The collision between India and Asia has been simulated with a variety of computational models that describe or predict the motions of the main faults of east Asia. Geological slip-rate estimates of 20-30 mm yr-1 suggest that the largest of these faults, the 2,000-km-long Altyn Tagh fault system on the northern edge of the Tibetan plateau, absorbs as much of the Indo-Asian convergence signal as do the Himalayas - partly by oblique slip and partly by contraction and mountain growth. However, the predictions of dynamic models for Asian deformation and the lower bounds of some geological slip-rates estimates (3-9 mm yr-1; refs 7, 8) suggest that the Altyn Tagh system is less active. Here, we report geodetic data from 89-91°E that indicate left-lateral shear of 9 ± 5 mm yr-1 and contraction of 3 ± 1 mm yr-1 across the Altyn Tagh system. This result - combined with our finding that, at 90°E, Tibet contracts north-south at 9 ± 1 mm yr-1 - supports the predictions of dynamic models of Asian deformation.