OBSERVATIONS ON THE TECTONIC EVOLUTION OF THE SOUTHERN ADELAIDE FOLD BELT

The Mount Lofty Ranges, Fleurieu Peninsula and Kangaroo Island regions of South Australia expose a section across the west and northwestern margin of the southern Adelaide Fold Belt that was deformed and metamorphosed during the Cambro-Ordovician Delamerian Orogeny. In the external parts of the belt, thrust complexes involving basement and a late Proterozoic platformal succession show a characteristic asymmetry reflecting west to northwest vergence towards the Australian craton. In the more internal zones, convergent deformation involving large-scale upright folding and high-T, low-P metamorphism of mainly Cambrian sediments, which were in part turbiditic, occurred between c. 516 and c. 490 Ma. In this part of the belt the distribution of heat and, possibly, strain reflects intimately the advective movement of magmas within the orogenic belt. Convergent deformation in the southern Adelaide Fold Belt was immediately preceded by rapid subsidence initiated during the deposition of the upper parts of the Normanville Group and which continued during deposition of the Kanmantoo Group. Mafic alkaline volcanism attendant with Normanville Group deposition is indicative of lithospheric thinning at c. 526 Ma.