Transpression and transection in the late Pan-African Vanrhynsdorp foreland thrust-fold belt, South Africa

Abstract

The Vanrhynsdorp foreland thrust-fold belt forms part of the polycychc Pan-African orogeny (∼650–450 Ma), which affected mobile belts and their foreland deposits on the African continent. The Vanrhynsdorp Group was deposited in a peripheral foreland basin cratonward of the Neoproterozoic Gariep belt. This belt is a transpressive collision orogen that developed when the southern African plate collided with the South American plate during the construction of the Gondwana supercontinent. The foreland basin was deformed by the same southeasterly transpression late during the orogeny leading to a NE-vergent, thin-skinned foreland thrust-fold belt. The thrust-fold belt shows progressive fold, thrust and cleavage development from northeast to southwest accompanied by an increase in metamorphic grade from diagenetic to biotite-grade greenwhist facies. The clockwise cleavage transection in the belt is ascribed to sinistral transpression, probably accompanied by reverse-slip reactivation of a basement fault that may have been the original rift-margin of the older Gariep trough. Shortening across this oblique basement ramp led to arcuation and the formation of a high strain zone and sirristral offset in the foreland deposits above. Lateral ramp style, thin-skinned step-thrusting and transected folding occur northeast of this zone. To the southwest, the imbricate thrust duplex contains wedges of the older Gariep belt that were thrust over the foreland margin and backfolded during progressive forward propagation of the thrust belt. Far to the southwest, along the coast, Kibaran basement slices overrode the Gariep metasediments. The minimum finite shortening across the belt is estimated to be ∼35%. Prograde metamorphism associated with the transpressive thrust-fold event in the foreland belt culminated at ∼496 Ma and was dated by 40 Ar 39 Ar step-heating of cleavage micas.