Evidence for an early Archaean transition to subduction

Modern plate tectonics is largely driven by slab-pull: a consequence of high-pressure, low-temperature metamorphism of the oceanic crust far from its origin at an oceanic ridge. As it ages, basaltic crust cools, become increasingly hydrated by hydrothermal circulation of seawater through it and its density increases. That is why the abyssal plains of the ocean floor are so deep relative to the shallower oceanic ridges where it formed. Due to the decrease in the Earth’s internal heat production by decay of radioactive isotopes, once oceanic lithosphere breaks and begins to descend high-P low-T metamorphism transforms the basaltic crust to a denser form: eclogite, in which the dense, anhydrous minerals garnet and sodium-rich pyroxene (omphacite) form. Depending on local heat flow, the entire oceanic slab may then exceed the density of the upper mantle to drag the plate downwards under gravity. Metamorphic reactions of any P-T regime creates minerals less capable of holding water and drive H2O-rich fluids upwards into the overriding lithosphere, thus inducing it to partially melt. Magmas produced by this create volcanism at the surface, either at oceanic island arcs or near to continental margins, depending on the initial position of the plate subduction.

A direct proof of active subduction in the geological record is the presence of eclogite and related blueschists. Such rocks are unknown before 2100 Ma ago (mid-Palaeoproterozoic of the Democratic Republic of Congo) but there are geochemical means of ‘sensing’ plate tectonic control over arc magmatism (See: So, when did plate tectonics start up? February 2016).  The relative proportions of rare-earth elements in ancient magmatic rocks that make up the bulk of continental crust once seemed to suggest that plate tectonics started at the end of the Archaean Eon (~2500 Ma). That method, however, was quite crude and has been superseded by looking in great detail at the geochemistry of the Earth’s most durable mineral: zircon (ZrSiO4), which began more than two decades ago. Minute grains of that mineral most famously have pushed back the geological record into what was long believed to be half a billion years with no suggestion of a history: the Hadean. Zircon grains extracted from a variety of ancient sediments have yielded U-Pb ages of their crystallisation from igneous magma that extend back 4.4 billion years (Ga) (see: Pushing back the “vestige of a beginning”;January 2001).  

Though simple in their basic chemical formula, zircons sponge-up a large range of other trace elements from their parent magma. So, in a sense, each tiny grain is a capsule of their geochemical environment at the time they crystallised. In 2020 Australian geochemists presented the trace-element geochemistry of 32 zircons extracted from a 3.3 Ga old sedimentary conglomerate in the Jack Hills of Western Australia, which lie within an ancient continental nucleus or craton. They concluded that those zircons mainly reveal that they formed in andesitic magmas, little different from the volcanic rocks that are erupted today above subduction zones. From those data it might seem that some form of plate tectonics has been present since shortly after the Earth’s formation. Oxygen-isotope data from zircons are useful in checking whether zircons had formed in magmas derived directly from partial melting of mantle rocks or by recycling of crustal magmatic rocks through subduction. Such a study in 2012 (see: Charting the growth of continental crust; March 2012) that used a very much larger number of detrital zircon grains from Australia, Eurasia, North America, and South America seemed, in retrospect, to contradict a subduction-since-the-start view of Earth dynamics and crust formation. Instead it suggested that recycling of crust, and thus plate-tectonic subduction, first showed itself in zircon geochemistry at about 3 Ga ago.

Detailed chemical and isotopic analysis of zircons using a variety of instruments has steadily become faster and cheaper. Actually finding the grains is much easier than doing interesting things with them. It is a matter of crushing the host rock to ‘liberate’ the grains. Sedimentary hosts that have not been strongly metamorphosed are much more tractable than igneous rocks. Being denser than quartz, the dominant sedimentary mineral, zircon can be separated from it along with other dense, trace minerals, and from them in turn by various methods based on magnetic and electrical properties. Zircons can then be picked out manually because of their distinctive colours and shapes. A tedious process, but there are now several thousand fully analysed zircons aged between 3.0 to 4.4 Ga, from eleven cratons that underpin Australia, North America, India, Greenland and southern Africa. The latest come from a sandstone bed laid down about 3.31 Ga ago in the Barberton area of South Africa (Drabon, N. et al. 2022. Destabilization of Long‐Lived Hadean Protocrust and the Onset of Pervasive Hydrous Melting at 3.8 GaAGU Advances, v. 3, article e2021AV000520; DOI: 10.1029/2021AV000520). The authors measured lutetium (Lu), hafnium (Hf) and oxygen isotopes, and concentrations of a suite of trace element in 329 zircons from Barberton dated between 3.3 to 4.15 Ga.

A schematic model of transition from Hadean-Eoarchaean lid tectonics to a type of plate tectonics that subsequently evolved to its current form, based on hafnium isotope data in ancient zircons (credit: Bauer et al. 2020; Fig 3)

The Hf isotopes show two main groups relative to the values for chondritic meteorites (assumed to reflect the composition of the bulk Earth). Zircons dated between 3.8 and 4.15 Ga all show values below that expected for the whole Earth. Those between 3.3 and 3.8 Ga show a broader range of values that extend above chondritic levels. The transition in data at around 3.8 Ga is also present in age plots of uranium relative to niobium and scandium relative to ytterbium, and to a lesser extent in the oxygen isotope data. On the basis of these data, something fundamentally changed in the way the Earth worked at around 3.8 Ga. Nadja Drabon and colleagues ascribe the chemical features of Hadean and Eoarchaean zircons to an early protocrust formed by melting of chemically undepleted mantle. This gradually built up and remained more or less stable for more than 600 Ma, without being substantially remelted through recycling back to mantle depths. After 3.8 billion years ago, geochemical signatures of the zircons start showing similarities to those of zircons derived from modern subduction zones. Hf isotopes and trace-element geochemistry in 3.6 to 3.8 Ga-old  detrital zircons from other cratons are consistent with a 200 Ma transition from ‘lid’ tectonics (see: Lid tectonics on Earth; December 2017) to the familiar tectonics of rigid plates whose basalt-capped lithosphere ultimately returns to the mantle to be involved in formation of new magmas from which continental crust stems. Parts of plates bolstered by this new, low density crust largely remain at the surface.

While Drabon et al. do provide new data from South Africa’s Kaapvaal craton, their conclusions are similar to earlier work by other geochemists based on data from other area (e.g. Bauer, A.M. et al. 2020. Hafnium isotopes in zircons document the gradual onset of mobile-lid tectonicsGeochemical Perspectives Letters, v. 14; DOI: 10.7185/geochemlet.2015), which the accompanying figure illustrates.

See also: Earliest geochemical evidence of plate tectonics found in 3.8-billion-year-old crystal. Science Daily, 21 April 2022. 3.8-Billion-Year-Old Zircons Offer Clues to When Earth’s Plate Tectonics Began. SciNews, 26 April 2022

Lower-mantle blobs may reveal relics of event going back to the Hadean

The World-Wide Standardised Seismograph Network (WWSSN) records the arrivals of waves generated by earthquakes that have passed through the Earth’s interior. There are two types of these body waves: S- or shear waves that move matter at right angles to their direction of movement; compressional or P-waves that are a little like sound waves as materials are compressed and expanded along the direction of movement. Like sound, P-waves can travel through solids, liquids and gases. Since liquids and gases are non-rigid they cannot sustain shearing, so S-waves only travel through the solid Earth’s mantle but not its liquid outer core. However, their speed is partly controlled by rock rigidity, which depends on the temperature of the mantle; the hotter the lower the mantle’s rigidity.

Analysis of the S-wave arrival times throughout the WWSSN from many individual earthquakes enables seismologists to make 3-D maps of how S-wave speeds vary throughout the mantle and, by proxy, the variation of mantle rigidity with depth. This is known as seismic tomography, which since the late 1990s has revolutionised our understanding of mantle plumes and subduction zones, and also the overall structure of the deep mantle. In particular, seismic tomography has revealed two huge, blob-like masses above the core-mantle boundary that show anomalously low S-wave speeds, one beneath the Pacific Ocean and another at about the antipode beneath Africa: by far the largest structures in the deep mantle. They are known as ‘large low-shear-wave-velocity provinces’ (LLSVPs) and until recently they have remained the enigmatic focus of much speculation around two broad hypotheses: ‘graveyards’ for plates subducted throughout Earth history; or remnants of the magma ocean thought to have formed when another protoplanet impacted with the early Earth to create the Moon about 4.4 billion years ago.

Three-dimensional rendition of seismic tomography results beneath Africa. Mantle with anomalously low S-wave speeds is show in red, orange and yellow. The faint grey overlay represents the extent of surface continental crust today – Horn of Africa at right and Cape Town at the lower margin – the blue areas near the top are oceanic crust on the floor od the Mediterranean Sea. (Image credit: Mingming Li/ASU)

Qian Yuan and Mingming Li of Arizone State University, USA have tried to improve understanding of the shapes of the two massive blobs (Yuan, Q. & Li, M. 2022. Instability of the African large low-shear-wave-velocity province due to its low intrinsic density. Nature Geoscience, v. 15  DOI: 10.1038/s41561-022-00908-3) using advanced geodynamic modelling of the seismic tomography. Their work reveasl that the Pacific LLSVP extends between 500 to 800 km above the core-mantle boundary. Yet that beneath Africa reaches almost 1000 km higher, at 1300 to 1500 km. Both of them are less rigid and therefore hotter than the surrounding mantle. In order to be stable they must be considerably denser than the rest of the mantle surrounding them. But, because it reaches much higher above the core, the African LLSVP is probably less dense than the Pacific one. A lower density suggests two things: the African blob may be less stable; the two blobs may have different compositions and origins.

Both the Pacific Ocean floor and the African continent are littered with volcanic rocks that formed above mantle plumes. The volcanic geochemistry above the two LLSVPs differs. African samples show signs of a source enriched by material from upper continental crust, whereas those from the Pacific do not. Yuan and Li suggest that the enrichment supports the ‘plate graveyard’ hypothesis for the African blob and a different history beneath the Pacific. The 3-D tomography beneath Africa (see above) shows great complexity, perhaps reflecting the less stable nature of the LLSVP. Interestingly, 80 % of the pipe-like African kimberlite intrusions that have brought diamonds up from mantle depths over that last 320 Ma formed above the blob.

But why are there just two such huge blobs of anomalous material that lie on opposite sides of the Earth rather than a continuous anomaly or lots of smaller ones? The subduction graveyard hypothesis is compatible with the last two distributions. In a 2021 conference presentation the authors suggest from computer simulations that the two blobs may have originated at the time of the Moon’s formation after a planetary collision (Yuan, Q. et al. 2021. Giant impact origin for the large low shear velocity provinces. Abstracts for the 52nd Lunar and Planetary Science Conference: Lunar and Planetary Institute, Houston). Specifically, they suggest that the LLSVPs originated from the mantle of the other planet (Theia) after its near complete destruction and melting, which sank without mixing through the magma ocean formed by the stupendous collision. Yet, so far, no geochemists have been bold enough to suggest that there are volcanic rocks of any age that reveal truly exotic compositions inherited from deep mantle material with such an origin. If Theia’s mantle was dense enough to settle through that of the Earth when both were molten, it would be sufficiently anomalous in its chemistry for signs to show up in any melts derived from it. There again, because of a high density it may never have risen in plumes to source any magma that reached the Earth’s surface …

Note added later: Simon Hamner’s Comment about alternative views on seismic tomography has prompted me to draw attention to something I wrote 19 years ago

Signs of massive hydrocarbon burning at the end of the Triassic

One of the ‘Big Five’ mass extinctions occurred at the end of the Triassic Period (~201 Ma), whose magnitude matches that of the more famous end-Cretaceous (K-Pg) event. It roughly coincided with the beginning of break-up of the Pangaea supercontinent that was accompanied by a major episode of volcanism preserved in the Central Atlantic Magmatic Province (CAMP). Eastern North America, West Africa and northern South America reveal scattered patches of CAMP flood basalts, swarms of dykes and large intrusive sills. Like all mass extinctions, that at the Triassic-Jurassic boundary left a huge selection of vacant or depleted ecological niches ready for evolution to fill by later adaptive radiation of surviving organisms. Because it coincided with continental break-up and drift, unlike other such events, evolution proceeded in different ways on the various wandering land masses and in newly formed seas (see  an excellent animation of the formation and break-up of Pangaea – move the slider to 3 minutes for the start of break-up). The Jurassic was a period of explosive evolution among all groups of organisms. The most notable changes were among marine cephalopods, to give rise to a bewildering variety of ammonite species, and on land with the appearance and subsequent diversification of dinosaurs.

Pangaea at the end of the Triassic (top) and in Middle Cretaceous times (Credit: screen shots from animation by Christopher Scotese)

Many scientists have ascribed the origin of these events to the CAMP magmatic activity and the release of huge amounts of methane to trigger rapid global warming. In October 2021 one group focused on a special role for the high percentages of magma that never reached the surface and formed huge intrusions that spread laterally in thick sedimentary sequences to ‘crack’ hydrocarbons to their simplest form, CH4 or methane. A sedimentary origin of the methane, rather than its escape from the mantle, is indicated by the carbon-isotope ‘signature’ of sediments deposited shortly after the Tr-J event. The lighter isotope 12C rose significantly relative to 13C, suggesting an organic source – photosynthesis selectively takes up the lighter isotope.

By examining the element mercury (Hg) in deep ocean sediments from a Tr-J sedimentary section now exposed in Japan, scientists from China, the US and Norway have added detail to the methane-release hypothesis (Shen, J et al. 2022. Mercury evidence for combustion of organic-rich sediments during the end-Triassic crisis. Nature Communications, v. 13, article 1307; DOI:10.1038/s41467-022-28891-8). The relative proportions of Hg isotopes strongly suggest that the mercury had been released, as was the methane, from organic-rich sediments rather than from the CAMP magmas (i.e. ultimately from the mantle) through gasification and then burning at the surface.

The hypothesis is enlivened by a separate study (Fox C.P. et al. 2022. Flame out! End-Triassic mass extinction polycyclic aromatic hydrocarbons reflect more than just fire. Earth and Planetary Science Letters, v. 584, article 117418; DOI: 10.1016/j.epsl.2022.117418) that sees magmatic heating as being not so important. Calum Fox and colleagues at Curtin University, Western Australia analysed sediments from a Triassic-Jurassic sedimentary sequence near the Severn Bridge in SW England, focusing on polycyclic hydrocarbons in them. Their results show little sign of the kinds of organic chemical remnants of modern wildfires. Instead they suggest a greater contribution from soil erosion by acid rain that increased input of plant debris to a late Triassic marine basin

See also: How a major volcanic eruption paved the way for the rise of the dinosaurs Eureka Alert 23 March 2022;  Soil erosion and wildfire: another nail in coffin for Triassic era. Science Daily, 21 March 2022

New ideas on how subduction works

Nowadays, plate tectonics is thought mainly to be driven by the sinking of old, relatively cold and dense oceanic lithosphere at subduction zones: slab-pull force dominates the current behaviour of the outermost Earth. At the eastern edge of Eurasia subduction beneath Japan has yet to consume Pacific Ocean lithosphere younger than 180 Ma (Middle Jurassic). The Pacific Plate extends eastwards from there for over 7000 km to its source at the East Pacific Rise. That spreading axis has disappeared quite recently beneath the North American Plate between Baha California and northern California. It has been subducted. Since, to a first approximation, sea-floor spreading is at the same pace either side of mid-ocean constructive plate margins, subduction at the western edge of the North America has consumed at least 7000 km of old ocean lithosphere. Slab-pull force there has been sustained for probably more than 250 Ma. As a result several former island arcs have been plastered onto the leading edge of the North American Plate to create the geological complexity of its western states. If at any time the weight of the subducting slab had caused it leading edge literally to snap and fall independently wouldn’t that have decreased slab-pull force or shut it off, and spreading at the East Pacific Rise, altogether? No, says the vast expanse of the West Pacific plate

That dichotomy once encouraged scientists of the plate-tectonic era to assume that a subducted slab remains as strong as rigid plates at the surface. They believed that subduction merely bends a plate so that it can slide into the mantle. The use of seismic waves (seismic tomography) to peer into the mantle has revealed a far more complex situation. Beneath North America traces of subducted slabs are highly deformed and must have lost their rigidity, yet they still maintain slab-pull force. Three geoscientists from the Swiss Federal Institute of Technology Zurich, Switzerland, and the University of Texas at Austin, USA (Gerya T. V., Becovici, D. & Becker, T.W. 2021. Dynamic slab segmentation due to brittle–ductile damage in the outer rise. Nature, v. 599, p 245-250; DOI: 10.1038/s41586-021-03937-x) used computer-generated models of how various forces and temperature conditions at small and large scales bear on the behaviour of slabs being subducted. Where a plate bends into a subduction zone its rigidity results in cracking and faulting of its no convex upper surface, while the base is compressed. Seismic anomalies in the descending slab reflect the formation of pulled-apart segments, similar to those in a bar of chocolate (for a possible example from an exhumed subduction zone see: A drop off the old block? May 2008). Thermo-mechanical modelling suggests that the slab becomes distinctly weakened through brittle damage and by reduction in grain size because of ductile deformation, yet each segment maintains a high viscosity relative to the surrounding mantle rocks. Under present conditions and those extrapolated back into the Proterozoic, where the slab is thinned between segments it remains sufficiently viscous to avoid segments detaching to sink independently of one another. Such delamination would reduce slab-pull force. Another process operates in the surrounding mantle. The occurrence of earthquakes in a subducted slab down to a depth of about 660 km – the level of a major discontinuity in the mantle where pressure induces a change in its mineralogy and density – confirms that a modern slab maintains some rigidity and deforms in a brittle fashion. But at this depth it cannot continue to descend steeply and travels horizontally along the discontinuity, pushed by the more shallow subduction. It can now become buckled as the mantle resists its lateral motion.

Left: the subduction zone beneath Japan defined by seismic tomography (yellow to red = lower seismic wave speeds – more ductile; yellow to blue = higher speeds – more rigid). Right: modelled evolution of viscosity in a similar subduction zone under modern conditions showing slab segmentation (blue to brown = increasing viscosity). (Credit: Gerya et al., Figs 4c & 1a-e)

Rather than trying to mimic the chaos beneath North America the authors compared their results with seismic tomography of the younger system of westward subduction beneath Japan. This allowed them to ‘calibrate’ their modelling against actual deep structure well-defined by seismic tomography. The tectonic jumble beneath North America probably resulted from a much longer history of eastwards subduction. The complexity there may be explained by successive foundering of deformed slabs into the deeper mantle looking a bit like a sheet of still viscous pie pastry dropped on its edge. This happened, perhaps, as island arcs that had formed in the eastern Pacific sporadically accreted to the continent as the intervening oceanic lithosphere was subducted.   

There is ample evidence that modern-style subduction was widespread back as far as the Palaeoproterozoic. But in the Archaean the evidence is fitful: some hints of subduction, but plenty of contrary evidence.  Gerya and co-workers suggest that higher heat production from radioactive decay mantle earlier in Earth’s history would have reduced plate strength and mantle resistance to slab penetration. Subduction may have occurred but was interrupted repeatedly by foundering/delamination of individual detached segments at much shallower depths. That implies weaker as well as intermittent slab pull, or even further back its complete absence, so that planetary recycling would then have required other mechanisms, such as ‘drip tectonics’.

See also: Crushed resistance: Tectonic plate sinking into a subduction zone and Fate of sinking tectonic plates is revealed, Science Daily, 11 November 2021

Nappe tectonics at the end of the Archaean

The beginning of modern-style plate tectonics is still debated in the absence of definite evidence. Because Earth’s mantle generates heat through radioactive decay and still contains heat left over from planetary accretion and core formation it must always have maintained some kind of heat transfer through some kind of circulatory motion involving the mantle and lithosphere. That must always too have involved partial melting and chemical differentiation that created materials whose density was lower than that of the mantle; e.g. continental crust. Since continental materials date back to more than 4 billion years ago and some may have been generated earlier in the Hadean, only to be lrgely resorbed, a generalised circulation and chemical differentiation have been Earth’s main characteristics from the start. One view is that early circulation was a form of vertical tectonics without subduction via a sort of ‘dripping’ or delamination of particularly dense crustal materials back into the mantle. A sophisticated model of how the hotter early Earth worked in this way has been called ‘lid tectonics’, from which plate tectonics evolved as the Earth cooled and developed a thicker, more rigid lithosphere. Such an outer layer would be capable of self-generating the slab pull that largely drives lateral motions of lithospheric plates. That process occurs once a slab of oceanic lithosphere becomes cool and dense enough to be subducted (see: How does subduction start?; August 2018).

The most convincing evidence for early plate tectonics would therefore be tangible signs of both subduction and large horizontal movements of lithospheric plates: common enough in the Neoproterozoic and Phanerozoic records, but not glaringly obvious in the earlier Archaean Eon. These unequivocal hallmarks have now emerged from studies of Archaean rocks in the Precambrian basement that underpins northern China and North Korea. The North China Craton has two main Archaean components: an Eastern Block of gneisses dated between 3.8 and 3.0 Ga and a Western Block of younger (2.6 to 2.5 Ga) gneisses, metavolcanics and metasediments. They are separated by a zone of high deformation. A key area for understanding the nature of the deformed Central Orogenic Belt is the Zanhuan Complex near the city of Kingtai (Zhong, YL. et al. 2021. Alpine-style nappes thrust over ancient North China continental margin demonstrate large Archean horizontal plate motions. Nature  Communications, v. 12, article6172, DOI: 10.1038/s41467-021-26474-7).

Schematic cross sections through the Zanhuan Complex of northern China, showing early and final development of the Central Orogenic Belt in the North China Block . (Credit: Zhong, YL. et al.;Figs 10b and c)

This small, complex area reveals that the older Eastern Block is unconformably overlain by Neoarchaean sediments, above which has been thrust a stacked series of nappes similar in size and form to those of the much younger Alpine orogenic belt of southern Europe. Though highly complex, the rocks involved having been folded and stretched by ductile processes, they are still recognisable as having originally been at the surface. Metavolcanics in the nappes can be assigned from their geochemistry to a late-Archaean fore-arc, through comparison with that of modern igneous rocks formed at such a setting in the Western Pacific. Thrust over the nappe complex is a jumble or mélange of highly deformed metasediments containing blocks of metabasalts and occasional ultramafic igneous rocks that geochemically resemble oceanic crust formed at a mid-ocean ridge. Some of them contain high-pressure minerals formed at depth in the mantle, indicating that they had once been subducted. The whole complex is cut by undeformed dykes of granitic composition dated at 2.5 Ga, confirming that the older rocks and the structures within them are Archaean in age. Thrust over the melange and tectonically underlying nappe complex are less-deformed volcanic rocks and granitic intrusions that closely resemble what is generally found in modern island arcs.

Orogenic belts bear witness to enormous crustal shortening caused by horizontal compressive forces. Assuming the average rate of modern subduction (2 cm yr-1) the 178 Ma history of the Zanhuan Complex implies more than 3,500 km of lateral transport. 2.5 billion years ago, higher radioactive heat production in the mantle would have made tectonic overturning considerably faster  The unconformity at the base of the complex suggests that it was driven over the equivalent of a modern passive, continental margin. So the complex provides direct evidence of horizontal plate tectonics and associated subduction during the latter stages of the Archaean that ranks in scale with that of many Phanerozoic orogenic belts, such as that of the European Alps. The Zanhuan Complex is a result of arc accretion that played a major role in many later orogens. The North China craton itself is reminiscent of continent-continent collision, as required in the formation of supercontinents.

Influence of massive igneous intrusions on end-Triassic mass extinction

About 200 Ma ago, the break-up of the Pangaea supercontinent was imminent. The signs of impending events are spread through the eastern seaboard of North America, West Africa and central and northern South America. Today, they take the form of isolated patches of continental flood basalts, dyke swarms – probably the feeders for much more extensive flood volcanism – and large intrusive sills. Break-up began with the separation of North America from Africa and the start of sea-floor spreading that began to form the Central Atlantic Ocean: hence the name Central Atlantic Magmatic Province (CAMP) for the igneous activity. It all kicked off at the time of the Triassic-Jurassic stratigraphic boundary, and a mass extinction with a similar magnitude to that at the end of the Cretaceous. Disappearances of animals in the oceans and on continents were selective rather than general, as were extinctions of land plants. The mass extinction is estimated to have taken about ten thousand years. It left a great variety of ecological niches ready for re-occupation. On land a small group of reptiles with a substantial destiny entered some of these vacant niches. They evolved explosively to the plethora of later dinosaurs as their descendants became separated as a result of continental drift and adaptive radiation.

Flood basalts of the Central Atlantic Magmatic Province in Morocco (Credit: Andrea Marzoli)

The end-Triassic mass extinction, like three others of the Big Five, was thus closely associated in time with massive continental flood volcanism: indeed one of the largest such events. Within at most 10 ka large theropod dinosaurs entered the early Jurassic scene of eastern North America. The Jurassic was a greenhouse world whose atmosphere had about five times more CO2, a mean global surface temperature between 5 and 10°C higher and deep ocean temperatures 8°C above those at present. Was mantle carbon transported by CAMP magmas the main source (widely assumed until recently) or, as during the end-Permian mass extinction, was buried organic carbon responsible? A multinational group of geoscientists have closely examined samples from a one million cubic kilometre stack of intrusive basaltic sills, dated at 201 Ma, in the Amazon basin of Brazil that amount to about a third of all CAMP magmatism (Capriolo, M. and 11 others 2021. Massive methane fluxing from magma–sediment interaction in the end-Triassic Central Atlantic Magmatic ProvinceNature Communications, v. 12, article 5534; DOI: 10.1038/s41467-021-25510-w).

The team focussed on fluid inclusions in quartz within the basaltic sills that formed during the late stages of their crystallisation. The tiny inclusions contain methane gas and tiny crystals of halite (NaCl) as well as liquid water. Such was the bulk composition of the intrusive magma that the presence of around 5% of quartz in the basalts would be impossible without their magma having assimilated large volumes of silica-rich sedimentary rocks such as shales. The host rocks for the huge slab of igneous sills are sediments of Palaeozoic age: a ready source for contamination by both organic carbon and salt. The presence of methane in the inclusions suggests that more complex hydrocarbons had been ‘cracked’ by thermal metamorphism. Moreover, it is highly unlikely to have been derived from the mantle, partly because methane has been experimentally shown not to be soluble in basaltic magmas whereas CO2 is. The authors conclude that both quartz and methane entered the sills in hydrothermal fluids generated in adjacent sediments. Thermal metamorphism of the sediments would also have driven such fluids to the surface to inject methane directly to the atmosphere. Methane is 25 times as potent as carbon dioxide at trapping heat in the atmosphere, yet it combines with the hydroxyl (OH) radical to form CO2 and water vapour within about 12 years. Nevertheless during continuous emission methane traps 84 times more heat in the atmosphere than would an equivalent mass of carbon dioxide.

Calculations suggest about seven trillion tonnes of methane were generated by the CAMP intrusions in Brazil. Had the magmas mainly been extruded as flood basalts then perhaps global warming at the close of the Triassic would have been far less. Extinctions and subsequent biological evolution would have taken very different paths; dinosaurs may not have exploded onto the terrestrial scene so dramatically during the remaining 185 Ma of the Mesozoic. So it seems important to attempt an explanation of why CAMP magmas in Brazil did not rise to the surface but stayed buried as such stupendous igneous intrusions. Work on smaller intrusive sills suggests that magmas that are denser than the rocks that they pass through – as in a large, thick sedimentary basin – are forced by gravity to take a lateral ‘line of least resistance’ to intrude along sedimentary bedding. That would be aided by the enormous pressure of steam boiled from wet sedimentary rocks forcing beds apart. In areas where only thin sedimentary cover rests on crystalline, more dense igneous and metamorphic rocks, basaltic magma has a greater likelihood of rising through vertical dyke swarms to reach the surface and form lava floods.

Subduction and continental collision in the Himalaya

The Indian subcontinent after it separated from Madagascar in the Late Cretaceous to move northwards to its destined collision with Eurasia and the formation of the Himalaya. (Credit: Frame from an animation ©Christopher Scotese)

During the Early Cretaceous (~140 Ma ago) India, Madagascar, Antarctica and Australia parted company with Africa after 400 Ma of unity as components of the Gondwana supercontinent. By 120 Ma Antarctica and Australia split from India and Madagascar, and the Indian Ocean began to form. India moved northwards , leaving Madagascar in its wake after about 70 Ma ago. By 50 Ma the subcontinent began to collide with Eurasia, its northward motion driving before it crustal materials that eventually formed the Himalaya. This highly complex process is wonderfully documented in an animation made in 2015 by Christopher Scotese, Emeritus Professor in the Department of Earth and Environmental Sciences, Northwestern University, USA. At the start of its journey India moved northwards at a slow rate of about 5 cm per year. After 80 Ma it speeded up dramatically to 15 cm per year, about twice as fast as any modern continental drift and a pace that lasted for over 30 Ma until collision began. How could that, in a geological sense, sudden and sustained acceleration have been induced? It would have required a change in the slab-pull force that is the primary driver of plate tectonics, suggesting an increase in the amount of subduction in the Tethys Ocean that formerly lay between India and Eurasia, probably at two, now hidden destructive plate margins.

A group of geoscientists from Canada, the US and Pakistan has documented that collision in terms of the record of metamorphism experienced beneath the Himalaya as slab after slab of once near-surface rocks were driven beneath the rising orogen (Soret, M. et al. 2021. How Himalayan collision stems from subduction. Geology, v. 49, p. 894-898; DOI: 10.1130/G48803.1). The Western Himalaya has trapped a deformed and tilted magmatic rock sequence of an island arc – the Kohistan Arc – between  the Eurasian plate and a zone of crustal thickening and shortening that was thrust southward over the ancient metamorphic basement of India itself. That crust was mantled by a variety of younger sediments deposited on the Tethyan continental shelf of the northern Indian plate which became involved in the process of crustal thickening. The Kohistan Arc probably formed above one of the destructive margins that consumed the oceanic lithosphere of the now vanished Tethys Ocean. Two distinct types of rock make up the slabs stacked-up by thrusting.

The uppermost, which also forms the highest part of the Western Himalaya in the form of Nanga Parbat (at 8,126 metres the world’s ninth highest mountain) comprises rocks thought to represent Tethyan oceanic lithosphere subducted perhaps at the second destructive margin. Their mineral assemblages, especially those of eclogites, indicate that they have been metamorphosed under pressures corresponding to depths of up to 100 km, but at low temperatures along a geothermal gradient of about 7°C km-1, i.e. in a low heat-flow environment. These ultra-high pressure (UHP) metamorphic rocks formed at the start of the India-Eurasia collision. The sequence of sedimentary slabs now overridden by the UHP slab were metamorphosed at around the same time, but under very different conditions. Their burial reached only about 35 km – the normal thickness of the continental crust – and a temperature of about 600°C on a 30°C km-1 geothermal gradient. Detailed mineralogy of the UHP slab reveals that as it was driven over the metasediments it evolved to the same geothermal conditions.

Matthew Soret and his colleagues explain how this marked metamorphic duality may have arisen in rocks that are now part of the same huge thrust complex. Their results are consistent with slicing together of oceanic lithosphere in a subduction zone to form a tectonic wedge of UHP mineral assemblages at the same time as continental shelf sediments were metamorphosed under more normal geothermal conditions. This was happening just as India came into contact with Eurasia. When crustal thickening began in earnest through the inter-slicing of the two assemblages, pressure on the UHP rocks fell rapidly as a result of their being thrust over the dominantly metasedimentary shelf sequence. It also moved into a zone of normal heat flow, first heating up equally quickly and then following a path of decreasing pressure and temperature as erosion pared away the newly thickened crust. Both assemblages now became part of the same metamorphic regime. In this way a subduction system evolved to become incorporated in an orogenic zone as two continents collided; a complex process that finds parallels in other orogens such as the Alps.

Tectonics on Venus

The surface of Venus is not easily observed because of the almost opaque nature of its atmosphere. The planet is veiled by a mixture of CO2 (96.5%) and nitrogen (3.5%), with a little sulfur dioxide and noble gases. The atmosphere’s mass is almost 100 times that of the Earth’s, and has a density about 6.5% that of liquid water at the surface. The opacity stems from a turbulent upper layer of mainly sulfuric acid. Venus is the victim of runaway greenhouse conditions. Despite that, radar can penetrate the atmosphere to reveal details of its surface morphology – roughness and elevation – at a spatial resolution of 150 m. Although coarser than that available from radar remote sensing of the Earth from orbit, the Magellan data are still geologically revealing.

Earlier interpretation of Venus radar images revealed the surface to be far simpler than that of the Earth, Mars and all other rocky bodies in the Solar System. Yet it has more volcanoes than does the Earth or Mars. However, despite being subject to very little erosion – Venus is a dry world – only around 1000 impact craters have been found: far short of the number seen on Mars or the Moon. This deficiency of evidence for bombardment suggests that Venus was ‘repaved’ by vast volcanic outpourings in the geologically recent past, estimated to have occurred 300 to 600 Ma ago. This early work concluded that plate tectonics was absent; indeed that for half a billion years the lithosphere on Venus had been barely deformed. It has been suggested that Venus has been involved in megacycles of sudden, planet-wide magmatic activity separated by long periods of quiescence. This could be attributed to the lack of plate tectonics, which is the principal means that Earth continuously rids itself of heat produced at depth by decay of radioactive isotopes in the mantle. Venus has been suggested to build up internal temperatures until they reach a threshold that launches widespread partial melting of its mantle. Planet-wide eruption of magma then reduces internal temperatures.

Polygonal blocks or ‘campuses’ on the lowland surface of Venus. Note the zones of ridges that roughly parallel ‘campus’ margins. Credit: Paul K. Byrne, North Carolina State University and Sean C. Solomon, Lamont-Doherty Earth Observatory

It comes as a surprise that 26 years after Magellan plunged into the Venusian atmosphere new interpretation of its radar images suggests a completely different scenario (it may be that academic attention generally switched to research on Mars because of all the missions to the ‘Red Planet’ since Magellan disappeared). It is based on features of the surface of Venus so large that their having been missed until now may be a planetary-scale example of ‘not seeing the woods for the trees’! Geoscientists from the US, Turkey, the UK and Greece have mapped out features ranging from 100 to 1000 km across that cover the lowland parts of Venus (Byrne, P.K. et al. 2021. A globally fragmented and mobile lithosphere on Venus. Proceedings of the National Academy of Science, v. 118, article e2025919118; DOI: 10.1073/pnas.2025919118). They resemble 1950s ‘crazy paving’ or floes in Arctic pack ice, but on a much larger scale. Extending the ice floe analogy, the polygonal blocks are separated by what resemble pressure ridges that roughly parallel the block margins. Paul Byrne of North Carolina State University, USA, and co-workers also found evidence that the large blocks of lithosphere had rotated and moved laterally relative to one another: they had ‘jostled’. Moreover, some of the movement has disturbed the youngest materials on the surface.

To distinguish what seem to be characteristic of Venus’s tectonics from Earthly tectonic plates, the team hit on the name ‘campus’, meaning ‘field’ in Latin. Rather than having remained a single spherical skin of lithosphere, the surface of at least part of Venus has broken into a series of ‘campuses’. It does display tectonics, but not as we know it on planet Earth. This could be ascribed to an outcome of stress transfer from deep convective motion in the Venerean mantle. Being in the virtually non-magmatic phase of Venus’s thermal cycling, there is neither formation of new lithosphere nor subduction of old, cold plates that characterise terrestrial plate tectonics. ‘Campus’ tectonics seems likely to be another form of planetary energy and matter redistribution, and Byrne et al. have likened it to how the Earth may have functioned during the ‘missing’ 600 Ma of the Hadean Eon on Earth. But perhaps not …

The runaway greenhouse has resulted in surface temperatures on Venus being 450°C higher than on Earth: enough to melt lead. It is not just solar heat that is trapped by the atmosphere, but that from the Venerean interior. This must result in a very different geotherm (the way temperature varies with depth in a planet) from that characterising the Earth. The temperature of the beginning of mantle melting – about 1200°C – must be much shallower on Venus. On Earth that is at depths between 50 and 100 km below active plate margins and within-plate hotspots, and is not reached at all for most of the Earth that lies beneath the tectonic plates. If the mantle of Venus contained a similar complement of heat-producing isotopes to that of Earth wouldn’t we expect continual volcanism on Venus rather than the odd dribble that has been observed by Magellan? Or does the jostling of ‘campuses’ absorb the thermal energy and help direct it slowly to space by radiation through the dense, greenhouse atmosphere. Here’s another poser: If the Earth and Venus are geochemically similar and Hadean Earth went through such a phase of ‘campus tectonics’ – perhaps our world had a CO2-rich atmosphere too – what changed to allow plate tectonics here to replace that system of thermal balance? And, why hasn’t that happened on Venus? Perhaps some light will be thrown on these enigmas once a series of new missions to Venus are launched between now and the 2030s, by NASA and the European Space Agency.

The subduction pulley: a new feature of plate tectonics

Geological map of part of the Italian Alps. The Sesia-Lanzo Zone is 6 in the Key: a – highly deformed gneisses; b – metasedimentary schists with granite intrusions; c – mafic rocks; d – mixed mantle and crystalline basement rocks. (Credit: M. Assanelli, Universita degli Studi di Milano)

To a first approximation, as they say, the basis of plate tectonics is that the lithosphere is divided up into discrete, rigid plates that are bounded by lines of divergent, convergent and sideways relative motions: constructive, destructive and conservative plate margins. These are characterised by zones of earthquakes whose senses of motion roughly correspond to the nature of each boundary: normal, reverse and strike-slip, respectively. The seismicity is mainly confined to the lithosphere in the cases of constructive and conservative boundaries (i.e. shallow) but extends as deep as 700 km into the mantle at destructive margins, thereby defining the subduction of lithosphere that remains cool enough to retain its rigidity. Although the definition assumes that there is no deformation within plates, in practice that does occur for a wide variety of reasons in the form of intra-plate seismicity, mainly within continental lithosphere. Oceanic plate interiors are much stronger and largely ‘follow the rules’; they are generally seismically quiet.

One important feature of plate tectonics is the creation of new subduction zones when an earlier one eventually ceases to function. Where these form in an oceanic setting volcanism in the overriding plate creates island arcs. They create precursors of new continental crust because the density of magmas forming the new lithosphere confers sufficient buoyancy for them to be more difficult to subduct. Eventually island arcs become accreted onto continental margins through subduction of the intervening oceanic lithosphere. Joining them in such ‘docking’ are microcontinents, small fragments spalled from much older continents because of the formation of new constructive plate margins within them. It might seem that arcs and microcontinents behave like passive rafts to form the complex assemblages of terranes that characterise continental mountain belts, such as those of western North America, the Himalaya and the Alps. Yet evidence has emerged that such docking is much more complicated (Gün, E. et al. 2021. Pre-collisional extension of microcontinental terranes by a subduction pulleyNature Geoscience, v. 14, online publication; DOI: 10.1038/s41561-021-00746-9).

Erkan Gün and colleagues from the University of Toronto and Istanbul Technical University examined one of the terranes in the Italian Alps – the Sesia-Lanzo Zone (SLZ) – thought to have been a late-Carboniferous microcontinental fragment in the ocean that once separated Africa from Europe. When it accreted the SLZ was forced downwards to depths of up to 70 km and then popped up in the latter stages of the Alpine orogeny. It is now a high-pressure, low-temperature metamorphic complex, having reached eclogite facies during its evolution. Yet its original components, including granites that contain the high-pressure mineral jadeite instead of feldspar, are still recognisable. Decades of geological mapping have revealed that the SLZ sequence shows signs of large-scale extensional tectonics. Clearly that cannot have occurred after its incorporation into southern Europe, and must therefore have taken place prior to its docking. Similar features are present within the accreted microcontinental and island-arc terranes of Eastern Anatolia in Turkey. In fact, most large orogenic belts comprise hosts of accreted terranes that have been amalgamated into older continents.

An ‘engineering’ simplification of the subduction pulley. Different elements represent slab weight (slab pull force) transmitted through a pulley at the trench to a weak microcontinent and a strong oceanic lithosphere. (Credit: Gün et al., Fig. 4)

Lithospheric extension associated with convergent plate margins has been deduced widely in the form of back-arc basins. But these form in the plate being underidden by a subduction zone. Extension of the SLZ, however, must have taken place in the plate destined to be subducted. Gün et al. modelled the forces, lithospheric structure, deformation and tectonic consequences that may have operated to form the SLZ, for a variety of microcontinent sizes. The pull exerted by the subduction of oceanic lithosphere (slab pull) would exert extensional forces on the lithosphere as it approached the destructive plate boundary. Oceanic lithosphere is very strong and would remain intact, simply transmitting slab-pull force to the weaker continental lithosphere, which ultimately would be extended. This is what the authors call a subduction ‘pulley’ system. At some stage the microcontinent fails mechanically, part of it being detached to continue with the now broken slab down the subduction zone. The rest would become a terrane accreted to the overriding plate. Subduction at this site would stop because the linkage to the plate has broken. It may continue by being transferred to a new destructive margin ‘behind’ the accreted microcontinent. This would allow other weak continental and island-arc ‘passengers’ further out on the oceanic plate eventually to undergo much the same process.

The observed complexity of tectonic terranes in other vast assemblies of them, such as the northern Pacific coast of North America and in many more ancient orogenic belts, is probably as much a result of extension before accretion as the compressional deformation suffered afterwards. The theoretical work by Erkan Gün and colleagues will surely spur tectonicians to re-evaluate earlier models of orogenesis.

Note: Figure 2 in the paper by Gün et al. shows how the width (perpendicular to the subduction zone) affects the outcomes of the subduction pulley. View an animation of a subduction pulley

When did supercontinents start forming?

Plate tectonics is easily thought of as being dominated by continental drift, the phenomenon that Alfred Wegener recognised just over a century ago. So it is at present, the major continents being separated by spreading oceans. Yet, being placed on a near-spherical planet, continents also move closer to others; eventually to collide and weld together. Part of Wegener’s concept was that modern continents formed from the breakup of a single large one that he called Pangaea; a supercontinent. The current drifting apart began in earnest around the end of the Triassic Period (~200 Ma), after 200 Ma  of Pangaea’s dominance of the planet along with a single large ocean (Panthalassa) covering 70% of the Earth’s surface. Wegener was able to fit Pangaea together partly on the basis of evidence from the continents’ earlier geological history. In particular the refit joined up zones of intense deformation from continent to continent. Although he did not dwell on their origin, subsequent research has shown these zones were the lines of earlier collisions between older continental blocks, once subduction had removed the intervening oceanic lithosphere; Pangaea had formed from an earlier round of continental drift. Even older collision zones within the pre-Pangaea continental blocks suggested the former existence of previous supercontinents.

Aided by the development of means to divine the position of the magnetic poles relative to differently aged blocks on the continents, Wegener’s basic methods of refitting have resulted in the concept of supercontinent cycles of formation and break-up. It turns out that supercontinents did not form by all earlier continental clanging together at one time. The most likely scenario is that large precursors or ‘megacontinents’ (Eurasia is the current example) formed first, to which lesser entities eventually accreted  A summary of the latest ideas on such global tectonic cycles appeared in the November 2020 issue of Geology (Wang, c. et al. 2020. The role of megacontinents in the supercontinent cycle. Geology, v. 49  p. 402-406; DOI: 10.1130/G47988.1). Chong Wang of the Chinese Academy of Sciences and colleagues from Finland and Canada identify three such cycles of megacontinent formation and the accretion around them of the all-inclusive supercontinents of Columbia, Rodinia and Pangaea since about 1750 Ma (Mesoproterozoic). They also suggestion that a future supercontinent (Amasia) is destined to agglomerate around Eurasia.

Known megacontinents in relation to suggested supercontinents since the Mesoproterozoic (credit: Wang et al.; Fig 2)

The further back in time, the more cryptic are ancient continent-continent collision zone or sutures largely because they have been re-deformed long after they formed. In some cases younger events that involved heating have reset their radiometric ages. The oldest evidence of crustal deformation lies in cratons, where the most productive source of evidence for clumping of older continental masses is the use of palaeomagnetic pole positions. This is not feasible for the dominant metamorphic rocks of old suture zones, but palaeomagnetic measurements from old rocks that have been neither deformed nor metamorphosed offer the possibility of teasing out ancient supercontinents. Commonly cratons show signs of having been affected by brittle extensional deformation, most obviously as swarms of vertical sheets or dykes of often basaltic igneous rocks. Dykes can be dated readily and do yield reliable palaeomagnetic pole positions. Some cratons have multiple dyke swarms. For example the Archaean Yilgarn  Craton of Western Australia, founded on metamorphic and plutonic igneous crust that formed by tectonic accretion between 3.8 to 2.7 Ga, has five of them spanning 1.4 billion years from late-Archaean (2.6 Ga) to Mesoproterozoic (1.2 Ga). Throughout that immense span of time the Yilgarn remained as a single continental block. Also, structural trends end abrubtly at the craton margins, suggesting that it was once part of a larger ‘supercraton’ subsequently pulled apart by extensional tectonics.  The eleven known cratons show roughly the same features.

On the strength of new, high quality pole positions from dykes of about the same ages (2.62 and 2.41 Ga) cutting the Yilgarn and Zimbabwe cratons, geoscientists from Australia, China, Germany, Russia and Finland, based at Curtin University in Western Australia, have attempted to analyse all existing Archaean and Palaeoproterozoic pole positions (Liu, Y. et al. 2021. Archean geodynamics: Ephemeral supercontinents or long-lived supercratons. Geology, v. 49  ; DOI: 10.1130/G48575.1). The Zimbabwe and Yilgarn cratons, though now very far apart, were part of the same supercraton from at least 2.6 Ga ago. Good cases can be made for several other such large entities, but attempting fit them all together as supercontinents by modelling is unconvincing. The modelled fit for the 2.6 Ga datum is very unlike that for 2.4 Ga; in the intervening 200 Ma all the component cratons ould have had to shuffle around dramatically, without the whole supercontinent edifice breaking apart. However, using the data to fit cratons together at two supercratons does seem to work, for the two assemblies remain in the same configurations for both the 2.6 and 2.4 Ga data.

Interestingly, all cratonic components of one of the supercratons show geological evidence of the major 2.4 Ga glaciation, whereas those of the other show no such climatic indicator. Yet the entity with glacial evidence was positioned at low latitudes around 2.4 Ga, the ice-free one spanning mid latitudes. This may imply that the Earth’s axial tilt was far higher than at present. The persistence of two similar sized continental masses for at least 200 Ma around the end of the Archaean Eon also hints at a different style of tectonics from that with which geologists are familiar. Only palaeomagnetic data from the pre 2.6 Ga Archaean can throw light on that possibility. That requires older, very lightly or unmetamorphosed rocks to provide palaeopole positions. Only two cratons, the Pilbara of Western Australia and the Kaapvaal of South Africa, are suitable. The first yielded the oldest-known pole dated at 3.2 Ga, the oldest from the second is 2.7 Ga. A range of evidence suggests that Pilbara and Kaapvaal cratons were united during at least the late Archaean.

The only answer to the question posed by this item’s title is ‘There probably wasn’t a single supercontinent at the end of the Archaean, but maybe two megacontinents or supercratons’. Lumps of continental lithosphere would move and – given time – collide once more than one lump existed, however the Earth’s tectonics operated …