David A.D. Evans

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.

A 90° spin on Rodinia: possible causal links between the Neoproterozoic supercontinent, superplume, true polar wander and low-latitude glaciation ☆

We report here new geochronological and paleomagnetic data from the 802±10 Ma Xiaofeng dykes in South China. Together with existing data, these results suggest that Rodinia probably spread from the equator to the polar region at ca. 800 Ma, followed by a rapid ca. 90° rotation around an axis near Greenland that brought the entire supercontinent to a low-latitude position by ca. 750 Ma. We propose that it was the initiation of a mantle superplume under the polar end of Rodinia that triggered an episode of true polar wander (TPW) which brought the entire supercontinent into equatorial latitudes. An unusually extensive emerged land area at the equator increased both atmospheric CO2 drawdown and global albedo, which, along with waning plume volcanism led directly to the low-latitude Sturtian glaciation at ca. 750–720 Ma.

True polar wander and supercontinents

Abstract I present a general model for true polar wander (TPW), in the context of supercontinents and simple modes of mantle convection. Old, mantle-stationary supercontinents shield their underlying mesosphere from the cooling effects of subduction, and an axis of mantle upwelling is established that is complementary to the downwelling girdle of subduction zones encircling the old supercontinent. The upwelling axis is driven to the equator by TPW, and the old supercontinent fragments at the equator. The prolate axis of upwelling persists as the continental fragments disperse; it is rotationally unstable and can lead to TPW of a different flavor, involving extremely rapid (≤m/year) rotations or changes in paleolatitude for the continental fragments as they reassemble into a new supercontinent. Only after several hundred million years, when the new supercontinent has aged sufficiently, will the downwelling zone over which it amalgamated be transformed into a new upwelling zone, through the mesospheric shielding process described above. The cycle is then repeated. The model explains broad features of the paleomagnetic database for the interval 1200–200 Ma. Rodinia assembled around Laurentia as that continent experienced occasionally rapid, oscillatory shifts in paleolatitude about a persistent axis on the paleo-equator, an axis that may have been inherited from the predecessor supercontinent Nuna. By 800 Ma, long-lived Rodinia stabilized its equatorial position and disaggregated immediately thereafter. Gondwanaland assembled as its constituent fragments documented rapid, oscillatory shifts of apparent polar wander, here interpreted as TPW. The Gondwanaland–Pangea centroid migrated to the equator immediately prior to Jurassic–Cretaceous breakup. Lack of substantial TPW since 200 Ma may indicate the stabilizing effects of specific plate boundary conditions (i.e., persistent convection patterns in the Tethys–Indian Ocean region), possibly superimposed on a secular geodynamic shift governed by increased lower-mantle viscosity associated with long-term planetary cooling. TPW is a significant geodynamic process that, in terms of continental motions, may even dominate plate tectonics for certain intervals of Earth history. The effects of such rapid TPW may be found among regional tectonics and sea-level changes, and possibly global climate change and biological evolution.

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.

Palaeoproterozoic supercontinents and global evolution: correlations from core to atmosphere

Abstract The Palaeoproterozoic era was a time of profound change in Earth evolution and represented perhaps the first supercontinent cycle, from the amalgamation and dispersal of a possible Neoarchaean supercontinent to the formation of the 1.9–1.8 Ga supercontinent Nuna. This supercontinent cycle, although currently lacking in palaeogeographic detail, can in principle provide a contextual framework to investigate the relationships between deep-Earth and surface processes. In this article, we graphically summarize secular evolution from the Earths core to its atmosphere, from the Neoarchaean to the Mesoproterozoic eras (specifically 3.0–1.2 Ga), to reveal intriguing temporal relationships across the various ‘spheres’ of the Earth system. At the broadest level our compilation confirms an important deep-Earth event at c. 2.7 Ga that is manifested in an abrupt increase in geodynamo palaeointensity, a peak in the global record of large igneous provinces, and a broad maximum in several mantle-depletion proxies. Temporal coincidence with juvenile continental crust production and orogenic gold, massive-sulphide and porphyry copper deposits, indicate enhanced mantle convection linked to a series of mantle plumes and/or slab avalanches. The subsequent stabilization of cratonic lithosphere, the possible development of Earths first supercontinent and the emergence of the continents led to a changing surface environment in which voluminous banded iron-formations could accumulate on the continental margins and photosynthetic life could flourish. This in turn led to irreversible atmospheric oxidation at 2.4–2.3 Ga, extreme events in global carbon cycling, and the possible dissipation of a former methane greenhouse atmosphere that resulted in extensive Palaeoproterozoic ice ages. Following the great oxidation event, shallow marine sulphate levels rose, sediment-hosted and iron-oxide-rich metal deposits became abundant, and the transition to sulphide-stratified oceans provided the environment for early eukaryotic evolution. Recent advances in the geochronology of the global stratigraphic record have made these inferences possible. Frontiers for future research include more refined modelling of Earths thermal and geodynamic evolution, palaeomagnetic studies of geodynamo intensity and continental motions, further geochronology and tectonic syntheses at regional levels, development of new isotopic systems to constrain geochemical cycles, and continued innovation in the search for records of early life in relation to changing palaeoenvironments.

Proterozoic low orbital obliquity and axial-dipolar geomagnetic field from evaporite palaeolatitudes

Palaeomagnetism of climatically sensitive sedimentary rock types, such as glacial deposits and evaporites, can test the uniformitarianism of ancient geomagnetic fields and palaeoclimate zones. Proterozoic glacial deposits laid down in near-equatorial palaeomagnetic latitudes can be explained by ‘snowball Earth’ episodes, high orbital obliquity or markedly non-uniformitarian geomagnetic fields. Here I present a global palaeomagnetic compilation of the Earth’s entire basin-scale evaporite record. Magnetic inclinations are consistent with low orbital obliquity and a geocentric-axial-dipole magnetic field for most of the past two billion years, and the snowball Earth hypothesis accordingly remains the most viable model for low-latitude Proterozoic ice ages. Efforts to reconstruct Proterozoic supercontinents are strengthened by this demonstration of a consistently axial and dipolar geomagnetic reference frame, which itself implies stability of geodynamo processes on billion-year timescales.

Reconstructing pre-Pangean supercontinents

Twenty-fi ve years ago, initial plans for reconstructing the Rodinia supercontinent were being drafted, based on the growing recognition of correlatable mid-Neoproterozoic (0.8–0.7 Ga) rifted passive margins, many of which were established on the eroded remnants of late Mesoproterozoic (1.3–1.0 Ga) orogenic belts. The 1990s witnessed a surge of interest in Rodinia, with many regional studies of tectonostratigraphy and U-Pb geochronology generally conforming to the “inside-out” reconstruction model: juxtaposition of west Laurentia with east Australia/ Antarctica, north Laurentia with Siberia, and east Laurentia with Baltica and cratons that would later form West Gondwana. This standard model of Rodinia appeared to be converging toward a solution with only minor variations by the turn of the millennium, but new paleomagnetic data and tectonostratigraphic information obtained in the succeeding decade chipped away at various aspects of the reconstruction; several cratons seemed to require exclusion from the supercontinent (thus questioning its very validity), or the landmass might have assembled much later (≤0.9 Ga) than originally envisaged (thus weakening the link to global Mesoproterozoic orogenesis). Although a consensus model of Rodinia’s assembly and fragmentation has arisen from the International Geoscience Programme Project 440 working group, the reconstruction is supported by rather sparse defi nitive-quality data. As the quest for Rodinia matures to a third decade of scrutiny, the search for its predecessor Nuna (a.k.a. Hudsonland or Columbia) is only now reaching a stage of global synthesis between tectonostratigraphic and paleomagnetic data. According to most defi nitions, Nuna assembled at 1.9–1.75 Ga, or perhaps as late as 1.6 Ga, and fragmented during the interval 1.5–1.2 Ga. Because mafi c dike swarms are ideal targets for paleomagnetic study, and because they are now amenable to routine dating by U-Pb on baddeleyite, the global abundance of Paleo-Mesoproterozoic dike swarms might make Nuna more imminently solvable than Rodinia. Prior to the assembly of Nuna, various “supercraton” connections such as Vaalbara, Superia, and Sclavia are only beginning to take form. Unmetamorphosed, early Paleoproterozoic (2.5–2.0 Ga) mafi c dike swarms are commonplace features across the interiors of Archean cratons, and their joint paleo magnetic and geochronologic study can help reassemble the cratons into their supercraton parent landmasses. Progressively older geologic times require consideration of a greater number of potentially independent terranes, each needing individual kinematic constraints. Furthermore, the initial stabilizing events of most extant cratons during Neoarchean time (3.0–2.5 Ga) therefore render global reconstructions older than that interval improbable.

Late Neoproterozoic 40° intraplate rotation within Australia allows for a tighter-fitting and longer-lasting Rodinia

Previous paleomagnetic work has appeared to demand the breakup of southwest United States−East Antarctic (SWEAT) type Rodinia reconstructions before ca. 750 Ma, significantly earlier than the stratigraphic record of rift-drift transition between 715 Ma and 650 Ma. Here we reanalyze Australian paleomagnetic and regional tectonic data to produce a model in which the Precambrian Australian continent had a slightly different configuration before the breakup of Rodinia. A cross-continental megashear zone developed along the Paterson and Petermann orogens at ca. 650–550 Ma, during or after the breakup of Rodinia, manifested as an ∼40° clockwise rotation of the South and West Australian cratons relative to the North Australian craton around a vertical axis in Central Australia. This model reconciles major paleomagnetic discrepancies within Australia, and allows for a longer lifespan of SWEAT-like reconstructions of Rodinia that are consistent with the Neoproterozoic stratigraphic records of Australia and Laurentia.

A fundamental Precambrian-Phanerozoic shift in earth's glacial style?

Abstract It has recently been found that Neoproterozoic glaciogenic sediments were deposited mainly at low paleolatitudes, in marked qualitative contrast to their Pleistocene counterparts. Several competing models vie for explanation of this unusual paleoclimatic record, most notably the high-obliquity hypothesis and varying degrees of the snowball Earth scenario. The present study quantitatively compiles the global distributions of Miocene–Pleistocene glaciogenic deposits and paleomagnetically derived paleolatitudes for Late Devonian–Permian, Ordovician–Silurian, Neoproterozoic, and Paleoproterozoic glaciogenic rocks. Whereas high depositional latitudes dominate all Phanerozoic ice ages, exclusively low paleolatitudes characterize both of the major Precambrian glacial epochs. Transition between these modes occurred within a 100-My interval, precisely coeval with the Neoproterozoic–Cambrian “explosion” of metazoan diversity. Glaciation is much more common since 750 Ma than in the preceding sedimentary record, an observation that cannot be ascribed merely to preservation. These patterns suggest an overall cooling of Earths longterm climate, superimposed by developing regulatory feedbacks involving an increasingly complex biosphere.

Geochronology of the Zambezi Supracrustal Sequence, Southern Zambia: A Record of Neoproterozoic Divergent Processes along the Southern Margin of the Congo Craton

The Zambezi supracrustal sequence (ZSC) of southern Zambia comprises a metasedimentary package of clastics and carbonates, with a thick sequence of basal volcanics and lavas. The sequence has traditionally been interpreted as a Neoproterozoic continental rift succession, but the lack of reliable age constraints hinders any tectonic interpretation. In this article, we date magmatic and detrital zircons using the U‐Pb SHRIMP method in order to better constrain the timing of rifting, volcanism, and basin deposition. The basal volcanoclastic Kafue Rhyolite and Nazingwe formations were erupted at ca. 880 Ma, and the sequence was intruded by the Lusaka Granite at ca. 820 Ma, providing lower and upper limits on the age of sedimentation. Whole‐rock Nd isotopic signatures of these volcanics indicate that they formed as a result of assimilation and recycling of basement gneisses, probably during crustal thinning and extension. We uphold the correlation between the ZSC and the Roan Group in the Zambian Copperbelt and suggest that both successions formed in discrete rift basins along the southern margin of the Congo‐Tanzania‐Bangweulu (CTB) Craton; however, extension at this time probably did not result in complete continental separation. If the CTB Craton were an integral part of Rodinia, then rifting at ca. 880 Ma would represent one of the first known records of attempted breakup of the supercontinent.