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Melt-weakened Crustal Flow Triggers Expansion of Tibetan Plateau

Jun 20, 2016     Email"> PrintText Size

The Tibetan Plateau is an area of anomalously thick (~ 50-90 km) continental crust and the highest and largest topographic feature on Earth. Three main mechanisms have been proposed to account for crustal thickening and the development of its high topography: thinning of thickened mantle lithosphere, intracontinental subduction, and crustal (channel) flow. 

The dispute about which mechanism is responsible for the thickening stems partially from the sparse data on the thermal evolution of the Tibetan deep crust and mantle lithosphere, and inconclusive interpretations of the geophysically determined low-velocity zones (LVZs) and high conductivity zones (HCZs) at depths of 15-50 km within the Tibetan crust (Figure 1). The diversity of models (e.g., mantle-derived melts, aqueous fluid and crustal shear zones) involving the LV-HCZs highlights that resolving their nature and origin requires petrological evidence on samples from the deep crust. 

Professor WANG Qiang and his research partners and team focused on Cenozoic volcanic rocks and entrained xenoliths in the Qiangtang, Songpan-Ganzi and Kunlun areas of central and northern Tibet (Figure 1) in order to unveil the formation of the LV-HCZs. In this study, Wang et al. report petrological and geochemical data on trachyandesites, dacites and rhyolites that erupted 4.7-0.3 million years ago and entrained xenoliths in central and northern Tibet. Based on detailed geochemical research (Figure 2) and previous melting experimental data for crustal rocks (Figure 3a), Wang et al. suggest that these Pliocene-Quaternary rocks were generated by partial melting of crustal rocks at temperatures of 700-1050 ºC and pressures of 0.5-1.5 GPa. 

The estimated temperatures and pressures required for the generation of the Pliocene-Quaternary felsic magmas are consistent with the present crustal geotherms for central and northern Tibet, based on geophysical, crustal xenolith and geophysical/petrological modeling studies (Figure 3b). This indicates that high temperatures in the mid-lower crust of central and northern Tibet were responsible for the fluid-absent partial melting. Simple batch melting models indicate that the crustal melts from central and northern Tibet reflect 8-22% partial melts, which are consistent with the amounts (5-23% and 8-23% based on previous magnetotelluric data and melting and numerical experiments, respectively) of melts required to explain the high conductivity zones (HCZs) in Tibet. 

Thus, Pliocene-Quaternary 8-22% melting of crustal rocks occurred at depths of 15-50 km in areas where the LV-HCZs have been recognized. This provides new petrological evidence that the LV-HCZs are sources of partial melt. 8-20% melts in the mid-lower crust in central and northern Tibet would cause the strength of the mid-lower crust beneath this area to have been markedly changed. This in turn would facilitate northward and eastward flow of melt-weakened mid-lower crust of central and northern Tibet. Crustal melt-enhanced ductile flow in the high-temperature, partially molten, mid-to-lower crust makes it easier to maintain a uniform elevation in the Tibetan Plateau and it accounts for the present expansion and frequent earthquakes along its northern and eastern margin. 

This paper was published in Nature Communications on 16 June 2016 (Wang, Q.*, Hawkesworth, C. J. *, Wyman, D., Chung, S.-L., Wu, F.-Y. Li, X.-H., Li, Z.-X., Gou, G.-N., Zhang, X.-Z., Tang, G.-J., Dan, W., Ma, L., Dong, Y.-H. 2016. Pliocene–Quaternary crustal melting in central and northern Tibet and insights into crustal flow. Nature Communications, 7:11888, doi: 10.1038/ncomms11888.) 

This research was supported by the Strategic Priority Research Program (B) of the Chinese Academy of Sciences (grant no. XDB03010600), National Natural Science Foundation of China (no. 41025006), Talent Project of Guangdong Province (2014TX01Z079) and GIGCAS 135 project (135TP201601).  

 

Figure 1 Integrated schematic cross section across Tibet (Image by WANG Qiang) 

  

Figure 2 Variations in selected magma characteristics with latitude: (1), Pliocene (2.97 Ma) adakitic trachyandesites in the Zhaixinshan area of the Central Kunlun Block; (2), Quaternary (1.08-0.3 Ma) non-adakitic trachyandesites in the Jindingshan area of the Central Kunlun Block; (3), Pliocene-Quaternary (4.0-1.5 Ma) rhyolites in the Songpan-Ganzi Block; (4), Pliocene-Quaternary (4.7-2.3 Ma) non-adakitic felsic volcanic lavas in the central-northern Qiangtang Block; 5, Pliocene (3.2-2.5 Ma) adakitic rhyolites in the Henglianghu area of the central-northern Qiangtang Block. (Image by WANG Qiang)

 

Figure 3 Crustal thermal state for central and northern Tibet: 1-4, pressure and temperature conditions for magma generation: (1), Quaternary (1.08-0.3 Ma) non-adakitic trachyandesites of the Songpan-Ganzi and Central Kunlun Blocks and Pliocene-Quaternary (4.7-2.3 Ma) non-adakitic felsic volcanic lavas from the central-northern Qiangtang Block; (2), Pliocene-Quaternary (4.0-1.5 Ma) rhyolites in the Songpan-Ganzi and Central Kunlun Blocks; (3), Pliocene (3.2-2.5 Ma) adakitic rhyolites in the Henglianghu area of the central-northern Qiang Block; (4), Pliocene (2.97 Ma) adakitic trachyandesites in the Zhaixinshan area of the Central Kunlun Block. (5), Adakitic magmas generated in the pressure range >1.2-1.5 GPa. (6), curves or lines for melting or mineral stability. (Image by WANG Qiang) 

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(Editor: CHEN Na)

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