Joseph G. Meert

A synopsis of events related to the assembly of eastern Gondwana

Abstract The assembly of the eastern part of Gondwana (eastern Africa, Arabian–Nubian shield (ANS), Seychelles, India, Madagascar, Sri Lanka, East Antarctica and Australia) resulted from a complex series of orogenic events spanning the interval from ∼750 to ∼530 Ma. Although the assembly of Gondwana is generally discussed in terms of the suturing of east and west Gondwana, such a view oversimplifies the true nature of this spectacular event. A detailed examination of the geochronologic database from key cratonic elements in eastern Gondwana suggests a multiphase assembly. The model outlined in this paper precludes the notion of a united east Gondwana and strongly suggests that its assembly paralleled the final assembly of greater Gondwana. It is possible to identify at least two main periods of orogenesis within eastern Gondwana. The older orogen resulted from the amalgamation of arc terranes in the Arabian–Nubian shield region and oblique continent–continent collision between eastern Africa (Kenya–Tanzania and points northward) with an, as of yet, ill-defined collage of continental blocks including parts of Madagascar, Sri Lanka, Seychelles, India and East Antarctica during the interval from ∼750 to 620 Ma. This is referred to as the East Africa Orogen (EAO) in keeping with both the terminology and the focus of the paper by Stern [Annu. Rev. Earth Planet. Sci. 22 (1994) 319]. The second major episode of orogenesis took place between 570 and 530 Ma and resulted from the oblique collision between Australia plus an unknown portion of East Antarctica with the elements previously assembled during the East African Orogen. This episode is referred to as the Kuunga Orogeny following the suggestion of Meert et al. [Precambrian Res. 74 (1995) 225]. Paleomagnetic data are currently too few to provide a rigorous test of this proposal, but the extant data do not conflict with the notion of a polyphase assembly of eastern Gondwana. The major conclusion of this paper is that east Gondwana did not exist until its Cambrian assembly.

Continental break-up and collision in the Neoproterozoic and Palaeozoic — A tale of Baltica and Laurentia

During the Neoproterozoic and Palaeozoic the two continents of Baltica and Laurentia witnessed the break-up of one supercontinent, Rodinia, and the formation of another, but less long-lived, Pangea. Baltica and Laurentia played central roles in a tectonic menage a trois that included major orogenic events, a redistribution of palaeogeography and a brief involvement of both with Gondwana. Many of these plate re-organisations took place over a short time interval and invite a re-evaluation of earlier geodynamic models which limited the speeds at which large continental plates could move to an arbitrarily low value. Baltica and Laurentia probably shared a common drift history for the time interval 750 – 600 Ma as they rotated clockwise and drifted southward from an equatorial position during the opening of the Proto-Pacific between Laurentia and East Gondwana (initial break-up of Rodinia). On their combined approach toward the south pole, Baltica and Laurentia were glaciated during the Varanger glaciations. Although the two continents drifted toward the south pole during the Late Proterozoic, they began to separate at around 600 Ma (rift to drift) to form the Iapetus Ocean through asymmetric rifting and relative rotations of up to 180°. Initiation of rifting on the Baltic margin is marked by the 650 Ma Egersund tholeiitic dykes (SW Norway) which contain abundant lower crustal zenoliths, and the tholeiitic magma was probably derived from a mantle plume. In latest Precambrian time, the final redistribution of Rodinia is characterised by high plate velocities. In particular, Laurentia began a rapid, up to 20 cm/yr, ascent to equatorial latitudes and essentially stayed in low latitudes throughout most of the Palaeozoic. The high velocities suggest either that Laurentia was pushed off a lower mantle heat anomaly originating from supercontinental mantle insulation or that Laurentia was pulled toward a subduction-generated cold spot in the proto-Pacific. Baltica, except for a short and rapid excursion to lower latitudes in the Late Vendian, remained mostly in intermediate to high southerly latitudes and closer to the Gondwana margin until Early Ordovician times. In Early Ordovician times, Arenig-Llanvirn platform trilobites show a broad distinction between the continents of Laurentia/Siberia/North China Block (Bathyurid), Baltica (Ptychopygine/ Megalaspid) and the areas of NW Gondwana/Avalonia/Armorica (Calymenacean-Dalmanitacean). During the Ordovician, Baltica rotated and moved northward, approaching close enough to Laurentia by the late Caradoc for trilobite and brachiopod spat to cross the intervening Iapetus Ocean. Docking appears to have been irregular both in time and manner: the collision between Scotland/Greenland and western Norway resulted in the early Scandian Orogeny in the Silurian (c. 425 Ma), but further south, there is evidence of late Silurian impingement with subduction of Avalonian continental crust (in England and Ireland) below the eastern edge of Laurentia until the Emsian. In the northern Appalachians the main time of collision appears to have been during the Emsian/Eifellian Acadian Orogeny. Recent analyses invalidates the traditional concept of a sustained orthogonal relationship between Baltica and Laurentia across a single Iapetus Ocean throughout the Caledonide evolution. The active margin of Baltica (Scandinavian Caledonides) faced Siberia during the Late Cambrian and Early Ordovician with oceanic separation between these landmasses in the order of 1200–1500 km. This may explain the local occureences of Siberia-Laurentian type Bathyarid tribobite faunas in Central Norwegian Caledonian nappes, earlier interpreted as Laurentia-Baltica trilobite mixing. Subsequent counterclockwise rotation of Baltica transferred the Caledonian margin in the direction of Laurentia by Silurian times, when the two continents once again started to collide to form Euramerica. This rotation, along with the strongly asymmetric opening of the Iapetus at around 600 Ma, demonstrates a complexity in Precambrian-Palaeozoic plate tectonics, i.e. a collage of metastable plate boundaries which have perhaps too often been simplified to an orthagonal Wilson cycle tectonic scenario.

The making and unmaking of a supercontinent: Rodinia revisited

Abstract During the Neoproterozoic, a supercontinent commonly referred to as Rodinia, supposedly formed at ca. 1100 Ma and broke apart at around 800–700 Ma. However, continental fits (e.g., Laurentia vs. Australia–Antarctica, Greater India vs. Australia–Antarctica, Amazonian craton [AC] vs. Laurentia, etc.) and the timing of break-up as postulated in a number of influential papers in the early–mid-1990s are at odds with palaeomagnetic data. The new data necessitate an entirely different fit of East Gondwana elements and western Gondwana and call into question the validity of SWEAT, AUSWUS models and other variants. At the same time, the geologic record indicates that Neoproterozoic and early Paleozoic rift margins surrounded Laurentia, while similar-aged collisional belts dissected Gondwana. Collectively, these geologic observations indicate the breakup of one supercontinent followed rapidly by the assembly of another smaller supercontinent (Gondwana). At issue, and what we outline in this paper, is the difficulty in determining the exact geometry of the earlier supercontinent. We discuss the various models that have been proposed and highlight key areas of contention. These include the relationships between the various ‘external’ Rodinian cratons to Laurentia (e.g., Baltica, Siberia and Amazonia), the notion of true polar wander (TPW), the lack of reliable paleomagnetic data and the enigmatic interpretations of the geologic data. Thus, we acknowledge the existence of a Rodinia supercontinent, but we can place only loose constraints on its exact disposition at any point in time.

Paleomagnetic constraints on timing of the Neoproterozoic breakup of Rodinia and the Cambrian formation of Gondwana

Paleomagnetic data from East Gondwana (Australia, Antarctica, and India) and Laurentia are interpreted to demonstrate that the two continents were juxtaposed in the Rodinia supercontinent by 1050 Ma. They began to separate after 725 Ma, allowing the formation of the Pacific Ocean. The low-latitude Rapitan and Sturtian glaciations occurred during the rifting that led to continental breakup. East Gondwana remained in low latitudes for the rest of the Neoproterozoic, while Laurentia moved to polar latitudes by 580 Ma. During the Vendian, a wide Pacific Ocean separated the two continental land masses. The younger Marinoan, Ice Brook, and Varangian glaciations in the early Vendian preceded a second continental breakup in the late Vendian, causing formation of the eastern margin of Laurentia and rejuvenation of its western margin. Paleomagnetic data indicate that Gondwana was not fully assembled until the end of the Neoproterozoic, possibly as late as Middle Cambrian.

The Proterozoic supercontinent Rodinia: paleomagnetically derived reconstructions for 1100 to 800 Ma

Abstract Well-dated paleomagnetic poles for the interval 1100–800 Ma have been compiled for the Laurentia, Baltica, Sao Francisco, Congo and Kalahari cratons in order to construct apparent polar wander paths (APWPs) for this interval. Laurentias APWP consists of a well-determined Keweenawan track for 1100–1000 Ma and a 1000–800 Ma Grenville loop. We use a counterclockwise APW loop for the Grenville poles based on ages for post-metamorphic cooling through ∼500°C for the Grenville Province between 1000 and 950 Ma, and the temporal and spatial similarities with Proterozoic counterclockwise APWPs for other cratons. Balticas APWP is comprised of seven dated poles that define a similar loop, counterclockwise and hinged at 950 Ma, that can be superimposed on the Laurentian Grenville loop. This loop is also seen in the seven poles of the APWP for the combined Sao Francisco–Congo craton; superposition of these loops leads to a reconstruction in which the Sao Francisco–Congo craton is to the south-southeast of Laurentia in present-day coordinates. A long 1090–985 Ma APWP track for the Kalahari is in reasonable agreement with the roughly coeval Keweenawan track, when the Kalahari craton is rotated ∼40° counterclockwise away from the Congo craton while remaining hinged at the Zambezi belt. The resulting Rodinia reconstruction resembles those previously proposed on geological grounds for Laurentia, East Gondwana, Baltica, Sao Francisco–Congo, and the Kalahari craton.

The assembly of Gondwana 800-550 Ma

Abstract The formation of the supercontinent Gondwana heralded the beginning of the Phanerozoic following a complex series of collisional events after the break-up of earlier supercontinental assemblages. Paleomagnetic data are used to help distinguish between these events and it appears that there are three critical periods of mountain building during Gondwana assembly. The first major orogenic event took place between 800 and 650 Ma and has been termed the East Africa Orogeny. This tectonic episode formed the Mozambique Belt and likely resulted from the collision of India, Madagascar and Sri Lanka with East Africa. The second and third orogenic periods during Gondwana assembly partially overlap in time. The Brasiliano orogeny (600–530 Ma) resulted in the amalgamation of the South American nuclei and Africa. The Kuunga Orogeny was proposed, in part, because of the recent collection of geochronologic data indicating a 550 Ma granulite forming event in East Gondwana and the observation that the apparent polar wander path for Gondwana does not form a spatially and temporally coherent pattern until roughly the same time. The Kuunga orogeny may have resulted from the collision between Australia and Antarctica with the rest of Gondwana.

Reconstructions of the continents around the North Atlantic at about the 60th parallel

Abstract Late Carboniferous–Early Tertiary apparent polar wander (APW) paths (300–40 Ma) for North America and Europe have been tested in various reconstructions. These paths demonstrate that the 500 fathom Bullard et al. fit is excellent from Late Carboniferous to Late Triassic times, but the continental configuration in northern Pangea changed systematically between the Late Triassic (ca. 214 Ma) and the Mid-Jurassic (ca. 170 Ma) due to pre-drift extension. Best fit North Atlantic reconstructions minimize differences in the Late Carboniferous–Early Jurassic and Late Cretaceous–Tertiary segments of the APW paths, but an enigmatic difference exists in the paths for most of the Jurassic, whereas for the Early Cretaceous the data from Europe are nearly non-existent. Greenland’s position is problematic in a Bullard et al. fit, because of a Late Triassic–Early Jurassic regime of compression (>300 km) that would be inherently required for the Norwegian Shelf and the Barents Sea, but which is geologically not defensible. We suggest a radically new fit for Greenland in between Europe and North America in the Early Mesozoic. This fit keeps Greenland ‘locked’ to Europe for the Late Paleozoic–Early Mesozoic and maintains a reconstruction that better complies with the offshore geological history of the Norwegian Shelf and the Barents Sea. Pre-drift (A24) extension amounted to approximately 450 km on the Mid-Norwegian Shelf but with peak extension in the Late Cretaceous.

Paleomagnetic Evidence for a Paleo-Mesoproterozoic Supercontinent Columbia

Abstract Pre-Pangea supercontinents have been proposed for Neoproterozoic and earlier times. Most of the configurations are based on analyses of geologic and structural evidence, but the only quantitative method for testing the proposed configurations is paleomagnetism. Unfortunately, the current paleomagnetic database is of limited use in evaluating the notion of a Paleo-Mesoproterozoic supercontinent due to a lack of well-dated sequential poles from the various cratonic nuclei. This paper examines the available data and shows that Laurentia could not have been a part of a supercontinent at 1.77 Ga, but it may have formed the core of a pre-Rodinia continent at 1.5 Ga. The available data do not preclude the existence of a Paleo-Mesoproterozoic supercontinent, but they do suggest that it must be younger than 1.77 Ga.

Paleomagnetic investigation of the Neoproterozoic Gagwe lavas and Mbozi complex, Tanzania and the assembly of Gondwana

The 810 Ma Gagwe-Kabuye lavas and the 743 Ma Mbozi gabbro-syenite complex of the Congo Craton in East Africa were sampled for paleomagnetic study in an effort to test a variety of tectonic models proposed for Neoproterozoic times. The paleomagnetic pole obtained from the Gagwe-Kabuye lavas falls at 25°S, 273°E (δp = 7°, δm = 12°) and compares favorably to a previously published paleomagnetic pole obtained from these rocks. The Mbozi complex pole yields a paleomagnetic pole at 46°N, 325°E (δp = 5°, δm = 9°) and differs significantly from a previously determined pole for the Mbozi complex. A comparison of these paleomagnetic poles to Laurentian poles of the same age suggests that the Congo Craton may not have constituted part of the Rodinia supercontinent in the configuration proposed by Dalziel (1992). An analysis of reliable paleomagnetic poles from the Gondwana blocks for the interval from 810 to 510 Ma reveals a coherent swathe of poles from 550 to 510 Ma and a scatter of pre-600 Ma poles. Our interpretation of the available paleomagnetic and tectonic data for this interval is consistent with the formation of Gondwana by two distinct orogenic events. This assembly resulted in the East Africa Orogen between 800 and 650 Ma and a younger Kuunga Orogen at 550 Ma outboard of the East Africa Orogen with possible sutures located in Sri Lanka, southern India and Enderby Land (East Antarctic Craton).

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.