Temperature beneath Tibet

- Shear heating between Tibet and the underthrust India warms the upper crust.
- This intraplate heating by viscous shear is both self-limiting and localizing.
- It produces a robust thermal structure insensitive to uncertain parameters.
- The thermal structure explains the bimodal distribution of earthquakes beneath southern Tibet.
- The thermal structure suggests substantial partial melting in the crust beneath central Tibet.

Temperature beneath Tibet is poorly understood, constituting a critical gap in understanding the dynamics of the most prominent case of active continental collision. Here we present results of numerical simulations to provide new insight into the thermal evolution of Tibet. We utilize collective constraints provided by several large-scale field experiments in the past 20 yr, and include recent rock physics data to fully account for important feedback processes among temperature, shear stress, shear heating, thermal conductivity and specific heat. We show that while the collision system as a whole is cooled by the northward-advancing Indian lithosphere beneath Tibet, the upper and middle crust is warmed by shear heating between the overlapping lithospheres. Such a thermal structure readily explains the longstanding enigma of a very warm upper crust over a cold upper mantle. Gradual northward warming of the system is also consistent with a bimodal distribution of seismicity in the upper crust and the upper mantle beneath southern Tibet and the absence of deep seismicity further north. We emphasize the localized nature of shear heating, which is self-sustaining yet self-limiting, therefore does not depend on precise values of various input parameters, such as the rate of convergence and the amount of radiogenic heating. This heat source may have further implications for late-stage magmatic activities and variations of crustal rheology under Tibet.