Source of the Lachlan fold belt flysch linked to convective removal of the lithospheric mantle and rapid exhumation of the Delamerian-Ross fold belt

During the Cambrian-Ordovician the paleo–Pacific margin of Gondwana underwent a rapid transition from a site of convergent deformation (Delamerian-Ross orogeny) to one of sedimentation during which the thick turbiditic sequences of the adjacent basal Lachlan fold belt were deposited. Laser-probe 40 Ar- 39 Ar data show that the synkinematic (523–486 Ma), mid-crustal I- and S-type granites and metamorphic country rocks of the Delamerian fold belt were rapidly cooled at 490–485 Ma, coincident with the intrusion of a suite of high-level, postkinematic A-type granites and gabbros (ca. 497–481 Ma). 40 Ar- 39 Ar data on detrital muscovites from the basal sections of the Lachlan fold belt provide information on exhumation rather than cooling but yield an age pattern (507–480 Ma) identical to that of the Delamerian fold belt, demonstrating that this was the source of the sediments. The combined data require that some 15 km of exhumation occurred very rapidly (∼ 5–15 mm ·yr −1 ), coincident (as implied by overlapping ages) with the cessation of convergent deformation and partial melting to form the postkinematic magmas. This scenario implies a causative link, which we infer to have been convective removal of lithospheric mantle following orogenic thickening. The model is analogous to the Tertiary uplift of the India-Asia orogen, which provided the sediment source for the Bengal fan. Our results suggest that similar processes were important in the evolution of mountain belts at least as far back as the early Phanerozoic.