By Robert J. Stern
Two of my favorite topics of geoconversation are how new subduction zones get started and when in Earth’s history did plate tectonics begin? Both are fascinating geoscientific questions but we seem to be making more progress on the first topic than on the second. The plate tectonic revolution changed our science forever but in the excitement of the late 1960’s when the paradigm shifted, the question of what makes the plates move was neglected. Yes it was mantle convection, but was convection driven by hot deep mantle rising or cold dense lithosphere sinking? Geodynamicists soon began investigating and now they tell us that it is mostly the sinking of dense lithosphere in subduction zones, pulling the plates and moving them. The most important consideration is that hotter asthenospheric mantle is slightly (~1%) less dense than colder overlying lithospheric mantle, so these want to change places. This sometimes happens during detachment and delamination of lithospheric mantle but generally happens by subduction, the end-on sinking of lithosphere beneath asthenosphere.
Our modern understanding of what drives the plates shows us that the key to understanding how subduction zones form is by understanding the density and strength of oceanic lithosphere. It also tells us that we should be thinking about lithospheric strength and density when we try to answer the question “When did plate tectonics start on Earth?” Certainly the Archean mantle 2.5 to 3.8 Ga was hotter than is the modern mantle. Consequently, Archean lithosphere would have thinner and more buoyant, and on this basis alone a reasonable person would conclude that plate tectonics must have been more difficult back then. In spite of this, most geoscientists think that plate tectonics was underway in Archean time. Regardless of your opinion on this matter, the question of when did plate tectonics start (WDPTS?) is one of the most important – and exciting – unresolved questions in the history of the solid Earth. I find this to be a particularly interesting question because EVERYONE can get involved: graduate students, undergraduate students, K-12 students, professors, amateurs, the media. We can’t agree on the answer yet so let’s discuss it!
The key to answering WDPTS? must be to reconstruct Earth’s tectonic history, using both first-order understanding of how large silicate bodies cool and proper interpretation of the rock record, particularly those mineral and rock assemblages that are diagnostic of plate tectonic records of independent plate motions, subduction and collision. One possibility is that Earth has always had plate tectonics. This follows from a strict interpretation of the Principle of Uniformity, which basically states that “the present is the key to the past”. Following strict Uniformitarianist logic, because we definitely have plate tectonics today, Earth must have always had plate tectonics. But strict adherence to Uniformitarianism is ridiculous, as Stephen Jay Gould pointed out in his first peer-reviewed paper (Gould, 1965). Uniformitarianism is very useful when you are trying to explain how the Earth came to be to a bunch of religious nuts who think the Earth is 6000 years old and that humans and dinosaurs coexisted, but it is not useful when trying to understand Earth’s tectonic history for the simple reason that it inhibits inquiry.
Earth is the only known silicate planet that has plate tectonics, so plate tectonics is probably a special way that viscous, rocky planets cool. Once we escape the Uniformitarianist straitjacket, we can see that a hotter early Earth may have cooled in a different way than the present Earth. Certainly we all know that there were different conditions in the Precambrian, which makes up 88% of all geologic time. We know that the interior of the early Earth was much hotter than that of today, for a number of reasons. For example, heat production due to radioactive decay at 4 Ga was ~3x that of today. Other causes of early heating include heat of accretion, the Sun’s T-tauri event (beginning of H fusion), core differentiation, and the Mars-size impact events. How much hotter was the early Earth? We don’t know but we do know that there are vanishingly few rocks from the first 800 Ma of Earth’s history, as expected for a hot early Earth.
Earth cooled sufficiently that 3.8 Ga rocks are fairly common (e.g. Greenland, Africa) but still, Earth must have been much hotter in the Archean than it is today. How did a hotter mantle affect our planet’s style of heat loss, i.e. tectonic style? Some conclude that a hotter mantle would have resulted in a greater total length of global spreading ridges, which means smaller plates and faster moving plates. Certainly a hotter Earth would have caused more extensive melting and thicker oceanic crust – komatiitic oceanic crust seems likely. It is also likely the oceanic lithosphere would have been thinner and more depleted and that the underlying asthenosphere would have been hotter. I surmise that Archean lithosphere would have been hotter, thinner, and less dense; it also would have been weaker and more prone to necking and breaking. These characteristics would have made it easier for sufficiently dense Archean lithosphere to trade places with buoyant Archean asthenosphere but this would have made subduction – which requires coherent plates – more difficult. We can (and should) stake out an opinion, but who knows for sure? Each of us should consider what we know about how our planet operates today and mentally explore how the hotter early Earth would have been similar or different than the plate tectonic Earth of today.
I discussed some of these issues with an eminent geoscientist, who argued that plate tectonics has always been operating on Earth. I asked him why he thought this and he replied “How else can you generate magmas and deform rocks?” There is no doubt that the Archean Earth witnessed a lot of igneous activity and deformation, maybe more than experienced by the modern Earth, but this does not require plate tectonics. This is vividly demonstrated by the examples of Venus and Mars, which today suffer intense deformation and magmatic activity but without plate tectonics.
For me, the most important evidence that Plate Tectonics operated at a given time interval is the preservation of ophiolites, blueschists, and ultra-high pressure (UHP) metamorphic terranes from a given time period somewhere on the globe. For those unfamiliar with what I call the “Smoking guns”* of Plate Tectonics’ (Fig. 1): ophiolites are fragments of oceanic crust and upper mantle (lithosphere) emplaced on continental crust (where geologists can study them). Ophiolites should have but sometimes lack extensive gabbros or sheeted dike complexes, but at a minimum an ophiolite should include tectonized harzburgitic mantle and pillowed tholeiite.
Blueschists are fragments of oceanic crust that have been metamorphosed 40-60 km deep in the distinctive high-P, low T environment of a subduction zone. This produces the diagnostic Na-amphibole known as glaucophane. UHP terranes are slivers of continental sediments which have been subducted even deeper than blueschists, to depths of 100 km. Pressures like this are required to produce UHP-diagnostic phases of diamond or a high-P polymorph of SiO2 known as coesite. Both blueschists and UHP terranes require a two way ticket, down to be metamorphosed in a subduction zone, and back to the surface to be greeted by enthusiastic geologists. Excepting a few 1.9 Ga ophiolites, all three ‘smoking guns’ first appear in Neoproterozoic time, less than 1 billion years ago. I am very impressed by the fact that the vast majority of these three primary indicators of plate tectonics are so young, other geoscientists are less impressed (Fig. 2). More details about the nature of these three petrotectonic indicators can be found in Stern (2005) and Stern (2008).
WTPTS? does not take up much of my research time but it is fun because it keeps me thinking about all the ways that Earth’s tectonic history can be interrogated. I wonder if there is some type of ore deposit or other rock association that could be used as a new plate tectonic indicator. Eclogites are also potential plate tectonic indicators. One type of eclogite forms when oceanic crust is metamorphosed at 50 km or more deep in a subduction zone but the term also can be used to describe any garnet-pyroxene rock produce by non-plate tectonic processes, for example in the lower continental crust as high-P cumulates or accompanying crustal thickening. Bob Coleman and colleagues wrote an interesting review entitled “Eclogites and Eclogites” that discussed some of these issues (Coleman et al., 1965). Subduction-related eclogites are a particular variety of clinopyroxene-garnet that contain Pyrope (Mg-Al) garnet and omphacite (jadeite-rich garnet). We need some kind of a “discrimination diagram” to distinguish subduction-related eclogites from those of other origins and then we could compile the distribution in time of subduction-related eclogites and use this as an independent petrotectonic indicator to help answer the question WDPTS? A few years ago, Tatsuki Tsujimori and co-authors looked at another subgroup of eclogites which must be subduction-related, those containing lawsonite (Tsujimori et al., 2006). Lawsonite is a hydrous calcium aluminum silicate that is typical of blueschist facies environments, and all known lawsonite-bearing eclogites are Phanerozoic (Fig. 1).
Another rock association that needs to be looked into for the purpose of addressing WTPTS? is the distribution of calc-alkaline batholiths through time. Batholiths mark the exhumed roots of magmatic arcs, exposed by a few km of erosion to remove the volcanic cover, and can be expected to persist as distinctive hallmarks of subduction until they are covered up by sediments. How can we recognize subduction-related batholiths in the rock record? We shouldn’t be happy with just a few trace element diagrams as sufficient to identify arc-like igneous rocks. Someone should develop a more robust set of characteristics and use these to define subduction-related batholiths. These characteristics should include a combination of geographic extent (how many km long and wide?), magmatic geochemical characteristics (e.g., K and isotopic gradients, and position relative to where the forearc basin and trench were (inferred from ophiolites, blueschists, and subduction-related eclogites), temporal features (subduction zones and thus magmatic arcs should be active for tens to hundreds of millions of years), and of course igneous rock compositions.
I continue to look for ways to interrogate the rock record for information about WDPTS? This investigation should be as broad as possible. I recently co-authored a Geology paper on the topic Plate Tectonic Gemstones (Stern et al., in press), which identified gemstones that are diagnostic of plate tectonic processes of subduction and collision. The subduction gemstone is Jade, which consists of nephrite (amphibole) and jadeite (pyroxene). Nephrite can form in other tectonic environments but jadeite only forms 25-70 km deep (0.8 – 2 GPa) under the cool (300-500°C) conditions found in subduction zones. All 19 known localities of jadeite are Phanerozoic in age (Fig. 1). The collision gemstone is ruby, which is gem corundum containing ~1% Cr2O3, an impurity that gives the gemstone its deep red color. Rubies form by hot metamorphism (500°- 800°C, 0.2 – 1.0 GPa), especially when passive margin sediments (esp. aluminous shales) get involved in continental collision. We summarized 32 ruby deposits and all but two are Neoproterozoic (Fig. 1). These gemstones are particularly useful because they form so deeply that erosion should reveal, not remove these. Our understanding of the global distribution of the gemstones ruby and jadeite are further indicators that subduction and collision – and therefore plate tectonics – are geologically young phenomena.
There should also be a way to use the temporal distribution of certain ore deposits to answer the question WDPTS? We haven’t made much progress in this aspect yet, but maybe someone will figure something out about this record. A while back I thought that porphyry copper deposits, which are clearly related to subduction, might be ‘smoking guns’ but now I understand that erosion is likely to remove all evidence of these deposits after a few tens of millions of years.
By now you have probably reached a point where you either think that there is some merit in these digital scribblings, or you may have concluded that I am full of unlithified coprolites. Regardless of what you think about WDPTS?, it must have begun at some time after Earth formed. I have shared my opinion about when this was, and some of the reasons for this opinion. Whenever “the great tectonic revolution” happened, there must have been a different tectonic style that it replaced. What was Earth’s pre-plate tectonic style?
To better understand Earth’s early tectonic style we must start from first principles. We know that the farther we go back in time, the hotter Earth’s mantle must have been. The lithosphere must have been correspondingly thinner and weaker and the asthenosphere must have been weaker and melted more extensively. Abundant mafic outpourings have loaded weak lithosphere, depressing it into the eclogite stability field (T>580°C, P>1.3 GPa) where the increase in density due to eclogitization would have stimulated further sinking, ultimately forming detached sinking diapirs, much as happens today during delamination. Archean greenstone belts must have been dominated by downwellings where weak lower crust delaminated and sank. The downwelling zones must have been flanked by mantle upwelling zones (Fig. 3). Hamilton (2007) concluded that dense mafic and ultramafic lavas erupted atop mobile felsic crust during the Archean produced a density inversion that led to the downfolding of volcanic rocks at the same time as domes of felsic middle crust flowed up and around the keel, resulting in the characteristic (keel-and-dome) structure of Archean greenstone belts. The lower panel on Fig. 3 summarizes one idea of what may have happened in the mostly “weak lithosphere vertical tectonics” of the early Earth.
OK, enough ramblings. This brief essay has hopefully stimulated the reader’s interest in the grand question of when Earth’s modern tectonic regime was established. I encourage the reader to join the fun and excitement of this investigation. It’s easy to join and contribute to the discussion; we are just feeling our way around this problem. One route forward is to identify those rocks that, in your opinion, most likely formed by plate tectonic processes, and make these your “smoking guns” for plate tectonics. The occurrence of these through time may be an important indicator. It will also be fun to watch how this line of inquiry evolves and what new ideas are advanced over the next few years.
*’The term “smoking gun” was originally, and is still primarily, a reference to an object or fact that serves as conclusive evidence of a crime. In addition, its meaning has evolved in uses completely unrelated to criminal activity: for example, scientific evidence that is highly suggestive in favor of a particular hypothesis is sometimes called smoking gun evidence. Its name originally came from the idea of finding a smoking (i.e., very recently fired) gun on the person of a suspect wanted for shooting someone, which in that situation would be nearly unshakable proof of having committed the crime (from Wikipedia).
Coleman, R.G., Lee, D.E., Beatty, L.B., and Brannock, W.W., 1965. Eclogites and Eclogites: Their Differences and Similarities. Bull. Geological Society America 76, 483-508.
Gould, S. J. 1965. Is Uniformitarianism Necessary? American Journal of Science 263, 223-238.
Hamilton, W.B., 2007, Earth’s first two billion years—The era of internally mobile crust, in Hatcher, R.D., Jr., Carlson, M.P., McBride, J.H., and Martínez Catalán, J.R., eds., 4-D Framework of Continental Crust: Geological Society of America Memoir 200, p. 233–296
Stern, R.J. 2005. Evidence from Ophiolites, Blueschists, and Ultra-High Pressure Metamorphic Terranes that the Modern Episode of Subduction Tectonics Began in Neoproterozoic Time. Geology 33,7, 557-560.
Stern, R.J. 2008. Modern-Style Plate Tectonics Began in Neoproterozoic Time: An Alternative Interpretation of Earth’s Tectonic History. Condie, K., and Pease, V., eds, When did Plate Tectonics Begin?: Geological Society of America Special Paper 440, 265-280.
Stern, R.J., Tsujimori, T., Harlow, G., and Groat, L. A., in press. Plate Tectonic Gemstones. Geology
Tsujimori, T., Sisson, V.B., Liou, J.G., Harlow, G.E., and Sorensen, S.S., 2006, Very low-temperature record in subduction process: a Review of worldwide Lawsonite eclogites. Lithos, doi:10.1016/j.lithos.2006.03.054.