TRANSPRESSION AND TRANSECTION IN THE LATE PAN-AFRICAN VANRHYNSDORP FORELAND THRUST-FOLD BELT, SOUTH-AFRICA

The Vanrhynsdorp foreland thrust-fold belt forms part of the polycycli c Pan-African orogeny (similar to 650-450 Ma), which affected mobile b elts and their foreland deposits on the African continent. The Vanrhyn sdorp Group was deposited in a peripheral foreland basin cratonward of the Neoproterozoic Gariep belt. This belt is a transpressive collisio n orogen that developed when the southern African plate collided with the South American plate during the construction of the Gondwana super continent. The foreland basin was deformed by the same southeasterly t ranspression late during the orogeny leading to a NE-vergent, thin-ski nned foreland thrust-fold belt. The thrust-fold belt shows progressive fold, thrust and cleavage development from northeast to southwest acc ompanied by an increase in metamorphic grade from diagenetic to biotit e-grade greenschist facies. The clockwise cleavage transection in the belt is ascribed to sinistral transpression, probably accompanied by r everse-slip reactivation of a basement fault that may have been the or iginal rift-margin of the older Gariep trough. Shortening across this oblique basement ramp led to arcuation and the formation of a high str ain zone and sinistral offset in the foreland deposits above. Lateral ramp style, thin-skinned step-thrusting and transected folding occur n ortheast of this zone. To the southwest, the imbricate thrust duplex c ontains wedges of the older Gariep belt that were thrust over the fore land margin and backfolded during progressive forward propagation of t he thrust belt. Far to the southwest, along the coast, Kibaran basemen t slices overrode the Gariep metasediments. The minimum finite shorten ing across the belt is estimated to be similar to 35%. Prograde metamo rphism associated with the transpressive thrust-fold event in the fore land belt culminated at similar to 496 Ma and was dated by Ar-40/Ar-39 step-heating of cleavage micas.