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A template for an improved rock-based subdivision of the pre-Cryogenian timescale
Abstract Pannotia is a hypothetical supercontinent that may have existed briefly during the Proterozoic–Cambrian transition. Various lines of evidence used to argue for its existence include global orogenesis in Ediacaran–Cambrian time, the development of Cambrian passive margins and some (but not all) tectonic reconstructions. Indirect measures used to infer Pannotia's veracity include patterns of biological diversity, palaeoclimate, sea level, magmatism and other palaeoenvironmental proxies. It is shown herein that neither the direct records nor the indirect proxies provide compelling support for Pannotia. If that ephemeral contiguous landmass existed at all, its effects on the broader Earth system are inextricably tied to the more fundamental processes of Gondwanaland assembly. This perspective emphasizes the remarkable consolidation of Gondwanaland as a semi-supercontinent within the early stages of the Pangaea cycle. Gondwanaland's size combined with its c. 300 myr longevity might have greater significance for mantle dynamics than the larger, but shorter-lived, Pangaea landmass.
Abstract It is likely that Archaean cratons of Laurentia had different palaeogeographic histories prior to their amalgamation. New palaeomagnetic, geochronological and geochemical evidence supports a reconstruction of the Wyoming craton adjacent to the southern margin of the Superior craton at 2.16 Ga, before rifting ( c. 2.1–2.0 Ga) and eventual reamalgamation after the Hudsonian Orogeny ( c. 1.8 Ga). U–Pb ages (TIMS on baddeleyite) from five dykes yield two groups of ages at c. 2164 and 2155 Ma. The younger group of ages defines the Rabbit Creek swarm at 2161–2152 Ma and precisely dates its palaeomagnetic pole. Two large and differentiated dykes (>100 m) in the Bighorn and Wind River uplifts are geographically related to the Rabbit Creek swarm but have slightly different orientations and yield slightly older ages at 2171–2157 Ma. These dykes may be parts of a single intrusion (the ‘Great Dyke of Wyoming’) that spans over 200 km between uplifts, possibly representing a different magmatic event. This older event does not have enough distinct intrusions to provide a correctly averaged palaeomagnetic pole, but correlates with magmatism in the Superior craton and has a palaeomagnetic remanence comparable to the Rabbit Creek dykes. With minor tilt corrections, the palaeomagnetic data from the Rabbit Creek swarm and Powder River–South Pass dykes support a reconstruction of the southeastern Wyoming craton against the southern Superior craton. This fit juxtaposes the Palaeoproterozoic Huronian and Snowy Pass Supergroups along two passive margins that experienced a prolonged period of mafic magmatism (>100 myr) and rift basin development. Although there are slight geochemical variations across the Rabbit Creek swarm, all dykes fit into two distinct groups that are independently dated and internally consistent. Supplementary material: Supporting figures and locality tables are available at www.geolsoc.org.uk/SUP18824
Abstract The link between observed episodicity in ore deposit formation and preservation and the supercontinent cycle is well established, but this general framework has not, however, been able to explain a lack of deposits associated with some accretionary orogens during specific periods of Earth history. Here we show that there are intriguing correlations between styles of orogenesis and specific mineral deposit types, in the context of the Nuna supercontinent cycle. Using animated global reconstructions of Nuna’s assembly and initial breakup, and integrating extensive databases of mineral deposits, stratigraphy, geochronology and palaeomagnetism we are able to assess spatial patterns of deposit formation and preservation. We find that lode gold, volcanic-hosted-massive-sulphide and nickel–copper deposits peak during closure of Nuna’s interior ocean but decline during subsequent peripheral orogenesis, suggesting that accretionary style is also important. Deposits such as intrusion-related gold, carbonate-hosted lead-zinc and unconformity uranium deposits are associated with the post-assembly, peripheral orogenic phase. These observations imply that the use of plate reconstructions to assess orogenic style, although challenging for the Precambrian, can be a powerful tool for mineral exploration targeting. Supplementary material: Supplementary material including (1) tables (S1–S3) of Euler poles and palaeopoles used, summary of Nuna orogens; (2) a figure (S1) of modelled plate velocities; (3) mp4 files (S1 & S2) of the model with age data; ore deposits and VGPs; and (4) a zip file (S1) of the Gplates model is available at http://www.geolsoc.org.uk/SUP18822 .
Abstract Redbeds of the Aubures Formation constitute the uppermost stratigraphic unit in the Mesoproterozoic Sinclair succession of southern Namibia. Aubures palaeomagnetic remanence vectors, held almost exclusively by hematite, document at least one geomagnetic polarity reversal in the stratigraphy, a positive intraformational conglomerate test indicating primary magnetization and greatest concentration of characteristic directions at 50–60% untilting, indicating that deformation was coincident with sedimentation. The new Aubures palaeomagnetic pole, at 56.4°N and 018.0°E with A 95 =11.3°, is located on the apparent polar wander path of the Kalahari craton, between poles of the 1110 Ma Umkondo igneous event and the c. 1090 Ma Kalkpunt redbeds of the Koras Group near Upington, South Africa. This distinctive concordance suggests that Aubures sediments have an age of approximately 1100 Ma, that the Sinclair region was probably part of Kalahari at that time and that the Aubures and Kalkpunt redbeds are broadly correlative. New laser-ablation inductively coupled plasma mass spectrometry detrital zircon results from the Aubures Formation, including a youngest age component (1108±9 Ma) that is coincident with the Kalahari-wide Umkondo large igneous province, conform to this interpretation. Palaeomagnetism and geochronology of the Sinclair succession can provide kinematic constraints on the tectonic evolution of Kalahari as it approached other cratons in the growing Rodinia supercontinent.
The palaeomagnetically viable, long-lived and all-inclusive Rodinia supercontinent reconstruction
Abstract Palaeomagnetic apparent polar wander (APW) paths from the world's cratons at 1300–700 Ma can constrain the palaeogeographic possibilities for a long-lived and all-inclusive Rodinia supercontinent. Laurentia's APW path is the most complete and forms the basis for superposition by other cratons' APW paths to identify possible durations of those cratons' inclusion in Rodinia, and also to generate reconstructions that are constrained both in latitude and longitude relative to Laurentia. Baltica reconstructs adjacent to the SE margin of Greenland, in a standard and geographically ‘upright’ position, between c . 1050 and 600 Ma. Australia reconstructs adjacent to the pre-Caspian margin of Baltica, geographically ‘inverted’ such that cratonic portions of Queensland are juxtaposed with that margin via collision at c . 1100 Ma. Arctic North America reconstructs opposite to the CONgo+São Francisco craton at its DAmaride–Lufilian margin (the ‘ANACONDA’ fit) throughout the interval 1235–755 Ma according to palaeomagnetic poles of those ages from both cratons, and the reconstruction was probably established during the c . 1600–1500 Ma collision. Kalahari lies adjacent to Mawsonland following collision at c . 1200 Ma; the Albany–Fraser orogen continues along-strike to the Sinclair-Kwando-Choma-Kaloma belt of south-central Africa. India, South China and Tarim are in proximity to Western Australia as previously proposed; some of these connections are as old as Palaeoproterozoic whereas others were established at c . 1000 Ma. Siberia contains a succession of mainly sedimentary-derived palaeomagnetic poles with poor age constraints; superposition with the Keweenawan track of the Laurentian APW path produces a position adjacent to western India that could have persisted from Palaeoproterozoic time, along with North China according to its even more poorly dated palaeomagnetic poles. The Amazonia, West Africa and Rio de la Plata cratons are not well constrained by palaeomagnetic data, but they are placed in proximity to western Laurentia. Rift successions of c . 700 Ma in the North American COrdillera and BRAsiliano-Pharuside orogens indicate breakup of these ‘COBRA’ connections that existed for more than one billion years, following Palaeoproterozoic accretionary assembly. The late Neoproterozoic transition from Rodinia to Gondwanaland involved rifting events that are recorded on many cratons through the interval c . 800–700 Ma and collisions from c . 650–500 Ma. The pattern of supercontinental transition involved large-scale dextral motion by West Africa and Amazonia, and sinistral motion plus rotation by Kalahari, Australia, India and South China, in a combination of introverted and extroverted styles of motion. The Rodinia model presented here is a marked departure from standard models, which have accommodated recent discordant palaeomagnetic data either by excluding cratons from Rodinia altogether, or by decreasing duration of the supercontinental assembly. I propose that the revised model herein is the only possible long-lived solution to an all-encompassing Rodinia that viably accords with existing palaeomagnetic data.
Abstract The IGCP 509 project is collating global information for the Palaeoproterozoic era through the activities of numerous international collaborators. A database system (StratDB) and web interface has been designed to facilitate this process with links to an existing geochronology database (DateView). As a result, all information captured will remain available in a digital format for future researchers. The philosophy and design of the database and some of the outputs available from it are described. One of the principal features of the system is that it facilitates the construction of time–space correlation charts using an innovative application of GIS technology to non-geographic information, which permits users to query a variety of attribute information associated with lithostratigraphic units, metamorphic and deformation episodes associated with user-selected tectonic domains, large igneous provinces and major ore deposits. In the process, much of the manual labour normally associated with the construction of such charts in standard graphical or drafting packages is avoided. Associations between units, deformation, metamorphism, large igneous provinces and ore deposits may become more apparent once linked information is available for querying and investigation. Geochronological information from the DateView database may also be linked to entities stored in StratDB. GIS maps may be linked to the attribute information in StratDB and DateView to construct a variety of time-slice maps or palaeogeographic reconstructions with the same symbology as is used in the time–space correlation charts. This database system will facilitate the dissemination of lithostratigraphic information for many countries to a broader community and will help non-specialists to easily view information for various Palaeoproterozoic tectonic domains. The system is illustrated using a preliminary compilation of information for the Palaeoproterozoic of southern Africa. The correlation charts and time-slice maps provide insights to the geological evolution of this region which emphasize some aspects and correlations which have not previously been extensively considered; for instance, possible correlation of units in the central and western zones of the Limpopo Belt (South Africa, Zimbabwe and Botswana) with the Magondi Belt of Zimbabwe and its extension into northern Botswana.
Correlations and reconstruction models for the 2500–1500 Ma evolution of the Mawson Continent
Abstract Continental lithosphere formed and reworked during the Palaeoproterozoic era is a major component of pre-1070 Ma Australia and the East Antarctic Shield. Within this lithosphere, the Mawson Continent encompasses the Gawler–Adélie Craton in southern Australia and Antarctica, and crust of the Miller Range, Transantarctic Mountains, which are interpreted to have assembled during c . 1730–1690 Ma tectonism of the Kimban–Nimrod–Strangways orogenies. Recent geochronology has strengthened correlations between the Mawson Continent and Shackleton Range (Antarctica), but the potential for Meso- to Neoproterozoic rifting and/or accretion events prevent any confident extension of the Mawson Continent to include the Shackleton Range. Proposed later addition ( c . 1600–1550 Ma) of the Coompana Block and its Antarctic extension provides the final component of the Mawson Continent. A new model proposed for the late Archaean to early Mesoproterozoic evolution of the Mawson Continent highlights important timelines in the tectonic evolution of the Australian lithosphere. The Gawler–Adélie Craton and adjacent Curnamona Province are interpreted to share correlatable timelines with the North Australian Craton at c . 2500–2430 Ma, c . 2000 Ma, 1865–1850 Ma, 1730–1690 Ma and 1600–1550 Ma. These common timelines are used to suggest the Gawler–Adélie Craton and North Australian Craton formed a contiguous continental terrain during the entirety of the Palaeoproterozoic. Revised palaeomagnetic constraints for global correlation of proto-Australia highlight an apparently static relationship with northwestern Laurentia during the c . 1730–1590 Ma time period. These data have important implications for many previously proposed reconstruction models and are used as a primary constraint in the configuration of the reconstruction model proposed herein. This palaeomagnetic link strengthens previous correlations between the Wernecke region of northwestern Laurentia and terrains in the eastern margin of proto-Australia.
Tempo and mode of early animal evolution: inferences from rocks, Hox, and molecular clocks
Models of Rodinia assembly and fragmentation
Abstract Amongst existing palaeogeographic models of the Rodinia supercontinent, or portions thereof, arguments have focused upon geological relations or palaeomagnetic results, but rarely both. A new model of Rodinia is proposed, integrating the most recent palaeomagnetic data with current stratigraphic, geochronological and tectonic constraints from around the world. This new model differs from its predecessors in five major aspects: cratonic Australia is positioned in the recently proposed AUSMEX fit against Laurentia; East Gondwanaland is divided among several blocks; the Congo-São Francisco and India-Rayner Cratons are positioned independently from Rodinia; Siberia is reconstructed against northern Laurentia, although in a different position than in all previous models; and Kalahari-Dronning Maud Land is connected with Western Australia. The proposed Rodinia palaeogeography is meant to serve as a working hypothesis for future refinements.
Abstract Recent plate tectonic models advocate assembly of Proterozoic Australia by tectonic processes that involved large-scale horizontal motions, whereas previous models suggested that the continent evolved as an essentially intact block of lithosphere. Geological and geochemical observations alone are insufficient to test whether the major cratonic blocks of Australia were together or widely separated during the Proterozoic; only palaeomagnetism can provide quantitative constraints on relative plate motions during the Precambrian. Despite deficiencies in the palaeomagnetic record for Proterozoic Australia, groups of overlapping palaeopoles for 1.7–1.8 and 1.5–1.6 Ga permit the North and West Australian cratonic assemblages to have occupied their present relative positions since at least c. 1.7Ga, and to have been joined to the South Australian cratonic assemblage since at least c. 1.5Ga. Nonetheless, additional geological, geochronological and palaeomagnetic data are required to test whether large oceans closed between any of the continental blocks.