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.

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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.

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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.

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