GP13C-0618 Indian Crustal Front in the Western Tibetan Plateau — the Evidence from Magnetotellurics Studies

Jiangfan Gu1, Sheng Jin1*, Hao Dong1**, Wenbo Wei1, Gaofeng Ye1, Letian Zhang1, Yaotian Yin1 1 University of Geosciences, Beijing First auther Email: [email protected]

Introduction Method The Plateau is known as the third pole of the world, and its formation has always been the focus of academic discus- In this study, we employed a 3D MT inversion algorithm ModEM utilizing the non- sion. It is believed that the collision and compression of the Indian plate and the Eurasian plate caused the uplift of the Qing- linear conjugate gradient (NLCG) optimizing method (Egbert and Kelbert 2012). hai-Tibet plateau. Tectonically, our research region(Fighure 1) could be divided into four parts, from south to north, of the Hi- The starting models were uniform 100 Ωm half space with 5km x 5km mesh in core malayas, Block, Qiangtang Block and Tianshuihai Block(=Songpan-Ganzi-Hoh Xil block to the east) ,and separated area. The final model’s global RMS is 0.87. The model and sensitive test result from each other by the Indus-Yarlung Zangbu suture(IYS), Bangong-Nujiang suture(BNS) and Longmu-Gozha Co fault(LF) could be seen in Figure3. (Yin and Harrison, 2000).There are several researchers who used different geophysical methods to analyze the north limit of Indian plates beneath the Tibet Plateau and give different conclusions. Discussion Data High conductivity analysis The MT data used in this paper(Figure 1) is mainly from the 102 broadband MT data of the array MT standard points As we all consider the resistances as the old and cold crystalline basement, conduc- (80°-81°E) of the “SinoProbe” project, collected in 2012-2013, using the periods of 0.01-3000 s. And 16 broad- tivities may have several explanations — deposit, graphite, fluid and partial melt. And band(0.01-2000s) MT data from the Zanda-Rutog profile (line 0) of the “INDEPTH” project was collected in 2006, to fill the the former 2 are out of our option. We use Archi’s law(ten Grotenhuis et al., 2005) to blank area. The data acquisition uses the MT-24NS system produced by EMI and the MTU-5 system produced by Phoenix, simply the possible partial melts/hydrous fluid condition by using longitudinal conduc- Canada. Two electric fields and three magnetic fields are collected, and the acquisition time of each site exceeds 18 hours. tivity from our model and the fluid resistivity resistivity is 0.2Ωm. According our calcula- Compared with the two-dimensional and three-dimensional magnetotelluric profiles of the former research(Jin et al., 2009; tion(Figure) and considering the data distribution, there are large space’s fluid content Xie et al., 2017), the data used in this study are more and extended in the north-south and east directions,· which is believed lower than 4% when it comes to 50-75km (lower crust) beneath Lhasa block, but when that it will have better resolution. it comes to mid-crust (20-50km), fluid content in many place are higher. But it still may not soppurt a horizentol flow layer in mid-crust as <1% area exists. And it seems like there is a “boundary” near the BNS in lower crust. Crustal front The typical resistivity of mid-lower crust of Lhasa block are 1-10Ωm(Zhang et Figure 4 Resistivity sections under profiles A–B,C–D shown on the right al.,2017) which is not shown in the lower crust of Lhasa block and already is proved that this kind of electrical structure of our model is controlled by the data we collect- ed,since when this part replaces by 5Ωm condector the global RMS >1. Combine with the former geophysics results, it might be the India lower crust(Figure7). The high par- tail melt area in lower crust under BNS may show the gap bewteen India lower crust and tibet crust which may give a channel to transfer the heat and materails between mantle and crust at present. The hot spring on the ground means the conductor in the upper crust may shows the still warm magma chambers and these chambers could be the source of volcanos about 16-26Ma ago shown by the known extrusive rocks with

high K2O which means it might have materails from mantle. What we will do next We have not discuss very clearly about the north of our research area and we are working on it. We welcome any advices, comments and discussions. Figure1 Simplified tectonic map of the study area with topography

Figure 5 Fluid content (porosity) of different layers required to account for electrical model 080313S10

low density high density high density

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Figure 6 Cartoon interpretation of the Indian crustal front beneath western Tibet References ACKNOWLEDGEMENTS Gilligan, Amy et al. 2015. “The Crustal Structure of the Western Himalayas and Tibet.” Journal of Geophysical Re- This study was funded by the SinoProbe search: Solid Earth 120(5): 3946–64. http://doi.wiley.com/10.1002/2015JB011891 (December 7, 2019). project (Sinoprobe -02-04), the National Natu- 08131S30 still worm place beated by ten Grotenhuis, Saskia M., Martyn R. Drury, Chris J. Spiers, and Colin J. Peach. 2005. “Melt Distribution in Olivine ral Science Foundation of China (4040406, movement and past heat Rocks Based on Electrical Conductivity Measurements.” Journal of Geophysical Research: Solid Earth 110(12): 1–11. 40674045). Some magnetotelluric data are Kelbert, Anna, Naser Meqbel, Gary D. Egbert, and Kush Tandon. 2014. “ModEM: A Modular System for Inversion of from the previous INDEPTH projects and the Electromagnetic Geophysical Data.” Computers and Geosciences 66: 40–53. Chinese Geological Survey. we used the GMT Shapiro, Nikolai M, Michael H Ritzwoller, Peter Molnar, and Vadim Levin. 2004. “Thinning and Flow of Tibetan Crust software package to produce some of the fig- Constrained by Seismic Anisotropy.” Science (New York, N.Y.) 305(5681): 233–36. http://www.ncbi.nlm.nih.gov/pub- ures med/15247475 (January 17, 2019). Xie, Chengliang et al. 2017. “Varying Indian Crustal Front in the Southern Tibetan Plateau as Revealed by Magnetotel- Any comments Figure 2 MT observed and predict- Figure 7 Schematic illustration for the formation and evolution of research luric Data.” Earth, Planets and Space 69(1). and advices Figure 3 Model horizontal slices and one site RMS when covered by 1000Ωm resistance from the Xu, Qiang et al. 2017. “Detailed Configuration of the Underthrusting Indian Lithosphere Beneath Western Tibet Re- ed data from station 08131S30 area please contect me (Himalayas), 08032 (Lhasa) and depth of 126km,115km,104km and 95km to the bottom of the model (colorbar in Figure4 page limit) vealed by Receiver Function Images.” Journal of Geophysical Research: Solid Earth 122(10): 8257–69. Zhang, Zhongjie et al. 2014. “The Moho beneath Western Tibet: Shear Zones and Eclogitization in the Lower Crust.” 080313S10 (Qiangtang) Earth and Planetary Science Letters 408: 370–77.