Results of fluid dynamical experiments on the behavior of subducted slabs are presented that address two important characteristics of subduction: slab penetration through the transition zone and horizontal slab migration. Cold, negatively buoyant molded slabs of concentrated sucrose solution, with viscosities of 3--5¿106 P were introduced into a more dilute, two-layered sucrose solution representing the upper and lower mantle.

The transition zone was modeled by a step increase in both density and viscosity. Ratios of slab thickness to upper layer depth, the relative viscosities in each layer, and the effective Rayleigh number were close to mantle values. The initial configuration consisted of two plates separated by a trengh gap, with one plate attached to a dipping slab, simulating a developing subduction zone with an overriding plate. Two different boundary conditions were used at the trailing (ridge) end of the subducting plate: (1) fixed end, corresponding to zero ridge motion, and (2) free end, corresponding to zero ridge resistance. Based on results from 15 experiments with various combinations of upper layer, lower layer, and slab densities ( &rgr;u, &rgr;t, &rgr;s) and viscosities, we find that slab penetration depth is sensitive to both the normalized slab density anomaly R =( &rgr;s-&rgr;g)/( &rgr;s-&rgr;u) and the dip angle.

Three penetration regimes are recognized: R>0.5, in which the slab sinks into the lower layer withoud distortion; -0.2≲R≲0.5, which permits limited penetration, the amount dependent on dip angle (in this regime the slab develops a thick root just beneath the transition zone); and R<-0.2, in which the slab is deflected by the transition, and little or no penetration occurs. Retrograde slab motion and trench migration occurred in nearly every case, with the overriding plate moving parallel to but faster than the trench, causing the gap between subducting and overriding plates to close with time. Prograde trench motion was never observed, and the trench remained stationary only in the extreme case (free ridge condition with equal upper and lower layer densities). These results suggest that the transient extension found in many back arc basins may be a direct consequence of the tendency for retrograde subduction and the amount of extension may be governed in part by the degree of slab penetration through the transition zone. ¿ American Geophysical Union 1987