The Shanxi rift in northern China is marked by intensive seismicity, including many devastating historic earthquakes. Geological and seismological evidence show 0.5--1.6 mm/yr extension across the rift, and previous GPS results indicated an extension rate of 4 ¿ 2 mm/yr. We show here newly compiled GPS data that indicate coherent crustal motion and no clear sign of extension across the rift. We reconcile the discrepancy between geological observations and GPS results in a simple viscoelastic finite element model with timescale-dependent crustal deformation. The GPS velocities can be fit by a model with a near elastic upper crust, consistent with predominantly interseismic deformation. The geological rate of extension is predicted when viscous creep of the crust is dominant, driven by the gravitational potential energy arising from the heterogeneity of crustal structure.