Seismic reflection profiling in the southern Lachlan fold belt, Australia, indicates geometrically complex mid- to lower crustal detachments possibly associated with crustal thickening by tectonic underplating of collided microplates and/or oceanic plateaus. Low-angle detachments either intersect or sole into the crust-mantle boundary, forming a crustal-stacking wedge typical of lithospheric- or A-type subduction. Upper crustal reflections show that major, moderately to steeply dipping reverse faults exposed at the surface flatten with depth and have listric form. The chevron-folded quartz-rich turbidite sequence of the Lachlan fold belt is therefore allochthonous with respect to the lower crust. Like the northern Appalachian maritimes, the Lachlan fold belt has a composite crust with diverse lower crustal blocks overlain by allochthonous, imbricated terranes of the surface rocks.