Wang et al. Earth, Planets and Space (2020) 72:40 https://doi.org/10.1186/s40623-020-01167-5 FULL PAPER Open Access 3D inversion of the Sichuan Basin magnetic anomaly in South China and its geological signifcance Jie Wang1,2,3, Changli Yao2*, Zelin Li4, Yuanman Zheng2, Xuhui Shen1, Zhima Zeren1 and Wenlong Liu3 Abstract The Sichuan Basin magnetic anomaly is the most striking regional aeromagnetic anomaly in South China. It is charac- terized by a continuous large-scale linear magnetic anomaly and contains long-wavelength constituents which could still be detected on satellites. However, Sichuan Basin is covered by 4 to 10 km thick non-magnetic sedimentary layers from Neoproterozoic to Cenozoic, which conceals the magnetic sources in the Precambrian basement and makes it difcult to explore the geological origin of the Sichuan Basin magnetic anomaly. In this study, we applied 3D inversion to the magnetic data, and then compared our inverted susceptibility model with geological maps. We found that the spatial distribution of the Sichuan Basin magnetic anomaly controls the range of the rigid basement beneath Sichuan Basin and it is surrounded by the widespread Neoproterozoic magmatic complexes. Based on our analysis, we pro- posed that the geological origin of the Sichuan Basin magnetic anomaly might be related to the craton-scale Neopro- terozoic magmatic event, which has played an important role in the stabilization of the basement and the evolution of the deep crust in Sichuan Basin. Keywords: Magnetic anomaly, Long-wavelength magnetic anomaly, 3D inversion, Sichuan Basin, South China, Neoproterozoic Introduction Sichuan Basin is covered by non-magnetic 4 to 10 km Tere is a large linear aeromagnetic anomaly in Sichuan thick sedimentary layers, and the magnetic source is not Basin, South China, extending continuously more than exposed on surface, so the strongly magnetized rocks 1000 km with a broad width of ca. 100 km. Sichuan Basin for the magnetic anomaly should be in the Precambrian magnetic anomaly contains long-wavelength signals basement of Sichuan Basin. Sichuan Basin magnetic which decay slowly with distance and could be detected anomaly reveals the magnetization diference of the base- on satellites. Te sources for long-wavelength magnetic ment, and it has attracted the interests of many research- anomalies are generally interpreted as large-scale mafc– ers to study its origin (Zhang et al. 1995; Luo 1998; Gu ultramafc rocks with strong magnetization in the mid- and Wang 2014; Xiong et al. 2015). Some researchers lower crust, e.g., the Bangui magnetic anomaly in Central thought it might refect the strongly magnetized intru- African Republic (Regan and Marsh 1982), Mackenzie sions beneath the basin (Luo 1998; Guo et al. 2016); how- River magnetic anomaly in western Canada (Pilkington ever, there is no direct proof to determine its geological and Saltus 2009) and the magnetic anomaly of the North origin. Tere are no drilling data on the magnetic anom- American Midcontinent Rift system (Hinze et al. 1992). aly to fgure out the properties of these strongly magnet- ized rocks. Te geological origin of the Sichuan Basin *Correspondence: [email protected] magnetic anomaly is still not clear. 2 School of Geophysics and Information, China University of Geosciences, Regional magnetic anomalies could refect the deep Beijing, China structure of the buried basement. Tey are always Full list of author information is available at the end of the article formed by large-scale magmatic activities and related © The Author(s) 2020. This article is licensed under a Creative Commons Attribution 4.0 International License, which permits use, sharing, adaptation, distribution and reproduction in any medium or format, as long as you give appropriate credit to the original author(s) and the source, provide a link to the Creative Commons licence, and indicate if changes were made. The images or other third party material in this article are included in the article’s Creative Commons licence, unless indicated otherwise in a credit line to the material. If material is not included in the article’s Creative Commons licence and your intended use is not permitted by statutory regulation or exceeds the permitted use, you will need to obtain permission directly from the copyright holder. To view a copy of this licence, visit http://creat iveco mmons .org/licen ses/by/4.0/. Wang et al. Earth, Planets and Space (2020) 72:40 Page 2 of 10 to signifcant tectonic events in geological history. formation of the uppermost crust of the Yangtze Ter- Terefore, the formation of the Sichuan Basin magnetic rane (Zheng et al. 2006). On surface, the outcrops of anomaly should be related to important Precambrian the Archean and Paleoproterozoic rocks are very lim- geological events and infuence the crustal evolution of ited in spatial distribution. Tere are widespread Neo- the basement. In this work, we studied the Sichuan Basin proterozoic magmatic rocks exposed in the periphery magnetic anomaly by 3D inversion; then we discussed of the Yangtze Terrane, which indicates that the Yang- our inversion results with other geodata and further ana- tze Terrane had experienced extensive remelting and lyze its geological signifcance. formed massive juvenile crust during Neoproterozoic, the same period as the formation and break-up of the Geological settings supercontinent Rodinia, generating a lot of magmatic Sichuan Basin is in the Yangtze Terrane of the South intrusions and volcanic eruptions around the Yangtze China Craton, which constitutes the Yangtze Terrane in Terrane (Li et al. 2003a, 2003b, 2008; Zheng et al. 2007; the northwest and the Cathaysia Terrane in the southeast Wang et al. 2009, 2010). (Fig. 1). Te basement of Sichuan Basin is formed in Pre- From the late Neoproterozoic, the Sichuan Basin cambrian period and covered by 4 to 10 km thick layered becomes a stabilized cratonic basin and acts as a rigid sedimentary successions from late Neoproterozoic to block resisting subsequent tectonic compressions. Cenozoic (Dong et al. 2013; Gao et al. 2016). Te sedimentary layers inside Sichuan Basin are less- During Precambrian Period, there are multi-stage deformed and kept sub-horizontal, but the crust sur- magmatic activities and tectono-thermal events hap- rounding the Sichuan Basin has gone through intense pened in the Yangtze Terrane, recording the forma- deformation from Mesozoic to Cenozoic: in the northern tion of the antient continent-nucleus, cratonization, margin of the Sichuan Basin, the Dabashan thrust fault crustal remelting and growth (Zheng and Zhang 2007). belt was formed during the N–S convergence between Te Archean Kongling complex lies near Yichang in the North and South China Craton in Jurrasic (Dong the eastern part of Sichuan Basin, which is the oldest et al. 2013); the westward subduction of the Paleo-Pacifc basement exposed in Yangtze Terrane and represents plate beneath the South China caused the broad thrust- the initial formation of the continent-nucleus of the fold belt in eastern Sichuan Basin (Zhang et al. 2011); the Yangtze Terrane (Zheng et al. 2006; Gao et al. 2011). eastward extrusion of Songpan-Ganzi Terrane of Himala- Te zircon data of the Kongling complex also refect yan–Tibetan plateau reactivated the Longmenshan thrust the crustal reworking in Paleoproterozoic and the fault belt in the western margin of Sichuan Basin, and Fig. 1 Topography in Sichuan Basin. The range of our study area is framed by a red rectangle in the simplifed tectonic maps of South China at the lower right corner. The main thrust faults in the western and northern Sichuan Basin are marked. The range of the Sichuan Basin is circled by a black dashed line Wang et al. Earth, Planets and Space (2020) 72:40 Page 3 of 10 the famous Longmenshan fault is still active and causes Reduction to the pole (RTP) is to reduce the infuences earthquakes until now (Zhang et al. 2009). of oblique magnetization. In oblique magnetization, the locations of magnetic anomalies would shift laterally over the sources and the shape of magnetic anomalies would Magnetic anomaly data be skewed (Blakely 1995). In our study area, the inclina- Figure 2a is the total-feld magnetic anomaly in Sichuan tion of geomagnetic feld is ca. 48° and the declination is Basin with a spatial resolution of 2 arc (≈ 3.5 km) at 4 km ca. 0°, i.e., the magnetic anomalies are caused by oblique altitude. Te Sichuan Basin magnetic anomaly is the larg- magnetization. It is noted that if we would like to com- est one in South China, which is a wide linear magnetic pare magnetic anomalies with geological maps or other anomaly extending continuously more than 1000 km with geodata, RTP is necessary so that the magnetic anoma- a main NE trending in the western to middle part and SE lies can better refect their source locations. Compared trending in the easternmost part. Te highest value of the to Fig. 2a, the location of the RTP magnetic anomaly in magnetic anomaly reaches 360 nT. Te magnetic high Fig. 2b migrates northward ca. 50 km. Te magnetic low anomaly has a broad width of ca. 100 km and changes accompanying the magnetic high on the north decreases slowly. Te obvious magnetic low is almost parallel to the a lot and the shape of the magnetic high becomes more magnetic high on the north with the lowest value -240 symmetrical. nT, which is mostly a manifestation of oblique magneti- Figure 2c is the total-feld long-wavelength magnetic zation. Te magnetic data are collected in EMAG2 (Earth anomaly in Sichuan Basin, obtained by satellite mag- Magnetic Anomaly Grid, 2 arc-minute resolution, https netic surveys. Te data in Fig. 2c are extracted from the ://www.ngdc.noaa.gov/geoma g/emag2 .html), which is MF7 model (http://geoma g.color ado.edu/magne tic-feld compiled from satellite, airborne and marine magnetic -model -mf7.html), which is produced using CHAMP data.