Tectonics

The ocean that tried to swallow itself

September 2008

Wegener’s famous supercontinent Pangaea lasted for about 200 Ma from the mid Carboniferous to the late Triassic, and formed a ‘slice world’ extending almost from pole to pole. Yet it had a vast spreading embayment on its eastern side around which wrapped two ‘horns’ of continental lithosphere: an ocean dubbed ‘Palaeotethys’. Another peculiarity is that at its core Pangaea is marked by a huge, orogenic belt that seems to have been buckled on a continental scale: the Iberian-Amorican Arc. Such refolded mountain chains are sometimes referred to as ‘oroclines’, and there is considerable debate about how they might have formed. The latest notion is that slab-pull at a north-dipping subduction zone at the northern edge of Palaeotethys not only caused its spreading centre to be consumed, but thereafter continued to suck at the remaining ocean lithosphere (Gutiérrez-Alonso, G. et al. 2008. Self-subduction of the Pangaean global plate. Nature Geoscience, v. 1, p. 549-553). The stresses involved in attempted closure of the wedge-shaped ocean spur on the otherwise elliptical supercontinent would explain several roughly radial rift systems with voluminous magmatism that formed in Pangaea in Permian times, such as the Oslo graben. Ever ready for a bit of fun, New Scientist has referred to Pangaea in terms of an aged, but well-known computer-game object that apparently turned on itself after consuming all lesser objects (Palmer, J. 2008. Pac-Man supercontinent ate itself to pieces. New Scientist.com News Service, 6 July 2008 http://environment.newscientist.com/article/dn14259-pacman-supercontinent-ate-itself-to-pieces.html).

Tibetan Plateau reviewed

September 2008

The roughly 5 km high Tibetan Plateau is not only the largest area of high elevations on Earth, it helps generate the monsoons of southern and SE Asia. Some have argued that it is a major climatic driver and may have been responsible for overall cooling of the Northern Hemisphere by diverting wind patterns once it had reached its present extent. Tibet may even have influenced global cooling through the Cenozoic by encouraging extraction of CO2 from the atmosphere by liberating enormous quantities of silicate minerals for chemical weathering. From a tectonic standpoint the Plateau is especially fascinating. In the mid-1970 Molnar and Tapponnier proposed that the near-doubling of Tibet’s crustal thickness had created unstable conditions and that crust was being extruded eastwards as a result of gravitational collapse: an evolving example of escape tectonics. There are hundreds of papers on or relating to the Tibetan Plateau, its origin and evolution, so a succinct review is welcome (Roydon, L.H. et al. 2008. The geological evolution of the Tibetan Plateau. Science, v. 321, p. 1054-1058). This centres on the development of the escape tectonics idea over 3 decades, and offers an important regional insight. Widening the context to include the evolution of the West Pacific oceanic lithosphere reveals a link between the timing of plate tectonic events there and changes in crustal collapse far to the west in eastern Tibet and adjacent lands. Soon after India began to collide with Eurasia in the Eocene, the subduction zones of the West Pacific and Indonesia migrated ridgewards, away from Eurasia as a result of trench rollback. This created space into which crustal collapse could spread as the Himalaya and Tibetan Plateau were thrown up. This trench migration stopped during the Miocene, severely interfering with the gravitational possibilities for escape tectonics. Effectively, the escape from Tibet was ‘dammed’, and it is from that date that the phenomenal rise to 5 km elevation has taken place. The authors even link this hindrance to the development of seismically hazardous conditions throughout western China, such as the Longmenshan mountains where the magnitude 7.9 12 May 2008 Wenchuan earthquake occurred (Burchfiel, B.C. et al 2008. A geological and geophysical context for the Wenchuan earthquake or 12 May 2008, Sichuan, People’s Republic of China. GSA Today, v. 18 (July 2008), p. 4-11)
See also: Kerr, R.A. 2008. Pumping up the Tibetan Plateau from the far Pacific Ocean. Science, v. 321, p. 1028-1029

A drop off the old block?

May 2008

It is not so long ago that detachment and foundering of material from lithospheric blocks began to be visualised as a means to explain large areas of recent, rapid uplift of the continental surface. Chunks falling from the subducted slabs beneath Tibet and Kamchatka (see Evidence for slab break-off in subduction zones in EPN September 2002) may have generated unusual magmatism or stopped volcanism respectively. Massive Himalayan uplift and that of areas such as the Sierra Nevada in the western US seem to indicate foundering of large masses of mafic rocks from the base of thickened lower crust (see Mantle dripping off mountain roots in EPN October 2004). Even the end-Miocene Messinian salinity crisis in the Mediterranean has been ascribed to uplift resulting from delamination (see When the Mediterranean dried up in EPN May 2003). Yet convincing evidence from seismic data are conspicuous by their rarity. A necking, or monstrous boudinage of the subducting slab beneath the Hindu Kush region of the Himalayan chain is convincingly demonstrated by geophysicists from the Australian National University (Lister, G. et al, 2008. Boudinage of a stretching slablet implicated in earthquakes beneath the Hindu Kush. Nature Geoscience, v. 1, p. 196-201).

The setting for this remarkable ‘caught in the act’ phenomenon is where a minor ocean basin closed when the Kohistan arc was accreted to Asia during the closure of the Tethys ocean, and is in the process of vanishing. Wherever such minor basins have been caught up in major destructive-margin tectonics they seem to coincide with markedly arcuate orogens characterised by high-P metamorphism and repeated stacking of thrust slices. Once school of thought seeks a solution by some kind of ductile ‘dripping’ of mantle, which the authors sought to test by looking at seismicity beneath the most prominent of these arcuate mini-orogens. What they found was a zone of ‘necking’ defined by clustered earthquakes on either side. Detailed analysis suggests that a drop-shaped mass is in the process of detaching itself by a combination of brittle and ductile deformation –a boudin several orders of magnitude than any the have previously been described.

Is plate tectonics a turn-on or a turn-off?

March 2008

The dominant force that helps to drive plate motions is the pull exerted by dense cold lithosphere descending subduction zones. If the total length of subduction zones were to increase or decrease, or some other factor affecting the global rate of subduction changed then plate movements overall would be affected. Yet it is plate tectonics that actually removes the bulk of heat continuously generated in the deep Earth by radioactive decay, the amount of which changes very slowly over periods of tens to hundreds of million years. Should plate movements slow or stop that heat would either build up at depth or would emerge in a way unrelated to the motion of plates, perhaps as within-plate magmatism.

Should the Pacific Ocean close, then a large proportion of modern subduction would stop, and some kind of thermal and mechanical compensation would cut-in. There were times in the past when vast oceans did close as supercontinents formed: the formation of Rodinia in the late Mesoproterozoic; the Pan African orogeny of the late Neoproterozoic; the mid-Phanerozoic assembly of Pangaea. Each would have resulted in an order-of-magnitude fall in the rate of subduction. Paul Silver and Mark Behn of the Carnegie Institution of Washington and Woods Hole Oceanographic Institution have attempted to judge what kind of thermal and mechanical compensation may have taken place (Silver, P.G & Behn. M.D. 2008. Intermittent plate tectonics. Science, v. 319, p. 85-88). They look at geochemical parameters that ought to act as proxies for subduction processes – the way certain element and isotope proportions in the mantle (Nb/Th and 4He/3He) are affected by the productivity of arc magmatism. Another proxy for subduction intensity is the rate of production of continental crust, assuming reasonably that most is produced from magmas generated at volcanic arcs.

It has become increasingly clear, as the number of absolute ages from the crust has steadily increased, that the continents have formed in a stop-start fashion. Convincingly, Silver and Behn’s synthesis of Nb/Th and 4He/3He ratios in basalts also shows a marked fluctuation in the rate of the mantle’s chemical depletion. It peaked at the end of the Archaean, declined to a minimum around 1 Ga and rose again with the formation of Pangaea at about 300 Ma. The link with supercontinent formation is not simple, although a pattern emerges. Pangaea and the suspected Nuna supercontinent of the Palaeoproterozoic link to peaks in mantle depletion rate, whereas the supercontinents Rodinia and Pannotia (arising from the Pan African) formed while depletion rates were low. Silver and Behn ascribe the differences to two kinds of closure of Pacific-sized oceans following their origination by rifting and drifting of supercontinents. One scenario involves closure of the ocean that once surrounded the supercontinent, as seems to be on the cards for the modern Pacific; P-type closure. The other when the ocean formed passively by break-up is ‘outgunned’ by sea-floor spreading in the once surrounding ocean. That would be the case had the spreading on the East Pacific Rise not involved double subduction around the Pacific margins – the Atlantic would have opened only for both its margins to become subduction zones; A-type closure. Pangaea and possibly Nuna resulted from A-type closure. On the other hand Rodinia and Pannotia seem to have involved circumnavigation of drifting continents to collide at roughly the antipode of the split in a preceding supercontinent by P-type closure.

The conclusion is that plate tectonics was active in the early Earth, becoming intermittent in its middle life and resurrected since a billion years ago. From an examination of the deep thermal consequences of changes in plate motions in the outer Earth, it appears that mantle temperature has fluctuated markedly through time, albeit with a net decrease due to decayed radioactivity. This may have partially ‘switched off’ the conditions for mantle convection that favours plate formation and motion to a more sluggish form. By way of confirmation of their theoretical work, Silver and Behn point to the vast emplacement on most modern continents of granitic and anorthositic plutons under tectonically quiescent conditions that characterised the Grenvillian events preceding the formation of Rodinia between 1.6 to 1.3 Ga.

Pacific plate about to split?

March 2008

The world’s largest lithospheric plate lies to the west of the East Pacific Rise spreading axis, and extends from 60˚N to 60˚S. A string of volcanic islands connects Easter Island close to the East Pacific Rise to Samoa on the northern end of the Tonga Trench. Each lies above a small hot spot, which collectively define the most densely packed area of active within-plate volcanism on the Pacific Plate. Associated with it is an area of anomalously shallow ocean floor: the South Pacific Superswell. North of this zone the plate velocity has been faster than that of the southern part of the Plate for the last 7 Ma. One explanation for the hot-spot cluster is that it lies above a ‘tear’ that is starting to develop in the Pacific lithosphere (Clouard, V. & Gerbault, M. 2008. Break-up spots: Could the Pacific open as a consequence of plate kinematics? Earth and Planetary Science Letters, v. 265, p. 195-208). Others dispute this conjecture, but Clouard and Gerbault have modelled strain patterns across the Plate, using plate speeds derived from magnetic stripes and GPS measurements, to predict where volcanism might arise in relation to a focussed shear zone in the lithosphere. The model points directly at the linear cluster of hot spots. Maybe this is the site of a future division of the Pacific Plate into two, the current magmatism perhaps to generate a new, E-W spreading axis. That would be 5 to 20 Ma off, so there is plenty of time to discuss the processes going on.
See also: Reilly, M. 2008. I the Pacific splitting in two. New Scientist, v. 197 (26 January 2008 issue), p. 10.

Lightened load speeded up India’s drift

November 2007

About 140 Ma ago India split from the other Gondwanan continents, and proceeded northwards eventually to collide with Eurasia around 50 Ma. Its progress was astonishing fast, as continental drift goes, at around 20 cm per year compared with Australia, Africa and Antarctica at little more than a tenth of that speed (Kumar, P. et al. 2007. The rapid drift of the Indian tectonic plate. Nature, v. 449, p. 894-897). Kumar from NGRI in India, and his Chinese and German colleagues, attribute this to the process of Gondwana break-up itself. It probably started when a large mantle plume arrived at the base of the supercontinent’s lithosphere; one now represented by the Marion, Kerguelen and Réunion plumes. The sluggish continents have thick lithospheric roots (180-300 km), whereas India (100 km) does not. So, the subcontinent was an especially light load to be dragged northwards by slab pull beneath Asia. The reason suggested for this is that the early Cretaceous plume destabilised the lithospheric root beneath India, causing it to founder. This might also explain another oddity about Southern India. The area is characterised by some of the largest extents of deep-crustal granulites in the world, which are also by far the highest: the Nilgiri granulites reach 2.6 km above sea level. Isostatic rebound from Cretaceous delamination would nicely explain this anomaly of mountains made of the densest continental materials.

Supercontinents of the past and future

November 2007

Vigorous plate tectonics on the Earth continually drives continents around on the surface. Inevitably, they will clang together sooner or later, along with new sialic crust formed in island arcs that older continental masses sweep up. The more continental crust the more likely it is that all of it will accumulate in a supercontinent. Geologists know for sure of three: Alfred Wegener’s Pangaea (250-200 Ma); Greater Gondwana that clumped together the modern southern continents at 600 Ma and ended up in Pangaea; Rodinia (1100-750 Ma). There are suspicions of earlier assemblies in the Palaeoproterozoic and Archaean, but the crucial palaeomagnetism method for confirming reconstructions breaks down before Rodinia’s times. Also, much has happened since 2 billion years ago so that all but shreds of geological evidence have become scrambled.

If plate tectonics is regularly paced over long periods, and that is quite likely as it is driven by slowly dwindling radioactive mantle heating, maybe there ought to be some kind of cycle of supercontinent assembly. But there again, once formed why shouldn’t sea-floor spreading in the corresponding super-ocean hold a supercontinent together and still efficiently dissipate the Earth internal heat production? The information to hand indicates that they can last a while (400 Ma in the case of Gondwana) but eventually break up. It has been pretty certain for some time that mantle plumes from the core-mantle boundary don’t follow the same motions as do surface plates, so sooner or later one pops up beneath a continent. The bigger the continent the greater the chance that a plume will perturb it, the more so as thick continental lithosphere will act as a thermal ‘lid’ because of the sluggishness of conductive heat transfer through rigid bodies. It is possible to model the two processes of continental drift/collision and the rise of plumes (Phillips, B.R. & Bunge, H-P. 2007. Supercontinent cycles disrupted by strong plumes. Geology, v. 35, p. 847-850). What the US and German co-authors discovered from their model is that planets like the Earth cannot settle into long-term tectonically stable modes. Periodicity is unlikely, and what might look like a kind of regularity – some authors, beginning with J. Tuzo Wilson, have argued for ~400 Ma cycles – is merely an artefact of there having been only a few supercontinents in the geological past.

This will come as a bit of blow to the authors of an entertaining speculation about the future of plate tectonics (Williams, C. & Nield, E. 2007. Pangaea, the comeback. New Scientist, v. 196 (20 October 2007), p. 36-40). Williams and Nield depend on there really being cyclicity in plate tectonics, and they are not the first to have a shot at predicting a world that will not harbour anything passably human; there has been NovoPangaea, Amasia and Pangaea Proxima scheduled for a quarter of a billion years hence. Yes, they would be odd worlds, in the same way as all supercontinents would be alien to those of us used to being no more than 2600 km from the seaside (at the continental pole of inaccessibility in NW China). There would be odd weather patterns, an odd climate overall and mountains where now there are plains and shallow seas. Evolution would slow down both in the seas and on the continents, since all life would potentially interact and there would be fewer ecological niches. A point that ought not to be lost, especially on the authors, is that we can only have such speculative fun by referring to what we do know from the past. So I am surprised that a major impact that wipes out all life except for prokaryotes isn’t built into the playful scenario.

The oldest ophiolite

May 2007

The Isua area of West Greenland is probably the most prowled over piece of geological real estate in the world, and is certainly one of the oldest, dated at ~3.8 Ga. Close to the Greenland ice cap, melting bared the fresh rock in the last few thousand years and there is little vegetation, soil or weathering. The area can be mapped and intricate details extracted at scales down to 1:100, and parts of it probably have been examined on hands and knees. The reason for the attention, apart from its antiquity, is that the rocks are recognisably metamorphosed volcanics and sediments, albeit quite highly deformed in places. There are conglomerates that prove deposition by moving water, fine iron-rich cherts that may have formed in submarine hot springs and pillow lavas effused underwater. The cherts would, if unmetamorphosed, be good places to look for signs of early life, and indeed carbon isotopes extracted from apatites in them have been suggested to show signs of life (more recent work failed to find any sign of carbon in such apatites, so the ancient-life aspect of Isua is somewhat tarnished). The lavas and accompanying igneous rocks attract geologists interested in ancient tectonics (Furnes, H. et al. 2007. A vestige of the Earth’s oldest ophiolite. Science, v. 315, p. 1704-1707). Furnes et al. are not the first to suggest that Isua preserves an ophiolite and evidence for early Archaean plate tectonics, that distinction having gone to a Japanese team 8 years before, who did map on their hands and knees. However, Furnes and colleagues did drive in the final nail—a sheeted-dyke component discovered in the complex—and describe the basalt geochemistry.

Geochemically, the Isua pillow lavas show affinities with both intra-oceanic island arc and mid-ocean ridge settings, their oxygen isotopes indicating extensive involvement of seawater in their alteration. Inevitably, geochemists will have another shot at signs of the earliest life, and perhaps evidence will be found for ‘black smokers’, the hydrothermal vents which today are colonised by weird and sometimes primitive microbial life forms. Yet Furnes et al. report, astonishingly, ‘…the strain history of these rocks is not yet sufficiently well-known to permit a detailed reconstruction of the Isua ophiolite complex’. On your knees, ladies and gentlemen…

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How the asthenosphere loses its strength

March 2007

The Earth is the only tectonically active planet, and indeed its plate movements are astonishingly rapid in a geological sense—about the same rate as toenails grow. Specifically, our planet's activity takes the form of thin, rigid slabs that move on top of a shallow layer of mantle called the asthenosphere (from the Greek a-sthenos meaning ‘without strength'). The reduced rigidity of the asthenosphere is signified by the low speeds at which seismic waves travel through it. It is tempting to regard this low-velocity zone as mantle ‘on the point of melting', perhaps even with isolated pockets of melt. A more widely held view is that it is due to an increased tendency over its range of depth for mantle minerals to contain defects, whose migration gives rise to deformation in the solid state, or creep. Both phenomena are enhanced by water, in the first case if it exists in molecular form, in the second when it dissolves as OH^- in otherwise anhydrous mantle minerals. Experimental work on how mantle minerals behave at different pressures in the presence of water is able to refine models for the asthenosphere (Mierdel, K. et al. 2007. Water solubility in aluminous orthopyroxene and the origin of the Earth's asthenosphere. Science, v. 315, p. 364-368).

Mierdel and her colleagues show that of the main mantle minerals, orthopyroxene shows a sharp decrease in water solubility from high at shallow-mantle pressures to low at depths within the asthenosphere. The dominant mineral, olivine, continuously increases its capacity to dissolve water as depth increases. This makes the depth range of the asthenosphere the least likely for water to be dissolved in the mantle. Instead it must exist in molecular form, able to stimulate incipient melting and therefore weakness. At pressures of the lithosphere, mantle orthopyroxene may ‘mop-up' water so that the dominant olivine is dried and thereby becomes stronger.

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Evidence supporting mantle plumes

January 2007

A variety of arguments have emerged that question the popular idea that hot-spots within plates and at constructive plate boundaries lie above zones of buoyant upwelling in the mantle—plumes for short. For instance, although seismic tomography has seemed an excellent means of demonstrating that plumes do exist, it has poor resolution, uses a proxy for temperature differences in the mantle and says little if anything about motion. Before the old concept of mantle plumes can be rehabilitated, it needs independent support as regards the necessary temperature differences and motions. One candidate approach, albeit complicated, uses intermediate steps in the ultimate decay of uranium isotopes to lead: uranium-series geochemistry (Bourdon, B. et al . 2006. Insights into the dynamics of mantle plumes from uranium-series geochemistry. Nature, v. 444, p. 713-717).

The approach uses ratios of ²³⁰Th/²³⁸U and ²³¹Pa/²³⁵U that show the isotopes' ‘activity' in terms of their decay constants and abundance in recently erupted lavas from hot-spot volcanoes. In the solid mantle, these activity ratios should be 1.0. Their difference from unity in basaltic lavas measures the degree to which there has been fractionation between U and Th and U and Pa, as a result of partial melting. The U-series data used by the authors show significant correlations between the degree of disequilibrium/ fractionation among the four isotopes and the speed, width and density difference (lumped together as the ‘buoyancy flux') associated with a hotspot. This, they claim, links to coupled variations in temperature and upwelling in the mantle, and constrains the widths of upwellings to the order of 250 km. That is they support the notion of mantle plumes (but not where they come from, which is the most interesting part of the plume hypothesis). The arguments are highly involved, and at first sight seem to involve sufficient circularity for opponents of plumes to easily side-step them…

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Clues to Archaean tectonics

October 2006

The Barberton Mountain land in South Africa has a special place in Archaean geology, because it was there that Carl Anhausser and the Viljoen brothers first demonstrated just how peculiar early Archaean conditions were. It preserves intricate detail about both sedimentation and volcanicity—komatiites and their odd spinifex textures were first thoroughly investigated there by this famous threesome—and superb exposures displaying tectonic features that are now recognised as those of an accretionary prism. During the Archaean more radioactive uranium, thorium and potassium was in the mantle than nowadays, since less time had elapsed for it to decay. That geothermal heat flow was a great deal higher than at present is an inescapable conclusion. The central question though is whether geothermal gradients were any higher than at present or whether heat production was dissipated mainly by increased magma upwellings so that temperate change with depth was little different than it is now. On that hangs the likely kind of tectonics at subduction zones. Some have argued that plate movements must have been faster, so that younger, warmer oceanic lithosphere was subducted then. If so, the result may have been a failure of crustal basaltic rocks to transform to eclogite, so that less dense material was subducted at gentle angles. The only way to answer these questions is to get some concrete evidence for the subduction zone geotherm from Archaean rocks. Barberton is probably the best place to look, and that has recently paid off (Moyen, J-F. et al. 2006. Record of mid-Archaean subduction from metamorphism in the Barberton terrain, South Africa. Nature, v. 442, p. 559-562).

Moyen and colleagues from the University of Stellenbosch, South Africa focused on rocks beneath the probably accretionary prism in Barberton. These are gneissic with isolated enclaves of supracrustal material, including epidotic amphibolites, that contain mineral assemblages suitable for geothermometry and geobarometry. Where later strain is low, there are patches in which garnet, clinopyroxene, sodic amphibole and Na feldspar are equilibrated. Microscopic textures still show intricate evidence for the metamorphic reactions that produced these assemblages. Using element partitioning between the phases, the authors have established metamorphic temperatures around 600°C at pressures around 14 kbar (equivalent to a depth between 35 to 40 Km). Those data suggest a geothermal gradient around 15°C km^-1; previously the lowest estimates for Archaean metamorphic conditions were around 25°C km^-1 (within the range of modern continental geotherms). The value established by Moyen et al. is within the range for modern subduction. So it seems likely that at least some Archaean subduction was little different from its modern equivalent, and old lithosphere at the leading edge of plates would have been cold and strong. Many, including me, need to revise long-held views of an alien world during the Archaean.

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Detection of rifting due to dyke emplacement

August 2006

The Afar Depression of NE Africa is a zone of complex continental rifting and nascent formation of new ocean floor that has been developing since the Late Oligocene, where the Red Sea, Gulf of Aden and Ethiopian rifts meet. Averaged out, the extension is at around 16 mm per year. In September and October 2005 small seismic events spread along about 60 km of a discrete segment of the Afar rifting system. Analysis of the vicinity of these earthquakes, using satellite-radar interferometry revealed an astonishing 8 m of extension in little more than a week (Wright, T.J. et al . 2006. Magma-maintained rift segmentation at continental rupture in the 2005 Afar dyking episode. Nature , v. 442 , p. 291-294). This could not be accounted for by extensional faulting alone, indeed that would only add up to less than 10% of the motion. It seems likely that sideways injection of around 2.5 km 3 of magma was responsible, forming a dyke extending from 2 to 9 km deep. Surface volcanism was barely noticeable, the event being represented by a small puff of felsic ash from a minor volcano while the dyke itself is twice the volume of the 1980 eruption of Mount St Helens.

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Folds in the mantle

June 2006

Seismic tomography – the processing of records of seismic waves from many earthquakes that arrive at the world-wide network of receiving stations – continues to add detail to structures in the mantle. It is based on 3-dimensional mapping of variations in wave speeds that gives clues to variations in temperature and rheological properties at depth. One of its most fascinating outcomes has been the detection of thick, steeply dipping sheets of anomalous material well below the 660 km mantle discontinuity where earthquakes cease to occur, i.e. where the whole mantle behaves in a ductile manner. These show signs of linkage to near-surface destructive plate margins, and have been ascribed to lithospheric slabs that continue to be subducted as discrete entities to as deep as the core-mantle boundary (CMB). If that were the case, it follows that their accumulation in this D" region might displace other deep material laterally, perhaps to set mantle-wide convective plumes in motion.

One such sheet occurs deep beneath the Caribbean, and is attributed to the remnants of a lithospheric plate, once forming the foundation of the eastern Pacific, which ceased to form once North America had overridden the East Pacific Rise. By analogy with the 160 Ma width of the West Pacific plate, this one would have been sufficiently extensive to reach the CMB once subducted. New tomography beneath the region no only suggests that it did, but that in doing so it accumulated as a heap of buckled material (Hutko, A.R. et al. 2006. Seismic detection of folded, subducted lithosphere at the core-mantle boundary. Nature, 441, p. 333-336). The reconstruction from tomographic results is highly reminiscent of the folding that occurs when honey or treacle is tipped into a tumbler of hot tea and falls to the bottom.. If the interpretation is correct, part of the D" zone is made up of gigantic recumbent folds of former oceanic lithosphere.

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Afar and the African superplume

Seismic tomography has also played a role in mapping zones in which hot, low-density mantle is likely to be rising – a contribution to understanding how plumes give rise to near-surface hot spots and major intra-plate volcanism. One of the largest active and long-lived zones of such thermal and magmatic activity is that of Ethiopia and Yemen , connected somehow with the opening of the Red Sea , Gulf of Aden and the East African Rift system; the Afar plume. This began about 45 Ma ago in Kenya and southern Ethiopia , reached its climax with the rapid extrusion of vast continental flood basalts of the Ethiopia-Yemen province around 30-26 Ma, and continues today in the Afar Depression. Thought by some to be a classic example of how a single upwelling of hot, low-density mantle generated a magmatic and tectonic hotspot, an alternative view is that the Afar plume is a mere near-surface part of a vast and complex system of anomalous mantle beneath the whole of southern and eastern Africa . Tomography based on the world-wide network of seismic observatories is unable to resolve the matter one way or the other. Geophysicists of the Pennsylvanian University and Carnegie Institution in the USA have analysed data from a more closely spaced network of temporary seismic stations around the famous RRR triple junction of Afar (Benoit, M.H. et al . 2006. Upper mantle P-wave speed variations beneath Ethiopia and the origin of the Afar hotspot. Geology , v. 34 , p. 329-332).

The results outline a wide (>500 km), elongated region of low P-wave speeds below 400 km that trends south-west from Djibouti, roughly parallel to the Ethiopian Rift. This is far too large to represent a classic plume, whose tails are thought to be no more than 100-200 km diameter, and whose heads on reaching the base of the lithosphere are no more than 100-200 km thick, despite spreading laterally to a radius of up to 2000 km. The huge structure is more consistent with a broad mantle upwelling that penetrates down to the lower mantle. Lower-resolution tomography does show anomalous low-speed mantle in a broad zone, which is deep in the mantle below southern Africa then rises obliquely towards the vicinity of Afar. The more detailed results support the influence of this African ‘superplume'.

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Crustal spreading from the Tibetan Plateau

In the mid 1970s Peter Molnar and Paul Tapponnier proposed that the active tectonics of eastern Asia were driven by gravitational collapse and lateral spreading of the huge mass of thickened crust that had accumulated beneath Tibet after India collided with Eurasia. The driving forces for such lateral spreading are variations in gravitational potential energy (GPE) due to regional differences in surface elevation. In the oceans, such GPE adds to plate driving forces as sliding from oceanic ridge systems that are elevated relative to abyssal plains because ridges are underlain by warmer, lower density oceanic lithosphere. Partly because the continental surface is not covered by water up to 4 km deep, the stresses resulting from GPE associated with Tibet 's high elevation are about twice as large as those connected with ridge slide. Computing the variations in GPE in eastern Asia and the adjoining oceans allows the magnitudes and directions of stresses due to gravitational spreading to be mapped ( Ghosh , A. et al . 2006. Gravitational potential energy of the Tibetan Plateau and the forces driving the Indian plate. Geology , 34 , p. 321-324).

One of the oddities discovered by Ghosh et al . is that the dominant stresses resulting from GPE differences in Tibet are oriented N-S and would tend to cause crustal spreading in those directions. Yet the surface of the Tibetan Plateau is riven with numerous N-S normal faults that indicate current spreading in E-W directions, as Molnar and Tapponier surmised. Somehow the N-S gravitational extension forces must be cancelled out, probably by traction between the lithosphere and motion of the underlying mantle driven by sea-floor spreading from the ridges in the Indian Ocean . One possibility is that the known buckling and thrusting within the oceanic part of the Indian Plate is a reflection of this balance. However, the stresses that emerge from the GPE calculations are simply not large enough to account for this intraplate deformation.

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Tibetan uplift: looking a gift horse in the mouth

May 2006

The old saying stems from it being possible to tell the age of a horse, indeed that of a number of herbivores, from the number of dark and light bands that show on the worn surface of its teeth. Because grasses contain abrasive material, such animals' teeth grow throughout their lives, different coloured material being laid down depending on the time of year. But there is a great deal more to this annual layering, from a chemical standpoint. By looking at various isotopes that are incorporated into enamel and dentine, it is possible to say where a horse—or a human for that matter—once lived (from variations in strontium-isotopes proportions for instance), and what it ate. The second forensic sign can be worked out from the carbon isotopes that a tooth has picked up during growth. Grasses have different proportions of carbon isotopes than those of other kinds of plans, such as shrubs and trees, the one depending on the so-called C3 type of photosynthesis and grasses on the C4 process. Each takes up carbon isotopes in measurably different proportion (d 13C in grasses is significantly lower than it is in C3 plants). Using carbon isotopes from teeth of fossil vegetarian animals is therefore a useful way of checking on the past proportions of grasses and other plants – often controlled in some way by climate. Neogene sediments of the Tibetan side of the High Himalaya contain abundant vertebrate faunas, and in view of the controversy over when the Tibetan Plateau began rapidly to rise (see When did Tibet Rise? in March 2006 issue of EPN) their dental geochemistry is a potentially useful approach to take. New results are somewhat at odds with those from other methods (Wang et al. 2006. Ancient diets indicate significant uplift of southern Tibet after ca. 7Ma. Geology, v. 34, p. 309-312).

Previous work using another approach (see When did Tibet Rise? in March 2006 issue of EPN) strongly suggests that southern Tibet was above 4 km elevation as far back as the Middle Eocene (40 Ma). Carbon isotopes in the teeth of Late Miocene Tibetan horses and rhinoceroses show that they ate a great deal of grass, unlike the modern yaks and wild herbivores that have to browse C3 plants. Wang and co-authors interpret this to signify that the southern Tibetan Plateau was considerably warmer than today, and also much lower: maybe around 2.5-3.5 km rather than the present 4 km or more. For elevation to change by 1-2 km in 7 million years suggests remarkably rapid uplift late in the evolution of the Plateau and adjoining Himalaya. Grasses, however, depend on both higher temperature and greater rainfall, but also on reduced CO2 in the atmosphere. They increased in their global cover only since about 8 Ma ago, when CO2 began to decline and climate cooled globally. Would it be possible for changes in the Asian monsoon to have had an effect on Tibetan vegetation, thereby explaining to dental evidence? Tibet is as dry as it is, because the monsoons now lose all their moisture in rising over the high Himalaya. If moist air and therefore cloud found its way into Tibet during the Miocene, maybe it would have been warmer too.

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When did Tibet rise?

March 2006

As plateaux go, that forming Tibet is by far the highest and the largest. Sitting at an average elevation above 5 km and spanning about 3500 x 1500 km, it dwarfs the next in the list, the Andean Altiplano (mean elevation 3.8 km). The position of the Tibetan Plateau, ahead of the Indian subcontinent's northward collision with Eurasia marks it obviously as being of tectonic origin. Some plateaux are possibly buoyed up by underlying thermal anomalies in the mantle (the Colorado Plateau of North America, underpinned by a subducted spreading centre), while others, such as that of northern Ethiopia, result partly from vast outpourings of flood basalts and partly from thermal effects of active mantle plumes and rebound associated with massive crustal extension.

There are two basic models for Tibet. It may have formed as a result of a near doubling of crustal thickness as Indian crust was driven beneath that of Asia, low density of the thickened continental crust acting to buoy up its vast area. If that is so, then as soon as India collided with Asia, around 40-50 Ma ago, Tibet would have steadily risen and its plateau would have grown in extent. There are however signs of sudden changes in thermal structure, marked by large-scale magmatism of roughly Late Miocene (8-10 Ma) age. That may have been induced by an extraordinary event, the detachment and foundering (delamination) of a large mass of underlying mantle, whose loss resulted in rapid uplift of the whole overlying region. Because Tibet is known to play a central role in the mechanism that drives the South Asian monsoon, assessing the timing of its formation is crucial to understanding the onset of the monsoon and the many phenomena of accelerated weathering and erosion associated with it. Cores from the floor of the Indian Ocean suggest that the monsoon suddenly increased in intensity at around 8 Ma. Both as a sink for carbon dioxide as a result of weathering of the continental crust, and as a means of obstructing and redirecting continental wind patterns, the growth of the Tibetan Plateau and the Himalaya in front of it have been assigned a major role in the decline of global mean temperatures that resulted in northern hemisphere glaciations. So establishing the timing of their formation makes or breaks two major geoscientific hypotheses of recent decades. The key is some form of proxy for past elevations in the area. One such proxy, the stomatal index of plant leaves found in Tibetan sediments of Miocene age, showed that 15 Ma ago the southern Plateau was just as high as today (see When did southern Tibet get so high? in March 2003 EPN). That cast doubt on a later cause of uplift, but remained unconfirmed.

Sediments deposited in lakes that periodically fill Tibet's many basins form a record that goes back at least 35 Ma. Carbonates in such lacustrine sediments offer a geochemical means of charting changes in elevation (Rowley, D.B. & Currie, B.S. 2006. Palaeo-altimetry of the late Eocene to Miocene Lunpola basin, Central Tibet. Nature, v. 439, p. 677-681). That depends on the proportion of 18O to the lighter 16O isotope of oxygen (Δ18O) in carbonate, which is believed to be inherited from rainwater that originally drained into the basins. The higher the elevation at which water falls as rain or snow, the less of the heavier oxygen isotope it contains, so Δ18O is a potential means of measuring the evolution of surface elevation. For central Tibet, this shows that the topography was at least 4 km high as early as 35 Ma ago. Results from other basins that span the Tibetan Plateau clearly suggest that 4 km elevation was achieved progressively later from south to north, anging from 40 to 10 Ma ago. So the delamination model for a sudden springing-up of the Plateau seems now to be a less plausible mechanism for the uplift than the simpler model of progressive crustal thickening following the collision of India. That does not entirely rule out an episode of delamination in the Miocene, for which geochemical evidence is fairly convincing. The implication of the new results is that if Tibet has been a major influence over climate, then it was one that developed progressively from the late Eocene.

See also: Mulch, A and Page Chamberlain, C. 2006. The rise and growth of Tibet. Nature, v. 439, p. 670-671. Kerr, R.A. 2006. An early date for aising the roof of the world. Science, v. 311, p. 758.

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Earth’s biggest ‘bull’s eye’

August 2005

Since astronauts and satellite imaging devices first made pictures from orbit, top of the list for oddness is the Richat structure of Mauritania. Sitting out in the Sahara is series of perfectly concentric rings that are almost circular. The structure is at least 40 km across, and even today, many geoscientists use images of Richat as a superb example of a meteorite impact. It is not (Matton, G. et al. 2005. Resolving the Richat enigma: Doming and hydrothermal karstification above an alkaline complex. Geology, v. 33, p. 665-668). Spectacular from space, Richat is not easily accessible. Early field work reported a breccia on a kilometric scale at its high-relief core, which unsurprisingly added to its designation as an impact structure. There are other possibilities: a structural dome, perhaps due to interference between open folds of a couple of generation; the result of upward forces from magmatic activity, such as an underlying plutonic diapir.

The rocks involved are Neoproterozoic to Ordovician sediments of various kinds, which dip radially outwards from Richat's core, so it is some kind of dome, rather than the sort of circular breach expected of an impact. Two large, basaltic ring dykes, whose centre coincides with that of the dome, cut the sediments. Other igneous materials are: carbonatites (formed from unusual carbonate-rich magmas) in dykes and sills; alkaline silicate-rich intrusions and flows occurring close to the central breccia; kimberlites in the form of plugs and sills. The central breccia is in fact a roughly horizontal lens, about 3 km across, that is made mainly of local sedimentary material, mainly once carbonates, set in a silica-rich matrix. The clasts range from highly angular to rounded, but show abundant evidence of some kind of corrosion and silicification. Matton et al. interpret the breccia as a zone of intense dissolution that caused the original sediments at the structure's core to collapse as volume was reduced as magmatic gases (supercritical fluids) rushed to the surface. So the Richat structure has all the hallmarks of doming above an alkaline igneous pluton, followed by intense hydrothermal activity that was able to dissolve carbonates and produce features akin to those formed by weathering in areas of karst. Rather than being particularly ancient, the igneous activity dates to the Middle Cretaceous. Richat is still unique. Diatremes (vertical breccia tubes) formed by explosive release of fluids from alkaline magmas are quite common, especially in areas dotted with kimberlites, but nowhere else have they produced doming on such a grand scale and with such a spectacular shape.

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Detecting the effects of slab to wedge fluid transfer in subduction zones

August 2005

A fundamental hypothesis concerning the formation of magmas above subduction zones is that partial melting in the over-riding wedge of mantle is induced by upward transfer of water vapour produced by dehydration of the descending lithospheric slab. Many aspects of the chemistry of igneous rocks in supra-subduction zone settings are explained by such dehydration-hydration. However, such fluid transfer is difficult to demonstrate, other than by its `second-hand' geochemical effects on crustal magmas. It should have another, physical effect: in the presence of water vapour, some of the dominant olivine in mantle rocks should break down to form hydrated minerals of the serpentine family. Since olivine is an iron-magnesium silicate, whereas serpentine contains only magnesium, the hydration reaction should release iron to crystallise in the form of iron oxide; specifically Fe3O4 or magnetite. Geophysicists at the US Geological Survey have been able to detect at first hand the effects of this process, thereby allowing zones of hydration in the mantle wedge to be mapped (Blakely, R.J. 2005. Subduction-zone magnetic anomalies and implications for hydrated forearc mantle. Geology, v. 33, p. 445-448). As well as finding substantial magnetic anomalies caused by the release of magnetite by olivine dehydration over the forearc of the Cascadia subduction zone in Oregon, they show gravity anomalies that reflect density variations in the underlying mantle. The other aspect of the olivine-serpentine transformation is a large decrease in density, which should result in a decrease in gravity anomaly should sufficient olivine have been transformed. The coincidence of gravity lows with magnetic highs allowed Blakely et al. to model the location of hydrated mantle wedge in the Cascadia subduction system: probably just above the zone where subducting oceanic crust is transformed to ecologite.

Serpentinite also has a marked effect on the rheology of mantle rocks, because of its ease of ductile deformation. It should allow subduction deformation to proceed in a continuous fashion within the part of the system where it occurs, yet may focus sudden strain in great earthquakes to shallow levels up-dip of its position.

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Erosion and plate tectonics

May 2005

For about 15 years geomorphologists have been conscious that landforms are not merely 3-D shapes left behind after the erosion of valleys, but are dynamic in the vertical dimension as well as those in plan view. Between 1990 and 1992 Peter Molnar and Phillip England developed ideas about the way in which erosion unloads the crust and triggers an isostatic response, if the mass removed is sufficient for buoyancy to overcome viscous forces. In a general sense there is nothing new in the idea of isostatic uplift in response to erosion, but Molnar and England looked closely at some specific predictions for areas of rapid erosion. If a peak is surrounded or a ridge is separated off by rapidly deepening valleys, they are likely to rise continuously. Simple examples are seen in the Ethiopian-Eritrean Escarpment that forms the western flank of the Red Sea and Afar Depression of NE Africa. Massive erosion has been in response to uplift over the last 25 Ma, following initiation of the Red Sea and Afar rifts. The uplift is preserved by the Ethiopian Plateau that ranges from 2.4 to 3 km above sea level. Beyond the rim of the escarpment are numerous peaks and ridges that out-top the nearby plateau, and moreover are made of ancient basement, while the Plateau is capped by Tertiary flood basalts, which must have been eroded from the outlying highs. Molnar and England believed that similar processes contribute to the rise of the world's highest peaks in the Himalaya, and indeed entire mountain ranges, provided climatic conditions resulted in rapid erosion.

Uplift must also be accompanied by deformation in the deeper crust, and that may have some influence on tectonics. A group of structural geologists from MIT and Dartmouth College, New Hampshire has looked for signs of such an influence in the foothills of the Himalaya, where there is probably a rapid change in the pace of erosion due to changing precipitation from the South Asian monsoon (Wobus, C. et al. 2005. Active out-of-sequence thrust faulting in the central Nepalese Himalaya. Nature, v. 434, p. 1008-1011). There, the overthrusting that helped thicken the crust after subduction of the Indian plate beneath Asia, has progressively migrated southwards from the Main Central Thrust, now inactive, that forms the southern flank of the Greater Himalaya. Most structural geologists believe that active faulting at the surface is concentrated on the southernmost, Main Frontal Thrust, not far north of the Gangetic plains. However, the greatest erosion today is taking place where monsoon prcipitation is most intense, between the two huge thrust faults where elevations start to rise rapidly towards the Greater Himalaya. Using a mixture of Ar-Ar dating of the cooling ages of micas, and cosmogenic dating of quartz grains in the active sediments of rivers, Wobus et al. have been able to plot changing uplift and erosion rates in a N-S traverse, for the past and in recent times respectively. Their results show a sudden fall in erosion and uplift about 100 km north of the active Main Frontal Thrust, over a mere 2 to 3 km at most, that coincides with a regional muting of topographic relief. Such an abrupt break most probably relates to an active thrust system that breaks the accepted N to S shift in regional thrusting through time: hence, out-of-sequence. Rapid erosion and rugged topography makes it difficult to spot dipping faults, in the field and from satellite images. The inferred major thrust system coincides well with the maximum monsoon precipitation, so it is likely that increased erosion resulting from that has cause a response in deeper active tectonics.

See also: Burbank, D.W. 2005. Cracking the Himalaya. Nature, v. 434, p. 963-964.

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The boys on the black stuff

November 2004

Tectonic activity continually re-paves the oceanic part of the Earth, though not in the manner of the awesome night-time machines seen frequently by owlish drivers as they negotiate the contraflows and cones on highways, large and small. Slab-pull helps ease plates apart, forcing asthenospheric mantle to rise and partially melt as pressure falls off. Or, at least that is widely believed, for active mid-ocean processes can only be observed at second-hand through samples scraped from the exposed ridge surface for analysis. What once lay at the guts of spreading centres emerges only when slabs of ocean lithosphere slide nicely over continental margins because of compressive forces related to plate subduction. Gravity demands that such obduction is a rare and special process, since oceanic lithosphere is denser than that of continents. Indeed, as ocean floor ages and cools it become increasingly likely to founder into the deep mantle. Ophiolites represent oddly buoyant parts of the ocean floor, almost certainly because they were once thermally anomalous or quite young at the time of their emplacement. There is no guarantee that they represent run-of-the-mill oceanic lithosphere. However, structures in them, especially a subsurface layer made of innumerable basaltic dykes and little else, show concretely that magmatism was dominated by continual extension; exactly as expected for a former spreading centre. The most studied ophiolite is that of the Semail Mountains in Oman , which exhibits every definitive layer of lithosphere that point to magmatism in an extensional oceanic environment. The crustal part is not the best guide to the ophiolite's genesis, because melt chemistry varies so much with pernickety vagaries of melting and fractionation. It is the mantle sequence that reveals what went on (Le Mée, L. et al . 2004. Mantle segmentation along the Oman ophiolite fossil mid-ocean ridge. Nature, v. 432, p.167-172). Laurent Le Mée and colleagues from the University of Nantes focus on chemistry and mineralogy of the well-preserved ultramafic rocks in the Oman ophiolite's mantle layers. Their results show how a whole number of petrogenetically important chemical features vary systematically parallel to the original axis of spreading, to define three distinct axial segments. Within each are other regular fluctuations that define segments of lesser magnitude. This along-axis chemical variability can be modelled in terms of large variations in the degree of mantle melting (between 10-30%), with the lowest degree coinciding with the major segment boundaries. Those discontinuities also tally with increased numbers of mantle-cutting dykes (not the crustal sheeted dykes). Major segments probably formed from regional upwellings of asthenosphere, whereas those with shorter wavelengths reflect individual diapirs. Along active spreading centres, segmentation of chemical affinities in basalt lavas seems to link with various magnitudes of transform faulting, and it is this local tectonics that shows up so nicely in the Oman mantle sample.

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Mantle dripping off mountain roots

September 2004

Continental arcs, such as the Andes, parts of the Himalaya and Tibetan Plateau and the Sierra Nevada of the western USA, are stuffed with granite intrusions. Large volumes coalesce to form classic batholiths. It is now well-accepted that very little of the granitic magma originated by melting of older continental crust, but by processes of fractionation from more mafic parent magmas. That presupposes a layer of dense, mafic to ultramafic cumulates below and complementing up to 30 km of batholithic crust. The overall density of the continental arc crust would be high relative to that of the granites themselves. So the fact that many batholithic cordilleras are topographically high suggests one of several processes: either the granitic part of the crust has become tectonically thickened relative to its denser root, or that root has separated from the continental lithosphere as a whole, and sunk into the mantle. Such decoupling, or delamination, would induce the remaining lithosphere to rise dramatically. Also, its descent could result in partial melting to produce peculiar potassium-rich basaltic magmas. The latter occur in Tibet and their presence there has been linked to foundering of deep lithosphere, that may have triggered the relatively recent surge in Himalayan uplift. Proving the existence of a descending lump of lithosphere is not easy, but developments in seismic processing can make a crucial contribution, if sufficient data are available for a suspected zone of delamination. The western USA is blessed with lots of seismic stations, so is a natural place to try out the new techniques as a test of the hypothesis. George Zandt of the University of Arizona, and other US colleagues have come up with interesting results (Zandt, G. et al. 2004. Active foundering of a continental arc root beneath the southern Sierra Nevada in California. Nature, v. 431, p. 41-46). Their analyses of seismic data shed light on a late stage in the development of the Sierra Nevada. During the Mesozoic Era, subduction beneath North America of the now disappeared Farallon plate of Pacific ocean lithosphere built up the Sierra Nevada batholith. About 10-16 Ma ago, subduction stopped and the plate margin became one of transpression, the most prominent feature of which is the San Andreas Fault. At that stage, a "drip" of dense cumulates began to form, and subsequently separated to descend into the mantle. Cruustal rebound was not simple but included zones of extension, as well as tell-tale high-K volcanism during the Pliocene.

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Quantifying motions inside continents

February 2004

If you are a member of the Geological Society of America you will either have heard or read the 2003 Address of its President (Burchfiel, B.C. 2004. New technology; new geological challenges. GSA Today, v. 14, p. 4-10). If not, get the February 2004 issue of GSA Today, if only for the wonderful illustrations in Burchfiel's paper. His topic is how the use of ever-increasing precision of satellite global positioning (GPS) has revolutionised continental neotectonics, since it began to be used by geoscientists in the late-1980s. The illustrations have a backdrop of what I suspect to be the 90m resolution Shuttle Radar Topography Mission (SRTM) digital elevation model (DEM), and show the fine topographic detail that stems very much from active tectonic movements. Superimposed on them are estimates of the speed at which points on the surface are moving and the directions of motion, gathered using GPS technology. Measured in mm per year, these velocities stem from the most precise positional measurements, with the degradation built into the GPS satellite signals for US military reasons (turned off in 2001) removed using differential processing. They are averages representing motions over the last 17 years or so. The most dramatic example covers the Tibetan Plateau and areas to the east of it, based on extensive work by Chinese scientists.. In general it shows a sort of clockwise swirling away of expelled crust east of the Eastern Himalayan Syntaxis (the "big bend" at the eastern termination of the Himalaya) in the ranges through which the headwaters of the Irrawaddy, Salween and Mekong rivers flow, rather than the eastward expulsion towards the China Sea first postulated by Tapponier in the early 1980s. Field studies suggest that this kind of motion has been going on for at least the last 4-6 Ma. Another conflict with expectation lies in the area of the Longmen Shan mountains and the huge Sichuan Basin of western China. A simple model of crust being expelled from the zone of the India-Asia collision suggests that Tibetan crust would be moving eastwards here to throw up the steep front of the Longmen Shan above the Sichuan Basin. There is in fact very little sideways movement at the surface. Explaining this requires deep crust from Tibet moving in a ductile manner far below, thereby "inflating" the Longmen Shan where entirely different kinds of crust are juxtaposed.. Many of the motions in East Asia can only be explained in terms of differential movements at different levels in the lithosphere, and the influence of subduction systems, such as the Indo-Burman and West Pacific, as well as the long-suspected expulsion of over-thickened crust in Tibet due to increased gravitational potential there.

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Geoscience consensus challenged

December 2003

The history of science shows that what is widely agreed is generally wrong. Yet, there is more than the temptation of cosiness, and the ease of publication that goes with it, that induces even the most imaginative scientists rarely to stick their necks out. In their overthrow of the geocentric view of the cosmos, both Copernicus and Kepler felt ideological pressures that we can only guess at. Colleagues of Copernicus had been burnt at the stake, so he hid himself for the 40 years of his life and only dared publish his ideas so late that the galleys arrived at his deathbed. Kepler, a Protestant in the Holy Roman Empire, kept one step ahead of trouble by networking that would done many a modern scientist proud, and a sort of Bowdlerisation of his ideas so that they merged almost seamlessly with the prevailing ideology of both sides of European Christianity. Even the bravest, most honest and gifted scientists generally agree with their peers, simply because they rarely know any better. If they do, they either keep or are kept quiet. There is very little, if any objectivity in the science of any age… because it is scientists who do it! Kepler cuddled up to Tycho de Brahe, he of the gold and silver nose (fitted after student duelling), in order to gain access to Tycho's observational data when the old feller died. He got them alright, and began to turn the universe back on its feet, thereby opening an avenue for Newton. Neither Kepler, an unstable hypochondiac who was good at geometry, but not much else, nor Tycho, an anal retentive maker of revolutionising instruments and the founder of empirical science, but devoid of ideas, would have been celebrated for four centuries if the one had not worked with the other. The evolution of science has been marked by the influence of non-conformists, but few worked in isolation against the mainstream.

One modern geoscientist who seems rarely to conform is Warren Hamilton of the Colorado School of Mines, and now he has gone for it big time (Hamilton, W.B. 2003. An alternative Earth. GSA Today, v. 13(11), p. 4-12). His starting point is to challenge the consensus among geophysicists and geochemists that the mantle has a still-unfractionated lower part beneath depleted upper mantle which has sourced oceanic and continental lithosphere progressively over time. Linked to that is the notion of easy circulation of material from top to bottom through descending, subducted slabs and plumes rising from the core-mantle boundary. Hamilton says that neither exists, and that upper and lower mantle are decoupled. His challenge stems from the certainty that the Earth accreted "hot, fast and violently", and the strong likelihood that its Moon originated after a titanic collision of Earth with a Mars-sized planet less than 100 Ma after accretion. Chances are it became wholly molten and suffered massive loss of volatiles. Such a body would have fractionated rapidly, to produce a lower mantle very unlike that imagined by most geochemists and geophysicists. Moreover, it would have remained so, partly due to its likely perovskite mineralogy, highly fractionated nature and phase-change barriers to transfer of matter – the 630, 1000 and 2000 km discontinuities. Such an early scenario would have transferred most potassium, uranium and thorium into the outermost Earth, where the generation of radiogenic heat would have concentrated. This is very similar to models proposed in the 1960s and early 70's by J.V. Smith and others, when lunar geochemistry, particularly that of the anorthositic highlands, set in motion ideas about a planet-wide magma ocean and global fractionation as it cooled. Like Smith and others, Hamilton considers continental crust to have formed rapidly, sequestering a large proportion of the elements that make mantle rocks "fertile". But only traces remain in the form of a small pinch of pre-4 Ga zircons, that could easily be lost in a single sneeze. Much of this early sial returned swiftly to the upper mantle to make it increasingly heterogeneous – fertile parts and some not so petrogenetically prone.

The current consensus has its roots, according to Hamilton, in much older ideas about the early phases of Earth's evolution. Harold Urey and others in the 1950s and early 60s considered the planet to have formed by slow, cold accretion of the most primitive meteoritic materials, chondrites, particularly those containing carbonaceous materials. They are petrogenetically highly fertile, and the radioactive heating of a chondritic Earth, plus that from core formation, would involve a continual, slow fractionation of the mantle that would probably still be going on today. That this fundamental set of assumptions still dominates, though is rarely mentioned, is down to the rapidly increasing number of mantle profiles based on seismic tomography, that are claimed to have imaged seismic-speed anomalies that could be explained by both slabs and plumes extending to the core-mantle boundary. Hamilton makes the reasonable point that the very irregular distribution of earthquakes in the top 600 km of the Earth leaves large volumes of the mantle in blind spots, and that the majority that are used are subduction related. That, he suggests, predestines tomograph images to create artifacts that just "look" like deep penetration of descending slabs. Moreover, stunning as they look in publications, there is much graphic sleight of hand that assigns primary colours to lower mantle anomalies that have an order of magnitude lower amplitude than those at shallower depths, as well as filling unimaged areas with average or interpolated values, placement of sections to look most plausible, and a great deal of data filtering. There is a "fudge factor" that hypes the hoped-for, and avoids alternative data analysis – you can't do this kind of thing on a PC. The plume hypothesis is falsified exactly where it ought not to be – in the Emperor-Hawaiian seamount chain (see Wandering hot spots in the September issue of EPN). There the great bend dated at 45 Ma is not matched by any known change in the direction of Pacific sea-floor spreading. The magma source for the chain might well be a restricted volume of mantle, but it didn't stay still as a plume must. Seismic tomography, at the time Hamilton's essay went to press, had not verified a single plume sourced in the lower mantle – there are many cases of volcanic hotspots without any plume, and tomographically inferred hot mantle doesn't always have a volcanic expression.

Hamilton's essay is worth reading in its entirety, as it reviews the whole of Earth's tectonic and magmatic evolution. I have just tried to pick out the critical aspects here.

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More, or less plumes

December 2003

In view of Warren Hamilton's questioning the existence of mantle plumes (Geoscience consensus challenged), in the same month as his essay appeared a team of seismologists from the universities of Princeton, California, Colorado and the National Taiwan University used a new approach to seismic tomography to seek evidence for plumes (Montelli, R. et al, 2003. Finite-frequency tomography reveals a variety of plumes in the mantle. Science Express www.sciencexpress.org, 4 December 2003, p, 1-10). They present evidence for 32 suspected plumes. Some have a seismic expression at shallower depths than 650 km in the mantle, such as beneath Iceland and the Galapagos. Others seem to reach as deep as the core-mantle boundary, as beneath Hawaii and the Kerguelen Plateau. In fact most of the classic volcanic hotspots that have associated chains appear to have plumes beneath them, with the exception of Yellowstone. An apparent duality of shallow and deep plumes suggests to the authors a two-tier division in vertically moving mantle, above and below the 660 km discontinuity. The long-suspected major plumes beneath Africaand the Pacific also appear to spawn lesser plumes, that in turn sometimes split.

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Wetting oceanic lithosphere

October 2003

Loss of watery fluids from downgoing subduction zones and their rise into the over-riding mantle wedge is the main reason why arc magmas form there by partial melting under high pH₂O conditions. It is usually assumed that all oceanic crust becomes thoroughly hydrated by circulation of seawater shortly after it forms at constructive plate margins. However, many oceanic basalts from ophiolites or dredged from the ocean floor are very fresh. It also seems that to explain the depth of fluid-influenced melting in some volcanic arcs, large amounts of water must be coming from the mantle part of the subducted slab. That is more difficult to hydrate by sea-floor hydrothermal processes. German and US geophysicists have found abundant evidence for faults oceanwards of where the Cocos Plate bends to descend below the Middle America Trench (Ranero, C.R. et al. 2003. Bending-related faulting and mantle serpentinization at the Middle America Trench. Nature, v. 425, p. 367-373). The faults show up clearly on detailed bathymetric images as wrinkles on the ocean floor off Nicaragua, and high-resolution seismic reflection profiles show that they penetrate deep into the mantle part of the Cocos Plate. Water can easily make its way down to form serpentinite from mantle peridotites just before the slab plunges down the subduction zone.

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Zircons that wander

August 2003

The crust beneath the British Isles is made up of several once widely separated terranes, parts of Laurentia, an arc segment called Avalonia that split from Gondwana around 500 Ma ago, and a similar terrane (Armorica) that followed Avalonia across the Iapetus Ocean to accrete to Laurentia at the end of the Palaeozoic Era. Because of its maritime position, modern Britain is cloaked in vegetation so that rock occurrences are few and far between by comparison with less humid areas. Conditions for geological investigations are made yet worse by a mantle of glacial sediments plastered on top of bedrock. So, although having been studied for longer than almost every other piece of continental crust, the evolution of that beneath the British Isles is a subject of continual controversy and surprises. Sitting at the interface between the Laurentian and Avalonian terranes, roughly where the Iapetus suture is thought to have consumed at least half of the eponymous ocean, sit the Lower Palaeozoic rocks of the Southern Uplands of Scotland. They are widely thought to have formed as an accretionary prism on the edge of the plate underidden by subducted Iapetus oceanic lithosphere until Avalonia collided with the north-British terranes at the close of the Silurian. Some of the Ordovician sediments in the pile contain clasts of volcanic rocks, which were long thought to be contemporary and giving evidence of the expected arc volcanism behind the prism. However, they turn out to be much older, now that zircons from the sediments have been dated using high-preciiision methods (Phiilips, E.R. and 7 others 2003. Detrital Avalonian zircons in the Laurentian Southern Uplands terrane, Scotland. Geology, v. 31, p. 625-628). The zircons yielded Neoproterozoic ages (557 to 613 Ma), with evidence that some had been assimilated from older crust (1043 Ma) during volcanism. Taken at face value, the Neoproterozoic ages are similar to those of volcanic rocks in England and Wales, which formed off Gondwana in an arc setting, when the terranes were widely separated. The problem is one of getting the material across the subduction zone that separates the accreted terranes, but that is the issue proposed by the authors (all from the Natural Environment Research Council. However, such a conclusion might stem from the authors' narrow context; that of British geology. Immediately to the north of the Southern Uplands terrane is another, poorly exposed crustal block that underlies the Scottish Midland Valley. It was directly involved in the Ordovician Grampian orogeny that formed the highly deformed Precambrian rocks of the Scottish Highlands. With a narrow view, that terrane is also a mystery, yet it has a counterpart in the Taconia terrane that is familiar to North American geologists, which was involved in orogenic events contemporary with the Grampian orogeny in Scotland. Taconia has late Neoproterozoic to Ordovician arc volcanics.

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Setting up subduction

July 2003

Although they have roughly the same size and overall density, and probably very similar bulk compositions, Earth and Venus behave in very different ways. The Earth has plate tectonics, whereas radar images how that Venus has no such phenomenon. For the most part, Earth loses its internal heat production steadily and plate movements are intimately bound up with that generalised convective heat transfer. The surface of Venus has seen no significant deformation in half a billion years. In fact, that surface was probably formed by a massive blurt of magma around late Cambrian times. In some respects that is similar to the roughly 30 Ma appearance of flood-basalt volcanism on Earth, but on a scale that dwarfs large igneous provinces such as the Deccan and Siberian Traps. Quite probably, Venus builds up thermal energy in its mantle, until its release by massive partial melting. The key to Earth's behaviour seems to be the fact that its oceanic lithosphere is able to break and descend into the mantle. The gravitational force down a subduction zone is sufficient to keep plate tectonics going. But why does it start? Oceanic lithosphere is as strong as that beneath continents, and the other main force involved in plate tectonics, due to the gravitational effect of deepening sea floor as it cools away from constructive margins, is so low that it is unlikely to result in lithospheric failure. This vital, but often overlooked topic is nicely reviewed by Stephen Battersby, a consultant to New Scientist (Battersby, S. 2003. Eat your crusts. New Scientist, 30 August 2003, p. 30-33).

A possible explanation lies in the way in which the strength of the main mantle mineral, olivine, varies with the presence of water. Even minute amounts of water allow hydrogen ions to enter the olivine molecular lattice, thereby creating defects that can migrate and result in softening of the mineral. Experimental deformation under mantle conditions, carried out at the University of Minnesota, show ten-fold decrease in olivine's strength with as little as 20 parts per million of available water. Subduction at continental margins might therefore be set in motion by the weight of sediments accumulating on the ocean floor, and with time that weight increases as the continents are eroded. The other factor, perhaps bearing on the start of intra-oceanic subduction that forms island arcs, is the effect of transform faults and fracture zones that separate segments of different age and therefore density. Maybe that sets up forces that stress the oceanic lithosphere. The big problem is that the bulk of the oceanic lithosphere, is mantle rock, and when it has been left as a residue by the basalt melting at constructive margins, it is well-nigh anhydrous. To soften it demands a source of water that permeates the peridotite. An obvious source is seawater penetration, but at the depths involved any pathways seal up tightly. Possibly there are wet masses in the deeper mantle, either as a result of earlier subduction or dating back to Earth's origin. Slow convection in the deep mantle could bring these into contact with the base of the oceanic lithosphere, where their water could permeate and weaken it to the point of failure. Just an idea, maybe. However, seismic tomography, so effective at charting the distribution of hot and cold (low- and high-velocity) mantle rocks, is also able to suggest places where damp, weak rock occurs in the deep mantle. One such low-velocity blob occurs beneath the eastern seaboard of North America (maybe a relic of the Palaeozoic Iapetus subduction zone that runs parallel to the present margin), where there is, as yet, no sign of subduction. But there is little sign that the blob is abnormally hot, and in all probability it is damp. The history of tectonics suggests that no ocean remains with passive margins forever, and inevitably subduction ends up devouring it, in 200 Ma at most (the greatest age of today's ocean floor). Given time the eastern USA may rank with the Andes!

So why does Venus behave so differently? Although we cannot yet analyse any Venus rock (there are no accredited Venusian meteorites!) there is a plausible scenario. Venus is the greenhouse planet. It is highly unlikely that it ever harboured life, particularly of a photosynthetic kind which could have produced free oxygen. In the Earth's atmosphere, it is the presence of ozone in the stratosphere that gives the atmosphere its peculiar thermal structure, especially the tropopause. That marks a sudden cooling that limits the height to which water vapour can rise before freezing out. In the stratosphere temperature warms up with height, due to the minor "greenhouse" effect of ozone. Venus probably never has a tropopause, so that clouds of water vapour could rise to the outer limits of the atmosphere warmed by high CO­₂ levels. In contact with ultraviolet light, water dissociates to hydrogen and oxygen, and at high levels the hydrogen leaks away to space. Any oxygen is quickly drawn down by oxidation of iron at its surface. So Venus has progressively lost all its water and as a result is a tough nut to crack, as regards forces in its interior. Earth on the other hand is a bit like a fondant chocolate…

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Wandering hot spots

July 2003

It was once an axiom of plate tectonics that volcanic-island and seamount chains provided robust evidence for sea-floor spreading. Jason Morgan in 1971 developed the notion, based on a pre-plate tectonic idea by John Tuzo Wilson, that within-plate oceanic volcanic islands derived their magma from upward moving plumes in the mantle below the lithosphere. Many of them in the Pacific have extinct volcanic islands and seamounts arranged in straight chains that parallel the direction of sea-floor spreading shown by magnetic stripes. He likened their formation to the burn mark on a sheet of paper passed slowly over a candle flame. The Hawaii-Emperor chain bucks this hypothesis, by being profoundly bent from a WNW trend in its youngest part to north for ages greater than about 50 Ma. The problem is that neither leg is at right angles to the magnetic stripes, which does rather suggest that hot spots move. Hot spots have long been used as a frame of reference for absolute plate motions, but if one has moved then so might all the rest, and how they have moved would probably be independently of one another. Absolute motions then are hard to judge. The key to checking on the suspected hot-spot drift is to look at the palaeolatitude of differently aged volcanic rock samples along a chain. This has been achieved using palaeomagnetic measurements from the S-N Emperor chain (Tarduno, J.A. et al. 2003. The Emperor seamounts: southward motion of the Hawaiian hotspot plume in Earth's mantle. Science, v. 301, p. 1064-1069). The test proved positive; the hotspot itself moved southwards between 81 to 47 Ma, while the Pacific plate was itself moving. Other tests suggest that hotspots in the Indian and Atlantic Oceans were indeed fixed for long periods, but the Pacific ones seem to have had a tendency to wander. Why that has happened is possibly connected to deep mantle flow, which might bend the plumes to which the hot spots owe their magmatic activity. Maybe their source region in the mantle shifts for entirely different reasons. Seismic tomography of the mantle has had some success in tracking the shapes of plumes, but not for relatively small ones because of its present poor resolution. One large plume that has an enormous tilt in the vertical dimension starts near the core-mantle boundary beneath the South Atlantic and hits the lithosphere in the Red Sea. No-one knows why, but its magmatic expression in the volcanic rocks of east Africa suggest that it too has moved from beneath Kenya about 50 Ma ago, across Ethiopia to its present position that fuels active volcanoes in the Afar Depression of NE Ethiopia, Djibouti and Eritrea.

See also: Stock, J. 2003. Hotspots come unstuck. Science, v. 301, p. 1059-1060.

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Rodinia muddles

July 2003

In the early 1990s, Ian Dalziel, Eldridge Moores and Paul Hoffman speculated on the former existence of a supercontinent comparable with Pangaea, between about 1100 and 750 Ma. The name Rodinia, from the Russian for Motherland, seemed appropriate. They based sketchy reconstructions on the way in which orogens formed almost globally between 1300 and 1000 Ma could be fitted together by shuffling older crustal fragments, along with evidence from sediments in North America, and Antarctica that the supercontinent began to disassemble around 800 Ma. A great conundrum of later Neoproterozoic times seemed to be partly resolved by what might have happened when Rodinia broke apart and its fragments drifted across the globe. This was the event that welded together the southern supercontinent of Gondwana between 800 to 500 Ma ago, forming the web of orogens known colloquially as the Pan African and Brazilide belts of Africa and South America. Palaeomagnetic pole positions for the 1200-750 Ma period, from the supposed components of Rodinia, were an obvious test of Rodinia's former existence and its gross structure. As they appeared the palaeomagnetic data seemed to confirm the early ideas that were based on Wegener's method of linking now far-separated orogens to reassemble his Carboniferous Pangaea supercontinent. A reasonable consensus existed by the early years of the 21^st century. One of the main contributors of palaeopole data for Rodinia reconstruction has been Trond Torsvik of the Geological Survey of Norway, so it is noteworthy that he has cast the first shadows of doubt on what seemed to be an elegant general solution to more than half a billion years of global tectonics (Torsvic, T.H. 2003. The Rodinia jigsaw puzzle. Science, v. 300, p. 1379-1381).

The problem that Torsvik recognises is that superficially convincing geological jigsaw fits are coming into increasing conflict with better evidence for the palaeolatitudes of different segments. This is compounded by a lack of palaeomagnetic data for some of the 13 major continental segments that had formed earlier in Precambrian times. The central element in the original Rodinia model was the way that India, Antarctica and Australia's 1300-1000 Ma orogens fitted in what appeared to be a rational reconstruction of East Gondwana. The first fly in the ointment is that revision of Australia's palaeolatitude seems to make its fit with India impossible. Likewise the position of the geologically fitted Congo and Kalahari cratons, that now make up West Africa, is less certain. Amazonia is also not "behaving" as expected, and Baltica may have been rotated by 180 degrees relative to its former orientation in the old Rodinia model. As well as varying quality of palaeomagnetic data, and its lack from crucial components such as Siberia and North China, their dates vary so much that it is impossible to allow for large-scale readjustments through the lifetime of the putative supercontinent. Torsvik figures a "worst case" scenario, in which the whole Rodinia concept becomes merely continents that were near one another and separated by a variety of active rifts; something of a dog's breakfast that should spur more dating, palaeomagnetism and tectonic research on the orogens that first suggested a grand unification. That is, if the main proponents do not become so profoundly depressed that they simply give up!

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Plume debates

June 2003

Jason Morgan's recognised in the early 1970-s that chains of volcanic islands and seamounts, such as the Hawaii-Emperor Chain, which cross sea-floor magnetic stripes, might have resulted from mantle "hot spots" that are fixed relative to motions of lithospheric plates. He went on to suggest that such magmatic anomalies might reflect narrow thermal upwellings within the deep mantle, and applied the term "plumes" to these notional convective zones. Geochemists have since flocked to active and extinct manifestations of within-plate magmatism, and developed a whole sub-culture of classification and hypotheses concerning their origin and inner workings. By the end of the 1990s over 5000 candidates for underlying plumes had been proposed, some still active and others inferred for past events, such as flood basalt provinces. Processing of seismic signals using supercomputers over the last few years has used them to map variations in P- and S-wave speeds at different depths in the mantle. Speeds below those expected are likely to reflect hot mantle relative to high-speed, colder regions. So seismic tomography potentially charts hot rising mantle and cool, descending parts; seemingly ideal for detecting mantle plumes and how deep they extend. Early results centred on proposed plumes were a mixed bag. Some seemed to have very deep origins, perhaps down to the core-mantle boundary, whereas others appeared to be above hardly anomalous mantle. Most exciting was a zone of hot, probably rising mantle with a source at the top of the core beneath the South Atlantic, yet whose upper parts sloped obliquely upwards towards the Red Sea. It seemed that the Afar plume, believed to have been responsible for continental flood volcanism in Kenya and the Ethiopian Plateau, and perhaps the East African Rift and opening of the Red Sea, still existed. Hot-spot activity is a minor aspect of global tectonics today, so it is not an ideal time to ponder on plumes. If they are real, then periods of massive flood volcanism would have been responses to superplumes, but the last in Ethiopia was 30 Ma ago.

Exciting as seismic tomography is, its resolution is currently too coarse to pick out the most revealing features of the plumes that potentially it could detect. To have sufficient gravitational potential energy to rise through the entire mantle, a very large volume is required, and that is assigned to the "plume head". Some hotspots are over large volumes of hot mantle, but they lie just beneath the lithosphere, and could have their origin at any level in the mantle. The tracks that they followed, if any, and which might continue to be a conduit for uprising material would be much narrower. Such predicted "plume tails" are too small for resolution by current tomography. A compilation and re-classification of hot spots (Courtillot, V. et al. 2003. Three distinct types of hotspots in the Earth's mantle. Earth and Planetary Science Letters, v. 205, p. 295-308) has whittled down candidates for mantle plumes to a mere 50 or so, with less than 10 likely to have risen from core depths. Two responses have arisen about this hugely popular topic: that Morgan's ideas are still basically valid, but need more work (DePaulo, D.J. & Manga, M. 2003. Deep origin of hotspots – the mantle plume model. Sciene, v. 300, p. 920-921); that hotspots might be linked to plate tectonics, and that mantle plumes are nothing more than a "belief system" (Fouger, G.R. & Natland, J.H. 2003. Is "hotspot" volcanism a consequence of plate tectonics? Science, v. 300, p. 921-922). A sensible aim that might resolve matters is to seek materials from the largest magmatic events – flood basalts – that should contain unambiguous geochemical signs that their parent mantle was at some stage exchanging matter with the core, if they had formed after rise of a superplume. But, every line of approach to deep-mantle processes relies on proxy evidence, several steps removed from actual events and properties. That makes David Stephenson's proposal for a mission to the core (above) so urgently in need of support!

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Hydrogeology of sea-floor cooling

March 2003

Much of the Earth's internal heat production escapes from the ocean floor, by a combination of direct cooling of new lavas at ridges, hydrothermal pumping of seawater through oceanic crust and conduction. These processes are responsible for the increase in density of oceanic lithosphere that causes the ocean floor to gradually deepen away from spreading axes, thereby adding a gravitational force (ridge-slide force) to help drive plate tectonics. The cooling also ensures that oceanic lithosphere is sufficiently cool at destructive margins for metamorphic processes in subduction zones to increase its density above that of the mantle, thereby largely driving plate tectonics through slab-pull force. More than 70% of internal heat loss through the oceans is dissipated through crust that is younger than 1 Ma. Much of that emanates from huge hydrothermal geysers, about which a great deal has been revealed in recent years. What of the other 30% that escapes through older crust? The older it is, the more it is literally blanketed by sediments that should act to block circulation of seawater, because they are so fine grained and impermeable. It might seem as if heat lost would have to be by conduction alone. That is not sufficient to explain the shape of the ocean basins. However, some recent work near the Juan de Fuca Ridge in the NE Pacific by a team from the USA, Canada and Germany (Fisher, A.T. and 12 others 2003. Hydrothermal recharge and discharge across 50 km guided by seamounts on a young ride flank. Nature, v. 421, p. 618-621) shows that basic principles of hydrogeology guide seawater to increase heat loss. Outflow is not through the sedimentary cover, but through seamounts, which are outcrops of the underlying igneous part of the crust. Like many springs on land, the water that flows from them can come from far afield. The sedimentary cover acts as an aquiclude, making the crystalline crust a confined aquifer, but for any flow to operate water must infiltrate the ocean floor. Fisher and colleagues have found that some seamounts have higher heat flow than others, and are sites of outflowing warm water. Some have anomalously low heat flow and may well be sites where seawater is infiltrating. Dating outflowing water using ¹⁴C reveals that it is very young, and must have flowed rapidly, yet in their study area there are no signs of significant recharge through the sediments. One seamount, 50 km from another which discharges water is the only likely source. So, it seems as if the distribution and number of sea mounts on the oceanic part of a plate might bear greatly on the processes that eventually take place when the plate is subducted. "Pimply" plates could have cooled more than smooth plates with an unbroken blanket of inefficiently conductive sediments.

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Eskola's mantled gneiss domes revisited

February 2003

The Finnish geologist Pentti Eskola famously recognised in the 1940s that many basement terrains throughout the world, particularly in Scandinavia, have large tracts of gneiss in the form of domal structures separated by synforms (mantles to the domes) of supracrustal rocks.  These mantled domes give a curious "egg-box" appearance to the geology of many shield areas, usually picked out by the conventional pink colours used to signify granitic rocks and greens for supracrustal belts.  Once it was recognised that interference between upright folds of different ages and with different axial trends could produce "egg-box" structures on the outcrop scale, many structural geologists turned to this as an explanation for the huge features recognised by Eskola, even suggesting that the "mantles" were above profound unconformities.  Eskola's view was that these regional features were due to differential uplift of low-density gneisses and more dense supracrustal rocks, and this view lingers with many other geologists.  Christain Teyssier and Donna Whitney, of the University of Minnesota, have reviewed the current state of knowledge for the phenomenon (Teyssier, C. & Whitney, D.L. 2002.  Gneiss domes and orogeny.  Geology, v. 30, p. 1139-1142), and conclude something more involved than either hypothesis.  Many of the gneiss domes show evidence for the involvement of crustal melting in response to decompression as orogens evolve, almost certainly resulting from removal of the upper crust, either by rapid erosion or extensional tectonics.  As well as forming bodies of melt or near-molten migmatites, such a process weakens the crust, allowing masses of low-density crust, including the partially melted bodies, to rise rapidly.  This feeds further decompression, the whole process becoming an effective means of advective heat transfer in large orogens.

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Archaean tectonics was different

February 2003

Higher mantle heat production in the past suggests that at some stage in the evolution of plate tectonics oceanic lithosphere would arrive at destructive margins too hot for oceanic basalt to dehydrate and form eclogite.  Without excess density over that of the mantle, conferred by subducted eclogite (3300 kg m^-3), the lithospheric slab would descend at a shallow angle, oceanic crust would probably undergo wet partial melting, and maybe slab pull force would be so low that subduction was a hit or miss affair.  The thermal state of the Archaean Earth might not have had plate tectonics as we know it today.  However, studies of the oldest probable ocean floor (the >3800Ma Akilia Association of West Greenland) looks for all the world as if it formed as an accretionary prism as a result of normal-seeming plate forces.  Previous speculation about Archaean tectonics assumed basaltic oceanic crust, much like today's.  High heat production also implies that Archaean constructive margins generated a great deal more magma by partial melting of mantle with higher potential temperature; probably more magnesian, picritic primary magma (Foley, S.F et al. 2003.  Evolution of the Archaean crust by delamination and shallow subduction.  Nature, v. 421, p. 249-252).  Instead of the lower oceanic crust being made from gabbroic cumulates, it was then probably dominated by ultramafic products of fractional crystallization.  Foley, and colleagues Stephan Buhre and Dorrit Jacob of the Universities of Greifswald and Franfurt in Germany, show from high-pressure experiments that such lower crust would form dense pyroxenites.  At destructive margins these might delaminate from the upper oceanic crust to subduct steeply, thereby conferring slab-pull force to drive tectonics.  Their eventual partial melting would source basaltic magmas to add to older oceanic crust that failed to subduct during the earliest Hadean times.  That would explain the lack of continental materials older than 4000 Ma.  .  The partial melting of garnet-bearing mafic materials (probably garnet amphibolite) that sourced Archaean continental crust would have had to await the end of such delamination, when the whole oceanic crust could descend, albeit with hot wet basalt in the upper part of the slab.  Interesting though the ideas in the paper are, apart from the authors suggestion of a connection with element depletion of the upper mantle progressively affecting an ever deeper zone, they hark back to thoughts on Archaean processes as early as the late 1970s.

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Mantle recycling

January 2003

Somewhere beneath the Americas there is a sizeable volume of what formerly constituted the East Pacific ocean lithosphere. It represents half the productivity of the East Pacific Rise over more than 100 Ma. Although there is still considerable uncertainty about where such subducted rugs end up, seismic tomography does suggest that a fair proportion may reach the core-mantle boundary. That region of the mantle also seems to be the source of at least some mantle plumes. So it would not be very surprising if lavas formed from some plumes carried a signature from much older lithosphere. Finding such signs is not so easy, but if one pops out of lava geochemistry it would indicate that mantle convection has not stirred up and chemically blended the mess of subducted material in the lower mantle; a "memory" of bygone tectonics. At least 3 billion years of plate tectonics has contributed to the geochemistry of the mantle, so finding such a memory has been just a matter of patience, developing a means of teasing it out and luck.

One such signature has emerged from the plume-related islands volcanic islands of the Azores, in the form of an anomalously low ¹⁸⁷Os/¹⁸⁸Os isotopic ratio (Schaefer, B.F. et al. 2002. Evidence for recycled Archaean oceanic mantle lithosphere in the Azores plume. Nature, v. 420, p. 304-307). The study shows that the parent isotope (¹⁸⁷Re) was depleted in the Azores source mantle up to 2500 Ma ago, perhaps before. Rhenium depletion is likely to occur in mantle rocks during partial melting, because it is incompatible, while osmium is compatible with mantle mineral assemblages that constitute the residue of melting. So the most likely explanation for unusually low ¹⁸⁷Os is that oceanic mantle lithosphere, depleted by late-Archaean melting events, has sat around somewhere without being blended with more primitive mantle. Lead isotopes in modern ocean-floor basalts suggest that recycling on timescales around 2 billion years has occurred, and the Os data from the Azores confirm that. However, this is the first swallow in what may (or may not) become an osmium-isotope summer for geochemists eager to map the mantle's evolution. And there is one big question: from what depth did the Azores plume rise? There is absolutely no evidence for it having risen from the core-mantle boundary (or anywhere else for that matter). So all the data really show is that Archaean materials have been incompletely mixed with their mantle surroundings. They could be products of Archaean subduction, but it requires special pleading to remove the possibility of Archaean lithosphere that resided just beneath the African or American continents before the Atlantic Ocean began to form.

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Beowulf and mapping the mantle

January 2003

Seismic tomography is a child of high-speed computing, of which we could barely dream only 10 years ago, as well as the world-wide network of seismic stations set up to detect nuclear tests. The grist to its mill is seismographic data supplied near instantaneously by modern broadband data telemetry. Mathematically it is not an easy subject, so an insight into how it is done is very welcome (Komatitsch, D. et al. 2002. The spectral-element method, Beowulf computing, and global seismology. Science, v. 298, p. 1737-1742). "Beowulf" refers to the use of clusters of ordinary PCs to perform the calculations, rather than single, main-frame supercomputing. The review outlines the theoretical approach of the spectral-element method (still beyond me!), but is most interesting in assessing the potential of future machines able to operate 100 times faster (petaflop machines) than even the most powerful today. It begins to look like geophysicists will unveil far more complexity in the mantle than geochemists have been able to sift from their analyses of exposed rocks at the surface.

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Orphan terranes and tectonic names

December 2002

The period from the Early Ordovician to the Late Silurian involved the assembly of much of the continental lithosphere that now surrounds the North Atlantic. British geologists refer to this as the Caledonian orogeny, a term coined long before the events that welded the bulk of the British Isles were even dreamt of, let alone understood. They are now in the embarrassing position (although most show few signs of grave discomfiture) of using the same term for at least two completely unrelated tectonic events. Clinging to the old name, they now refer to mountain-building events around 470 Ma, during which accretion of an arc terrane to Laurentia resulted in the famous "fountain of nappes" of the Dalradian and Moinian Supergroups, as the "Grampian phase of the Caledonian orogeny". Now, I am all in favour of retaining a sense of history in nomenclature, but the fact is that northern Scotland is now known to have been part of Laurentia for a good billion years before this event. Moreover, the offending island arc was first recognised on the eastern seaboard of North America, where it was dubbed the Taconic Arc; hence the Taconic orogeny there. About 60 to 70 Ma later, the Avalonia terrane (also named first by North American geologists from a peninsula in Newfoundland) collided with this earlier orogenic belt in Laurentia. North American geologists, for reasons of their own, refer to the deformation and metamorphism that ensued as the Acadian orogeny. The British Isles experienced exactly the same event, yet it is referred to as the "Acadian phase of the Caledonian orogeny" - not the Cumbrian, as one might expect from the parochial considerations that prefer "Grampian" to Taconic, for the Iapetus suture that divides terranes north and south in Britain probably lies beneath northern Cumbria. How confusing this is, and how unnecessary!

The plot thickens in Scandinavia, long renowned for the pandemonium of orogenies dating from Palaeoproterozoic times. There, tectonic events around 470 Ma are the "Finnmarkian phase of the Caledonian orogeny", and those which closed the Lower Palaeozoic are the "Scandian phase". Norse, Swedish and Finnish geologists can be excused for sticking with their palaeotoponymy, because Scandinavian lithosphere was a separate entity from Laurentia during these times—Baltica. The comforting isolation of Baltica had been thought to have ended with its accretion to Laurentia when the "Old Red" continent (Laurussia) formed. Not entirely so. Norway is now the proud custodian of a bit of the Taconian orogen (Yoshinobu, A.S. et al. 2002. Ordovician magmatism, deformation, and exhumation in the Caledonides of central Norway: An orphan of the Taconic orogeny. Geology, v. 30, p.883-886). However, that does not make a unification of Baltica's tectonic nomenclature with Laurentia sensible, because the sliver seems to have travelled a vast distance from its parent. Hence "orphan", because it was emplaced as one of the many nappes of western Scandinavia. British geologists should take no comfort from this, and it is about time that they accepted a common tectonic history for the whole of Laurentia, otherwise their parochially-named orogenies might justifiably be called "bastards"!

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Slab pull versus subduction suction

November 2002

The dominant forces that drive plate tectonics are those created by subduction. Slab pull is transmitted throughout a plate system when subducted oceanic lithosphere remains mechanically attached to its parent plate. However, detached slabs that descend into the mantle, excite viscous flow that might exert traction on the base of the lithosphere, thereby sucking plates along. Attached slabs also create suction. The relative influence of the two forces is an important input to global dynamics that prevail today. Slab pull operates to draw subducting plates towards destructive margins, whereas subduction suction should act on both the under- and over-riding plates to drive them towards subduction zones.

Using plate motions, estimated from Mesozoic to Recent magnetic stripes, and subduction history at nine destructive margins, Clinton Conrad and Caroline Lithgow-Bertelloni of the University of Michigan compared them with motions predicted from slab-pull and subduction suction (Conrad, C.P. & Lithgow-Bertelloni, C. 2002. How mantle slabs drive plate tectonics. Science, v. 298, p. 207-209). Much simplified, their findings suggest that slab-pull forces account for around half of the driving force of plate tectonics, with a nearly equal contribution from subduction suction induced by subducting slabs. However, both attached and detached portions of lithosphere that descend beneath the 660 km deep mantle transition zone probably do not transmit stresses into higher-level slabs, and only their suction effect adds to plate motions.

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Continental insulation at the Precambrian-Cambrian boundary

November 2002

Shortly before the Neoproterozoic ended with the Cambrian Explosion of animals with hard parts, much continental lithosphere clumped together in a Vendian supercontinent, called Pannotia by some geologists. If the idea described in Empirical geochemistry points to continents’ role in mantle dynamics (earlier) is realistic, that surely would have created "pressure-cooker" conditions in the mantle beneath it. Possibly piled with as much as 2 km of ice sheet during a "Snowball Earth" episode, this assembly of cratons would also have been somewhat depressed. From about 650 to 500 Ma Pannotia experienced generally outward extensional forces. The Pan-African and Braziliano orogens, formed slightly earlier, underwent widespread magmatism unrelated to any crustal thickening and deposition in many sedimentary basins. Spanish and Moroccan geologists have tried to explain this evolution in terms of the blanketing effects of Pannotia (Doblas, M. et al. 202. Mantle insulation beneath the West African craton during the Precambrian-Cambrian transition. Geology, v. 30, p. 839-842). Pan African and Braziliano orogens surround the West African craton, and the authors opinion is that their anorogenic magmatism stemmed from a build-up of heat resulting from insulation by thick continental lithosphere. More controversially, they see this as an escape mechanism from Snowball Earth conditions, through the associated magmatic release of CO₂. In turn, they see this addition leading to increased flux of calcium to the oceans, toxic stress from this spurring an evolutionary response by soft-bodied metazoans in the form of carbonate secretion by their cells; hence continental clustering leads to the Cambrian Explosion!

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The lost world of the Galápagos hotspot track

November 2002

The Galápagos islands straddle both a hotspot and the active spreading centre that generates the Cocos and Nazca Plates in the Easter Pacific. Consequently, both those plates have topography owed to former activity at the Galápagos hotspot, the Cocos Ridge and associated seamount chain, and a set of ocean-floor uplands that resulted from complex evolution of the Nazca Plate. Both plate vectors drive this topography towards subduction zones beneath Central America and the Andes. Unsurprisingly, this gives rise to a kind of inverse tectonic constipation, as both subduction zones attempt to consume awkward knobbles on top of the downgoing slabs. Detailed seismic profiles have revealed the current state of affairs, which has been going on for around 71 Ma. Some of the seamount and aseismic ridge materials parted company with the downgoing slab, to be obducted onto the Central American arc. These ophiolites represent the lost history of the Gala Galápagos hotspot, from about 71 to 16 Ma ago, and information from them has allowed a team of German and Cost Rican geoscientists to piece together an enthralling tale that feeds into the evolution of the Central American land bridge (Hoernle, K. et al. 2002. Missing history (16-71 Ma) of the Galápagos hotspot: Implications for the tectonic and biological evolution of the Americas. Geology, v. 30, p. 795-798). Central America not only formed a land bridge that allowed the Late Tertiary mingling of faunas from South and North America, but by disconnecting the Atlantic and Pacific Oceans it transformed low-latitude ocean currents, and probably set in motion the climatic cooling towards the Great Ice Age. However, the story now seems considerably more complex.

Galápagos-related igneous rocks bear strong geochemical similarities to those of the 90 Ma Caribbean Large Igneous Province (CLIP), now to the east of Central America. This supports a long-held view that the CLIP formed during the initial evolution of the Galápagos hotspot, and was driven eastwards by spreading from the predecessor of the East Pacific Rise. Being a huge, low-density patch of ocean floor, it failed to subduct, but passed between North and South America when it encountered the volcanic arc of the Greater Antilles, channelled by two large fracture zones. Subduction flipped beneath the Antilles, to consume Atlantic lithosphere westwards, while Pacific subduction restarted in the "lee" of the CLIP and began to generate the Central American arc. This Late Cretaceous to Palaeogene transformation formed the first land bridge connecting both continents, allowing terrestrial fauna and flora to mingle, including late dinosaurs. Emplacement of the CLIP in its present position removed the land bridge of the Antilles Arc, again separating both continents for most of the Tertiary. Assorted sloths, armadillos, elephants, ferocious cats and the like, eagerly awaited the next chance to rampage, evolving awhile. By the Early Pliocene, the growing Central American arc slid in to fill the gap, and biotic pandemonium ensued. This signal event of recent geological times was itself encouraged by the continued magmatic productivity of the Galápagos hotspot, and the failure of its low-density products to return to the mantle.

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Cunning means of estimating uplift

November 2002

Rises and falls of the continental surface have frustrated geologists trying to assess their timing and rates, largely because the available methods ar