Seismic tomography results show a large variety in upper mantle structure along convergent plate boundaries. We numerically investigate the effect of trench migration on the evolution of a slab (with temperature dependent viscosity) encountering a viscosity interface. We find that subduction zone geometry is sensitive to even small rates (1 cm/yr) of retrograde motion: increase in trench migration rate decreases the dip angle of the slab and its ability to penetrate the lower mantle. Upon including a background mantle flow it turns out that trench migration relative to the upper mantle flow is more decisive than the absolute plate velocities. Subduction zone geometry appears to be strongly time-dependent. We conclude that individual tectonic setting and time-dependent slab behaviour can account for many different types of observable subduction zone anomalies. Our model temperature can adequately account for magnitudes and patterns of seismic anomalies as obtained from seismic tomography.¿ 1997 American Geophysical Union