LATE-OROGENIC TO POSTOROGENIC BASINS OF THE PAN-AFRICAN-BRASILIANO COLLISION OROGEN IN SOUTHERN AFRICA AND SOUTHERN BRAZIL

Late- to post-orogenic basins formed on both sides of the Pan-African - Brasiliano orogen when the Congo and Kalahari Cratons collided with the Rio de la Plata Craton during the formation of western Gondwana. T race fossil evidence and radiometric age dating indicate that deposits on both sides are coeval and span the Cambrian-Precambrian boundary. A peripheral foreland basin, the Nama Basin, developed on the subducti ng southern African plate. Lower, craton-derived fluviomarine clastics are overlain by marine platform carbonates and deltaic flysch derived in part from the rising subduction complex along the northern (Damara Belt) and western (Gariep Belt) orogenic margins. Rare, thin volcanic ash layers (tuffs and cherts) are present. Upper sediments consist of unconformable red molasse related to collisional orogenesis. Orogenic loading from the north and west led to crustal flexure and the format ion of a remnant ocean that drained to the south and closed progressiv ely from north to south. During final collision SE-, E- and NE-verging nappes overrode the active basin margins. Although younger than most of the post-orogenic magmatism, its setting on the cratonic edge of th e subducting plate precluded marked volcanism or granitic intrusion, t he only exception being the youngest intrusions of the Kuboos-Bremen S uite dated at 521 +/- 6 Ma to 491 +/- 8 Ma. Two foreland-type basins, perhaps faulted remnants of a much larger NE-SW elongated retroarc for eland basin, are found west of the Dom Feliciano Belt on the edge of t he Rio de la Plata Craton in southern Brazil. In the southern Camaqua Basin, basal fluvial deposits are followed by cyclical marine and coar sening-up deltaic deposits with a notable volcanic and volcaniclastic component. This lower deformed succession, comprising mainly red beds, contain stratabound Cu and Pb-Zn deposits and is overlain unconformab ly by a fluviodeltaic to aeolian succession of sandstones and conglome rates (with minor andesitic volcanics), derived primarily from an east ern orogenic source and showing southerly longitudinal transport. In t he northern Itajai Basin, sediments range from basal fluvial and platf orm sediments to fining-up submarine fan and turbidite deposits with i ntercalated acid tuffs. The Brazilian basins had faulted margins off w hich alluvial fans were shed. They also overlie parts of the Ribeira B elt. Thrust deformation along the orogenic margin bordering the Dom Fe liciano Belt was directed westward in the Camaqua and Itajai basins, b ut reactivated strike-slip and normal faults are also present. Late- t o post-orogenic granitoids and volcanics of the Dom Feliciano Belt, ra nging in age from 568 +/- 6 Ma to 529 +/- 4 Ma, occur in the foreland basins and are geochemically related to some of the synsedimentary vol canics.