Magmatic Underplating Thickening of the Crust of the Southern Tibetan Plateau Inferred From Receiver Function Analysis

• Published in: Geophysical research letters Vol. 48; no. 17
• Main Authors: Liu, Zhen, Tian, Xiaobo, Liang, Xiaofeng, Liang, Chuntao, Li, Xin
• Format: Journal Article
• Language: English
• Published: 16.09.2021
• Subjects: crustal thickness; magmatic underplating; receiver functions; Tibetan Plateau; Vp/Vs ratio
• ISSN: 0094-8276; 1944-8007
• DOI: 10.1029/2021GL093754

How the crust thickens to double the normal continental crust is a key issue in the formation of the Tibetan Plateau. Using data recorded by a new 2‐D broadband seismic array, we extracted the crustal thicknesses and average VP/VS ratios in central Tibet from receiver functions. In our results, the crustal thicknesses increase southward, while the crustal average VP/VS ratios generally decrease southward. The lower VP/VS ratios in the crust seem to be produced by higher silica contents. We suggest that upper mantle magmas induced by the subduction of Indian lithospheric and Tethyan oceanic slabs underplating the overriding crust differentiate into felsic melts and mafic residues. Mafic residues delaminate into the mantle due to their high density, while felsic magmas are retained in the crust. As a result, crustal thickening in southern Tibet is accompanied by increasing silica contents. Plain Language Summary The Tibetan crust has thickened to double the thickness of normal continental crust. How the crust thickens is a key issue in the formation of the Tibetan Plateau. Different thickening modes lead to variations in the rock composition within the crust. Laboratory studies indicate that the VP/VS ratio is more sensitive to the silica content than VP or VS alone. Using the waveform data recorded by a new 2‐D broadband seismic array in central Tibet, we extracted the crustal thickness and average VP/VS ratio at each seismic station. Our results show that the crustal thicknesses increase and that the crustal average VP/VS ratios decrease southward. Underplated magma, induced by the subduction of the Indian and Tethyan oceanic slabs, differentiates into felsic melts and mafic residues. Mafic residues accumulate and delaminate into the mantle due to their high density. Felsic melts retained in the crust reduce the crustal average VP/VS ratio. Therefore, the crustal thickening in the plateau should be attributed considerably to magmatic underplating induced by the subducting Indian and Tethyan oceanic slabs. This process is the third main mode of crustal thickening in the plateau after crustal shortening and Indian lower crustal underthrusting. Key Points Crustal thicknesses and VP/VS ratios in central Tibet are estimated by receiver functions extracted from a new 2‐D broadband seismic array The crust thickens, and the crustal average VP/VS ratio decreases southward in southern Tibet, correlated with increasing silica contents Magmatic underplating followed by differentiation and delamination of mafic accumulations contributes to crustal thickening and more silica