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.

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.

How does plate tectonics work?

Well, surely we ought to know, 52 years after W. Jason Morgan proposed that the Earth’s surface consists of 12 rigid plates that move relative to each other. But that is not completely true, although most of its mechanisms expressed by external and internal Earth processes are known in great detail. It is still a ‘chicken and egg’ issue: do convective motions in the mantle drive the superficial plates around by dragging at the base of the lithosphere or is it the subduction of plates and slab-pull force that result in overturn of the mantle? Nicolas Coltice of the University of Paris and colleagues from those of Grenoble, Rome and Texas consider that posing plate tectonics in such a manner is an abstraction; rather like the plot for a novel that is yet to be written (Coltice, N. et al. 2019. What drives tectonic plates? Science Advances, v. 5, online eaax4295; DOI: 10.1126/sciadv.aax4295). Instead, all the solid Earth’s vagaries and motions have to be considered as an indivisible whole rather than the traditional piecemeal approach of focussing on the forces that act on the interfaces between plates.

Their approach is to model a combination of mechanisms throughout the Earth as a single, evolving three-dimensional system without the constraint of perfectly rigid plates, which of course they are not. The physical parameters boil down to those involved in relative buoyancy, viscosity, and gradients of temperature, pressure and gravitational potential energy within a spherical planet. Designing the algorithms and running the model on a supercomputer took 9 months to reconstruct the evolution of the planet over 1.5 billion years.

4-whatmakesthe
Still from a movie of simulated breakup of a supercontinent, in bland blue-grey, showing what happens at the surface (left) and, at the same time, in the mantle (right): note the influence of rising plumes (credit: Nicolas Coltice)

The result is a remarkable series of unfolding scenarios. In them, 2/3 of the planet’s surface moves faster than does the underlying mantle, suggesting that the surface is dragging the interior. For the remainder, mantle motions exceed those of the surface. Continents are dragged by the underlying mantle to aggregate in supercontinents, which in turn are torn apart by the sinking of cold oceanic slabs. The model takes on a highly visual form, showing in 3-D, for instance: ocean closure and supercontinent assembly; and example of continental breakup; how subduction is initiated.

It will be fascinating to see the reaction of the authors’ peers to their venture, and the extent to which the technicalities of the paper are translated into a form that is suitable for teaching. My suspicion is that most Earth scientists will be happy to stay with the old conceptions until the latter is achieved, and laptops are able to run the model(!)

Metamorphic evidence of plate tectonic evolution

The essence of plate tectonics that dominates the Earth system today is the existence of subduction zones that carry old, cold oceanic lithosphere to great depths where they become denser by the conversion of the mineralogy of hydrated basalt to near-anhydrous eclogite. Such gravitational sinking imparts slab-pull force that is the largest contributor to surface plate motions. Unequivocally demonstrating the action of past plate tectonics is achieved from the striped magnetic patterns above yet-to-be-subducted oceanic lithosphere, the oldest being above the Jurassic remnant of the West Pacific. Beyond that geoscientists depend on a wide range of secondary evidence that suggest the drifting and collision of continents and island arcs, backed up by palaeomagnetic pole positions for various terranes that give some idea of the directions and magnitudes of horizontal motions.

Occasionally – the more so further back in time – metamorphic rocks (eclogites and blueschists) are found in linear belts at the surface, which show clear signs of low-temperature, high pressure metamorphism that created the density contrast necessary for subduction. Where such low T/P belts are paired with those in which the effects of high T/P metamorphism occurred they suggest distinctly different geothermal conditions: low T/P associated with the site of subduction of cold rock; high T/P with a zone of magmagenesis – at island- or continental arcs – induced by crustal thickening and flux of volatiles above deeper subduction. Such evidence of geothermal polarity suggests a destructive plate margin and also the direction of relative plate motions. The oldest known eclogites (~2.1 Ga) occur in the Democratic Republic of the Congo, but do they indicate the start of modern-style plate tectonics?

Interestingly, ‘data mining’ and the use of statistic may provide another approach to this question. Determination of the temperatures and pressures at which metamorphic rocks formed using the mineral assemblages in them and the partitioning of elements between various mineral pairs has built up a large database that spans the last 4 billion years of Earth history. Plotting each sample’s recorded pressure against temperature shows the T/P conditions relative to the thermal gradients under which their metamorphism took place. Robert Holder of Johns Hopkins University and colleagues from the USA, Australia and China used 564 such points to investigate the duration of paired metamorphism (Holder, R.M. et al. 2019. Metamorphism and the evolution of plate tectonics. Nature, v. 572, p. 378–381; DOI: 10.1038/s41586-019-1462-2).

The 109 samples from Jurassic and younger metamorphosed terranes that demonstrably formed in arc- and subduction settings form a benchmark against which samples from times devoid of primary evidence for tectonic style can be judged. The post-200 Ma data show a clear bimodal distribution in a histogram plot of frequency against thermal gradient, with peaks either side of a thermal gradient of 500°C GPa-1 (~17°C km-1); what one would expect for paired metamorphic belts. A simple bell-shaped or Gaussian distribution of temperatures would be expected from metamorphism under a similar geothermal gradient irrespective of tectonic setting.

Metc PvT
Pressure-temperature data from Jurassic and younger metamorphic rocks (a) pressure vs temperature plot; (b) Frequency distribution vs log thermal gradient. (Credit: Holder et al. 2019, Fig. 1)

Applying this approach to metamorphic rocks dated between 200 to 850 Ma; 850 to 1400 Ma; 1400 to 2200 Ma, and those older than 2200 Ma, Holder and colleagues found that the degree of bimodality decreased with age. Before 2200 Ma barely any samples fell outside a Gaussian distribution. Also, the average T/P of metamorphism decreased from the Palaeoproterozoic to the present. They interpret the trend towards increased bimodality and decreasing average T/P as an indicator that the Earth’s modern plate-tectonic regime has developed gradually since the end of the Archaean Eon (2500 Ma). Their findings also tally with the 2.1 Ga age of the oldest eclogites in the DRC.

Plate tectonics is primarily defined as the interaction between slabs of lithosphere that are rigid and brittle and move laterally above the ductile asthenosphere. Their motion rests metaphorically on the principle that ‘what comes up’ – mantle-derived magma – ‘must go down’ in the form of displaced older material that the mantle resorbs. That is more likely to be oceanic lithosphere whose bulk density is greater than that supporting the thick, low-density continental crust. Without the steeper subduction and slab pull conferred by the transformation of hydrated basalt to much denser eclogite, subduction would not result in low T/P metamorphism paired with that resulting from high T/P conditions in magmatic arcs. But, while ever lithosphere was rigid and brittle, plate tectonics would operate, albeit in forms different from that which formned terranes younger than the Jurassic

The effect of surface processes on tectonics

Active sedimentation in the Indus and Upper Ganges plains (green vegetated) derived from rapid erosion of the Himalaya (credit: Google Earth)

The Proterozoic Eon of the Precambrian is subdivided into the Palaeo-, Meso- and Neoproterozoic Eras that are, respectively, 900, 600 and 450 Ma long. The degree to which geoscientists are sufficiently interested in rocks within such time spans is roughly proportional to the number of publications whose title includes their name. Searching the ISI Web of Knowledge using this parameter yields 2000, 840 and 2700 hits in the last two complete decades, that is 2.2, 1.4 and 6.0 hits per million years, respectively. Clearly there is less interest in the early part of the Proterozoic. Perhaps that is due to there being smaller areas over which they are exposed, or maybe simply because what those rocks show is inherently less interesting than those of the Neoproterozoic. The Neoproterozoic is stuffed with fascinating topics: the appearance of large-bodied life forms; three Snowball Earth episodes; and a great deal of tectonic activity, including the Pan-African orogeny. The time that precedes it isn’t so gripping: it is widely known as the ‘boring billion’ – coined by the late Martin Brazier – from about 1.75 to 0.75 Ga. The Palaeoproterozoic draws attention by encompassing the ‘Great Oxygenation Event’ around 2.4 Ga, the massive deposition of banded iron formations up to 1.8 Ga, its own Snowball Earth, emergence of the eukaryotes and several orogenies. The Mesoproterozoic witnesses one orogeny, the formation of a supercontinent (Rodinia) and even has its own petroleum potential (93 billion barrels in place in Australia’s Beetaloo Basin. So it does have its high points, but not a lot. Although data are more scanty than for the Phanerozoic Eon, during the Mesoproterozoic the Earth’s magnetic field was much steadier than in later times. That suggests that motions in the core were in a ‘steady state’, and possibly in the mantle as well. The latter is borne out by the lower pace of tectonics in the Mesoproterozoic. Continue reading “The effect of surface processes on tectonics”

How does subduction start?

Robert Stern of the University of Texas at Dallas, USA, and Taras Gerya of ETH, Zurich, have produced a masterly review of how subduction gets started from place to place, and from time to time in geological history (Stern, R.J. & Gerya, T. 2018. Subduction initiation in nature and models: A review. Tectonophysics, v. 744 (in press); (PDF). It is the foundering of oceanic lithosphere into the mantle and gravity that give modern plate tectonics the bulk of energy that drives it along by slab pull. Yet the mantle’s consumption of a lithospheric slab somehow has to be set in motion from the symmetrical spreading of ocean floor as occurs either side of a constructive margin. It could not happen were the lithosphere to retain its low bulk density relative to mantle peridotite for all time. Moreover, it wouldn’t last for long were the lithosphere not to retain its strength through hundreds of kilometres depth as it sinks into the mantle. Active subduction zones have consumed vast amounts of oceanic lithosphere, for more than 65 million years, especially in fast-spreading ocean basins such as the western and eastern Pacific. The record is held by the destructive margin on the west flank of South America where more than 150 million years-worth of eastern Pacific lithosphere has been swallowed. Yet in order for oceanic lithosphere, which is stronger than that beneath the continents, somehow to fail and begin to sink a linear weak zone must develop at the interface between two incipient new plates. On top of that, all subduction on Earth is one-sided. A simple mechanism involving just thermal convection predicts that both plates either side of a break would have similar density so both should sink, more or less symmetrically.

subduction types
Various ways in which subduction may start. (Credit: Stern and Gerya 2018 – in press – Figure 4)

Geophysical observations reveal that terrestrial subduction can be divided into that which is induced by plate motions and changes in force balance within spreading plates, or spontaneously due to unique conditions developing along the line of initiation. In the first class are cases where a microcontinent is driven into another continental margin and extinguishes the subduction responsible, while spreading continues behind the accreted microcontinent drive older lithosphere beneath the suture (this may have happened in the past but is not seen today). Another, similar, induced case occurs where an oceanic island arc accretes by subduction beneath it so that subduction flips in polarity to consume the driving sea-floor spreading. The loading of oceanic lithosphere by sediments piled onto it by erosion of a continental margin may spontaneously collapse to result in subduction beneath the sedimentary wedge and the continent (again, not happening today, but inferred from examples inferred by earlier geological history). Spontaneous failure may also occur where old, cold lithosphere is juxtaposed with younger by transform faulting, or where a mantle plume heats up lithosphere to create a thermally weakened zone.

Stern and Gerya do not leave the issue at simple mechanics but discuss how plates may develop weak zones or inherit them from earlier tectonic events. The role of water released by metamorphism of descending materials may encourage the observed one-sidedness of subduction by reducing frictional resistance and plate strength and make the process self-sustaining. The paper also discusses the various permutations and combinations that affect the style of induced destructive margins in compressional and extensional environments and a whole variety of nuanced cases of spontaneous initiation. Numerical modelling of the subduction process plays an important, though somewhat bewildering role in discussion, as do considerations of the forces likely to be at play. Applying theoretical considerations to actual examples from the geological record are sublimely enlivening, as are speculations about the future evolution of the passive margins of the Atlantic. Clearly, there is a healthy future for field and mathematical study on the processes at destructive plate margins, such as building in the aspects of magmagenesis. Since Stern has built his career on study of long dead collusions zones, products of arc accretion etcetera, development of their understanding is undoubtedly the main thrust of his and Gerya’s tour de force. Stern provides a full PDF at his University of Texas website for the benefit of anyone who wants to delve deeper than space at Earth-pages and my limited intellect permit!

A fully revised edition of Steve Drury’s book Stepping Stones: The Making of Our Home World can now be downloaded as a free eBook

Snowball Earth: A result of global tectonic change?

The Snowball Earth hypothesis first arose when Antarctic explorer Douglas Mawson (1882-1958)speculated towards the end of his career on an episode of global glaciations, based on his recognition in South Australia of thick Neoproterozoic glacial sediments. Further discoveries on every continent, together with precise dating and palaeomagnetic indications of the latitude at which they were laid down, have steadily concretised Mawson’s musings. It is now generally accepted that frigid conditions enveloped the globe at least twice – the Sturtian (~715 to 660 Ma) and Marinoan (650 to 635 Ma) glacial episodes – and perhaps more often during the Neoproterozoic Era. Such an astonishing idea has spurred intensive studies of geochemistry associated with the events, which showed rapid variations in carbon isotopes in ancient seawater, linked to the terrestrial carbon cycle that involves both life- and Earth processes. Strontium isotopes suggest that the Neoproterozoic launched erratic variation of continental erosion and weathering and related carbon sequestration that underpinned major climate changes in the succeeding Phanerozoic Eon. Increased marine phosphorus deposition and a change in sulfur isotopes indicate substantial change in the role of oxygen in seawater. The preceding part of the Proterozoic Eon is relatively featureless in most respects and is known to some geoscientists as the ‘Boring Billion’.

Untitled-1
Artist’s impression of the glacial maximum of a Snowball Earth event (Source: NASA)

Noted tectonician Robert Stern and his colleague Nathan Miller, both of the University of Texas, USA, have produced a well- argued and -documented case (and probably cause for controversy) that suggests a fundamental change in the way the Precambrian Earth worked at the outset of the Neoproterozoic (Stern, R.J. & Miller, N.R. 2018. Did the transition to plate tectonics cause Neoproterozoic Snowball Earth. Terra Nova, v. 30, p. 87-94). To the geochemical and climatic changes they have added evidence from a host of upheavals in tectonics. Ophiolites and high-pressure, low-temperature metamorphic rocks, including those produced deep in the mantle, are direct indicators of plate tectonics and subduction. Both make their first, uncontested appearance in the Neoproterozoic. Stern and Miller ask the obvious question; Was this the start of plate tectonics? Most geologists would put this back to at least the end of the Archaean Eon (2,500 Ma) and some much earlier, hence the likelihood of some dispute with their views.

They consider the quiescent billion years (1,800 to 800 Ma) before all this upheaval to be evidence of a period of stagnant ‘lid tectonics’, despite the Rodinia supercontinent having been assembled in the latter part of the ‘Boring Billion’, although little convincing evidence has emerged to suggest it was an entity formed by plate tectonics driven by subduction. But how could the onset of subduction-driven tectonics have triggered Snowball Earth? An early explanation was that the Earth’s spin axis was much more tilted in the Neoproterozoic than it is at present (~23°). High obliquity could lead to extreme variability of seasons, particularly in the tropics. A major shift in axial tilt requires a redistribution of mass within a planetary body, leading to true polar wander, as opposed to the apparent polar wander that results from continental drift. There is evidence for such an episode around the time of Rodinia break-up at 800 Ma that others have suggested stemmed from the formation of a mantle superplume beneath the supercontinent.

Considering seventeen possible geodynamic, oceanographic and biotic causes that have been plausibly suggested for global glaciation Stern and Miller link all but one to a Neoproterozoic transition from lid- to plate tectonics. Readers may wish to examine the authors’ reasoning to make up their own minds –  their paper is available for free download as a PDF from the publishers.

A fully revised edition of Steve Drury’s book Stepping Stones: The Making of Our Home World can now be downloaded as a free eBook

Hot-spot track beneath the Greenland ice cap

Around 63 Ma ago, during the Palaeocene Epoch, major igneous activity broke out in what are now both sides of the North Atlantic Ocean. After initial sputtering it culminated massively between 57 and 53 Ma. Relics are to be seen in Baffin Island, West and East Greenland, the Faeroes and north-western parts of the British Islands, in the form of flood basalts, dyke swarms and scattered remnants of central volcanoes. Offshore drilling on the North Atlantic’s continental shelves suggests that the volcanism extended over 1.3 million km2 and blurted out around 6.6 million km3 of magma. Not for nothing have the products of this event been categorised as a Large Igneous Province. Its formation took place before the North Atlantic existed. It began to form as this precursor magmatic paroxysm waned.  Continued basaltic magma production created the ocean floor each side of the mid-Atlantic Ridge system to divide North America and Greenland from northern Europe. Sea floor spreading continues, rising above sea level in Iceland, which is underlain by a large mantle plume.

The plume beneath Iceland may have been present at a fixed position in the mantle for tens of million years. A hot spot over which plate movements have shifted lithosphere to be heated in a similar way to a sheet of paper dragged slowly over a candle flame. The Iceland plume may have left a hot-spot track similar to that involved in the Hawaiian island chain. The ocean floor to the east and west of Iceland is shallower and forms broad rides at right angles to the trend of the Mid-Atlantic Ridge system, judged to be such tracks that are still warm and buoyant after formation over the plume. But are there traces of earlier passage of drifting lithosphere over the plume. A way to detect older hot-spot tracks is through variations in geothermal heat flow through the continental surface, a linear pattern raising suspicions of such trace of passage. There is no sign to the east beneath Europe, so what about to the west. Greenland, being mainly blanketed in ice, is not a good place to conduct such a search as it would involve deep drilling through the ice at huge cost for each hole. But there is a roundabout way of obtaining geothermal information without even setting foot on Greenland’s icy wastes.

The geomagnetic field measured at the surface records anomalies in rock magnetisation in the solid Earth beneath. Near-surface variations due to large variations in rock types that comprise the continental crust appear as sharp, high frequency signals. Aeromagnetic surveys over Greenland are characterised by such noisy patterns because the subsurface geology is extremely complicated. However, the underlying upper mantle beneath all continents is geologically quite bland, but being uniformly rich in iron it contains a high proportion of magnetic minerals such as magnetite (Fe3O4). The upper mantle should therefore leave a signal in the surface geomagnetic field, albeit a commensurately bland one. Like radio signals that span a large range of wavelengths, Earth properties that vary spatially, such as the geomagnetic field, may be analysed using filters. Once the high-frequency geomagnetic features of the crust are filtered out what should remain is a signal that reflects the magnetic structure of the upper mantle. It should be more or less featureless, yet beneath Greenland it isn’t.

greenland hot spot
Estimated Curie depth variation below Greenland (left) converted to geothermal heat flow variation (right). (Credit: Martos et al. 2018; Figures 1b and 1c)

Magnetic anomalies are created by magnetisation induced in magnetic minerals in rocks by the Earth’s magnetic field. Yet minerals lose their ability to be magnetised at temperatures above a threshold known as the Curie point, which is 580 °C for magnetite, the most abundant magnetic mineral. Depending on the geothermal heat flow the Curie point is exceeded at some depth in the lithosphere. So magnetic anomalies can safely be assumed to be produced only by rocks above the so-called Curie depth. Yasmina Martos of the British Antarctic Survey (now at the University of Maryland) and scientists from Britain, the US and Spain used a complex procedure, including gravity data and a few direct measurements of heat flow below Greenland as well as filtered aeromagnetic data, to estimate the variation in Curie depth beneath the ice cap. (Martos, Y.M. et al. 2018. Geothermal heat flux reveals the Iceland hotspot track underneath Greenland. Geophysical Research Letters, v. 45, online publication; doi: 10.1029/2018GL078289). Using that as an inverse proxy for heat flow they were able to map the likely geothermal variation beneath the island. Rather than a random and narrow variation in depth, as would be expected for roughly uniform heat flow, the Curie depth varied in a non-random way by over 20 km, equivalent to roughly 20 mW m-2.

The shallowest Curie depth and highest estimated heat flow occurs in East Greenland around Scoresby Sund where the largest sequence of Palaeocene flood basalts occur. It is also on a line perpendicular to the mid-Atlantic Rift system that meets the active Iceland plume. Running north-west from Scoresby Sund is a zone of locally high estimated heat flow. Martos et al. suggest that this is the track of Greenland’s motion over the Iceland hot spot from about 80 Ma to the period of maximum on-shore volcanism and the start of sea-floor spreading at around 50 Ma.

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Recycling of continental crust through time

Because continental crust is so light – an average density of 2700 kg m-3 compared with the mantles’ value of 3300 – it has been widely believed that continents cannot be subducted en masse. Yet it is conceivable that sial can be ‘shaved’ from below during subduction and from above by erosion and added to subductable sediment on the ocean floor. Certainly, there is overwhelming evidence for the net growth of continents through time and plenty for periods of increased and dwindling growth in the past. In some ancient orogens there are substantial slabs of continental composition whose mineralogy bears witness to ultra-high pressure metamorphism at depths greater than that of the base of continents. These slabs had been caught-up in subduction but never reached sufficiently high density to be retained by the mantle; they eventually ‘bobbed up’ again. On the other hand, if early continents were less silica rich through incorporation of substantial proportions of rock with basaltic composition parts of them could founder if subjected to high-pressure, low-temperature metamorphism. But not all crustal recycling to the mantle is through subduction. Some abnormally highly elevated parts of the continents that rose quickly in geological terms, such as the Tibetan Plateau, may have formed by lower crustal slabs becoming detached or delaminated from their base. Again modelling can help assess the past magnitude of continental recycling (Chowdhury, P. et al. 2017. Emergence of silicic continents as the lower crust peels off on a hot plate-tectonic Earth. Nature Geoscience, v. 10, p. 698-703; DOI: 10.1038/NGEO3010).

Various lines of evidence suggest that between 65 to 70% of the present continental volume existed by 3 billion years ago, yet that does not manifest itself in the rock record; perhaps a sign that some has returned to the mantle. It is also widely suggested that plate tectonics in the modern style began at about that time. Pryadarshi Chowdhury and colleagues simulate what may happen at depth in continent-continent collision zones – the classic site of orogenies –at different times in the past. Under the hotter conditions in the early Archaean mantle delamination would have been more likely than it has been during the Phanerozoic; i.e. the peeling off and sinking of the denser, more mafic lower crust and the attached upper mantle. The authors show that increased mantle temperature further back in time increases the likelihood and extent of such delamination. It also encourages partial melting of the descending continental material so creating rising bodies of more silicic magma that add to the remaining continent at the surface. Together with the lower crust’s attachment of to a mantle slab, this ensures that the peeled off material is able to descend under its own load. Once below a depth of 250 km felsic rocks are doomed to further descent. Waning of radiogenic mantle heat production encourages descending slabs to fail and break from the connection with lithosphere at higher levels so that a smaller proportion of the lower crust becomes detached and recycled. This evolution suggests that less and less continental crust is recycled with time. This broadly fits with current geochemical ideas based on the record of radiogenic Nd-, Sr- and Pb-isotopes in rocks ranging from early Archaean to Phanerozoic age.