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 …

The oldest known impact structure (?)

That large, rocky bodies in the Solar System were heavily bombarded by asteroidal debris at the end of the Hadean Eon (between 4.1 to 3.8 billion years ago) is apparent from the ancient cratering records that they still preserve and their matching with dating of impact-melt rocks on the Moon. Being a geologically dynamic planet, the Earth preserves no tangible, indisputable evidence for this Late Heavy Bombardment (LHB), and until quite recently could only be inferred to have been battered in this way. That it actually did happen emerged from a study of tungsten isotopes in early Archaean gneisses from Labrador, Canada (see: Tungsten and Archaean heavy bombardment, August 2002; and Did mantle chemistry change after the late heavy bombardment? September 2009). Because large impacts deliver such vast amounts of energy in little more than a second (see: Graveyard for asteroids and comets, Chapter 10 in Stepping Stones) they have powerful consequences for the Earth System, as witness the Chicxulub impact off the Yucatán Peninsula of Mexico that resulted in a mass extinction at the end of the Cretaceous Period. That seemingly unique coincidence of a large impact with devastation of Earth’s ecosystems seems likely to have resulted from the geology beneath the impact; dominated by thick evaporite beds of calcium sulfate whose extreme heating would have released vast amounts of SO2 to the atmosphere. Its fall-out as acid rain would have dramatically affected marine organisms with carbonate shells. Impacts on land would tend to expend most of their energy throughout the lithosphere, resulting in partial melting of the crust or the upper mantle in the case of the largest such events.

The further back in time, the greater the difficulty in recognising visible signs of impacts because of erosion or later deformation of the lithosphere. With a single, possible exception, every known terrestrial crater or structure that may plausibly be explained by impact is younger than 2.5 billion years; i.e. they are post-Archaean. Yet rocky bodies in the Solar System reveal that after the LHB the frequency and magnitude of impacts steadily decreased from high levels during the Archaean; there must have been impacts on Earth during that Eon and some may have been extremely large. In the least deformed Archaean sedimentary sequences there is indirect evidence that they did occur, in the form of spherules that represent droplets of silicate melts (see: Evidence builds for major impacts in Early Archaean; August 2002, and Impacts in the early Archaean; April 2014), some of which contain unearthly proportions of different chromium isotopes (see: Chromium isotopes and Archaean impacts; March 2003). As regards the search for very ancient impacts, rocks of Archaean age form a very small proportion of the Earth’s continental surface, the bulk having been buried by younger rocks. Of those that we can examine most have been subject to immense deformation, often repeatedly during later times.

The Archaean geology of part of the Akia Terrane (Manitsoq area) in West Greenland. The suggested impact structure is centred on the Finnefjeld Gneiss (V symbols) surrounded by highly deformed ultramafic to mafic igneous rocks. (Credit: Jochen Kolb, Karlsruhe Institute of Technology, Germany)

There is, however, one possibly surviving impact structure from Archaean times, and oddly it became suspected in one of the most structurally complex areas on Earth; the Akia Terrane of West Greenland. Aeromagnetic surveys hint at two concentric, circular anomalies centred on a 3.0 billion years-old zone of grey gneisses (see figure) defining a cryptic structure. It is is surrounded by hugely deformed bodies of ultramafic and mafic rocks (black) and nickel mineralisation (red). In 2012 the whole complex was suggested to be a relic of a major impact of that age, the ultramafic-mafic bodied being ascribed to high degrees of impact-induced melting of the underlying mantle. The original proposers backed up their suggestion with several associated geological observations, the most crucial being supposed evidence for shock-deformation of mineral grains and anomalous concentrations of platinum-group metals (PGM).

A multinational team of geoscientists have subjected the area to detailed field surveys, radiometric dating, oxygen-isotope analysis and electron microscopy of mineral grains to test this hypothesis (Yakymchuck, C. and 8 others 2020. Stirred not shaken; critical evaluation of a proposed Archean meteorite impact in West Greenland. Earth and Planetary Science Letters, v. 557, article 116730 (advance online publication); DOI: 10.1016/j.epsl.2020.116730). Tectonic fabrics in the mafic and ultramafic rocks are clearly older than the 3.0 Ga gneisses at the centre of the structure. Electron microscopy of ~5500 zircon grains show not a single example of parallel twinning associated with intense shock. Oxygen isotopes in 30 zircon grains fail to confirm the original proposers’ claims that the whole area has undergone hydrothermal metamorphism as a result of an impact. All that remains of the original suggestion are the nickel deposits that do contain high PGM concentrations; not an uncommon feature of Ni mineralisation associated with mafic-ultramafic intrusions, indeed much of the world’s supply of platinoid metals is mined from such bodies. Even if there had been an impact in the area, three phases of later ductile deformation that account for the bizarre shapes of these igneous bodies would render it impossible to detect convincingly.

The new study convincingly refutes the original impact proposal. The title of Yakymchuck et al.’s paper aptly uses Ian Fleming’s recipe for James Bond’s tipple of choice; multiple deformation of the deep crust does indeed stir it by ductile processes, while an impact is definitely just a big shake. For the southern part of the complex (Toqqusap Nunaa), tectonic stirring was amply demonstrated in 1957 by Asger Berthelsen of the Greenland Geological Survey (Berthelsen, A. 1957. The structural evolution of an ultra- and polymetamorphic gneiss-complex, West Greenland. Geologische Rundschau, v. 46, p. 173-185; DOI: 10.1007/BF01802892). Coming across his paper in the early 60s I was astonished by the complexity that Berthelsen had discovered, which convinced me to emulate his work on the Lewisian Gneiss Complex of the Inner Hebrides, Scotland. I was unable to match his efforts. The Akia Terrane has probably the most complicated geology anywhere on our planet; the original proposers of an impact there should have known better …

Weak lithosphere delayed the formation of continents

There are very few tangible signs that the Earth had continents at the surface before about 4 billion years (Ga) ago. The most cited evidence that they may have existed in the Hadean Eon are zircon grains with radiometric ages up to 4.4 Ga that were recovered from much younger sedimentary rocks in Western Australia. These tiny grains also show isotopic anomalies that support the existence of continental material, i.e. rocks of broadly granitic composition, only 100 Ma after the Earth formed (see: Zircons and early continents no longer to be sneezed at; February 2006). So, how come relics of such early continents have yet to be discovered in the geological record? After all granitic rocks – in the broad sense – which form continents are so less dense than the mantle that modern subduction is incapable of recycling them en masse. Indeed, mantle convection of any type in the hotter Earth of the Hadean seems unlikely to have swallowed continents once they had formed. Perhaps they are hiding in another guise among younger rocks of the continental crust. But, believe me; geologists have been hunting for them, to no avail, in every scrap of existing continental crust since 1971 when gneisses found in West Greenland by New Zealander Vic McGregor turned out to be almost 3.8 Ga old. This set off a grail-quest, which still continues, to negate James Hutton’s ‘No vestige of a beginning …’ concept of geological time.

There is another view. Early continental lithosphere may have returned to the mantle piece by piece by other means. One that has been happening since the Archaean is as debris from surface erosion and its transportation to the ocean floor, thence to be subducted along with denser material of the oceanic lithosphere. Another possibility is that before 4 Ga continental lithosphere had far less strength than characterised it in later times; it may have been continually torn into fragments small enough for viscous drag to defy buoyancy and consign them into the mantle by convective processes. Two things seem to confer strength on continental lithosphere younger than 4 billion years: its depleted surface heat flow and heat-production that stem from low concentrations of radioactive isotopes of uranium, thorium and potassium in the lower crust and sub-continental mantle; bolstering by cratons that form the cores of all major continents. Three geoscientists at Monash University in Victoria, Australia have examined how parts of early convecting mantle may have undergone chemical and thermal differentiation (Capitanio, F.A. et al. 2020. Thermochemical lithosphere differentiation and the origin of cratonic mantle.  Nature, v. 588, p. 89-94; DOI: 10.1038/s41586-020-2976-3). These processes are an inevitable outcome of the tendency for mantle melting to begin as it becomes decompressed when pressure decreases when it rises during convection. Continual removal of the magmas produced in this way would remove not only much of the residue’s heat-producing capacity – U, Th and K preferentially enter silicate melts – but also its content of volatiles, especially water. Even if granitic magmas were completely recycled back to the mantle by the greater vigour of the hot, early Earth, at least some of the residue of partial melting would remain. Its dehydration would increase its viscosity (strength). Over time this would build what eventually became the highly viscous thick mantle roots (tectosphere) on which increasing amounts of the granitic magmas could stabilise to establish the oldest cratons. Over time more and more such cratonised crust would accumulate, becoming increasingly unlikely to be resorbed into the mantle. Although cratons are not zoned in terms of the age of their constituent rocks, they do jumble together several billion years’ worth of continental crust in what used to be called ‘the Basement Complex’.

Development of depleted and viscous sub-continental mantle on the early Earth – a precedes b – TTG signifies tonalite-trondhjemite-granodiorite rocks typical of Archaean cratons (Credit, Capitanio et al.; Fig 5)

Early in this process, heat would have made much of the lithosphere too weak to form rigid plates and the tectonics with which geologists are so familiar from the later parts of Earth’s history. The evolution that Capitanio et al. propose suggests that the earliest rigid plates were capped by Archaean continental crust. That implies subduction of oceanic lithosphere starting at their margins, with intra-oceanic destructive plate margins and island arcs being a later feature of tectonics. It is in the later, Proterozoic Eon that evidence for accretion of arc terranes becomes obvious, plastering their magmatic products onto cratons, further enlarging the continents.

Kerguelen Plateau: a long-lived large igneous province

It’s easy to think of the Earth’s largest outpourings of lava as being restricted to the continents; continental flood basalts with their spectacular stepped topography made up of hundreds of individual massive flows and intervening soil horizons. The Deccan Traps of western India are the epitome, having been so named by natural scientists of the late 18th century from the Swedish word for ‘stairs’ (trappa). Examples go back to the Proterozoic Era, younger ones still retaining much of their original form as huge plateaus. All began life within individual tectonic plates, although some presaged continental break-up and the formation of new oceanic spreading centres. They must have been spectacular events, up to millions of cubic kilometres of magma belched out in a few million years. They have been explained as manifestations of plumes of hot mantle rock rising from as deep as the core-mantle boundary. Unsurprisingly, the biggest continental flood-basalt outpourings coincided with mass extinction events. Otherwise known as large igneous provinces (LIPs), they are not the only signs of truly huge production of magma by partial melting in the mantle. The biggest LIP, with an estimated volume of 80 million km3, lies deep beneath the Western Pacific Ocean. To the northeast of New Guinea, the Ontong Java Plateau formed over a period of about 3 Ma in the mid-Cretaceous (~120 Ma) and blanketed one percent of the Earth’s solid surface with lavas erupted at a rate of 22 km3 per year. Possibly because this happened on the Pacific’s abyssal plains beneath around 4 km of sea water, there is little sign of any major perturbation of mid-Cretaceous life, but it is associated with evidence for global oceanic anoxia. Ontong Java isn’t the only oceanic LIP. Bearing in mind that oceanic lithosphere only goes back to the start of the Jurassic Period (200 Ma) – earlier material has largely been subducted – they are not as abundant as continental flood-basalt provinces. One of them is the Kerguelen Plateau 3000 km to the SE of Australia, which is about three times the area of Japan and the second largest LIP of the Phanerozoic Eon. The Plateau was split into two large fragments while sea-floor spreading progressed along the Southeast Indian Ridge.

Bathymetry of the Indian Ocean south-west of Australia, showing the Kerguelen Plateau and South-east Indian Ridge. The red arrows show the amount of sea-floor spreading on either side of the Ridge since it began to open. The pale blue area at the NE end of the arrow was formerly part of the Plateau (credit: Google Earth)

Long regarded as a microcontinental  fragment left when India parted company with Antarctica – based on isolated occurrences of gneisses – there is evidence that during the formation of the Kerguelen LIP the basalts rose above sea level. Because earlier radiometric dating of basalts from ocean-floor drill cores were of low quality, an Australian-Swedish group of geoscientists have re-evaluated those data and supplemented them with 25 new Ar-Ar dates from 12 sites (Jiang, Q. et al. 2020. Longest continuously erupting large igneous province driven by plume-ridge interaction. Geology, v. 48, online; DOI: 10.1130/G47850.1). Rather than a cluster of ages around a short time range as expected from the short life of most other LIPs, those from Kerguelen span 32 Ma during the Cretaceous (from 122 to 90 Ma). The magmatic pulse began at roughly the same time as that of Ontong Java, but continued for much longer. Smaller oceanic LIPs do seem to have lingered for unusually lengthy periods, but all seem to have constructed in several separate pulses. Large-volume eruption at Kerguelen was continuous for at least 32 Ma; the drilling did not penetrate the oldest of the plateau basalts. It seems that the Kerguelen LIP is unique in that respect and requires an explanation other than simply a mantle plume, however large.

Jiang et al. suggest a model of continuous interaction between a long-lived plume and the development of the Southeast Indian Ridge oceanic spreading centre. Their model involves the line of continental splitting between India and Antarctic taking place close to a major deep-mantle plume at around 128 Ma. There is nothing unique about that; incipient ocean rifting in the Horn of Africa and formation of the Red Sea and Gulf of Aden ridges is currently associated with the active Afar plume. This was followed by a kind of tectonic shuffling of the Ridge back and forth across the head of the Kerguelen plume: not far different from the Palaeogene North Atlantic LIP, where the mid-Atlantic Ridge and the still-active Iceland plume, except the ridge and plume seem more intimately involved there. However, there are probably many subtle relationships between plumes and various kind of oceanic plate margins that are still worth exploring. Since the first discovery of mantle plumes as an explanation for volcanic island chains (e.g. the Hawaiian chain) where volcanism becomes progressively older in the direction of plate movement, there is still much to discover.

See also: Magma .conveyor belt’ fuelled world’s longest erupting supervolcanoes (Science Daily, 4 November 2020)