Consumption of an ocean basin by subduction commonly brings a sea-floo
r-spreading ridge toward a deep-sea trench. If plate divergence and co
nvergence continue after the ridge intersects the subduction zone, a s
lab window forms between the subducted parts of the diverging oceanic
plates, producing anomalous thermal, physical and chemical effects in
the surrounding asthenospheric mantle. In turn, these conditions alter
the tectonic and magmatic evolution of the overriding plate, usually
disturbing ordinary fore-are and are regimes. Differential lithospheri
c stresses on opposite sides of the triple junction contribute to dist
urbances in the overriding plate. Anomalous magmatism from fore are to
back are may be accompanied by fore-are metamorphism, strike-slip fau
lting, uplift, extension and, in extreme cases, rifting. The shape and
size of the window are controlled mainly by the pre-subduction ridge-
transform-trench configuration, slab dip angles and vectors of plate c
onvergence. Subducted ridge segments expand into windows whose margins
approximately parallel the motion vectors between the triple junction
and the subducting plates. Subducted transform faults continue to be
active, usually as oblique-slip faults, until the plates separate. As
transform faults subduct, they become longer on the plate which occupi
es the acute angle between ridge and trench, and shorter on the other
plate. Trains of isolated windows produced by subduction of a segmente
d ridge-transform system progressively expand during descent, commonly
merging together to form a composite slab window. Oblique subduction
of a highly segmented ridge is likely to produce two or more fraternal
slab windows, one at each site of ridge-trench intersection. Above a
slab window, are volcanism diminishes and may be replaced by volcanism
of mid-ocean ridge or rift affinity. The change in chemical character
reflects various processes including elevated heat flow, decreasing h
ydration of the upper mantle, juxtaposition of supra- and sub-slab man
tle reservoirs, asthenospheric upwelling and melting of the trailing p
late edges. If the slab window migrates, the anomalous magmatic regime
may be replaced by renewed are volcanism. Identifying the effects of
slab windows in ancient convergent margin assemblages requires an unde
rstanding of slab window principles and implications.