Adamastor Ocean Research Articles

The transpressional connection between Dom Feliciano and Kaoko Belts at 580–550 Ma

A new U–Pb SHRIMP age of 551 ± 4 Ma on a mylonitic porphyry that intruded into the Sierra Ballena Shear Zone (Southernmost Dom Feliciano Belt, Uruguay) and a review of relevant published data make possible a more refined correlation and reconstruction of Brasiliano/Pan-African transpressional events. Paleogeographic reconstruction, kinematics and timing of events indicate a connection between the shear systems of the Dom Feliciano and Kaoko Belts at 580–550 Ma. Sinistral transpression recorded in shear zones accommodates deformation subsequent to collision between the Congo and Río de la Plata Cratons. The correlation is strengthened by the similarity of magmatic and metamorphic ages in the Coastal Terrane of the Kaoko Belt and the Punta del Este Terrane of the Dom Feliciano Belt. This post-collisional sinistral transpression brought these units near to their final position in Gondwana and explains the different evolution at 550–530 Ma. While in the Kaoko Belt, an extensional episode resulted in exhumation as a consequence of collision in the Damara Belt, in the Dom Feliciano Belt, sinistral transpression occurred associated with the closure of the southern Adamastor Ocean due to Kalahari-Río de la Plata collision.

Correlation of Neoproterozoic terranes between the Ribeira Belt, SE Brazil and its African counterpart: comparative tectonic evolution and open questions

Abstract Four main classes of tectonic entities may be considered for the Ribeira Belt and southwest African counterparts: (1) cratonic fragments older than 1.8 Ga and their passive margin successions, (2) reworked basement terranes with Mesoproterozoic and/or Neoproterozoic deformed cover, (3) magmatic arc associations, (4) terranes with Palaeoproterozoic basement and deformed Neoproterozoic back-arc successions. Based on comparative investigation, a tectonic model of polyphase amalgamation is proposed with c . 790 and 630–610 Ma major episodes of intra-oceanic and cordilleran arc magmatism along both sides of the Adamastor Ocean. Subsequent diachronous collision of the arc terranes and small plates followed at c . 630, 600, 580 and 530 Ma. The tectonic complexity reflects an accretionary evolution from Cryogenian to Cambrian times. The São Francisco–Congo and Angola palaeo-continents did probably not behave as one consolidated block, but rather may have accommodated considerable convergence during the Brasiliano/Pan-African episodes. The final docking of Cabo Frio and Kalahari in the Cambrian was coeval with the arrival of Amazonia on the opposite side, resulting in lateral reactivation and displacement between the previously amalgamated pieces. The transition between the Cambrian and the Ordovician is marked by the extensional collapse of the metamorphic core zones of the orogens.

A Damara orogen perspective on the assembly of southwestern Gondwana

Abstract The Pan-African Damara orogenic system records Gondwana amalgamation involving serial suturing of the Congo–São Francisco and Río de la Plata cratons (North Gondwana) from 580 to 550 Ma, before amalgamation with the Kalahari–Antarctic cratons (South Gondwana) as part of the 530 Ma Kuunga–Damara orogeny. Closure of the Adamastor Ocean was diachronous from the Araçuaí Belt southwards, with peak sinistral transpressional deformation followed by craton overthrusting and foreland basin development at 580–550 Ma in the Kaoko Belt and at 545–530 Ma in the Gariep Belt. Peak deformation/metamorphism in the Damara Belt was at 530–500 Ma, with thrusting onto the Kalahari Craton from 495 Ma through to 480 Ma. Coupling of the Congo and Río de la Plata cratons occurred before final closure of the Mozambique and Khomas (Damara Belt) oceans with the consequence that the Kuunga suture extends into Africa as the Damara Belt, and the Lufilian Arc and Zambezi Belt of Zambia. Palaeomagnetic data indicate that the Gondwana cratonic components were in close proximity by c. 550 Ma, so the last stages of the Damara–Kuunga orogeny were intracratonic, and led to eventual out-stepping of deformation/metamorphism to the Ross–Delamerian orogen ( c. 520–500 Ma) along the leading edge of the Gondwana supercontinental margin.

The Coastal Terrane of the Kaoko Belt, Namibia: Outboard arc-terrane and tectonic significance

The Coastal Terrane, or westernmost part of the Kaoko Belt outboard of the Three Palms Mylonite Zone, has distinct feldspathic arenite sedimentary sequences, no basement, distinct ɛ Nd sediment signatures excluding the Archaean and suggesting Mesoproterozoic–Neoproterozoic provenance sources, and primitive arc-like geochemical signatures for I-type granitoids. It contains evidence for an older metamorphic event at ∼650–645 Ma not present in any other part of the Kaoko Belt. This metamorphism (M1) was of high- T/low- P granulite to upper-amphibolite facies and includes migmatisation associated with I-type granitic magmatism of arc affinity. Arc growth at 650–640 Ma took place outboard within the Adamastor Ocean while Swakop Group facies turbidite sedimentation continued inboard along the passive margin. Overprinting M1 and M2 fabrics and metamorphic assemblages constrain the docking to have occurred between 650 and 580 Ma. Lack of ophiolite fragments and evidence of intermediate- to high- P metamorphism along the Three Palms Mylonite Zone suggest that it is not a suture and more likely to be part of an arc–backarc wrench–shear system that developed inboard of an E-dipping subduction system where the Adamastor Ocean was subducted beneath the leading edge of the attenuated Congo Craton. Oblique collision between 650 and 580 Ma, due to docking of this outboard Coastal Terrane with arc affinities, caused: (i) crustal scale oblique-overriding of the arc terrane over the passive margin of the Congo Craton; (ii) crustal scale sinistral shear partitioned into two major ductile shear zones; (iii) high- T granulite facies metamorphism in an extruded, shear zone-bounded core; and (iv) outwards- or cratonwards-verging basement-cored fold nappes.

Stratigraphic setting of the metalliferous Rosh Pinah Formation and the Spitzkop and Koivib Suites in the Pan-African Gariep Belt, southwestern Namibia

In this paper a stratigraphic scheme is proposed for the Neoproterozoic Rosh Pinah Formation in southwestern Namibia. The formation forms part of the Port Nolloth Zone in the Pan-African Gariep Belt and has become famous for hosting two major economic base metal deposits at Rosh Pinah and Skorpion. It is characterised by the presence of volcanic rocks in an otherwise clastic-dominated sediment succession. Genetically, it is related to progressive opening of a failed intracratonic rift graben in the east (Rosh Pinah Graben) that was separated by a basement horst from a half graben to the west, which developed into the Adamastor Ocean. During early rifting, sedimentation started along the western flank of an actively rising fault scarp as alluvial fans, alluvial plains and fan deltas. The main rock types are arkose, dolomitised limestone, calcarenite, siltstone, mudstone and a variety of volcanic and pyroclastic rocks related to bimodal, predominantly felsic ~750 Ma magmatism. Sedimentation of the Rosh Pinah Formation began during a glacial period that is correlated with the global 720 to 750 Ma Sturtian glaciation. The Rosh Pinah Formation displays repetitive sedimentary cycles that reflect rapid deposition with intervening quiescent periods related to reactivation on basin bounding faults with subsequent thermal and mechanical subsidence. Sedimentary facies play an important role in the localization of sedimentary-exhalative and hydrothermal replacement mineral deposits as fine-grained anoxic sediments. While deposition of the Rosh Pinah Formation volcano-sedimentary successions was sustained in the more active parts of the basin, background sedimentation to the west is reflected by more distal deposits, ranging from planar laminated allodapic limestone to mudstone, followed by platform carbonates. In proximal positions, tilting of the rift shoulders led to partial erosion of the Rosh Pinah Formation and thus to intraformational breccias and olistostromes. The Rosh Pinah Formation is overlain by a regressive, siliciclastic succession, which has been interpreted as reflecting eustatic seawater fall in preparation of a major, probably syn-Marinoan (~580 Ma) glaciation. Finally the original geometry of the Rosh Pinah Formation depository was largely overprinted by intense ductile deformation during the Gariepian orogeny, which started ~580 Ma and reached its peak at ~545 Ma.

Late Vendian Closure of the Adamastor Ocean: Timing of Tectonic Inversion and Syn-orogenic Sedimentation in the Gariep Basin

New Pb-Pb age data obtained on stromatolitic dolomite from the oceanic Marmora Terrane in the Pan-African Gariep Belt support previous Ar-Ar age data on early amphibole, that has been related to sea floor metamorphism, and indicate formation of oceanic crust in the Neoproterozoic Adamastor Ocean (Gariep Basin) around 600 Ma. A maximum age for oceanic crust in the Gariep Basin is given by 741±6 Ma, which is the best available minimum constraint on continental rifting in the Gariep Belt. Tectonic inversion to basin closure overlaps in time with the end of a younger glacial period that is recorded both in the oceanic and continental realms and is correlated with the global c. 580 Ma Marinoan glaciation. The rock record up to that time in the Marmora Terrane is completely different to that in the para-autochthonous, continental Port Nolloth Zone in the east of the belt, but the post-glacial sediment successions bear many lithological and geochemical similarities. These younger successions are therefore interpreted as reflecting syn-orogenic deposits in a foredeep, the age of which is constrained between c. 580 and 550 Ma. Comparison of three chemostratigraphic profiles through the post-glacial carbonate succession shows that enrichment in carbonate 13C is, in the first instance, a function of proximity to the basin margin and thus of the extent of ocean water stratification.

Geochronological constraints on the evolution of the Embu Complex, São Paulo, Brazil

Petrologic and geochronological work was carried out on a roadside outcrop of amphibolite facies orthogneisses near São Lourenço da Serra, about 50 km southwest of São Paulo City. These orthogneisses belong to the Embu Complex, within the Neoproterozoic Brasiliano Orogenic Cycle mobile belts of SE Brazil. The outcrop consists of predominantly foliated biotite tonalites and granodiorites, which were cut by granitic veins and pegmatites prior to final deformation. SHRIMP U/Pb measurements on zircons from one granodioritic–tonalitic gneiss indicate magmatic crystallization of the protolith at 811±13 Ma (MSWD=1.0). Zircons with dates of ca. 2000 and ca. 1000 Ma in this rock are interpreted as inherited from older crust. One zircon analyzed from the gneiss and three zircons from a discordant pegmatitic vein indicate an event at 650–700 Ma, perhaps related to the intrusion of the pegmatites. A regression of Rb–Sr whole rock data for four biotite gneisses yielded an imperfect isochron, giving an apparent age of 821±68 Ma and an elevated initial 87Sr/ 86Sr ratio of 0.719±0.005. The elevated initial 87Sr/ 86Sr ratio and the inherited zircons indicate involvement of older crust in the genesis of the gneisses. Rb–Sr feldspar and whole rock pairs yield ca. 560 Ma tielines, giving the time of final cooling below 300–350 °C, and the cessation of medium-grade metamorphism and ductile deformation. These results document a series of tectono-thermal events spanning 250 million years during the Brasiliano Orogenic Cycle. They relate to ca. 800 Ma magmatic arc activity and later allochthonous terrane assembly during closure of the Adamastor Ocean, resulting in the accretion of Western Gondwana.

The Araçuaı́-West-Congo Orogen in Brazil: an overview of a confined orogen formed during Gondwanaland assembly

The Neoproterozoic Adamastor-Brazilide Ocean was generated during the breakup of the Rodinia supercontinent, and remnants of its oceanic lithosphere have been found in the Brasiliano-Pan African orogenic system that includes the Araçuaı́, West-Congo, Brası́lia, Ribeira, Kaoko, Dom Feliciano, Damara and Gariep belts. The Araçuaı́ and the West-Congo belts are counterparts of the same Neoproterozoic orogen. The first belt comprises two thirds of the Araçuaı́-West-Congo Orogen. This orogen is rather unique owing to its confined nature within the embayment outlined by the São Francisco and Congo cratons. In spite of this, the presence of ophiolitic remnants, and a calc-alkaline magmatic arc, indicate that the basin/orogen evolution comprise both oceanic spreading and consumption. It is assumed that coeval Paramirim and Sangha aulacogens played a key role by making room for the Araçuaı́-West-Congo Basin. Sedimentary successions record all major stages of a basin that evolved from continental rift, when glaciation-related sedimentation was very significant, to passive margin. Rifting started around 1.0–0.9 Ga. The oceanic stage is constrained by an ophiolitic remnant dated at 0.8 Ga. If the cratonic bridge that once linked the São Francisco and Congo palaeocontinental regions did not hinder the opening of an ocean basin, it certainly limited its width. As a consequence, only a narrow oceanic lithosphere was generated, and it was subducted afterwards. This is also suggested by orogenic calc-alkaline granitoids occuping a small area of the orogen. Geochronological data for pre-, syn- and late-collisional granitoids indicate that the orogenic stage lasted from 625 Ma to 570 Ma. A period of magmatic quiescence was followed by intrusion of postcollisional plutons at 535–500 Ma. The features of the Araçuaı́-West-Congo Orogen suggest the development of a complete Wilson Cycle in a branch of the Adamastor Ocean, which can be interpreted as a gulf with limited generation of oceanic lithosphere.

Tectono-sedimentary evolution of sedimentary basins from Late Paleoproterozoic to Late Neoproterozoic in the São Francisco craton and Araçuaı́ fold belt, eastern Brazil

Two first-order regional unconformities separate Proterozoic supracrustal rocks of the São Francisco craton and the Araçuaı́ fold belt (eastern Brazil) into three genetic units. These constitute three superimposed first-order basin-fill cycles: the Espinhaço, Macaúbas-Salinas and Bambuı́ megasequences. The Paleo-Mesoproterozoic Espinhaço megasequence (ca. 1730–1500 Ma) was deposited in an intracratonic rift-sag basin in four stages (pre-rift, rift, transitional and flexural). Each of these stages is bounded by regional, second-order unconformities. Basin evolution was controlled primarily by tectonics, although sea-level variations were also important in the transitional and flexural stages. After a hiatus of several hundred million years, subsidence renewed during extension related to the early Neoproterozoic breakup of the supercontinent Rodinia with rift to passive-margin sedimentation in the Macaúbas-Salinas Basin (ca. 950–700 Ma). Initial glaciogenic sedimentation, possibly induced by uplift related to a mantle plume, occurred in a rift setting. Subsequent a shelf/slope/deep-sea passive-margin deposits, represent opening of a northern branch of the Adamastor Ocean. Tectonic, magmatic, climatic and eustatic processes all combined to control basin evolution. The Bambuı́ megasequence (ca. 800–650 Ma) was deposited in a foreland basin during convergent and collisional tectonics related to closure of the Brazilide Ocean and generation of the Brası́lia fold belt west of the São Francisco paleocontinental region. The evolution of the Bambuı́ Basin, whose fill is characterized by three transgressive–progradational, shallowing–upward sequences, was mainly controlled by tectonic processes.

New U-Pb zircon ages for the Kuboos pluton in the Pan-African Gariep belt, South Africa: Cambrian mantle plume or far field collision effect?

The age of the Kuboos pluton, which forms part of a series of alkaline, apparently post-orogenic intrusive bodies along the Kuboos-Bremen line, was re-assessed using single zircon U-Pb isotope analyses. A new age of 507 ± 6 Ma obtained for the youngest intrusive phase, a monzogranite from the Kuboos composite pluton is clearly younger than the main phase of deformation in the Gariep Belt (545 ± 2 Ma). This age corresponds to a major thermal event, as reflected by Rb-Sr and Ar-Ar mineral ages and by coeval post-orogenic granite in the Damara Belt, and also with Ar-Ar ages of syn-tectonic muscovite, related to a late tectonic pulse in the Pan-African Gariep and Saldania Belts, their basement and corresponding foreland basins. While the earlier 545 Ma tectonothermal event is explained by the closure of the Adamastor Ocean between the Kalahari and the Rio de la Plata cratons, the younger 500 Ma tectonic pulse can be linked to subduction beneath the western margin of Gondwana. The alkaline magmatism, which is represented by the Kuboos pluton and which is contemporaneous with the latter tectonic event, can be explained either by a mantle plume underneath southern Africa or as a distant effect of collision along the Gondwana margin. The occurrence of a 1.1 - 1.2 Ga xenocrystic zircon in the Kuboos pluton hints at the external Gariep Belt being floored by Kibaran crust.

Marine evaporites from an oceanic island in the Neoproterozoic Adamastor ocean

We report a hitherto unknown occurrence of ancient Neoproterozoic evaporite deposits from an allochthonous terrane in the Pan-African Gariep belt in Namibia. Low contents of Rb, Cs, Ba, Zr, Hf, Th, and U, flat chondrite-normalised rare earth element (REE) patterns, 87Sr/ 86Sr ratios as low as 0.7075, Na–Cl–Br systematics of fluid inclusion leachates, and high δ 11B values for stratiform tourmalinites, together with geologic evidence, such as association with oceanic basalt, gabbro, and stromatolitic dolomite, point to a marine evaporitic origin. An atoll environment on an oceanic island is envisaged as a likely depositional setting. In the associated mafic sequence we found a diamictite with metre-sized ice rafted detritus, suggesting the presence of sea ice cover at relatively low latitude around the time of evaporite deposition. Based on chemostratigraphic ( 87Sr/ 86Sr, δ 13C) comparison with passive continental margin sediments in the para-autochthonous external part of the Gariep belt, a correlation of the mafic diamicitite with the global Varangian (590–560 Ma) glaciation is proposed.

Tectonics of the southern portion of the Ribeira Belt (Apiaı́ Domain)

The name Ribeira Belt was originally used in a reference to a broad belt of deformed crust, which strikes subparallel to the southeastern Brazilian coastline. It was originally believed to be Neoproterozoic (Brasiliano–Panafrican), but present investigations indicate that this age seems to be mainly related to the extensive arc related plutonism and tectono–metamorphism that affected its most representative formations, whereas the time of deposition of the supracrustals would be significantly older. In this study, the main tectono-stratigraphic features of the Apiaı́ Domain of the Ribeira Belt are described. The lithostratigraphy is reviewed, as well the paleogeographic, structural, geochemical and radiometric data, and their implications for the tectonic evolution models for the region. The marine sediments of the Apiaı́ Domain were laid down on the borders of the Paraná and Luı́s Alves cratons, and on an intervening Mesoproterozoic (1.8–1.1 Ga) ocean crust. They were intensively deformed by ocean closure and collision of the cratonic fragments, and they constitute the majority of the local supracrustals designated as the Açungui Supergroup. Symmetrical continental passive margin deposits are represented to the northwest by a more proximal facies (Itaiacoca Group), probably related to the Paraná paleocontinent, and by the Capiru Formation to the southeast. Between them lies the Central Domain formed by the Votuverava Group, characterized by the remains of gradually deepening carbonatic platform sediments of the Lajeado Subgroup, which grade eastward into distal turbiditic sequences of the Ribeira Subgroup, where ocean floor and immature island arc magmatic rocks are present. During the Neoproterozoic, the Apiaı́ Domain behaved as a single continental mass into which basic rocks were intruded between 1.1 and 0.85 Ga. Between 0.7 and 0.6 Ga, the Apiaı́ Domain evolved into a Cordilleran-type magmatic arc, related to the evolution of a subduction zone, possibly situated to the southeast of the Luı́s Alves cratonic fragment. This subduction caused metamorphism and migmatization of the supra- and infra-crustal rocks of the region, and produced the extensive calc-alkaline granitoid plutonism. Between 0.6 and 0.45 Ga, the majority of the southeastern continental blocks amalgamated during closure of the Adamastor Ocean, followed by development of a system of regional dextral transcurrent shear zones related to oblique collision and/or escape tectonics. Pull-apart basins opened up and were filled with molasse sequences. The region was intruded by post-tectonic granitoids and then underwent regional cooling.

Neoproterozoic to Early Cambrian Crustal Evolution of the Pan-African Saldania Belt, South Africa

The Saldania Belt forms part of a system of Neoproterozoic mobile belts that surround and weld older cratons on the African continent. It is a poorly exposed, low-grade orogenic belt composed of a number of inliers unroofed in mega-anticlinal hinges of the Permo-Triassic Cape Fold Belt, along the southern tip of Africa. Deformed meta-volcanosedimentary units are contained within tectonostratigraphic terranes separated by deep-rooted, sinistral/dextral strike–slip fault zones, which display evidence of prolonged reactivation. Sedimentary rocks and volcanics of the Boland terrane (Malmesbury Group), lower Kango Group and Gamtoos Group are considered to be distal facies of rift successions, overlying the mid-Proterozoic crystalline basement on the southwestern edge of the Kalahari Craton. These lithologies have been deposited in semi-pull-apart basins that formed in response to the breakup of Rodinia and progressive opening of the proto-Atlantic (Adamaster Ocean) from 780–750 Ma. Fragments of juvenile Pan-African crust with WPB–MORB characteristics occur in the Bridgetown Formation (Swartland terrane, Malmesbury inlier) and attest to ocean floor spreading in this belt. The thick turbidite successions of the Swartland and Tygerberg terranes (Malmesbury Group) reflect deeper water conditions and were deposited partly on oceanic crust in an evolving ocean/continental margin basin marked by increasing sea levels and reduction of continental freeboard. Reversal of spreading and closing of the Adamastor Ocean, possibly initiated by the opening of Iapetus, occurred in the period 600–570 Ma. Although this movement vector caused sinistral transpressional reactivation of the Gariep and Saldanian margins, there is no proper collision orogen developed in the Saldania Belt. The Bridgetown Formation represents the suture between the southern, rifted margin succession of the Kalahari Craton and a poorly developed southern orogenic belt represented in part by the Swartland and Tygerberg terranes. This geosuture is similar to that of the South Atlantic, which formed in the analogous Gariep Belt between 575 and 543 Ma. Multiphase S-, I- and A-type granites of the Cape Granite Suite intruded the Saldania Belt in a pervasive transpressive regime between 550 and 510 Ma, comparing well with the timing of peak metamorphism/collision in the Gariep Belt (545 Ma). The absence of a proper collisional orogen and the strongly developed northwesterly structural grain in the Saldania Belt seem to suggest that the Cape Granite Suite was generated in a strike–slip regime. A discrepancy of approximately 50 Ma between the age of the collision granites of the Dom Feliciano Belt and Cape Granite Suite suggests that they are genetically unrelated. Similarities in structural styles, magmatism and dated events between the Saldania–Ross–Delamarian chain of orogens, however, indicate a common history. The evolution of this chain and that of the Gariep–Damara–Dom Feliciano Belts can be accommodated in a single plate tectonic model of simultaneous sinistral transpressive subduction driven by the rifting of Laurentia from South America and opening of the Iapetus Ocean. Loading and depression of the western and southern Kalahari Craton margin by thrust stacks of the Late Neoproterozoic orogens led to the formation of syn- to post-orogenic peripheral foreland basins, such as those represented by the Nama and Vanrhynsdorp Groups, lowering of sea level and increased continental freeboard. Deposition of the unconformity based upper Kango Group, and possibly the molassic Franschhoek Formation, occurred in intra-orogen pull-apart basins, which possibly developed between 530 and 510 Ma in the Saldania Belt.

The break-up of Rodinia, birth of Gondwana, true polar wander and the snowball Earth

A major global plate reorganisation occurred between ∼750 and ∼550 Ma. Gondwana was assembled following the dispersal of Rodinia, a supercontinent centred on Laurentia in existence since ∼1050 Ma. The reorganisation began when tectonic elements, later composing East Gondwana, rotated piecemeal away from the Pacific margin of Laurentia. These elements swept across the ancestral Pacific (Mozambique) Ocean that lay between Laurentia and the combined African cratons of Congo and Kalahari, which were loosely joined after ∼820 Ma. Simultaneously, the Adamastor (Brasilide) Ocean closed by subduction bordering the West Gondwana cratons, drawing virtually all of Gondwana together by ∼550 Ma. The final assembly of Gondwana occurred contemporaneously with the separation of Laurentia from West Gondwana. It has been postulated that the imprint of Rodinia's long-lived existence on lower mantleconvection produced a prolate ellipsoidal geoid figure. This could give rise to episodic inertial interchange true polar wander (IITPW), meaning that the entire silicate shell of the Earth (above the core-mantle boundary) rolled through 90° with respect to the diurnal spin axis in ∼15 Ma (equivalent to an apparent polar wander velocity of ∼66 cm a −1. Although empirical arguments for IITPW of Cambrian age appear to be flawed, evidence for an ultra-fast ( > 40 cm a −1) meridional component of apparent polar wander for Laurentia between 564 and 550 Ma suggests that IITPW might have occurred at that time. The break-up of Rodinia increased the continental margin area and preferential organic C burial globally, which is reflected by high δ 13C values in seawater proxies. The consequent drawdown of CO 2 is implicated in a succession of runaway ice-albedo catastrophes between ∼750 and ∼570 Ma, during each of which the oceans completely froze over. Each “snowball” Earth event must have lasted for millions of years because their terminations depended on extreme CO 2 levels, built up by subaerial volcanic outgassing in the absence of sinks for C. A succession of ice-albedo catastrophes, each terminated under ultra-greenhouse conditions, must have imposed an intense environmental filter on the evolution of life. They may have triggered the radiation of Ediacaran fauna in the aftermath of the final snowball event. It is increasingly recognised that the Late Neoproterozoic was one of the most remarkable periods in Earth history, and it appears to exemplify the interplay of tectonics, the environment and biology in deep time.

A structural transect across the coastal mobile belt in the Brazilian Highlands (latitude 20°S): the roots of a Precambrian transpressional orogen

We present results of a detailed structural analysis from a 250 km-long, east–west-trending transect crossing the Coastal Mobile Belt, a part of the Precambrian orogen which lies between the eastern edge of the São Francisco craton and the Atlantic coast. The region exposes amphibolite–granulite grade metamorphic rocks and migmatites which formed at mid-lower crustal depths during the Brasiliano orogeny (0.63–0.52 Ga). This event marked the closure of the northernmost Adamastor Ocean and collision between the São Francisco and Congo Cratons during West Gondwana assembly. Brasiliano deformation resulted in W-vergent structures including thrust faults which accommodated kilometer-scale transport of crystalline basement, overturned kilometer-scale folds, sheath folds and penetratively developed gneissosity and schistosity. Isolated relics of an older folded fabric occur locally and may represent Transamazonian (2.2–2.0 Ga) deformation. The orogen is kinematically partitioned with the eastern 175 km dominated by moderately- to steeply dipping north-trending dextral strike-slip and oblique-slip faults and associated flower structures, whereas the western 75 km is dominated by W-vergent shallowly- to moderately east-dipping thrust faults. The boundary between these two provinces may mark a Brasiliano suture. Throughout the transect, quartzite and metasedimentary belts form strongly deformed zones between massive crystalline basement thrust sheets. The granulite-cored Serra do Caparaù massif, the highest mountains in South America outside of the Andes and Guyana shield, occupies a restraining bend between two Brasiliano dextral shear zones. The W-vergent Coastal Mobile Belt formed contiguously with the E-vergent Pan-African West Congo orogen now exposed along the conjugate margin of Africa. Thus an important late Precambrian boundary between structurally linked but kinematically opposed structural provinces must lie hidden in the extended offshore continental margins of either continent. Cretaceous opening of the South Atlantic and separation of the West Congo belt from the Coastal Mobile Belt may have been structurally influenced by this boundary.

Neoproterozoic tectono-thermal evolution of the Gariep Belt and its basement, Namibia and South Africa

Within the Pan-African Gariep Belt in southwestern Africa a para-autochthonous, predominantly sedimentary sequence, deposited in a rift graben and subsequently on a passive continental margin with a Kibaran basement, is distinguished from an allochthonous, predominantly oceanic terrane that was thrust on top of the former during collision of the Rio de la Plata and the Kalahari plates. 40Ar/ 39Ar mineral cooling ages presented here set the following constraints: after Kibaran high-grade metamorphism, the consolidated basement cooled through ∼300°C at 1006 Ma. Rifting between 781 and 741 Ma was followed by the formation of oceanic crust, the lower parts of which cooled through 500°C sometime between 630 and 600 Ma. After inversion from extension to compression, some of the oceanic crustal material was shredded off and incorporated into an accretionary wedge—a process that is indirectly dated between 573–576 Ma by hornblende plateau ages. Continent–continent collision and thrusting of the internal onto the external zones of the tectonic belt culminated between 547–543 Ma as indicated by further hornblende plateau ages. Muscovite cooling ages for the internal zone are between 526 and 529 Ma, those for the tectonically deeper, external zone are 483 Ma in the north and between 495–506 Ma in the south of the belt. In conjunction with published data, these results suggest successive closure of first the northern Adamastor ocean (Kaoko Belt), followed by the Khomas sea (intracontinental Damara Belt), and finally the southern Adamastor ocean (Gariep Belt).

Paleomagnetic Constraints on the Rodinia Supercontinent: Implications for its Neoproterozoic Break-up and the Formation of Gondwana

An interpretation of available paleomagnetic data from the Laurentia, Congo-Sao Francisco, Kalahari, and Amazonia cratons favors the hypothesis that these units were juxtaposed in a supercontinent by 1000 Ma. This supercontinent is similar to Hoffman's (1991) Rodinia, except for the Kalahari craton, whose 1300 to 1000 Ma Namaqua-Natal mobile belt is now juxtaposed against the correlated 1300 to 1000 Ma Grenville belt in eastern Laurentia, Our model suggests that a continuous 1300 to 1000 Ma orogenic belt, formed by the Grenville, Sunsas, Kibaride-Irumide-Lurio, Namaqua-Natal, and Dronning Maud Land-Coats Land belts, represents the suture zone between the Amazonia, Congo-Sao Francisco, Kalahari-Grunehogna, and Laurentia blocks. The formation of western Gondwana (from our Rodinia supercontinent) may be accomplished by the closure of the large Mozambique Ocean and the more restricted Adamastor Ocean, combined with some counterclockwise rotation of the Congo-Sao Francisco craton. Rotation of the Congo-Sao Francisco craton can explain the observed oblique convergence and wrench tectonics of Pan African-Brasiliano mobile belts that encircle this craton. The model is also consistent with the synchroneity between the Rodinia break-up and the assembly of Gondwana, as suggested by several authors (Laurentia began to separate from Rodinia at ∼625 Ma or later).

Late Precambrian (850-800 Ma) palaeomagnetic pole for the south Indian Shield from the Harohalli alkaline dykes: geotectonic implications for Gondwana reconstructions : Radhakrishna T. & Mathew J., Precambrian Research, 1996, 80/1–2 (77–87)

The Saldania Belt forms part of a system of Neoproterozoic mobile belts that surround and weld older cratons on the African continent. It is a poorly exposed, low-grade orogenic belt composed of a number of inliers unroofed in mega-anticlinal hinges of the Permo-Triassic Cape Fold Belt, along the southern tip of Africa. Deformed meta-volcanosedimentary units are contained within tectonostratigraphic terranes separated by deep-rooted, sinistral/dextral strike–slip fault zones, which display evidence of prolonged reactivation.Sedimentary rocks and volcanics of the Boland terrane (Malmesbury Group), lower Kango Group and Gamtoos Group are considered to be distal facies of rift successions, overlying the mid-Proterozoic crystalline basement on the southwestern edge of the Kalahari Craton. These lithologies have been deposited in semi-pull-apart basins that formed in response to the breakup of Rodinia and progressive opening of the proto-Atlantic (Adamaster Ocean) from 780–750 Ma. Fragments of juvenile Pan-African crust with WPB–MORB characteristics occur in the Bridgetown Formation (Swartland terrane, Malmesbury inlier) and attest to ocean floor spreading in this belt. The thick turbidite successions of the Swartland and Tygerberg terranes (Malmesbury Group) reflect deeper water conditions and were deposited partly on oceanic crust in an evolving ocean/continental margin basin marked by increasing sea levels and reduction of continental freeboard.Reversal of spreading and closing of the Adamastor Ocean, possibly initiated by the opening of Iapetus, occurred in the period 600–570 Ma. Although this movement vector caused sinistral transpressional reactivation of the Gariep and Saldanian margins, there is no proper collision orogen developed in the Saldania Belt. The Bridgetown Formation represents the suture between the southern, rifted margin succession of the Kalahari Craton and a poorly developed southern orogenic belt represented in part by the Swartland and Tygerberg terranes. This geosuture is similar to that of the South Atlantic, which formed in the analogous Gariep Belt between 575 and 543 Ma. Multiphase S-, I- and A-type granites of the Cape Granite Suite intruded the Saldania Belt in a pervasive transpressive regime between 550 and 510 Ma, comparing well with the timing of peak metamorphism/collision in the Gariep Belt (545 Ma). The absence of a proper collisional orogen and the strongly developed northwesterly structural grain in the Saldania Belt seem to suggest that the Cape Granite Suite was generated in a strike–slip regime.A discrepancy of approximately 50 Ma between the age of the collision granites of the Dom Feliciano Belt and Cape Granite Suite suggests that they are genetically unrelated. Similarities in structural styles, magmatism and dated events between the Saldania–Ross–Delamarian chain of orogens, however, indicate a common history. The evolution of this chain and that of the Gariep–Damara–Dom Feliciano Belts can be accommodated in a single plate tectonic model of simultaneous sinistral transpressive subduction driven by the rifting of Laurentia from South America and opening of the Iapetus Ocean.Loading and depression of the western and southern Kalahari Craton margin by thrust stacks of the Late Neoproterozoic orogens led to the formation of syn- to post-orogenic peripheral foreland basins, such as those represented by the Nama and Vanrhynsdorp Groups, lowering of sea level and increased continental freeboard. Deposition of the unconformity based upper Kango Group, and possibly the molassic Franschhoek Formation, occurred in intra-orogen pull-apart basins, which possibly developed between 530 and 510 Ma in the Saldania Belt.

Relationship between northwestern Tasmania and east Gondwanaland in the late Cambrian/early Ordovician: paleomagnetic evidence : Li Z.X., BAillie P.W. & Powell C.M., Tectonics, 1997, 16/1 (161–171)

The Saldania Belt forms part of a system of Neoproterozoic mobile belts that surround and weld older cratons on the African continent. It is a poorly exposed, low-grade orogenic belt composed of a number of inliers unroofed in mega-anticlinal hinges of the Permo-Triassic Cape Fold Belt, along the southern tip of Africa. Deformed meta-volcanosedimentary units are contained within tectonostratigraphic terranes separated by deep-rooted, sinistral/dextral strike–slip fault zones, which display evidence of prolonged reactivation.Sedimentary rocks and volcanics of the Boland terrane (Malmesbury Group), lower Kango Group and Gamtoos Group are considered to be distal facies of rift successions, overlying the mid-Proterozoic crystalline basement on the southwestern edge of the Kalahari Craton. These lithologies have been deposited in semi-pull-apart basins that formed in response to the breakup of Rodinia and progressive opening of the proto-Atlantic (Adamaster Ocean) from 780–750 Ma. Fragments of juvenile Pan-African crust with WPB–MORB characteristics occur in the Bridgetown Formation (Swartland terrane, Malmesbury inlier) and attest to ocean floor spreading in this belt. The thick turbidite successions of the Swartland and Tygerberg terranes (Malmesbury Group) reflect deeper water conditions and were deposited partly on oceanic crust in an evolving ocean/continental margin basin marked by increasing sea levels and reduction of continental freeboard.Reversal of spreading and closing of the Adamastor Ocean, possibly initiated by the opening of Iapetus, occurred in the period 600–570 Ma. Although this movement vector caused sinistral transpressional reactivation of the Gariep and Saldanian margins, there is no proper collision orogen developed in the Saldania Belt. The Bridgetown Formation represents the suture between the southern, rifted margin succession of the Kalahari Craton and a poorly developed southern orogenic belt represented in part by the Swartland and Tygerberg terranes. This geosuture is similar to that of the South Atlantic, which formed in the analogous Gariep Belt between 575 and 543 Ma. Multiphase S-, I- and A-type granites of the Cape Granite Suite intruded the Saldania Belt in a pervasive transpressive regime between 550 and 510 Ma, comparing well with the timing of peak metamorphism/collision in the Gariep Belt (545 Ma). The absence of a proper collisional orogen and the strongly developed northwesterly structural grain in the Saldania Belt seem to suggest that the Cape Granite Suite was generated in a strike–slip regime.A discrepancy of approximately 50 Ma between the age of the collision granites of the Dom Feliciano Belt and Cape Granite Suite suggests that they are genetically unrelated. Similarities in structural styles, magmatism and dated events between the Saldania–Ross–Delamarian chain of orogens, however, indicate a common history. The evolution of this chain and that of the Gariep–Damara–Dom Feliciano Belts can be accommodated in a single plate tectonic model of simultaneous sinistral transpressive subduction driven by the rifting of Laurentia from South America and opening of the Iapetus Ocean.Loading and depression of the western and southern Kalahari Craton margin by thrust stacks of the Late Neoproterozoic orogens led to the formation of syn- to post-orogenic peripheral foreland basins, such as those represented by the Nama and Vanrhynsdorp Groups, lowering of sea level and increased continental freeboard. Deposition of the unconformity based upper Kango Group, and possibly the molassic Franschhoek Formation, occurred in intra-orogen pull-apart basins, which possibly developed between 530 and 510 Ma in the Saldania Belt.

Alleghanian paleostress reconstruction in the northern Appalachians: intraplate deformation between Laurentia and Gondwana : Faure S., Tremblay A. & Angelier J., Geological Society of America Bulletin, 1996, 108/11 (1467–1480)

The Saldania Belt forms part of a system of Neoproterozoic mobile belts that surround and weld older cratons on the African continent. It is a poorly exposed, low-grade orogenic belt composed of a number of inliers unroofed in mega-anticlinal hinges of the Permo-Triassic Cape Fold Belt, along the southern tip of Africa. Deformed meta-volcanosedimentary units are contained within tectonostratigraphic terranes separated by deep-rooted, sinistral/dextral strike–slip fault zones, which display evidence of prolonged reactivation.Sedimentary rocks and volcanics of the Boland terrane (Malmesbury Group), lower Kango Group and Gamtoos Group are considered to be distal facies of rift successions, overlying the mid-Proterozoic crystalline basement on the southwestern edge of the Kalahari Craton. These lithologies have been deposited in semi-pull-apart basins that formed in response to the breakup of Rodinia and progressive opening of the proto-Atlantic (Adamaster Ocean) from 780–750 Ma. Fragments of juvenile Pan-African crust with WPB–MORB characteristics occur in the Bridgetown Formation (Swartland terrane, Malmesbury inlier) and attest to ocean floor spreading in this belt. The thick turbidite successions of the Swartland and Tygerberg terranes (Malmesbury Group) reflect deeper water conditions and were deposited partly on oceanic crust in an evolving ocean/continental margin basin marked by increasing sea levels and reduction of continental freeboard.Reversal of spreading and closing of the Adamastor Ocean, possibly initiated by the opening of Iapetus, occurred in the period 600–570 Ma. Although this movement vector caused sinistral transpressional reactivation of the Gariep and Saldanian margins, there is no proper collision orogen developed in the Saldania Belt. The Bridgetown Formation represents the suture between the southern, rifted margin succession of the Kalahari Craton and a poorly developed southern orogenic belt represented in part by the Swartland and Tygerberg terranes. This geosuture is similar to that of the South Atlantic, which formed in the analogous Gariep Belt between 575 and 543 Ma. Multiphase S-, I- and A-type granites of the Cape Granite Suite intruded the Saldania Belt in a pervasive transpressive regime between 550 and 510 Ma, comparing well with the timing of peak metamorphism/collision in the Gariep Belt (545 Ma). The absence of a proper collisional orogen and the strongly developed northwesterly structural grain in the Saldania Belt seem to suggest that the Cape Granite Suite was generated in a strike–slip regime.A discrepancy of approximately 50 Ma between the age of the collision granites of the Dom Feliciano Belt and Cape Granite Suite suggests that they are genetically unrelated. Similarities in structural styles, magmatism and dated events between the Saldania–Ross–Delamarian chain of orogens, however, indicate a common history. The evolution of this chain and that of the Gariep–Damara–Dom Feliciano Belts can be accommodated in a single plate tectonic model of simultaneous sinistral transpressive subduction driven by the rifting of Laurentia from South America and opening of the Iapetus Ocean.Loading and depression of the western and southern Kalahari Craton margin by thrust stacks of the Late Neoproterozoic orogens led to the formation of syn- to post-orogenic peripheral foreland basins, such as those represented by the Nama and Vanrhynsdorp Groups, lowering of sea level and increased continental freeboard. Deposition of the unconformity based upper Kango Group, and possibly the molassic Franschhoek Formation, occurred in intra-orogen pull-apart basins, which possibly developed between 530 and 510 Ma in the Saldania Belt.