Seismic tomography results show a large variety in upper mantle struct
ure along convergent plate boundaries. We numerically investigate the
effect of trench migration on the evolution of a slab (with temperatur
e dependent viscosity) encountering: a viscosity interface. We iind th
at 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 migra
tion relative to the upper mantle flow is more decisive than the absol
ute 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 obser
vable subduction zone anomalies. Our model temperature can adequately
account for magnitudes and patterns of seismic anomalies as obtained f
rom seismic tomography.