• 标题：The groundwater deep circulation and large-scale geothermal deposition in response to the extension of the Yadong–Gulu rift, South Tibet, China
• 关键词：Travertine, U–Th dating, Geothermal spring, Extension fault, Tibet, Seismic
• 作者：Jinbao Su, Hongbing Tan, Xi Chen

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The Yadong–Gulu rift (YGR) is famous for its extensional structure in the compressional background of Tibet. The mid-crust of the rift is partially molten, which has attracted considerable attention regarding whether it is associated with the regional tectonics and hydrothermal system. The Cs contents of the travertines of the YGR have attained the standard of ―Chinese pay grade‖ (Cs2O: 100 ppm), however, their element sources are uncertain. This paper displays the trace element and rare earth element (REE) contents of the travertines located at the Ningzhong, Gaxi, and Gulu hot springs of the YGR. The samples are  enriched in Cs, W, Be, Sb, and Sr, and depleted in Ta, Ha, Nb, Zr, Ti, Ga, and Cd. The travertines are likely from multiple sources, originating from water-rock interaction and deep aquifer supercritical fluid. The deep melting fluid initially provided high Cs concentrations to the Gulu geothermal spring, and then mixed with shallower groundwater in Ningzhong and Gaxi springs, leading to wide variation in Cs contents. Trace elements indicate transilient, abnormal contents in the same travertine beds at different depths, which may be related to regional earthquakes and faulting action. Because of these earthquakes, Ningzhong travertines show diachronous U–Th ages, which range from 0.18 ± 0.44 to 11.94 ± 1.53 ka and from 1.25 ± 0.2 to 122.8 ± 1.5 ka in different sections. These ages are comparable with moraine exposure times. Considering the depth and permeability of the mid-crust, a shallower magma is speculated to exist and provide the heat resource and hydrothermal system. These results combined with the extension history of the rift, the timing of geothermal groundwater circulation to the surface, and the age of the area’s oldest travertine may reveal the initiation of partially melting of the mid-crust of the YGR.