THE Tien Shan--a high, seismically active intracontinental mountain belt, 1,000-2,000 km north of the Himalaya--has grown as a result of India's collision with Asia1 . The crustal shortening (~ 200 ± 50 km; refs 2, 3) and thickening that gave rise to the Tien Shan accommodates only a small fraction of India's total penetration into Asia (2,000--3,000km), and the temporal relationship of deformation in this belt to the India-Asia collision remains unclear. Here we report geodetic measurements of the Tien Shan, using the Global Positioning System (GPS), that indicate that the current crustal shortening rate is nearly half of India's convergence rate with Eurasia in this area4 . We infer a total shortening rate for the Tien Shan of ~20 mm yr-1 , which is approximately twice that inferred previously from the extrapolation of slip rates in the Holocene3 and earthquake-induced displacements during this century5 , suggesting that the rate of mountain building in this region has accelerated several-fold since the onset of collision ~50-55 Myr ago6,7 . If, as we argue, the current shortening rate can be extrapolated to geological timescales, then our results suggest that most of the Tien Shan has been constructed during the past 10 Myr, perhaps in response to an increased horizontal force following an abrupt rise of the Tibetan plateau8,9 .