As the "roof of the world", the formation and evolution of the Tibetan Plateau, especially the internal crustal deformation, thickening and uplift, and outward expansion, are the focus of research in the field of earth science. In view of the special geological phenomena on the eastern edge of the Tibetan Plateau, such as large-scale strike-slip faults, significant crustal thickening and lack of Cenozoic crustal shortening, predecessors have proposed three classic geological models, namely rigid block extrusion. (Tapponnier et al., 1982), lithospheric-scale continuous deformation (England and Houseman, 1986), and large-scale mid- to lower crustal flows (Royden et al., 1997; Clark and Royden, 2000). Among them, large-scale (1000km) middle and lower crustal flows have received widespread attention and support from many scholars in the past 20 years. This model believes that the crustal thickening on the eastern edge of the Tibetan Plateau is caused by crustal material flowing eastward from the interior of the Tibetan Plateau (North Qiangtang Massif) at a depth of 25-40km and joining the eastern edge of the Tibetan Plateau. Geophysical observation results show that there is an extensive low-velocity and high-conductivity layer on the eastern edge and interior of the Tibetan Plateau, supporting the flow of materials in the middle and lower crust (e.g., Bai et al., 2010; Wang et al., 2010; Bao et al., 2015 ). However, some scholars have questioned this, believing that this model lacks geological evidence (Searle et al., 2016), and some recent research results do not seem to support the existence of such large-scale crustal flows (Wang et al., 2012; Zhang et al., 2016; Bao et al., 2020).
According to the large-scale middle and lower crustal flow model, the significant uplift and lack of crustal shortening deformation in the eastern margin of the Tibetan Plateau during the Late Miocene-Pliocene was due to crustal flow. According to the prediction of this model, it will take about 20 million years to complete the thickening and uplift of the crust, so the initiation time of crustal flow should be between 30-40 Ma. Secondly, for the crustal flow model, the pressure gradient is the initial and necessary condition for its formation, and the central Tibetan Plateau already had a higher altitude in the Eocene (Xu et al., 2013; Hu et al., 2020 ), therefore, during the Eocene period, there may have been a pressure gradient between the central and eastern edges of the Tibetan Plateau. In addition, the currently observed higher geothermal gradient in the central and eastern edges of the Tibetan Plateau is also a necessary condition for the formation of large-scale crustal flow. From this, the conditions required for large-scale crustal flow appear to have been met.
In order to explore whether large-scale crustal flow shaped the eastern edge of the Tibetan Plateau, Hu Fangyang, a distinguished associate researcher at the Key Laboratory of Mineral Resources, Institute of Geology and Geophysics, Chinese Academy of Sciences, and Academician Wu Fuyuan of the State Key Laboratory of Lithosphere Evolution Wu Fuyuan , University of ArizonaProfessor Mihai N. Ducea, University of WyomingAssociate Professor James B. Chapman, Chengdu University of TechnologyDr. Yang Lei, etc., collaborated to study the largest Cenozoic intrusion on the eastern edge of the Tibetan Plateau-Gongga Mountain-Zheduo Mountain Flower The ganglitic intrusive complexes have been systematically studied. The complex is located on the channel of large-scale crustal flow proposed by predecessors. According to previous research results, the crustal composition characteristics of the North Qiangtang Massif are similar to those of the eastern edge of the Qinghai-Tibet Plateau (Songpan-Ganzi Massif) A distinction can be made. Therefore, if there was a large-scale eastward flow of materials within the crust during the Cenozoic, since these granitic rocks mainly come from partial melting of the crust, the magma source area they reflect should change (Figure 1).
Figure 1 A simplified geological map of the Qinghai-Tibet Plateau and the Zheduo Mountains in the Gongga Mountains and a conceptual model map. The yellow arrow in the picture indicates the direction of potential crustal flow.
The research team conducted detailed field surveys and indoor analysis, combined with previous research results, and found that the Gonggashan-Zheduoshan intrusive complex recorded the Mesozoic Era (215-172Ma ) and Cenozoic (50-3Ma) two stages of multi-stage magmatism . In particular, there were three stages of magmatism in the Cenozoic period, namely 50-30Ma, 20-10Ma and 5-3Ma. According to the isotope composition characteristics of rocks of different stages, it was found that the source area of these granitic rocks has not changed from the Mesozoic to the Cenozoic. The magma mainly comes from the basic lower crust of the Songpan-Ganzi massif and Metamorphic sedimentary rocks (Figure 2).In order to determine whether the source areas of these rocks can represent the layers of middle and lower crustal flow, the research team used Perple_X phase equilibrium simulation and trace element simulation methods to perform partial melting simulations assuming different source areas. The simulated melt composition was consistent with the results. The observed sample compositions are basically consistent, and the temperature and pressure conditions for the formation of Cenozoic magma rock are thus obtained. Based on the obtained temperature and pressure conditions, the research team found that these Cenozoic magmas were mainly formed at 30-40 km, which is basically consistent with the depth proposed by the crustal flow model (Figure 2). Therefore, the results of this study show that there is no crustal material from the interior of the Tibetan Plateau at the depth of the crust where this crustal flow should exist, which obviously does not conform to the predictions of the large-scale crustal flow model.
Figure 2 Isotope characteristics of Gongga Mountain-Zheduo Mountain granite rocks and Cenozoic rock thermodynamics and trace element simulation analysis results. Open symbols represent Mesozoic samples, and solid symbols represent Cenozoic samples. The simulation analysis results show that the formation depth of Cenozoic rocks is consistent with the crustal flow model, but the isotope composition shows that these rocks come from the partial melting of in-situ crustal materials, and their source area composition is consistent with Mesozoic rocks.
The research team further found that the Cenozoic magma There are staged changes in the source area, that is, the 50-30Ma and 5-3Ma granitic rocks come from the partial melting of metamorphic basic rocks, while the 20-10Ma granitic rocks come from the partial melting of metasedimentary rocks, which indicates that Two transformations of magma source areas. These two transitions can correspond to two uplift events on the eastern edge of the Tibetan Plateau, and can correspond to regional mantle-derived magmatism (Fig. 3). Combining the results of regional and numerical simulations, the research team believes that the upwelling of the asthenosphere mantle may be an important factor leading to the uplift of the eastern edge of the Tibetan Plateau. In addition, long-term crustal magmatism may have changed the rheological properties of the earth's crust. , under the continuous impact of the India-Eurasian continent collision, the current geological characteristics and landform characteristics of the eastern edge of the Tibetan Plateau have been shaped.
Figure 3 Changes in isotope composition of Cenozoic granitic rocks in Gongga Mountain-Zheduo Mountain over time and its relationship with regional magma-tectonic processes. The Cenozoic magma source area has undergone two transformations, which can be compared with the history of regional tectonic uplift and magmatism, indicating that magmatism and tectonic processes on the eastern edge of the Tibetan Plateau are closely linked. The upwelling of the asthenosphere mantle and the continuous compression and collision jointly shape the eastern edge of the Tibetan Plateau.
Research results were published in the international academic journal Geophysical Research Letters (Hu F Y*, Wu F Y, Ducea M N, Chapman J B, Yang L . Does Large-Scale Crustal Flow Shape the Eastern Margin of the Tibetan Plateau? Insights From Episodic Magmatism of Gongga-Zheduo Granitic Massif[J].Geophysical Research Letters, 49: e2022GL098756. DOI: 10.1029/2022GL098756). The research is jointly funded by the Basic Science Center Project of the National Natural Science Foundation of China (41888101), the National Natural Science Foundation of China Youth Science Fund Project of China (41902055), and the General Project of the China Postdoctoral Science Foundation (2018M640177).
Artist: Fu Shixu
Proofreading: Wanpeng
