Abundant granitic rocks exposed in ancient mountain belts suggest that crustal melting plays a major role in orogenic processes. However, complex field relations and superposition of multiple tectonic events make it difficult to determine the role of melting in orogenesis. In contrast, geophysical measurements image present-day crustal conditions but cannot discriminate between partial melt and aqueous fluids. Here we connect pressure–temperature paths of Himalayan Miocene crustal rocks to the present-day conditions beneath the Tibetan plateau imaged with geophysical data. We use measurements of electrical conductivity to show that 4–16% water-rich melt is required to explain the crustal conductivity in the north-western Himalaya. In southern Tibet, higher melt fractions >30% reflect a crust that is either fluid-enriched (+1% H2O) or hotter (+100 °C) compared to the Miocene crust. These melt fractions are high enough for the partially molten rocks to be significantly weaker than the solid crust.
Bibliographical noteFunding Information:
Part of this study was financially supported by the Special Fund for Earthquake-Scientific Research in the Public Interest (Grant no. 20150818), the National Natural Science Foundation of China (Grant nos. 41372202 and 41672197), while the experimental work was supported by the European Research Council (Grant no. 279790). J.C. acknowledges the China Scholarship Council (CSC) for a visiting scholarship to Université d’Orléans, Institut des Sciences de la Terre d’Orléans (ISTO). A.V. acknowledges funding from Labex VOLTAIRE (ANR-10-LABX-100-01) and the ANR program VARPEG (ANR-15-CE01-0001). We thank Qingbao Duan and Lining Cheng for providing us samples of leucogranitic bulk rocks and the thin sections of gneisses from Purang county and Nyalam county in Tibet of China P.R., David Sifré for helping us with the experiments of electrical conductivity measurements, and Giada Iacono Marziano and Ida Di Carlo for analytical assistance.