Evidence of the Separation of Laurentia From Australia During the Rifting of the Coulmbia Sueprcontinent found?

Rifting of Columbia to form a deep-water siliciclastic to carbonate succession: The Mesoproterozoic Pinguicula Group of northern Yukon, Canada

Authors:

Medig et al

Abstract:

The Mesoproterozoic Pinguicula Group (less than 1.38 Ga) is exposed in the Wernecke and Hart River inliers in northern Yukon, Canada. The Pinguicula Group records deposition of non-cyclic siliciclastic and carbonate strata on low-energy slopes affected by rare high-energy deposits in a tectonically active epicratonic setting. The succession is ∼1.4 km thick at its measured type sections and comprises three newly formalised formations: the Mount Landreville, Pass Mountain, and Rubble Creek formations (formerly units A, B, and C, respectively). The Mount Landreville Formation is a predominantly siltstone succession with minor conglomerate and sandstone deposited below storm wave-base on a relatively gentle slope. The Pass Mountain Formation is a wispy- to planar-laminated carbonate succession deposited on a low-energy slope mostly below storm wave-base and is punctuated by rare high-energy gravity-flow deposits including debrites, grain-flows, turbidites, and micro-turbidites. The Rubble Creek Formation is dominated by repetitive centimetre- to decimetre-scale lime mudstone beds; it is distinguished from the Pass Mountain Formation by abundant zebra texture (an alternating dark grey and white banding caused by late diagenetic or hydrothermal fluid influx) and a lack of sediment gravity-flow deposits.

The Pinguicula Group is the middle of five, unconformity-bounded, Proterozoic stratigraphic successions deposited on the northwestern margin of Laurentia (ancestral North America). The Pinguicula basin was epicratonic and deepened to the south (present coordinates). The basin formed during an amagmatic extensional event that contributed to the break-up of Columbia and the separation of Laurentia from Australia.

Whereas most preserved Mesoproterozoic basins are dominated by shallow-water lithofacies deposited in rift and epicratonic settings, with few deep-water lithofacies preserved, the carbonate strata of the Pinguicula Group provide a rare insight into deeper-water carbonate environments.

Evidence of a Rhyacian Paleoproterozoic Supercontinent From Brazil

Crustal evolution of the northern Brasília Belt basement, central Brazil: a Rhyacian orogeny coeval with a pre-Rodinia supercontinent assembly

Authors:

de Sousa et al

Abstract:

The Brasília thrust and fold belt developed in the course of Neoproterozoic Brasiliano orogeny, during the convergence of São Francisco, Amazonian and Paranapanema cratons, in an early phase of Gondwana agglutination. A Paleoproterozoic terrane cropping out in its northern segment -represents the basement of the belt and corresponds to a magmatic arc. The arc developed during Rhyacian, possibly on the margin of an Archean-Paleoproterozoic block that is today represented by the São Francisco craton. The Paleoproterozoic arc evolution starts following a calc-alkaline trend between 2.30 and 2.16 Ga, changing from a typical juvenile to crustal contaminated terrane with slightly negative ɛNd(t) values, as revealed by tonalites and granodiorites of Conceição do Tocantins Suite. By c. 2.17-2.15 Ga, a collisional event took place, triggering crustal melting and generating peraluminous granites. These syn-collisional granites present strongly negative ɛNd values and belong to Aurumina Suite. Natividade, Almas-Conceição do Tocantins and Cavalcante-Arraias domains within the basement do not represent distinct tectonic blocks. The first two are areas where mainly plutonic arc rocks crop out and in the last one, syn-collisional granites are the most common lithology. Syn-collisional granites characterize granitogenesis produced by collision of another landmass against São Francisco landmass. The arc development is contemporary with collisional orogens that culminated in a conjectural Paleoproterozoic pre-Rodinia supercontinent. Since Rhyacian, these rocks are part of the landmass that would become the São Francisco craton and compose Brasília Belt basement.

How Australia and Antarctica Transitioned From Being Part of the Supercontinent Columbia to Gondwana

The Australo-Antarctic Columbia to Gondwana transition

Authors:

Aitken et al

Abstract:

From the Mesoproterozoic to Cambrian, Australo-Antarctica was characterised by tectonic reconfiguration as part of the supercontinents Columbia, Rodinia and Gondwana. New tectonic knowledge of the Wilkes Land region of Antarctica allows Australo-Antarctic tectonic linkages to be resolved through reconstruction into ca. 160 Ma Gondwana. We also resolve 330 ± 30 km of sinistral strike-slip offset on the greater than 3000 km long Mundrabilla-Frost Shear Zone and 260 ± 20 km of dextral offset on the greater than 1000 km long Aurora Fault to reconstruct the ca. 1150 Ma geometry of Australo-Antarctica. Using this revised geometry, we derive the first model of the Columbia to Gondwana reconfiguration process that is geometrically constrained to ~ 100 km scale. In this model, early Mesoproterozoic tectonics is driven by two opposing subduction systems. A dominantly west-dipping subduction zone existed at the eastern margin of Australo-Antarctica until ca. 1.55–1.50 Ga. A predominantly east-dipping subduction zone operated at the western margin of the Mawson Craton from ca. 1.70 Ga to ca. 1.42 Ga. The latter caused gradual westwards motion and clockwise rotation of the Mawson Craton relative to the West and North Australian Craton and the accretion of a series of continental ribbons now preserved in the Musgrave Province and its southern extensions. A mid-Mesoproterozoic switch to predominantly west-dipping subduction beneath the West Australian Craton brought about the final closure of the Mawson Craton with the North and West Australian Craton along the Rodona-Totten Shear Zone. Convergence was achieved prior to 1.31 Ga, but final collision may not have occurred until ca. 1.29 Ga. Post-1.29 Ga intraplate activity involved prolonged high-temperature orogenesis from 1.22 to 1.12 Ga, and significant movement on the Mundrabilla-Frost Shear Zone between 1.13 and 1.09 Ga, perhaps in response to the assembly of Rodinia at ca. 1.1 Ga. The Australo-Antarctic Craton was amalgamated with Indo-Antarctica along the Indo-Australo-Antarctic Suture (IAAS) and Kuunga Orogeny, probably in the latest Neoproterozoic to early Cambrian.

The PaleoPosition of the Amazonian Craton in the Calymmian MesoProterozoic

Reassessment of Aguapeí (Salto do Céu) Paleomagnetic pole of the Amazonian Craton and implications for Proterozoic supercontinents

Authors:

D’Agrella-Filho et al

Abstract:

The Aguapei paleomagnetic pole obtained for mafic sills and dykes from Salto do Céu region at the western margin of the Amazonian Craton constrains its links with Baltica and Laurentia in Rodinian reconstructions. A new U-Pb age on baddeleyites at 1439 ± 4 Ma for the intrusives, constrasts strongly with a previous Ar-Ar age at 981 ± 2 Ma, with important consequences for paleogeographic reconstructions. We report new paleomagnetic and magnetic anisotropy results for sills from the Salto do Céu region and reassess the paleomagnetic data in view of the new geochronological age. A total of 155 samples were collected from thirteen new paleomagnetic sampling sites, five of them corresponding to sedimentary rocks located at the borders of the sills in an attempt to perform baked contact tests. After thermal and alternating field demagnetization, the sills provided a characteristic magnetic component at Dm = 208.2°, Im = 68.5° (N = 8, α95 = 6.4°), with a corresponding paleomagnetic pole at 46.4°S; 277.0°E (A95 = 10.2°). Directions obtained in this study are similar to those reported previously for other mafic sills and dykes in the same region. A pole integrating the new results for the sills with those of the previous Aguapeí pole is situated at 56.0°S; 278.5°E (A95 = 7.9°). This new combined Salto do Céu pole supersedes the previous Aguapeí pole. Magnetic mineralogy studies, optical and electronic microscopy indicates PSD magnetite as the main magnetic carrier in these rocks. The baked contact tests were inconclusive, but the similarity between Salto do Céu pole and other high-quality poles with ages around 1420-1430 Ma suggests they carry a primary thermoremanence of that age. The Salto do Céu and other coeval poles are compatible with a connection between Amazonia and Baltica at the Mesoproterozoic in a paleogeographic configuration slightly different from SAMBA (South America-Baltica). At the same time, the new geochronological and paleomagnetic data imply that all paleogeographic interpretations for the position of Amazonia in Rodinia based on the previously published Aguapeí pole (and now renamed as Salto do Céu pole) must be revised.

Academic Bun Fight: Panama Isthmus Closed Earlier Than Currently Thought (or did it)

Biological evidence supports an early and complex emergence of the Isthmus of Panama
Authors:

Bacon et al

Abstract:

The linking of North and South America by the Isthmus of Panama had major impacts on global climate, oceanic and atmospheric currents, and biodiversity, yet the timing of this critical event remains contentious. The Isthmus is traditionally understood to have fully closed by ca. 3.5 million years ago (Ma), and this date has been used as a benchmark for oceanographic, climatic, and evolutionary research, but recent evidence suggests a more complex geological formation. Here, we analyze both molecular and fossil data to evaluate the tempo of biotic exchange across the Americas in light of geological evidence. We demonstrate significant waves of dispersal of terrestrial organisms at approximately ca. 20 and 6 Ma and corresponding events separating marine organisms in the Atlantic and Pacific oceans at ca. 23 and 7 Ma. The direction of dispersal and their rates were symmetrical until the last ca. 6 Ma, when northern migration of South American lineages increased significantly. Variability among taxa in their timing of dispersal or vicariance across the Isthmus is not explained by the ecological factors tested in these analyses, including biome type, dispersal ability, and elevation preference. Migration was therefore not generally regulated by intrinsic traits but more likely reflects the presence of emergent terrain several millions of years earlier than commonly assumed. These results indicate that the dramatic biotic turnover associated with the Great American Biotic Interchange was a long and complex process that began as early as the Oligocene–Miocene transition.

That was a bad analysis!!!

Appearance of an early closure of the Isthmus of Panama is the product of biased inclusion of data in the metaanalysis

Author:

Lessios

Extract:

In their PNAS article “Biological evidence supports an early and complex emergence of the Isthmus of Panama,” Bacon et al. (1) use data from molecular comparisons of terrestrial and marine organisms taken from the literature to estimate dates of rate shifts in migration. One of their conclusions is that “events separating marine organisms in the Atlantic and Pacific oceans [occurred] at ca. 23 and 7 Ma” (1). The authors base this conclusion on two kinds of molecular dating: (i) 31 dates from phylogenies with evolutionary rates calibrated from fossils at one or more nodes, and (ii) 52 dates from mitochondrial divergence between sister species on either side of the Isthmus taken from the review by Lessios (2) (note: complete data are available from the Dryad Digital Repository). For the latter, divergence was converted to time by assuming a mitochondrial DNA divergence rate of 2% per million years. Unfortunately, Bacon et al.’s (1) metaanalysis of separations of marine organisms contains unexplained omissions of data and mistakes. Nine of the fossil calibrated divergence values are wrong, and three are omitted (though present in publications used to derive other dates). Thirty-eight comparisons from Lessios (2) are excluded. Criteria for inclusion of data are not stated but, judging from the estimated dates, only data from Cytochrome c oxidase subunit 1 were taken into account, even though Lessios (2) presents data for multiple mitochondrial genes. This selectivity in the marine dataset of Bacon et al. (1) is hard to explain, because the terrestrial data come from various genes, and because eight comparisons of Cytochrome c oxidase subunit 1 of marine species are among those excluded.

no!  it wasn't, damnit!

Reply to Lessios and Marko et al.: Early and progressive migration across the Isthmus of Panama is robust to missing data and biases

Authors:

Bacon et al

Extract:

The emergence of the Isthmus of Panama left a major imprint on the biodiversity of the Americas. The connection between South and North America facilitated dispersal of terrestrial and freshwater organisms, while separating marine species between the eastern Pacific and Caribbean seas. Recent geological data have questioned the long-standing view of a Pliocene emergence of the Isthmus (1) and show that the Central American Seaway, defined as the deep oceanic seaway along the tectonic boundary of the South American plate and Panama arc, was already closed by 15–13 Ma (2). Caribbean–Pacific shallow water exchange probably continued, albeit intermittently, until a full closure at 3.5 Ma (1–3). Recently Bacon et al. (3) used molecular and fossil data to evaluate the timing, tempo, and directionality of biotic exchange and vicariance across the Isthmus, and tested whether biological data are congruent with recent geological evidence. Significant increases in terrestrial dispersals were found at ca. 20 and 6 Ma, and increases in marine vicariance at ca. 23 and 7 Ma. Similar patterns prevailed despite intrinsic differences among the taxonomic groups surveyed. This led Bacon et al. (3) to reject the assumption of a single closure of the Isthmus at ca. 3.5 Ma in favor of an older, more complex model of land emergence and biotic interchange.

The Role of Continental Drift & Glaciations on Carbonifierous PaleoClimates

Impact of continental motion and dynamic glaciations on low-latitude climate during the Carboniferous: The record of the Wyoming Shelf (Western United States)

Authors:

Blanchard et al

Abstract:

The dynamic character of the Late Paleozoic Ice Age is evident from glacial deposits, but its impact on tropical climate is not well constrained. Global changes in climate are overprinted on longer-term paleogeographic variations, resulting in a complex time–space distribution of climate-sensitive lithologies. The significance of such lithologies in Carboniferous successions of the western United States has not been fully explored. In this study, we provide new interpretations for the paleoclimatic context of the Amsden and Tensleep Formations (Pennsylvanian, Northern Wyoming, USA). The Amsden Formation consists of a basal sandstone member overlain by red siltstones containing pisolites. Very large-scale (~ 10 m) cross-bedding within the basal sandstone indicates deposition in an erg environment. Iron pisoid-rich layers in the overlying member suggest an evolution toward more humid conditions. Persistent arid conditions during the middle Pennsylvanian are suggested by eolian sandstones and calcretes in the overlying Tensleep Formation. These formations were deposited on the karst topography that developed on top of the lower to middle Mississippian Madison Group. Although the development of karstic features implies that humid conditions prevailed during the late Mississippian, evaporites and evidence for early dolomitization within the formation suggest that it was deposited under arid conditions. These relationships argue for a long-term climate evolution from arid to humid during the Mississippian, and a return to arid conditions during the Pennsylvanian. This trend can be explained by the northward drift from ~ 15°S to ~ 12°N. A comparison with contemporaneous records reveals a diachronous evolution across western Pangaea, with the climatic conditions documented on the Wyoming Shelf being reached later in eastern North America. These relationships indicate that plate motion considerably overprints long-term climatic records. Departures from this trend, suggested by the presence of erg deposits in the basal Amsden Formation, record the overprinting of shorter periods of climate change.

Evidence from Baltica Supports Warm Equator, Cold Polar Ediacaran NeoProterozoic PaleoClimates

Baltica during the Ediacaran and Cambrian: A paleomagnetic study of Hailuoto sediments in Finland

Authors:

Klein et al

Abstract:

We present a new Late Neoproterozoic paleomagnetic pole for Baltica from an inclined 272 m deep oriented sedimentary drill core in Hailuoto, Western Finland. The depositional age of the Hailuoto sediments is poorly constrained at 570–600 Ma. Three components of magnetization were isolated with thermal and alternating field (AF) demagnetization treatments. The ChRM (characteristic remanence magnetization) component is a high coercivity/unblocking temperature dual polarity component that passes a reversal test. The combined observed ChRM component of the Hailuoto sediments (D = 334.2°; I = 44.4°; α95 = 7.2°; k = 16.5) yields a paleomagnetic pole of Plat = 48.7° N and Plon = 241.1° E with A95 = 8.1°. The inclination corrected direction (f = 0.6) of D = 334.4°; I = 57.7°; α95 = 5.8°; k = 25.2 yields a paleomagnetic pole of Plat = 60.5° N and Plon = 247.9° E with A95 = 7.6°. As it is a dual-polarity ChRM carried by both magnetite and hematite, with no resemblance to younger events, we interpret it as a primary component. A paleolatitude for Hailuoto of 38.3° was calculated from the ChRM. Two secondary components were identified. The first is a low coercivity/blocking temperature component with a remanent magnetization of D = 239.0°; I = 67.3°; α95 = 8.7° (N = 13 samples), which we interpret as drilling-induced remanent magnetization (DIRM). The second secondary component has a remanent magnetization of D = 49.4°; I = 34.9°; α95 = 8.6° (N = 5 samples) and is commonly seen in Fennoscandian formations.

The ChRM Hailuoto pole adds to the scattered Ediacaran paleomagnetic data of Baltica and indicate large distances between other late Neoproterozoic and early Cambrian paleomagnetic poles. We present reconstructions of Baltica and Laurentia between 616 and 550 Ma which move Baltica from high latitudes (615 Ma), over the polar region, to low latitudes (550 Ma), and Laurentia from low latitudes (615 Ma) to a polar position (570 Ma) and back to an equatorial position (550 Ma). A low to mid latitude position of Baltica determined by the Hailuoto paleomagnetic pole, and the lack of glaciogenic sediments determined in an earlier study of Hailuoto sediments indicate a warm deposition environment.

Australia's Lake Eyre Caused by Fossil Subduction Zone, to Disappear in 30 Million Years

Geoscientists have, for the first time, discovered the origins of Australia's two largest basins: Lake Eyre and the Murray-Darling Basin. The research also implies that in 30 million years' time both basins will cease to exist.

Monash University geoscientist Associate Professor Wouter Schellart, and his colleague Professor Wim Spakman from Utrecht University, have discovered how the floor of an entire ocean basin that was destroyed 70 to 50 million years ago off the North coast of New Guinea is currently located at 800-1200km depth below Central and South-eastern Australia.

Using supercomputers, the researchers found that this dense piece of ocean floor material (called a lithospheric slab) is slowly sinking into the Earth's mantle and is responsible for the formation of the Lake Eyre Basin, one of the Earth's largest internally drained basins and home to the lowest point in Australia at 15m below sea level, as well as the Murray-Darling Basin, home to the largest river system in Australia. With a combined surface area exceeding 2 million square kilometres, both basins are located directly above the deep mantle slab.

The research also predicts that in 30 million years from now, when Australia has moved about 1500km northwards, the fossil slab will be located below the Southern Ocean and, as a consequence, the Lake Eyre Basin and Murray Darling Basin will cease to exist.

Using geological and geophysical data from the New Guinea region, Schellart was able to reconstruct the geological evolution of the region over the last 70 million years, including the motion of the tectonic plates and plate boundaries. He discovered that the occurrence of deep ocean floor rocks, volcanic rocks and deformed rocks, which are currently found in the mountain ranges of New Guinea, point to the existence of a 4000km wide subduction zone. At subduction zones such as these, an oceanic tectonic plate sinks (subducts) into the Earth's interior, the mantle.

With these plate tectonic reconstructions Schellart was able to predict where the fossil subduction zone was during its lifetime some 50-70 million years ago, and therefore where the lithospheric slab disappeared into the mantle. With a global seismic tomography model that makes use of seismic waves to map the internal structure of the Earth's mantle, Schellart and Spakman were able to identify the fossil slab structure below central and south-eastern Australia at a location and depth predicted by the reconstructions.

link.

Coryphoid Palms Spread to India Prior to Maastrichtian Cretaceous, Before Asiastic Collision

Coryphoid Palm Leaf Fossils from the Maastrichtian–Danian of Central India with Remarks on Phytogeography of the Coryphoideae (Arecaceae)

Authors:

Srivastava et al

Abstract:

Premise of research

A large number of fossil coryphoid palm wood and fruits have been reported from the Deccan Intertrappean beds of India. We document the oldest well-preserved and very rare costapalmate palm leaves and inflorescence like structures from the same horizon.

Methodology

A number of specimens were collected from Maastrichtian–Danian sediments of the Deccan Intertrappean beds, Ghughua, near Umaria, Dindori District, Madhya Pradesh, India. The specimens are compared with modern and fossil taxa of the family Arecaceae.

Pivotal results

Sabalites dindoriensis sp. nov. is described based on fossil leaf specimens including basal to apical parts. These are the oldest coryphoid fossil palm leaves from India as well as, at the time of deposition, from the Gondwana- derived continents.

Conclusions

The fossil record of coryphoid palm leaves presented here and reported from the Eurasian localities suggests that this is the oldest record of coryphoid palm leaves from India and also from the Gondwana- derived continents suggesting that the coryphoid palms were well established and wide spread on both northern and southern hemispheres by the Maastrichtian–Danian. The coryphoid palms probably dispersed into India from Europe via Africa during the latest Cretaceous long before the Indian Plate collided with the Eurasian Plate.

Cambrian Radiation Linked to an Iapetus-Pacific Oceanic Connection?

Cambrian transgression and radiation linked to an Iapetus-Pacific oceanic connection?

Author:

Dalziel

Abstract:

The geologically abrupt appearance in the fossil record of almost all animal phyla is referred to as the Cambrian radiation or "explosion" of life on Earth. Also known as "Darwin's dilemma," because it seemingly posed a major problem for his theory of gradual evolution, it coincided with the initiation of the first of the two principal global marine transgressions of the Phanerozoic. Although now seen as more protracted, it is still one of the most striking and critical events in the history of the biosphere. Almost all paleogeographic reconstructions for the early Cambrian feature a previously isolated Laurentia, the core of ancestral North America. Yet geological evidence from five continents, integrated here for the first time, indicates that the present-day "southern cone" of Laurentia was still attached to the newly amalgamated supercontinent of Gondwanaland into Cambrian times. Laurentia was then isolated by the development of a major deep oceanic connection between the opening Iapetus Ocean basin and the already well-developed paleo-Pacific. As the marine transgression advanced, major changes in ocean chemistry occurred, upwelling generated phosphorite deposits, and the number of fossilized metazoan phyla "exploded" with morphologic disparity between Laurentia and Gondwanaland already established. The development of this deep oceanic gateway, and of an ocean floor–consuming and arc-generating subduction zone along virtually the entire margin of Gondwanaland shortly thereafter, need to be taken into account in consideration of the global environmental and biotic changes associated with the Neoproterozoic-Phanerozoic transition.

Gulden Draak Knoll Microcontinent Found Off Western Australia

Discovery of a microcontinent (Gulden Draak Knoll) offshore Western Australia: Implications for East Gondwana reconstructions

Authors:

Gardner et al

Abstract:

Analysis of dredged samples from Gulden Draak Knoll demonstrate it is a submarine rifted continental fragment that lies at the boundary between the western Perth Abyssal Plain and Wharton Basin, Indian Ocean. The Knoll comprises a granulite facies basement, including pelitic paragneiss and mafic orthogneiss, with a Cambrian granite inferred to intrude the other rocks. Boulders and cobbles of felsic gneiss with Mesoproterozoic and Cambrian protolith ages were also sampled likely reflecting a complex basement to variable sedimentary and volcanic rocks. The U-Pb isotopic system in Archean and Mesoproterozoic zircon is significantly disturbed, reflecting Cambrian orogenesis that affected all samples. The protolith to garnet-sillimanite-biotite paragneiss has a maximum deposition age of 1163 ± 24 Ma and includes older detrital zircon grains with populations at c. 2.65 Ga and between 1.4-1.1 Ga. A younger population in this sample is interpreted as a mix of newly grown metamorphic zircon and isotopically reset zircon, implying the granulite facies metamorphism occurred at c. 511 ± 5 Ma. Protracted Cambrian orogenesis is indicated by a metamorphic age in the mafic orthogneiss of 530 ± 6 Ma and isotopic disturbance shortly following emplacement of granite (c. 540 Ma with zircon ages disturbed to 509 ± 7 Ma) and the protolith to the felsic orthogneiss (c. 528 Ma with zircon ages disturbed to 510 ± 3 Ma). Xenocrystic zircon grains in Cambrian rocks include Archean (c. 2839 ± 9 Ma) and Mesoproterozoic (1230–1370 Ma) populations also isotopically disturbed during Cambrian orogenesis. Igneous Cambrian zircon grains have less radiogenic Hf-isotope compositions (Hfi = 0.281821-0.281367) than Mesoproterozoic xenocrysts (Hfi = 0.282267-0.281993), indicating limited involvement of the Mesoproterozoic crust in granite production. A more likely source includes Archean crust represented by xenocrysts with Hfi = 0.281399-0.280863. The Gulden Draak Knoll is reconstructed in Gondwana (‘Leeuwin’ full-fit model) along strike of a major structure termed the Indo- Australo- Antarctic Suture (IAAS), recently mapped from geophysical interpretations in Wilkes Land, Antarctica. New isotopic data suggest basement rocks from the Gulden Draak Knoll have affinity to crust exposed either side of the IAAS. Determining if this structure is a suture zone sensu stricto remains to be tested.

Evidence From China PaleoProterozoic Plate Tectonics did NOT Shutdown

Early Paleoproterozoic (2.45–2.20 Ga) magmatic activity during the period of global magmatic shutdown: Implications for the crustal evolution of the southern North China Craton

Authors:

Diwu et al

Abstract:

A global database of zircon ages from both granitoids and detrital sediments shows an exceptionally and robust ages gap between 2.45 and 2.20 Ga. The early Paleoproterozoic magmatism dramatically decreased on the Earth, which was proposed to relate to the global plate tectonic shutdown. However, the available data indicate that 2.45–2.20 Ga magmatic rocks are widespread in the Taihua Complex during the quiet interval. The Taihua Complex is located in both the Lushan and Xiaoqinling areas along the southern segment North China Craton (NCC). The Complex is composed mainly of a gneiss series and the Khondalite-dominated supracrustal rock. The latter, occurs as a linear structural belt and was named the Khondalite Belt. The gneiss series in the Lushan area has an age of 2.85–2.72 Ga, whereas the Xiaoqinling region contains widespread tonalitic–trondhjemitic–granodioritic gneisses with ages of 2.45–2.20 Ga. Significant variation of Hf isotopes in zircons and whole-rock ɛNd(t) values suggest that these rocks were produced by variable mixing of a juvenile materials with older crust in an Andean-type continental margin arc or island arc setting. The depositional age of the sedimentary protoliths of the khondalite series can roughly constrained to between 2300 and 1970 Ma. It has been considered that they were deposited on a stable continental margin environment. The U–Pb dating of metamorphic zircons from the Taihua Complex suggest that the southern NCC underwent a polyphase tectonic evolution during the period 1.97–1.82 Ga, the peak metamorphism coeval with crustal thickening has occurred at ∼1.94 Ga.

Evidence of a Continental Collision/Subduction at the NeoArchean/PaleoProterozoic Boundary

Neoarchean to Paleoproterozoic high-pressure mafic granulite from the Jiaodong terrain, North China Craton: Petrology, zircon age determination and geological implications

Authors:

Liu et al

Abstract:

The North China Craton is an ideal place for studying the transition of the Earth's thermal structure and tectonics at the Archean-Proterozoic boundary due to its good preservation of the ~ 2.5 Ga tectono-thermal events. We report the discovery of a high-pressure mafic granulite from the Jiaodong Terrain in the North China Craton. The mafic granulite occurs as garnet-clinopyroxene-orthopyroxene-hornblende gneiss enclaves within a late-Archean trondhjemite-tonalite-granodiorite (TTG) gneiss. Typical high-pressure mineral assemblage of garnet - clinopyroxene - plagioclase - quartz ± rutile has been identified. Plagioclase + clinopyroxene ± orthpyroxene ± hornblende symplectite surrounding garnet ("white eye") is also observed. Using the conventional geothermobarometry and the pseudosection modeling, a clockwise metamorphic P-T path with the peak conditions at ~ 17 kbar and ~ 880 °C was determined. Zircon U-Pb analyses (SHRIMP) on the overgrowth rim of zircon grains of two samples from the same outcrop yielded a metamorphic age of 2473 ± 6 Ma (MSWD = 0.8). The analyses on magmatic core gave a probable magmatic age of 2527 ± 12 Ma (MSWD = 1.9). The high-pressure granulite facies metamorphism corresponds to a collisional event between the ~ 2.5 Ga crust and ~ 2.9 Ga crust at the dawn of Paleoproterozoic in the North China Craton. It also represents a new but rare case of a subduction-collision tectonics at the Archean-Proterozoic transition and provides insight into the change of the Earth's thermal structure.

Is the Wilson Cycle/Supercontinent Assembly/Plate Tectonics Speeding UP?

Is the rate of supercontinent assembly changing with time?

Authors:

Condie et al

Abstract:

To address the question of secular changes in the speed of the supercontinent cycle, we use two major databases for the last 2.5 Gyr: the timing and locations of collisional and accretionary orogens, and average plate velocities as deduced from paleomagnetic and paleogeographic data. Peaks in craton collision occur at 1850 and 600 Ma with smaller peaks at 1100 and 350 Ma. Distinct minima occur at 1700–1200, 900–700, and 300–200 Ma. There is no simple relationship in craton collision frequency or average plate velocity between supercontinent assemblies and breakups. Assembly of Nuna at 1700–1500 Ma correlates with very low collision rates, whereas assemblies of Rodinia and Gondwana at 1000–850 and 650–350 Ma, respectively correspond to moderate to high rates. Very low collision rates occur at times of supercontinent breakup at 2200–2100, 1300–1100, 800–650, and 150–0 Ma. A peak in plate velocity at 450–350 Ma correlates with early stages of growth of Pangea and another at 1100 Ma with initial stages of Rodinia assembly following breakup of Nuna. A major drop in craton numbers after 1850 Ma corresponds with the collision and suturing of numerous Archean blocks.

Orogens and passive margins show the same two cycles of ocean basin closing: an early cycle from Neoarchean to 1900 Ma and a later cycle, which corresponds to the supercontinent cycle, from 1900 Ma to the present. The cause of these cycles is not understood, but may be related to increasing plate speeds during supercontinent assembly and whether or not long-lived accretionary orogens accompany supercontinent assembly. LIP (large igneous province) age peaks at 2200, 2100, 1380 (and 1450?), 800, 300, 200 and 100 Ma correlate with supercontinent breakup and minima at 2600, 1700–1500, 1100–900, and 600–400 Ma with supercontinent assembly. Other major LIP age peaks do not correlate with the supercontinent cycle. A thermochemical instability model for mantle plume generation can explain all major LIP events by one process and implies that LIP events that correspond to the supercontinent cycle are independent of this cycle.

The period of the supercontinent cycle is highly variable, ranging from 500 to 1000 Myr if the late Archean supercratons are included. Nuna has a duration of about 300 Myr (1500–1200 Ma), Rodinia 100 Myr (850–750 Ma), and Gondwana–Pangea 200 Myr (350–150 Ma). Breakup durations are short, generally 100–200 Myr. The history of angular plate velocities, craton collision frequency, passive margin histories, and periodicity of the supercontinent cycle all suggest a gradual speed up of plate tectonics with time.

A Single Superplume is Splitting Africa

Africa is splitting in two. The reason: a geologic rift runs along the eastern side of the continent that one day, many millions of years in the future, will be replaced with an ocean. Scientists have argued for decades about what is causing this separation of tectonic plates. Geophysicists thought it was a superplume, a giant section of the earth's mantle that carries heat from near the core up to the crust. As evidence, they pointed to two large plateaus (one in Ethiopia and one in Kenya) that they said were created when a superplume pushed up the mantle. Geochemists were not able to confirm that theory. Instead they thought there might be two small, unrelated plumes pushing up the plateaus individually. The theories did not align, says David Hilton, a geochemist at the Scripps Institution of Oceanography in La Jolla, Calif. “There was a mismatch between the chemistry and the physics.”

So in 2006 and 2011 Hilton headed to East Africa to see whether he could lay the argument to rest. He and his team decided to use gases emanating from the rift to determine how it was created. Donning gas masks, they hiked to the tops of volcanoes in Tanzania and Ethiopia and climbed into mazuku (the Swahili word for “evil wind”)—geothermal vents and depressions where deadly gases accumulate and often kill animals. At these locations, the team collected samples of rocks deposited during eruptions, including olivines, crystals that trap volcanic gases like a bottle.

Back home in California, Hilton crushed the rocks inside a vacuum to release their gases. He was looking for helium 3, an isotope of helium present when the planet was forming that was trapped in the earth's core. Hilton figured that if rocks around both the Ethiopian and Kenyan plateaus contained this primordial gas, that would at least confirm that underground mantle plumes created them. The readings showed that, indeed, both plateaus contained helium 3. But Hilton and his group still had to wonder: Was one superplume behind it all? Or were there a couple of lesser plumes?

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Well, James, there's no need to go to the Moon!  Let's strip mine the Olduvai Gorge for Helium-3!

Evidence of Continental Rifting in the Columbia Supercontinent

Constraints on the Statherian evolution of the intraplate rifting in a Paleo-Mesoproterozoic paleocontinent: New stratigraphic and geochronology record from the eastern São Francisco craton

Authors:

Fiho et al

Abstract:

An integrated approach of stratigraphic analysis and U-Pb age dating reveals some information on the tectono-sedimentary evolution of the Statherian cover of the São Francisco craton in the so-called Espinhaço basin (Atlantic shield in eastern Brazil). Here, continental sedimentation patterns, such as alluvial fan, braided-plain and lacustrine facies associations, with associated volcanic rocks are documented in two superposed basin fill-successions, which are defined as the Algodão and Sapiranga Synthems and grouped in the Botuporã Supersynthem. Both studied units consist mainly of conglomerates and cross-bedded sandstones and minor amounts of mudstones, sedimentary breccias, volcanic lava beds and volcaniclastic rocks, which were deposited in a rift basin – the Botuporã rift – during two syn-rifting phases. The Algodão Synthem represents the first rifting phase. The basal synsedimentary conglomerates of this unit were deposited mainly by subaerial debris flows, most likely along and near a rift border fault. The framework of this rock consists of only crystalline rock clasts from the basement and no fragments of volcanic rocks. Detrital zircons grains that were extracted from this facies show ages older than 2.05 Ga. The remainder of the section is dominated by fluvial sandy lithofacies with minor conglomerate lenses and sandstone-mudstone heterolithic lithofacies, which represent distal, waning-flood deposits in a lacustrine environment. The upper section also contains hummocky cross-stratified sandstone lithofacies, which are related to a storm-influenced deposition. On top of the Algodão succession, the volcanic rocks were dated at 1775 ± 7 Ma, which was interpreted as the near final age of the first rift-phase. Representing the second rift-phase, the Sapiranga Synthem shows similar sedimentation patterns to the Algodão Synthem. The Sapiranga Synthem rests directly on the volcanic rocks of the Algodão Synthem, and its basal conglomerates (which are most likely also related to a master fault) contain voluminous clasts of volcanic rocks, sandstones and crystalline rocks. The detrital zircons grains that were extracted from this facies show ages of 1741 ± 14 Ma. The volcanic rocks on the upper succession of the Sapiranga Synthem record ages of 1740 ± 10 Ma, which finalized the Botuporã rift evolution. A preliminary geochemical study of volcanic rocks from the Botuporã Supersynthem showed that these rocks are alkaline rocks with high K2O/Na2O ratios, which belong to an ultrapotassic suite. The low concentrations of MgO wt% suggest a felsic ultrapotassic character. The Botuporã Supersynthem is unconformably covered by a volcano-sedimentary rift-succession of EoCalymmian age – the Pajeú Synthem, which represents the second rifting stage of the Espinhaço basin. Several Statherian-related volcano-sedimentary sequences and anorogenic granitoids were dispersed in the São Francisco block, which requires a regional geologic model to explain the extensional and magmatism process during this time. We used the last Columbia paleocontinental reconstruction to constrain these processes by relating them to far-field continental extensional and magmatic record as part of a silicic LIP, which was triggered by the convection-driven tectonic-plate motion on the western border of the Atlantica block inside of the Columbia supercontinent.

Did Plate Tectonics Shutdown During the Siderian/Rhyacian PaleoProterozoic?

Did plate tectonics shutdown in the Paleoproterozoic? A view from the Siderian geologic record

Authors:

Pehrsson et al

Abstract:

The early Proterozoic Era between 2.45-2.2 Ga is well known for a distinct minima in juvenile magmatism and detrital zircon abundance, an intriguing observation given its coincidence with many fundamental changes in Earth processes. A recent hypothesis seeks to explain this Siderian ‘quiet interval’ as the result of a plate tectonic shutdown in which extended tectonic quiescence is due to widespread lithospheric stagnation in an episodic mantle overturn regime. The model suggests this period characterizes a ‘pre-modern’ geodynamic style and has profound implications for many geodynamic processes.

We use spatially-linked chronostratigraphic and paleomagnetic databases to assess the major predictions of the model and find six of its key predictions are not supported by current data. The quiet interval includes a greater extent of contractional orogenesis and a broader range of paleopressures than previously known and is not characterized by LP-HT metamorphism proposed to have been related to higher upper mantle temperatures from decreased upper mantle cooling. Glacial conditions do not appear to have been triggered by the coincidence of the onset of magmatic shutdown with the end of mass-independent sulphur isotope fractionation and oxygenation of the atmosphere, as the initial glacial episodes predate this time. The glacial record, moreover, requires four episodes of climatic amelioration during the proposed shutdown, for which a mechanism appears lacking. A purported gap in Large-Igneous-Province formation, related to decreased mantle vigour, is not apparent. Quiet interval magmatism includes juvenile, arc-type and TTG magmatism, supporting significant crustal additions on a number of cratons. The prediction of negligible plate velocities during shutdown is not borne out by the well-constrained Superior Province paleomagnetic record. We suggest plate tectonics did not shut down but that the Siderian quiet interval represents overall diminished tectonic activity during peripheral orogenesis, as is known for other relatively quiet periods following supercontinent or supercraton amalgamation.

The Rate of Continental Drift has Varied Wildly From the Precambrian to the Present

Two studies show that the movement rate of plates carrying the Earth's crust may not be constant over time. This could provide a new explanation for the patterns observed in the speed of evolution and has implications for the interpretation of climate models. The work is presented today at Goldschmidt 2014, the premier geochemistry conference taking place in Sacramento, California, USA.

The Earth's continental crust can be thought of as an archive of Earth's history, containing information on rock formation, the atmosphere and the fossil record. However, it is not clear when and how regularly crust formed since the beginning of Earth history, 4.5 billion years ago.

Researchers led by Professor Peter Cawood, from the University of St. Andrews, UK, examined several measures of continental movement and geologic processes from a number of previous studies. They found that, from 1.7 to 0.75 billion years ago (termed Earth's middle age), Earth appears to have been very stable in terms of its environment, with little in the way of crust building activity, no major fluctuations in atmospheric composition and few major developments seen in the fossil record. This contrasts markedly with the time periods either side of this, which contained major ice ages and changes in oxygen levels. Earth's middle age also coincides with the formation of a supercontinent called Rodinia, which appears to have been stable throughout this time.

Professor Cawood suggests this stability may have been due to the gradual cooling of the earth's crust over time. "Before 1.7 billion years ago, the Earth's crust would have been substantially hotter, meaning that continental plate movement may have been governed by different rules to those that operate today," said Professor Cawood. "0.75 billion years ago, the crust reached a point where it had cooled sufficiently to allow modern day plate tectonics to start working, in particular allowing subduction zones to form (where one plate of the crust moves under another). This increase in activity could have kick-started a myriad of changes including the break-up of Rodinia and changes to levels of key elements in the atmosphere and seas, which in turn may have induced evolutionary changes in the life forms present."

This view is backed up by work from Professor Kent Condie from New Mexico Tech, USA, which suggests the movement rate of the Earth's crust is not constant but may be speeding up over time. Professor Condie examined how supercontinents assemble and break up. "Our results challenge the view that the rate of plate movement is stable over time," said Professor Condie. "The interpretation of data from many other disciplines such as stable isotope geochemistry, palaeontology and paleoclimatology in part rely on the assumption that the movement rate of the Earth's crust is constant."

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Was the Grawler Craton a Continental Margin During the NeoArchean Through PaleoProterozoic

SHRIMP U-Pb zircon age constraints on the tectonics of the Neoarchean to early Paleoproterozoic transition within the Mulgathing Complex, Gawler Craton, South Australia

Authors:

Reid et al

Abstract:

The Mulgathing Complex within the Gawler Craton, South Australia, preserves evidence for magmatism, sedimentation and metamorphism spanning the transition between the Neoarchean and Paleoproterozoic (ca. 2555–2410 Ma). Prior to this study, limited data were available to constrain the timing of these tectonothermal events. We report SHRIMP zircon U-Pb dating of metamorphosed sedimentary and magmatic rocks from the Mulgathing Complex that shows metasedimentary gneisses (Christie Gneiss) have maximum depositional ages ca. 2480 Ma, in contrast to previous studies that have suggested deposition had occurred ca. 2510 Ma. The oldest metamorphic zircons in our data are ca. 2465 Ma, thus indicating there was a time interval of less than 15 Myr between the cessation of sedimentation and the occurrence of high-grade metamorphism. Metamorphic zircons have a range of ages, from ca. 2465 to ca. 2415 Ma, consistent with a period of ∼50 Myr during which high-grade metamorphism occurred. Mafic and felsic intrusions have ages that range from ca. 2520 Ma to 2460 Ma, indicating magmatism occurred during sedimentation and continued during the early stages of metamorphism and deformation of these rocks. The presence of bimodal magmatism with continental arc affinity (Devils Playground Volcanics) together with mafic intrusions showing temporal overlap with sedimentation within the Mulgathing Complex suggests that the overall tectonic regime prior to deformation likely involved lithospheric extension, possibly related to a continental magmatic arc. The Mulgathing Complex shows similarities in the nature and timing of these tectonothermal processes with several other Neoarchean - Paleoproterozoic terranes, in particular the Terra Adelie Craton in Antarctica, the Sask Craton, Canada, and regions within the North China Craton and the North Australian Craton. It is possible that similarities in process could imply along-strike relationships between these terranes, for example as laterally continuous terranes at the margin of a former continental domain (or domains). Basement rocks with ages of ca. 3250–3150 Ma, ca. 2950–2800 Ma and 2600–2550 Ma within the Gawler Craton, as inferred from magmatic and inherited zircons, are temporally equivalent to some of the main basement units within the Pilbara Craton, and basement rocks in both the Gawler and Pilbara cratons are overlain by sedimentary rocks at ca. 2480 Ma that include banded iron formations. It is possible to envisage the Gawler Craton as representing an extended continental margin adjacent to a larger, more intact continental domain, such as the Pilbara.

Evidence From Ediacaran Urals With Implications for the Paleogeography of Baltica and Opening of the Iapetus Ocean

Paleomagnetism of upper Ediacaran clastics from the South Urals: Implications to paleogeography of Baltica and the opening of the Iapetus Ocean

Authors:

Levashova et al

Abstract:

The progress in understanding the evolution of the Earth during the Ediacaran-Cambrian is greatly hindered by the scarcity and inconsistency of paleomagnetic data for this time interval. In order to acquire new data and clarify the confusing situation, Upper Ediacaran clastic rocks of the Basu Formation were sampled at several localities in the westernmost parts of the South Urals that is the deformed margin of Baltica at least since the beginning of the Neoproterozoic. With the aid of stepwise thermal demagnetization, a dual-polarity high-temperature component (HTC) was reliably isolated from grey and maroon sandstones and siltstones at 34/49 sites. The HTC mean direction D° = 55, inclination I° = -35, (k = 31, α95° = 4.5) corresponds to a paleolatitude of 19° ± 3°. The reversal and fold tests are positive for the HTC. The slump test on two meter-sized slumps shows that the HTC predates slumping in one case and is coeval with it, in the other, thus convincingly indicating the primary origin of the HTC. Also, we demonstrate that inclination shallowing is either absent altogether, or, at worst, less than 10°, in these rocks; hence the position of Baltica can be reliably reconstructed for time 560-575 Ma. We reviewed paleomagnetic data with ages from 615 to 530 Ma for Baltica and Laurentia and come to the conclusion that there is still no uncontestable scenario for the opening of the Iapetus Ocean that is based on non-controversial geologic and paleomagnetic data.