Several geophysical surveys carried out in southern Tibet have revealed the occurrences of bright spots of high electrical conductivity at 15–20 km depth, corresponding to zones of seismic attenuation and correlated to high heat flow at the surface. Such zones have been variably interpreted as intruding magmas, partially melted middle crust, or concentrations of free fluids. Here we show that experimentally derived electrical conductivities of hydrous granite melts under pressure-temperature-H2O conditions appropriate for crust-derived magmas are in perfect agreement with those inferred from magnetotelluric data, strongly suggesting that the Tibetan bright spots image present-day pluton assembly in the middle crust, in a manner analogous to what happened during the Miocene to form the leucogranite plutons now exposed southward in the High Himalaya range.

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