Changing conditions of metamorphism since the Archaean

Metamorphic petrologists have known since their branch of geology emerged that the intensity or ‘grade’ of metamorphism varies with position in an orogenic belt. This is easily visualised by the sequence mudstone-shale-slate-phyllite-schist-gneiss that results from a clay-rich starting material as metamorphic grade increases. Very roughly speaking, the sequence reflects burial, heat and pressure, and must have been controlled by temperature increasing with depth and pressure: the geothermal gradient. In turn, that depends on internal heat production, geothermal heat flow and the way in which heat is transferred through the deep crust: by thermal conduction or mechanical convection. A particular rock composition gives rise to different metamorphic mineral assemblages under different temperature and pressure conditions.

George Barrow was the first to recognise this in the Southern Highlands of Scotland as a series of zones marked by different index minerals. For instance, in once clay-rich sediments he recognised a succession of new minerals in the sequence chlorite; biotite; garnet; staurolite; kyanite; sillimanite in rocks of progressively higher metamorphic grade. Barrow found that once basaltic lavas interleaved with the sediments displayed zones with different characteristic minerals. Other metamorphic terrains, however, revealed different index minerals. Experimental mineralogy eventually showed that Barrow’s zones and others reflected a wide range of chemical reactions between minerals that reach equilibrium over different combinations of pressure and temperature. This enabled geologists to distinguish between metamorphism that had occurred under conditions of high-pressure and low-temperature, low-P and high-T and intermediate conditions (see diagram). This suggested that metamorphic rocks can form in areas with different heat flow and geothermal gradients. Geochemical means of assessing the actual temperatures and pressures at which particular rocks had reached mineralogical equilibrium, known as ‘thermobarometry’, now enable such variations to be assessed quantitatively.

The latest division in pressure-temperature space of different styles of metamorphism (colours) and the main mineral equilibria (dashed lines) that define them

It has long been suspected that the average T/P conditions revealed by metamorphic rocks have varied over geological time, as well as from place to place at any one time. A recent paper has analysed thermobarometric data from the earliest Archaean to recent times (Brown, M. et al. 2020, Evolution of geodynamics since the Archean: Significant change at the dawn of the Phanerozoic: Geology, v. 48, p. 488–492; DOI: 10.1130/G47417.1) They conclude that from the Archaean to the start of the Neoproterozoic the average P/T ratio was more than twice as high as it was in the following billion years. At about 2 Ga they suggests a relatively sudden decrease that correlates with what they regard as the first major assembly of continental crust: the Columbia (Nuna) supercontinent. The Mesoproterozoic Era, occupied by the disassembly of Columbia and the eventual creation of the Rodinia supercontinent, retained a high mean T/P. That began to decline with the break-up of Rodinia and a succession of tectonic cycles of ocean opening and closing during the Neoproterozoic and the Phanerozoic. This phase of truly modern plate textonics saw first the assembly of Gondwana and then the all-encompassing Pangaea, followed by its break up as we witness today. There are other correlations with the T/P variations, but they need not detain us.

The raw metamorphic data (564 points spanning 3.5 Ga) are by no means evenly spaced in time, and four dense clusters of points show a very wide spread of T/P – up to 2 orders of magnitude. Yet the authors have used locally weighted scat­terplot smoothing (LOWESS) to reduce this to a smoothed curve with a zone of uncertainty that is a great deal narrower than the actual spread of data. Frankly, I do not believe the impression of systematic change that this approach has produced, though I am not a statistician. To a lesser extent than me, it seems that neither does Peter Cawood, who comments on the paper in the same issue of Geology: more clearly than do the authors themselves.

Peter Cawood’s ‘take’ on the relationship between tectonic development and other important variables in the Earth-system with the estimate by Brown et al. of the mean metamorphic T/P (‘thermobaric’) variation through Earth history

Cawood’s view is that it was all due to a steady fall in mantle temperature and related broad changes in tectonic processes. But metamorphic rocks form in only the outermost 100 km of the Earth. The post-800 Ma examples include a much greater proportion of those formed under high- and ultrahigh pressures – blueschists and various kinds of eclogite – than do the earlier metamorphic belts. This weights the post-800 Ma record to lower mean T/P. Such rocks form in subduction zones and their high density might seem to doom them to complete resorption into the deep mantle. Yet large chunks now end up embedded in continents, interleaved with less extreme materials. Cawood suggests, as do others, that cooling of the mantle has enabled deeper break-off of subducted slabs to meet their end at the core-mantle boundary. The retained low T/P lithosphere since 800 Ma may have been sliced into the continents by increased underthrusting during continent-continent collisions that dominate the more modern orogenic-metamorphic belts.

See also:  Cawood, P.A. 2020 Earth Matters: A tempo to our planet’s evolution: Geology, v. 48, p. 525–526; DOI: 10.1130/focus052020.1

Earliest direct evidence of plate motions

There are two ways that we recognise the movement of tectonic plates. Since the latter half of the Mesozoic Era, following break up of the Pangaea supercontinent, it bests manifests itself in the magnetic ‘stripes’ on the ocean floor. They result from alternating polarisation of the geomagnetic field as new oceanic lithosphere is generated at constructive plate boundaries to drive sea-floor spreading. The oldest remaining stripes date back to the early Jurassic. For earlier times geologists have to turn to the continental crust.  Lavas and some sedimentary rocks undergo magnetisation at the time of their formation and retained that imprint. Such remanent, palaeomagnetism reveals the original latitude at which it was imprinted, together with the subsequent rotation of a drifting continent relative to an assumed N to S axis joining the opposed magnetic poles. The apparent ‘wandering’ of the pole through time when successive ancient pole positions of different ages are plotted in relation to the present position of a continent is a good guide to its history of drifting as a result of plate tectonics. Comparing the polar-wander paths of two continents allows the time when they were formerly united to be estimated. So palaeomagnetic pole data makes it possible to reconstruct not just Pangaea but a whole series of earlier supercontinents, ancient magnetic data being supplemented by other geological evidence such as reconnecting the trends on different continents of ancient mountain belts.

Apparent polar wander paths for two continents for a period when they were united then split and were separated by sea-floor spreading, eventually to collide and reunite

The further back in time the fewer palaeomagnetic pole positions have been estimated, and the more uncertain are the apparent polar wander paths and the more complex each continent’s accumulated geological history. One of the reasons for such uncertainty is that episodes of metamorphism can reset a rock’s remanent magnetisation, hundreds of million years after it originally formed. Thus, the harder it becomes to be certain about early supercontinents that have been suggested, of which there are quite a few. The earliest that has been proposed is Vaalbara, albeit on grounds of geological similarity, that supposedly united the Kaapvaal and Pilbara Cratons of southern Africa and Western Australia, respectively. Its duration is suggested to have been between 3.6 to 2.8 Ga (billion years ago). The oldest supercontinents with sound palaeomagnetic records date from the end of the Archaean Eon (2.5 Ga). It is the lack or uncertainty of earlier palaeomagnetic evidence that makes the start of plate tectonics the subject of so much debate.

However, geophysicists continually strive to improve the detection of ancient magnetisation, and advances have been made recently to unravel original magnetisation signals from those that have been superimposed later. The fruits of these developments are borne out by a study of a sequence of mafic lavas from the Pilbara Craton that formed about 3.2 Ga ago (Brenner, A.R. et al. 2020. Paleomagnetic evidence for modern-like plate motion velocities at 3.2 Ga. Science Advances, v. 6, article eaaz8670; DOI: 10.1126/sciadv.aaz8670). Alec Brenner and colleagues from several US universities measured palaeomagnetism in more than 200 diamond drill cores from two localities in this sequence and combined their data with others from the Pilbara to cover a roughly 600 Ma period between 3.35 to 2.77 Ga. The palaeopoles form a polar wander path that spans roughly 50 degrees of palaeolatitude. From this they have been able to estimate, in considerable detail, the rate at which the Pilbara Craton had moved in Mesoarchaean. In the first 170 Ma the average horizontal motion was about 2.5 cm per year, falling rapidly to 0.4 cm per year over the following 410 Ma. The earlier speed is comparable with the average of modern plate motions. Data from the later period suggests relative stagnation. Motions over the entire ~600 Ma could be due to episodic operation of plate tectonics on the global scale, or a local slowing in the rate of plate growth.

Earliest plate tectonics tied down?

Papers that ponder the question of when plate tectonics first powered the engine of internal geological processes are sure to get read: tectonics lies at the heart of Earth science. Opinion has swung back and forth from ‘sometime in the Proterozoic’ to ‘since the very birth of the Earth’, which is no surprise. There are simply no rocks that formed during the Hadean Eon of any greater extent than 20 km2. Those occur in the 4.2 billion year (Ga) old Nuvvuagittuq greenstone belt on Hudson Bay, which have been grossly mangled by later events. But there are grains of the sturdy mineral zircon ZrSiO4)  that occur in much younger sedimentary rocks, famously from the Jack Hills of Western Australia, whose ages range back to 4.4 Ga, based on uranium-lead radiometric dating. You can buy zircons from Jack Hills on eBay as a result of a cottage industry that sprang up following news of their great antiquity: that is, if you do a lot of mineral separation from the dust and rock chips that are on offer, and they are very small. Given a laser-fuelled SHRIMP mass spectrometer and a lot of other preparation kit, you could date them. Having gone to that expense, you might as well analyse them chemically using laser ablation inductively coupled plasma mass spectrometry (LA-ICP-MS) to check out their trace-element contents. Geochemist Simon Turner of Macquarie University in Sydney, Australia, and colleagues from Curtin University in Western Australia and Geowissenschaftliches Zentrum Göttingen in Germany, have done all this for 32 newly extracted Jack Hills zircons, whose ages range from 4.3 to 3.3 Ga (Turner, S. et al. 2020. An andesitic source for Jack Hills zircon supports onset of plate tectonics in the HadeanNature Communications, v. 11, article 1241; DOI: 10.1038/s41467-020-14857-1). Then they applied sophisticated geochemical modelling to tease out what kinds of Hadean rock once hosted these grains that were eventually eroded out and transported to come to rest in a much younger sedimentary rock.

Artist’s impression of the old-style hellish Hadean (Credit : Dan Durday, Southwest Research Institute)

Zircons only form duuring the crystallisation of igneous magmas, at around 700°C, the original magma having formed under somewhat hotter conditions – up to 1200°C for mafic compositions. In the course of their crystallising, minerals take in not only the elements of which they are mainly composed, zirconium, silicon and oxygen in the case of zircon , but many other elements that the magma contains in low concentrations. The relative proportions of these trace elements that are partitioned from the magma into the growing mineral grains are more or less constant and unique to that mineral, depending on the particular composition of the magma itself. Using the proportions of these trace elements in the mineral gives a clue to the original bulk composition of the parent magma. The Jack Hills zircons  mainly  reflect an origin in magmas of andesitic composition, intermediate in composition between high-silica granites and basalts that have lower silica contents. Andesitic magmas only form today by partial melting of more mafic rocks under the influence of water-rich fluid driven upwards from subducting oceanic lithosphere. The proportions of trace elements in the zircons could only have formed in this way, according to the authors.

Interestingly, the 4.2 Ga Nuvvuagittuq greenstone belt contains metamorphosed mafic andesites, though any zircons in them have yet to be analysed in the manner used by Turner et al., although they were used to date those late-Hadean rocks. The deep post-Archaean continental crust, broadly speaking, has an andesitic composition, strongly suggesting its generation above subduction zones. Yet that portion of Archaean age is not andesitic on average, but a mixture of three geochemically different rocks. It is referred to as TTG crust from those three rock types (trondhjemite, tonalite and granodiorite). That TTG nature of the most ancient continental crust has encouraged most geochemists to reject the idea of magmatic activity controlled by plate tectonics during the Archaean and, by extension, during the preceding Hadean. What is truly remarkable is that if mafic andesites – such as those implied by the Jack Hills zircons and found in the Nuvvuagittuq greenstone belt – partially melted under high pressures that formed garnet in them, they would have yielded magmas of TTG composition. This, it seems, puts plate tectonics in the frame for the whole of Earth’s evolution since it stabilised several million years after the catastrophic collision that flung off the Moon and completely melted the outer layers of our planet. Up to now, controversy about what kind of planet-wide processes operated then have swung this way and that, often into quite strange scenarios. Turner and colleagues may have opened a new, hopefully more unified, episode of geochemical studies that revisit the early Earth . It could complement the work described in An Early Archaean Waterworld published on Earth-logs earlier in March 2020.

An Early Archaean Waterworld

In Earth-logs you may have come across the uses of oxygen isotopes, mainly in connection with their variations in the fossils of marine organisms and in ice cores. The relative proportion of the ‘heavy’ 18O isotope to the ‘light’ 16O, expressed by δ18O, is a measure of the degree of fractionation between these isotopes under different temperature conditions when water evaporates. What happens is that H216O, in which the lighter isotope is bound up, slightly more easily evaporates thus enriching the remaining liquid water in H218O. As a result the greater the temperature of surface water and the more of evaporates, the higher is its δ18O value. Shells that benthonic (surface-dwelling) organism secrete are made mainly of the mineral calcite (CaCO3). Their formation involves extracting dissolved calcium ions and CO2 plus an extra oxygen from the water itself, as calcite’s formula suggests. So plankton shells fossilised  in ocean-floor sediments carry the δ18O and thus a temperature signal of surface water at the place and time in which they lived. Yet this signal is contaminated with another signal: that of the amount of water evaporated from the ocean surface (with lowered  δ18O) that has ended up falling as snow and then becoming trapped in continental ice sheets. The two can be separated using the δ18O found in shells of bottom-dwelling (benthonic) organisms, because deep ocean water maintains a similar low temperature at all time (about 2°C). Benthonic δ18O is the main guide to the changing volume of continental ice throughout the last 30 million year or so. This ingenious approach, developed about 50 years ago, has become the key to understanding past climate changes as reflected in records of ice volume and ocean surface temperature. Yet these two factors are not the only ones at work on marine oxygen isotopes.

Artistic impression of the Early Archaean Earth dominated by oceans (Credit: Sci-news.com)

When rainwater flows across the land, clays in the soil formed by weathering of crystalline rocks preferentially extract 18O and thus leave their own δ18O mark in ocean water. This has little, if any, effect on the use of δ18O to track past climate change, simply because the extent of the continents hasn’t changed much over the last 2 billion years or so. Likewise, the geological record over that period clearly indicates that rain, wet soil and water flowing across the land have all continued somewhere or other, irrespective of climate. However, one of the thorny issues in Earth science concerns changes of the area of continents in the very long term. They are suspected but difficult to tie down. Benjamin Johnson of the University of Colorado and Boswell Wing of Iowa State University, USA, have closely examined oxygen isotopes in 3.24 billion-year old rocks from a relic of Palaeoarchaean ocean crust from the Pilbara district of Western Australia that shows pervasive evidence of alteration by hot circulating ocean water (Johnson, B.W. & Wing, B.A. 2020. Limited Archaean continental emergence reflected in an early Archaean 18O-enriched ocean. Nature Geoscience, v. 13, p. 243-248; DOI: 10.1038/s41561-020-0538-9). Interestingly, apart from the composition of the lavas, the altered rocks look just the same as much more recent examples of such ophiolites.

The study used many samples taken from the base to the top of the ophiolite along some 20 traverses across its outcrop. Overall the isotopic analyses suggested that the circulating water responsible for the hydrothermal alteration 3.2 Ga ago was much more enriched in 18O than is modern ocean water. The authors’ favoured explanation is that much less continental crust was exposed above sea level during the Palaeoarchaean Era than in later times and so far less clay was around on land. That does not necessarily imply that less continental crust existed at that time compared with the Archaean during the following 700 Ma , merely that the continental ‘freeboard’ was so low that only a few islands emerged above the waves. By the end of the Archaean 2.5 Ga ago the authors estimate that oceanic δ18O had decreased to approximately modern levels. This they attribute to a steady increase in weathering of the emerging continental landmasses and the extraction of 18O into new, clay-rich soils as the continents emerged above sea level. How this scenario of a ‘drowned’ world developed is not discussed. One possibility is that the average depth of the oceans then was considerably less than it was in later times: i.e. sea level stood higher because the volume available to contain ocean water was less. One possible explanation for that and the subsequent change in oxygen isotopes might be a transition during the later Archaean Eon into modern-style plate tectonics. The resulting steep subduction forms deep trench systems able to ‘hold’ more water. Prior to that faster production of oceanic crust resulted in what are now the ocean abyssal plains being buoyed up by warmer young crust that extended beneath them. Today they average around 4000 m deep, thanks to the increased density of cooled crust, and account for a large proportion of the volume of modern ocean basins.

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”

Tectonics and glacial epochs

Because the configuration of continents inevitably affects the ocean currents that dominate the distribution of heat across the face of the Earth, tectonics has a major influence over climate. So too does the topography of continents, which deflects global wind patterns, and that is also a reflection of tectonic events. For instance, a gap between North and South America allowed exchange of the waters of the Pacific and Atlantic Oceans throughout the Cenozoic Era until about 3 Ma ago, at the end of the Pliocene Epoch, although the seaway had long been shallowing as a result of tectonics and volcanism at the destructive margin of the eastern Pacific. That seemingly minor closure transformed the system of currents in the Atlantic Ocean, particularly the Gulf Stream, whose waxing and waning were instrumental in the glacial-interglacial cycles that have persisted for the last 2.5 Ma. This was partly through its northward transport of saltier water formed by tropical evaporation that cooling at high northern latitudes encouraged to sink to form a major component of the global oceanic heat conveyor system.   Another example is the rise of the Himalaya following India’s collision with Eurasia that gave rise to the monsoonal system  dominating the climate of southern Asia. The four huge climatic shifts to all-pervasive ice-house conditions during the Phanerozoic Eon are not explained so simply: one during the late-Ordovician; another in the late-Devonian; a 150 Ma-long glacial epoch spanning much of the Carboniferous and Permian Periods, and the current Ice Age that has lasted since around 34 Ma. Despite having been at the South Pole since the Cretaceous Antarctica didn’t develop glaciers until 34 Ma. So what may have triggered these four major shifts in global climate?

Five palaeoclimatologists from the University of California and MIT set out to find links, starting with the most basic parameter, how atmospheric greenhouse gases might have varied. In the long term CO2 builds up through its emission by volcanoes. It is drawn down by several geological processes: burial of carbon and carbonates formed by living processes; chemical weathering of silicate minerals by CO2 dissolved in water, which forms solid calcium carbonate in soil and carbonate ions in seawater that can be taken up and buried by shell-producing organisms. Rather than comparing gross climate change with periods of orogeny and mountain building, mainly due to continent-continent collisions, they focused on zones that preserve signs of subduction of oceanic lithosphere – suture zones (Macdonald,F.A. et al. 2019. Arc-continent collisions in the tropics set Earth’s climate state. Science, v. 363 (in press); DOI: 10.1126/science.aav5300 ). Comparing the length of all sutures active at different times in the Phanerozoic with the extent of continental ice sheets there is some correlation between active subduction and glaciations, but some major misfits. Selecting only sutures that were active in the tropics of the time – the zone of most intense chemical weathering – results in a far better tectonic-climate connection. Their explanation for this is not tropical weathering of all kinds of exposed rock but of calcium- and magnesium-rich igneous rocks; basaltic and ultramafic rocks. These dominate oceanic lithosphere, which is exposed to weathering mainly where slabs of lithosphere are forced, or obducted, onto continental crust at convergent plate margins to form ophiolite complexes. The Ca- and Mg-rich silicates in them weather quickly to take up CO2 and form carbonates, especially in the tropics. Through such weathering reactions across millions of square kilometres the main greenhouse gas is rapidly pulled out of the atmosphere to set off global cooling.

407458aa.2
Top – variation in the total length of active, ophiolite-bearing sutures during the Phanerozoic; middle – length of such sutures in the tropics; bottom – extent of Phanerozoic glaciers. (Credit: Macdonald et al. 2019; Fig.2

Rather than the climatic influence of tectonics through global mountain building, the previous paradigm, Macdonald and colleagues show that the main factor is where subduction and ophiolite obduction were taking place. In turn, this very much depended on the configuration of continents on which ophiolites can be preserved. The most active period of tectonics during the Mesozoic, as recorded by the global length of sutures, was at 250 Ma – the beginning of the Triassic Period – but they were mainly outside the tropics, when there is no sign of contemporary glaciation. During the Ordovician, late-Devonian and Permo-Carboniferous ice-houses active sutures were most concentrated in the tropics. The same goes for the build-up to the current glacial epoch.

Read more on Palaeoclimatology and 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’.

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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

Evolution of the River Nile

The longest river in the world, the Nile has all sorts of riveting connotations in terms of archaeology, Africa’s colonial history, the romance of early exploration and is currently the focus of disputes about rights to its waters. The last stems from its vast potential for irrigation and for hydropower. It is probably the most complex of all the major rivers of our planet because it stretches across so many climatic zones, topographic systems geological and tectonic provinces. Mohamed Abdelsalam of Oklahoma State University, who was born in the Sudan and began his career at the confluence of the White and Blue Nile in its capital Khartoum, is an ideal person to produce a modern scientific summary of how the Nile has evolved. That is because he has studied some of the key elements of the geology through which the river and its major tributaries travel, but most of all because he is a leading geological and geomorphological interpreter of remotely sensed data. Only space imagery can let us grasp the immense span and complexity of the Nile system. His recent review of its entirety (Abdelsalam, M.G. 2018. The Nile’s journey through space and time: A geological perspective. Earth Science Reviews, v. 177, p. 742-773; doi: 10.1016/j.earscirev.2018.01.010) is a tour de force, many years in the compilation, and it makes fittingly compulsive reading.

Abdelsalam lays out the geomorphology, underlying geology and regional tectonics of the Nile drainage basin, synthesized from publications over the last century, including his own work on the evolution of the Blue Nile in Ethiopia. On the regional scale elements of its complexity can be ascribed to the upwelling of mantle plumes beneath the Ethiopian Highlands and Red Sea, and under the Lake Plateau centred on Kenya, Tanzania, Rwanda and Burundi. These plumes are part of a much larger mantle mass rising from the core-mantle boundary beneath the African continent. Their influence on the lithosphere of north-east Africa began over 30 million years ago, producing vast outpourings of flood basalts followed by regional doming, the formation of large shield volcanoes and rifting to transform a once muted surface to one with a topographic range of up to 5 kilometres in the Nile’s two main source regions in Ethiopia and the Lakes Plateau.

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The geological underpinnings of the Nile system (Credit: Abdelsalam 2018; Fig. 5)

The basin can be divided into six distinct provinces, from south to north the Lakes, Sudd, Central Sudan, Ethiopia – East Sudan, Cataract and Egyptian Niles. Each of them has had a different history; in fact, the making of the Nile system as we know it has taken at least 6 million years and probably longer. For instance, the Lakes Nile basin, founded mostly on Precambrian crystalline basement, seems original to have drained westward through the Congo system to the Atlantic Ocean. Sometime between 20 and 12 Ma the western branch of the East African Rift System began to form along with slow, broad uplift, hindering westward flow to create the forerunners of the Great Lakes. The flow was reversed around 2.5 Ma ago by the rise of the Rwenzori and Virunga massifs on the western rift flank and eventually forced northwards into the low-lying Sudd, breaching a major divide in Northern Uganda. The vast swamps there have acted as a buffer for sediment supply, other than the finest silts and clays, into the northern stretches of the White Nile. The Blue Nile’s tortuous trajectory evolved as the Ethiopian flood basalt province rose after 30 Ma, rifted to form the Lake Tana Basin and drained to initiate erosion into the rising plateau with the interference of huge shield volcanoes that formed as uplift proceeded.

Other events are recorded along the Nile’ general trajectory by huge, abandoned alluvial fans, relics of now vanished lakes and evidence from satellite radar of palaeo-drainages with reversed flow beneath the surface of the eastern Sahara. The system evolved episodically, in five or more steps, at the whim of broad tectonic processes that affected flow direction and erosive capacity. The Cataract Nile that cuts through hard basement rocks perhaps records the increase in energy added by the Blue Nile which, which in turn may have encouraged the drainage of the huge Sudd swamps that established the White Nile’s course. Even the Mediterranean Sea played a role: the Egyptian Nile may have formed when the sea vanished to expose a deep saline basin during the Messinian Salinity Crisis 5.5 Ma ago. This reduction in the regional base level of erosion possibly directed drainage into the present course of the Nile. The various provinces only became a unified drainage system during the last half million years, and that emerged in its present form as recently as 15 thousand years ago.  But as Abdelsalam points out, there is a great deal to learn about the fabled river system. Hopefully his review will encourage others to take investigations forward and into previously unstudied regions.