Sedimentology and structural geology of the Gariep belt in Southern Namibia.

[摘要] The Gariep Belt is an arcuate north-south trending tectonic unit extending ;along thewestern coast of southern Namibia and northwestern South Africa. It forms part of thePan-African Damara Oragert, which consists of a north-south trending coastal branch(Kaoko and Gariep Belts) and an east-west trending inland branch. The Gariep Beltis subdivided into an eastern parautochthonous passive continental margin on thewestern edge of the Kalahari Craton, the Port Nolloth Zone, and a westernallochthonous ophiolitic terrane, the Marmora Superterrane, thrusted on the PortNolloth Zone.The sedimentary evolution of the Gariep Belt was initiated with the deposition ofsediments of the Rosh Pinah Formation into fluvial, alluvial and lacustrine depositionalsystems, which are associated with bimodal volcanic activity and related Zn-Pb-Cu-Ag-(+Ba) sulphide mineralization. The overlying mixed continental/shallow marinedeposits of the Gumchavib Formation are overlain by the carbonate dominatedPickelhaube Formation, which was deposited in shallow marine to pelagic depositionalsettings. The Rosh Pinah, Gumchavib and lower Pickelhaube Formations are intrudedby basic volcanic sills of the Gannakouriep dyke swarm, The conformably overlyingObib Peak Formation was deposited into a fluvial/alluvial palaeoenvlronrnent, TheNumees Formation unconformably overlies the underlying stratigraphic units andconsists predominantly of glaciomarine massive diamictites and occasionallyinterbedded iron formations, which are occasionally interbedded with interglacialsediments and volcanics.The regional structural pattern is characterized by three phases of deformation. Theearliest deformational event, D1, comprises intrafolial small scale recumbent andisoclinal F1 folds, a penetratlve beddlnq-subparatel 81 cleavage and a preferredelongation of boulders, pebbles, grains and minerals (11). Associated with D1 arebedding-subparallel thrust faults. The D2 deformational phase is characterized by smallto large scale, north to northwest trending F2 folds with a generally easterly vergence,which can change into a westerly direction due to backfolding. The F2 folds areassociated with a penetrative axial planar S2 cleavage. Thrusting continued during theD2 deformational phase. The latest deformational event, D3, is characterized by smallto large scale open F3 folds with southerly to south-westerly trending fold axes. Themetamorphic evolution of the Gariep Belt during the Damara Orogeny was associatedwith barrovtan-type metamorphism with a geothermal gradient of about 20oC/km,reaching greenschist to lower amphibolite facies metamorphism.The sedimentological and structural features are explained in a tectono-sedimenterymodel, which started with rift initiation during the deposition of Rosh Plnah Formationlithologies along old tectonic weakness zones of Middle Proterozoic age at about780Ma. Supercontinenta! breakup is documented by the initial transgression ofGumchavib Formation sediments and the continuous trensgression of the PickelhaubeFormation, which is related to thermal subsidence after the initiation of oceanicspreading of the Adamastor Ocean. Renewed rifting and/or glacial outwash heralding'the advance of the Numees glaciation and associated seale': :'changes may beresponsible for the sudden break in marine sedimentation during the deposition of theOhio Peak Formation. The Numees glaciation is probably contemporaneous with the670Ma old Varangian glacial and the Rapitan glacial episodes and provides a minimumage for the deposition of Gariep Group lithologies. Rifting and spreading was followedby the closure of the Adamastor Ocean, associated with. a southeastward directedsubduction. of oceanic crust under the Kalahari Craton and was accompanied byintense southeastward and subsequently eastward directed tectonic transport during01 and D2. During D3, a late Sinistral movement developed along tre defined faultzones in the late stages of the Adamastor Orogeny.Metamorphic ages from the Namaand Vanrhynsdorp Groups indicate that that the tectonic evolution of the Gariep Belt onlyceased at about 500Ma.

[发布机构] University of the Witwatersrand