing

Table 1 Data sampling parameters

震源类型	炸药量/kg	炮间距/m	井深/m	井数	道间距/m	接收道数
中炮	150	960	45	2	40	960
小炮	48	240	33	1	40	960

Fig.4 First arrival pickup of single shot records. Red line indicates the first arrival time of the pickup for all channels

Fig.5 First-arrival pickup results of all single-shot records

Fig.6 Initial 1D velocity model

Fig.7 Iterative convergence curve for tomographic inversion

Fig.8 Plot of the elevation along the survey line (a), the velocity structure of the tomographic inversion (b) and the ray density distribution map of the tomographic inversion (c)

Fig.9 Plots of first arrival travel time (a) and fitted travel time from the final model (b) vs. survey distance, and superposition of the two plots (c)

Fig.10 (a) Topographic map along the survey line, (b) velocity profile obtained by inversion, and (c) interpretation of the velocity profile. In c the black dotted line indicates the inferred position of the top of the crystalline basement in the basin, and red solid line the inferred fault.

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