Thin slabs of oceanic pillow basalt formed at ridge axes with associated pelagic sediments are common features in ancient accretionary complexes. Estimates of the average thicknesses of scraped-off basaltic sections of oceanic crust in accretionary prisms vary from a few tens of metres to 300 m, with a mean thickness of 80–100 m in the complexes studied. The permeability contrast within the uppermost part of the oceanic crust is an important factor governing the thickness of delaminated oceanic crust. This mechanism is an effective means of separating oceanic crust altered at low temperatures from the remainder of the downgoing slab and thus controlling geochemical budgets in subduction zones.