Magnetic Structure and its Tectonic Implication Around Longmenshan Fault Zone Revealed by EMAG2v3

Yu Lei,Jiyao Tu,Qinghua Huang,Liguo Jiao,Junhao Zhao

doi:10.3389/feart.2022.848824

Yu Lei, Jiyao Tu + Show 3 more

Open Access

https://doi.org/10.3389/feart.2022.848824

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Abstract

The formation of magnetic minerals is bound up with the tectonic evolution history, whereupon the distribution of magnetic anomalies has great meanings for regional tectonics. In this study, we use the latest global lithospheric magnetic field model EMAG2-v3, processed by various techniques including reduction to the pole (RTP), upward continuation, derivations, Euler deconvolution, estimation of total magnetization direction, and Curie point depth (CPD), to unveil the tectonics around Longmenshan fault zone (LFZ). LFZ is clearly displayed as a positive and negative anomaly transition zone in RTP anomalies and acts as a magnetic basement boundary. The Sichuan Basin (SB), located to the east of LFZ, is marked by strong magnetic basement and NE-strike banded induced positive anomalies which are associated with the Neoproterozoic magmatic activity. The banded shape, absence of radial pattern of anomalies, and existence of fossil subduction zone supports that the magnetic basement was formed in arc environment. The CPD in SB estimated by radial average power spectral is 30–51 km, which allows magnetic minerals in deep crust or even in lithospheric mantle to exhibit high magnetizations. The Songpan-Ganzi fold belt (SGFB), in contrast, is located to the west of LFZ and covered by thick and low-susceptibility Triassic deposits of flysch, manifests as weak negative anomalies caused by relatively shallow CPD and widespread remanent magnetization. Significant positive anomalies, appearing around the Manai and Rilonguan granitic massifs, indicate a strong magnetic basement beneath SGFB, which is conjectured as westward extension of the Yangtze Block at depth.

Highlights

The Longmenshan Fault Zone (LFZ) is one of the most active seismogenic zones in Mainland China, originated from the stress transfer of collision between the Indian and Eurasian plate
Good correspondence exists between the reduction to the pole (RTP) anomalies and tectonic structure, in that the positive anomalies delineate the shape of magnetic basement beneath Sichuan Basin (SB)
The Curie point depth (CPD) of SB estimated by power density spectrum (PDS) shows that the depth to the bottom of the magnetic basement is 30–51 km, which allows strong magnetized rocks to be preserved and exhibit strong anomalies

Summary

INTRODUCTION

The Longmenshan Fault Zone (LFZ) is one of the most active seismogenic zones in Mainland China, originated from the stress transfer of collision between the Indian and Eurasian plate. Xiong et al (2016b), Wang et al (2018b), Gao et al (2015) used magnetic data derived from the aeromagnetic survey, EMAG2 grid model, and NGDC-720 model to investigate the CPD They all showed depression area of CPD in SB corresponding to the stable Yangtze Block, the range and distribution pattern are different. Magnetotoluric and seismological studies indicated that there is a channel flow in the middle and lower crust of SGFB (Royden et al, 1997; Bai et al, 2010; Wang et al, 2015b, 2017a), where the Tibetan Plateau material moves, but is blocked by the cold YB, and shifts to southeast and northeast corresponding to the shallow CPD of Qinling orogenic belt on northern SB (27–38 km) and the western part of Xianshuihe fault (30–36 km). The stable magnetic basement of SB is largely dominated by induced magnetization

DISCUSSION

CONCLUSION

DATA AVAILABILITY STATEMENT