The Main Central thrust zone is the major structural feature of the Himalayas. It was active during Miocene time and accommodated at least 100 km, and possibly as much as 300 km, of convergence between India and Tibet. We present evidence that late in the tectonic history, another large shear zone, located north of the High Himalayas (the North Himalayan shear zone), underwent a phase of dextral strike-slip motion. The most plausible explanation for this phase of motion is that it reflects the onset of extension in Tibet. It is usually thought that extension began about 2.5 Ma. The phase of dextral shear we report occurred between 25 and 15 Ma. If our explanation for this movement is correct, it places the onset of extension at least 12 m.y. earlier than previously thought.