Richard E. Hanson

Proterozoic geochronology and tectonic evolution of southern Africa

Abstract The Precambrian foundation of southern Africa consists of Archaean cratonic nuclei surrounded by belts formed during three separate Proterozoic orogenic episodes. Crust that formed or was reworked at 2.05–1.8 Ga is represented by orogenic belts (e.g. Kheis-Magondi, Ubendian-Usagaran) that partly wrap the ancient craton margins or form extensive basement in younger belts. Orogenic belts formed at 1.35–1.0 Ga record arc magmatism and collisional events during assembly of the Rodinia supercontinent. The Namaqua-Natal Belt defines a major convergent plate boundary active at this time along the southern margin of the Archaean Kaapvaal Craton, and the western part of this belt is inferred to link with a largely buried, NE-trending orogen present in the Kalahari region. Orogenesis in the same time frame farther north is recorded by the Kibaran and Irumide Belts, which are separated by the Palaeoproterozoic Bangweulu Block but are inferred to have undergone a linked tectonic evolution. East of the Irumide Belt, extensive Mesoproterozoic arc crust occurs in Malawi and Mozambique, although strong Neoproterozoic overprinting of the arc rocks makes their original relations unclear. Breakup of Rodinia is signalled by widespread Neoproterozoic alkaline and bimodal magmatism associated with rift zones which, in some cases, evolved into major ocean basins. Subsequent amalgamation of Gondwana led to collisional orogenesis culminating at 575–505 Ma in the Mozambique and West Congo-Kaoko-Gariep-Saldania Belts along the present eastern and western margins of southern Africa. Coeval deformation in the interconnecting, transcontinental Damara-Lufilian-Zambezi Orogen may reflect destruction of linked rifts and narrow ocean basins driven by farfield stresses from the collisional plate margins.

Were aspects of Pan-African deformation linked to Iapetus opening?

The convergence recorded in some Pan-African deformational belts (sensu lato) in South America, Africa, Madagascar, southern India, Sri Lanka, and Antarctica is temporally correlated with opening of the Iapetus ocean. We propose a model in which continent-continent collision and closure of the Adamastor ocean between the Amazon–West African–Rio de La Plata cratons and the Sao Francisco–Congo–Kalahari cratons in the late Neoproterozoic are linked to rifting and orthogonal spreading between Laurentia and the South American cratons. By the Early Cambrian, the cratons in South America and Africa were assembled as West Gondwana. Closure of the Mozambique ocean, which appears to have extended across Antarctica between Lutzow-Holm Bay and the Shackleton Range, resulted in continued convergence between the Congo–Kalahari–Queen Maud Land block and East Gondwana in the Cambrian. Coeval deformation in the Transantarctic Mountains may be related to the obliquity of the Antarctic margin relative to Iapetus spreading directions. Initiation of voluminous arc magmatism along the paleo-Pacific margin of Gondwana in the Early Cambrian is broadly synchronous with the cessation of intra-Gondwana Pan-African deformation, possibly reflecting a change in plate motions at the time of final Gondwana assembly. The new subduction regime along the Gondwana margin in the Early Cambrian may be linked to the closure of the Iapetus ocean basin.

Paleoproterozoic intraplate magmatism and basin development on the Kaapvaal Craton: Age, paleomagnetism and geochemistry of ~1.93 to ~1.87 Ga post-Waterberg dolerites

We report U–Pb baddeleyite crystallization ages of ~1927 and ~1879 to ~1872 Ma for dolerite sills intruding the Waterberg Group in Botswana and South Africa. These data increase the known extent of ~1.9 Ga intraplate magmatism in southern Africa and place tighter age constraints on the Waterberg Group than previously available. In South Africa, ~1.88 Ga dolerite intrudes upper Waterberg strata, constraining most, if not all, of the succession to have accumulated between ~2.06 Ga (age of the underlying Bushveld Complex) and ~1.88 Ga. This is consistent with derivation of much of the group from uplifted sources in reactivated segments of the Limpopo Belt. The dolerites are typical continental tholeiites, but their trace-element contents discriminate them from dolerite sills of the 1.1 Ga Umkondo Igneous Province, which occur in the same region. Paleomagnetic samples from dolerite intrusions in the Waterberg Group in South Africa (including one sill with a U–Pb baddeleyite age of ~1872 Ma), and from dolerite sills and basalt flows in the Soutpansberg Group to the east-northeast, yield antipodal directions with a site mean pole at 15.6°north, 17.1°east, A95 = 8.9°. These data are interpreted to indicate that the ~1879 to ~1872 Ma dolerites were intruded into the Waterberg Group during voluminous magmatism associated with development of the Soutpansberg rift basin. Older, ~1927 Ma dolerite in Botswana is similar in age and geochemistry to basalts in the craton-margin Olifantshoek Supergroup, suggesting that the mafic magmatism in those two regions is genetically related.

U-Pb zircon age for the Umkondo dolerites, eastern Zimbabwe: 1.1 Ga large igneous province in southern Africa–East Antarctica and possible Rodinia correlations

We report a U-Pb zircon age of 1105 ± 2 Ma for the extensive Umkondo dolerites in eastern Zimbabwe, which are part of a large igneous province that can be traced over much of southern Africa and originally contiguous parts of East Antarctica. Other members of the province include widespread tholeiitic intrusions in Botswana and South Africa, bimodal volcanic rocks in Botswana and Namibia, and dolerites and flood basalts in Dronning Maud Land, Antarctica. Available data indicate that substantial parts of the province were emplaced in a restricted interval at ca. 1.1 Ga and originated from a large-scale mantle thermal anomaly inboard of a coeval continental-margin orogen. Striking similarities in age and tectonic setting between the Umkondo igneous province and widespread 1.1 Ga within-plate magmatism in Laurentia are consistent with reconstructions of the early Neoproterozoic Rodinia supercontinent that place southern Africa and Dronning Maud Land off the southern tip of Laurentia.

Geologic evolution of the neoproterozoic Zambezi orogenic belt in Zambia

The Neoproterozoic Zambezi belt links with the Mozambique belt, Lufilian arc, and the inland branch of the Damara belt within the regional Pan-African tectonic framework of southern Africa. The belt contains a thick supracrustal sequence deposited on older sialic basement and penetratively deformed with it during Neoproterozoic (Pan-African) orogenesis. In Zambia, where the entire width of the orogen is exposed, local bimodal volcanic rocks at the base of the sequence are overlain by psammites and pelites, which in turn are succeeded by extensive carbonate and calc-silicate rocks. Abundant scapolite in metamorphic assemblages within the belt is taken as evidence for the original presence of evaporites. The nature of the rock types and the inferred stratigraphic sequence are consistent with deposition in an intracontinental rift basin invaded by marine waters. Available isotopic age brackets for the timing of supracrusta deposition show that the basin developed between 880 nad 820 Ma. Main-phase deformation in the belt involved both transcurrent shearing and south- to southwest-vergent thrusting and was associated with predominantly amphibolite-facies regional metamorphism. Mineral assemblages throughout much of the belt in Zambia, together with limited thermobarometric data, indicate typical Barrovian-type intermediate P/T conditions during metamorphism. Eclogites and other high-pressure metamorphic assemblages in parts of the belt, however, provide evidence that significant crustal thickening occurred, presumably in relation to thrusting. Reworked basement and syntectonic granite were subjected to extensive mylonitization related to strike-slip and oblique, reverse-slip shearing. The major orogenic event is dated at c. 820 Ma, based on an igneous age for a sheet-like, syntectonic batholith injected into a transcurrent shear zone within the central part of the belt. Pan-African orogenesis along the Zambezi-Lufilian-Damara trend was diachronous and records closure of intracratonic basins in the Zambezi belt and Lufilian arc, with evidence for the involvement of oceanic lithosphere present only in the Damara belt.

UPb zircon ages from the Hook granite massif and Mwembeshi dislocation: constraints on Pan-African deformation, plutonism, and transcurrent shearing in Central Zambia

The Hook granite massif in the inner part of the Lufilian arc in central Zambia generally has been interpreted as granitized Archean or Paleoproterozoic sialic basement partly remobilized during Pan-African orogenesis. Reconnaissance field studies and UPb zircon geochronology reveal no evidence for exposed basement within the massif, which instead is shown to be a large, syn- to post-tectonic, composite batholith intrusive into upper Katangan (Kundelungu) strata. Syntectonic parts of the massif contain a single, moderate- to high-temperature, solid-state foliation that is continuous with the regionally developed S1 fabric in the adjacent lower-grade Kundelungu metasedimentary rocks. Two separate phases of syntectonic granite yield UPb zircon upper intercept ages of 559±18 and 566±5 Ma. Both samples show normal, linear discordance patterns, and zircon residues from acid leaching plot nearer concordia. An undeformed rhyolite dike intruding a Katangan pendant in the central part of the massif shows more complex zircon isotopic systematics, but a nearly concordant fraction has an age of 538±1.5 Ma. Post-tectonic granite yields an upper intercept age of 533±3 Ma. These ages constrain Kundelungu deposition in central Zambia as pre-570 Ma and show that regional deformation and voluminous syntectonic granite plutonism occurred in the inner part of the Lufilian arc at 560–570 Ma, with the major tectonic activity ending by 530–540 Ma. The Mwembeshi dislocation, a regionally significant Pan-African transcurrent shear zone, runs along the southern margin of the Hook massif. Syntectonic rhyolite intruded in the dislocation yields an upper intercept age of 551±19 Ma, showing that transcurrent shearing occurred in the same time frame as batholith emplacement, probably within an overall transpressive regime. Deformation and batholith emplacement in the inner part of the Lufilian arc were synchronous with a major pulse of syn- to post-tectonic granite plutonism in the Damara belt of Namibia, supporting a direct link between these regions during Pan-African orogenesis. In contrast, tectonothermal activity in central Zambia is unrelated to main-phase orogenesis in the Zambezi belt to the east, dated at ∼820 Ma. Models that propose a direct kinematic link between deformation in the Lufilian arc, shearing along the Mwembeshi dislocation, and main-phase orogenesis in the Zambezi belt are not supported by the present age data.

Timing and structural expression of the Nevadan orogeny, Sierra Nevada, California

The Nevadan orogeny was a very short-lived event in the Late Jurassic that involved the deformation of a great variety of rock types and Paleozoic and Mesozoic terranes throughout the extent of the Sierra Nevada. The Nevadan structures show great variation in style but relatively constant orientations. These relations can be explained by considering the prior histories of the various terranes. Slaty cleavages and tight folds are the characteristic main-phase structures in the western belt of Jurassic island-arc volcanic rocks and flysch-type sedimentary rocks. A strip of phyllites and greenschists along the eastern edge of the belt apparently represents similar Jurassic rocks that were deformed and metamorphosed at greater depths, probably during underthrusting of the western belt beneath the central belt. The central belt of Paleozoic metasedimentary and metavolcanic rocks shows the most extreme variation in style of main-phase structures, from weak, spaced to crenulation cleavages in the south, where polyphase deformed rocks formed a structural basement, to slaty and phyllitic cleavages and asymmetric to isoclinal folds in the north, where most of the Paleozoic basement rocks lack penetrative pre-Nevadan fabrics. Eastward-directed thrust faulting apparently was important only in the northern part of the range, where main-phase deformation was most intense. The eastern belt of Jurassic and Triassic magmatic arc-volcanic and sedimentary rocks defines the core of a major synclinorium, and the rocks contain penetrative slaty cleavages and asymmetric, tight to isoclinal folds. A late phase of Nevadan structures, consisting of northeast-trending cleavages and minor folds, also shows a marked variation in style, from relatively intensely developed in the north to very weakly developed in the south. The regional extent and geometry of the Nevadan structures indicate that the Nevadan orogeny involved underthrusting of island-arc rocks on the west and significant crustal shortening in the central and eastern belts. These features suggest that the Nevadan orogeny resulted from the collision of the island arc (western belt) with an andean-type arc (eastern belt) situated at the western edge of North America.

Large-scale rhyolite peperites (Jurassic, southern Chile)

Development of a Jurassic volcano-tectonic rift basin in the southern Andes created a setting in which thick, rhyolitic volcaniclastic sequences accumulated in submarine environments and were penetrated by hypabyssal intrusions during or shortly after deposition. In the Ultima Esperanza District of southern Chile, extensive masses of peperite were produced when rhyolite magma underwent quenching, disruption, and commingling with wet, unconsolidated sediments during intrusion at shallow levels beneath the sea floor. The peperite forms discordant intrusive masses with volumes of up to several cubic kilometers, in which large, widely spaced, coherent rhyolite feeder pods are surrounded by, and grade into closely packed and dispersed peperite. Closely packed peperite consists of tightly fitting clasts separated by sediment-filled fractures. In dispersed peperite, the sediment forms a matrix surrounding large masses of fractured rhyolite and smaller more widely separated rhyolite clasts; evidence of in situ quench fragmentation is well preserved on both outcrop and thin-section scales. Thin sections show that clast margins and, in some cases, entire small clasts underwent cooling-contraction granulation, releasing shards of quenched rhyolite and fragments of phenocrysts into the adjacent sediment. Interaction between magma and wet sediment was non-explosive and involved fluidization of the host sediments, creating space for the intruding magma and causing pervasive injection of highly mobile sediment along thermal contraction cracks in quench-fragmented rhyolite. The ability of the magma to undergo complex intermixing with large volumes of sediment, with widespread preservation of in situ fragmentation textures, is interpreted to reflect a relatively low magma viscosity, presumably caused by retention of volatiles in the magma at the ambient pressures involved. Beds of redeposited peperite within the rift-basin fill indicate that some of the intrusive peperite masses reached the sea floor, undergoing slumping and mass flow. The peperites were thus an important local source of coarse-grained debris during the evolution of the basin.

Gondwana assembly: The view from Southern Africa and East Gondwana

Abstract Neoproterozoic-Cambrian tectonism in Africa, Antarctica and formerly adjacent parts of East Gondwana records the final assembly of the Gondwana supercontinent. Here we compile recent data regarding the timing and kinematics of tectonism for critical parts of southern Africa and East Gondwana, focusing on the period between ~ 650-500 Ma, and integrate these data into a regional framework in order to investigate aspects of the assembly of this portion of Gondwana. We use this information to address the following questions. What cratons remained coherent continental blocks between Rodinia break-up and Gondwana assembly? What were the configurations of Neoproterozoic-Cambrian oceans that separated these coherent blocks? When did the cratons reassemble into the Gondwana supercontinent? In southern Africa, the apparent continuity of older terranes across the Neoproterozoic Zambezi belt, together with other evidence, suggest that the Congo and Kalahari cratons have been a coherent block since ~ 1.1 Ga. The Khomas/Adamastor ocean did not transect the Congo/ Kalahari block; instead any tectonic connection with the Neoproterozoic-Cambrian collisional plate boundary in the Mozambique belt must have been via shear zones such as the Mwembeshi dislocation, which was active at ~ 550 Ma. Age and structural data from Queen Maud Land in East Antarctica indicate that this area was also linked with the Congo/Kalahari block and remained attached to it during the breakup of Rodinia. A suture must thus separate this block from the main part of the East Antarctic craton and likely represents the southern continuation of the Mozambique Ocean. Accumulating evidence for Neoproterozoic-Cambrian orogenesis (~ 550-520 Ma) in the Lutzow-Holm Bay and Queen Maud Land regions of East Antarctica suggest the postulated continuation of the Mozambique suture zone may lie beneath the ice, where it separates the main part of the East Antarctic craton from the Congo-Kalahari block and contiguous sectors of Queen Maud Lond. There is also increasing evidence for Neoproterozoic-Cambrian tectonism in the Enderby Land and Wilkes Land sectors of East Antarctica, and in formerly adjacent portions of western Australia and India. Although the regional tectonic significance of this activity with regard to Gondwana assembly remains uncertain, its widespread nature is additional evidence that East Gondwana did not remain an entirely coherent, rigid entity since ~ 1.1 Ga. Final assembly of East and West Gondwana occurred in the Cambrian along boundaries that only partially coincide with the Mesozoic continental margins of the southern continents.

Deformed batholiths in the Pan-African Zambezi belt, Zambia: Age and implications for regional Proterozoic tectonics

The Zambezi belt makes up part of the Late Proterozoic (Pan-African) network of orogenic belts in southern Africa. In Zambia, supracrustal rocks within the Zambezi belt are in contact with two extensive units of granitic augen gneiss, which have previously been interpreted as uplifted masses of remobilized sialic basement. Field relations and U-Pb zircon geochronologic data demonstrate that the gneisses are deformed batholiths that intruded the supracrustal sequence during two separate episodes of plutonism at ca. 1100 and 820 Ma. The Proterozoic tectonothermal evolution of the Zambezi belt thus involved production of voluminous granitic magmas. Isotopic ages for the batholiths place new constraints on the timing of major depositional and orogenic events within the belt and enable comparisons to be made with other parts of the regional Pan-African network.