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Earthquake data collected by the INDEPTH-II Passive-Source Experiment show that there is a substantial south to north variation in the velocity structure of the crust beneath southern Tibet. North of the Zangbo suture, beneath the southern Lhasa block, a midcrustal low-velocity zone is revealed by inversion of receiver functions, Rayleigh-wave phase velocities, and modeling of the radial component of teleseismic P-waveforms. Conversely, to the south beneath the Tethyan Himalaya, no low-velocity zone was observed. The presence of the midcrustal low-velocity zone in the north implies that a partially molten layer is in the middle crust beneath the northern Yadong-Gulu rift and possibly much of southern Tibet.

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Abstract. Mantle convection on Mars and Venus is likely to occur in the regime known as stagnant lid convection. We perform thermal boundary layer analyses as well as finite element simulations of stagnant lid convection with non-Newtonian viscosity (which is believed to be more appropriate for the lithosphere and upper mantle) and discuss one particular application of the results, the efficiency of heat transport on the terrestrial planets. As in the case of Newtonian viscosity, the efficiency of heat transfer in the stagnant lid regime is extremely low compared to plate tectonics: For example, in the absence of plate tectonics, the mantle temperature on Earth, which is already close to the solidus, would be about 700-1500 K higher for the present-day value of the surface heat flux.

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