Category: Tectonics

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.

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.

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

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.

Being able to picture Earth features far beneath the surface is what makes seismic tomography such an exciting tool, even though it is in its infancy. It shows variations in the velocity of P and S waves in 3-D. Regions of fast waves are likely be cooler than those in which wave speeds are relatively slow.  The detail depends on the spacing between seismic recorders and the distribution of natural seismic events, whose interactions produce tomographic data.  Despite being rarely affected by seismicity themselves, the British Isles have a remarkably dense network of seismic stations that was developed for research.  Given arrival times at the different stations by waves from earthquakes that occurred over a wide range of epicentral angles from the British Isles, it becomes possible to probe in detail what lies beneath.  Exploiting the potential to the full, a group of British and US geophysicists has shown that the ‘British’ mantle is far from boring (Arrowsmith, S.J. et al. 2005. Seismic imaging of a hot upwelling beneath the British Isles.  Geology, v. 33, p. 345-348).

Down to a depth of 600 km, Britain is underlain by a series of significantly slow and fast mantle ‘blobs’.  The seismically slow, probably warm mantle zones seem to follow large features last active during Early Palaeogene magmatism that affected the Hebrides and Northern Ireland, and roughly parallel the 60 Ma dyke swarms that radiate from these centres.  They also correlate with regions of anomalously high gravity.  It seems highly likely that both features are long-lived relics of a spur of the still active Iceland plume that is intimately associated with spreading on the Mid-Atlantic Ridge.  The warm zones also underlie those parts of the British Isles that were most affected by uplift and erosion during the Cenozoic: as much as 3 km in the case of the Irish Sea.  Such areas also focused extension at the time of the magmatism, and they are still most affected by minor seismicity.

Estimates of the magnitude of the temperature anomaly associated with the slowing of P-waves are as much as 200 °C above ambient mantle temperature; sufficient to be associated with partial melts.  That Britain might once more have active volcanoes is highly unlikely, and the anomalies are probable parts of the Iceland plume system that became trapped beneath zones of crustal thinning. Their loss of heat is sufficiently slow for them to have bolstered areas of uplift and erosion for tens of million years.  There is even a chance that some form of convection might yet be going on.

This item can be read at Earth-logs in the Geomorphology archive for 2005

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.

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.

Plate tectonics’ basic tenet is that discrete segments of the lithosphere behave as rigid bodies, whose motion is accommodated by extensional, overriding and strike-slip faulting at equally discrete boundaries.  That is true to a first approximation for the parts of plates made up from oceanic lithosphere, which is rheologically strong because of its mineralogical composition.  Continental lithosphere is weakened by its quartz-rich crust, which tends to behave plastically at high temperatures deep within it.  So it is no surprise that opposed motions of plates induce large-scale shortening and thickening of continental lithosphere that they carry, but there are no orogens in the ocean basins.  The largest site of active continental shortening and thickening is, of course, the Alpine-Carpathian-Himalayan orogen.  The Tibetan Plateau is underpinned by continental crust that is in the process of being thickened as India drives north-eastwards into Asia, at about 4 to 5 cm per year.  Consequently it the largest area of high-elevations on the planet.  In the 1973 John Dewey and Kevin Burke speculated that forces involved in continent-continent collisions with irregular margins might expel thickened continental lithosphere sideways, at right angles to the opposed plate motions.  Peter Molnar of the University of Colorado in Boulder and Paul Tapponnier of the Institute of Global Physics in Paris applied this on a grand scale to the neotectonics of the Tibetan Plateau and East Asia in 1975.  They considered that south-eastward expulsion was channelled by the many enormous strike-slip faults in the region.  In a sense, this notion considers the continental tectonics to be akin to the rigid-body behaviour of oceanic parts of plates.  If the overall motions involving Tibet and continental lithosphere to the east was dominated by plastic deformation in the deep crust and mantle, the motion would be taken up by a host of smaller faults in the brittle upper crust.  Geodetic measurements using GPS over the last 17 years do conflict with the movement of discrete blocks of East Asian crust (see Quantifying motions inside continents, March 2004 EPN).  Two papers published in July 2004 also lean towards plastic behaviour of the bulk continental lithosphere.  One uses data from surface seismic waves to show about 30% ductile thinning in the middle and lower crust beneath Tibet (Shapiro, N.M. et al. 2004.  Thinning and flow of Tibetan crust constrained by seismic anisotropy.  Science, v. 305, p. 233-236).  The other is based on interferometric analysis of radar data from satellites, which involves measuring signal differences between radar data captured on different dates, in this case between 1992 and 1999 (Wright, T.J. et al. 2004.  InSAR observations of low slip rates on the major faults of western Tibet. Science, v. 305, p. 236-239).  The technique has mainly been used to look for vertical displacements associated with earthquakes and volcanoes.  By eliminating the effects on signals by terrain, using an accurate digital elevation model, InSAR results can estimate the motion of the surface along and opposite to the illumination direction of the radar pulses, thereby detecting horizontal ground movements over a period of several years with sub-centimetre precision.  Rather than revealing large movements in the two opposed directions that are expected on either side of  large strike-slip faults, such as the Karakorum and Altyn Tagh Faults, there was none.  In a zone crossing western Tibet from NNE to SSW, much of the orogen appears to be moving slowly eastwards, irrespective of the large faults.  Tapponnier still maintains the importance of the big faults, and perhaps the InSAR survey coincided with a period of tectonic quiescence.

Early Earth’s Nemesis

William Hartmann’s proposal that, shortly after it formed, the Earth suffered  impact by a planet about as big as Mars has become a central feature on ideas about our planet’s evolution and the origin of the Moon.  The problem with the theory is a conundrum that lies in quite esoteric geochemistry.  Studies of meteorites show that the oxygen isotopes in them vary considerably, and that almost certainly resulted from their forming at varying distances from the Sun where fractionation among oxygen’s stable isotopes had different effects on their proportions.  So it is possible to judge the original orbits in the solar system of meteorites’ parent bodies.  Martian meteorites are identified on this basis.  The difficulty with Hartmann’s idea is that rocks from the Earth and Moon have nearly identical oxygen isotope proportions.  There seems no way that an errant planet that crashed into the Earth could not have left its mark in oxygen isotopes, particularly in those of the Moon, for debris flung off from the Earth would have mixed with that from the colliding body.  It turns out that there is a possible explanation (Chown, M.  2004.  The planet that stalked the Earth.  New Scientist, 14 August 2004, p. 26-30).  The Earth’s orbit could have involved the accretion of more than one planet from interstellar dust.  This can happen once a planet has grown until it has sufficient gravitational potential to interact with solar gravity.  The result is a series of points in the orbit (Lagrange points) where the two gravitational fields exactly balance.  Matter that drifts into Lagrange points accumulates there rather than being swept up by the growing, larger planet.  So considerable mass can build up, even enough to make a small companion planet.  While all the main planets were growing, gravitational fields were continually changing, so the Lagrange points would not remain as stable as they are today.  A small planet formed at one of them would begin to move erratically within the Earth’s orbit.  Eventually it would be caught up by mutual attraction between the two, and then would collide with the Earth, but not at immense speed.  So far, the hypothesis based on complex modelling of Lagrange accretion seems plausible.  Geochemists will be pleased because it resolves their fundamental conundrum about the similar chemistries of the Earth and its Moon.

Uranium in the core?

The constant, but complex circulation in the Earth’s liquid outer core almost certainly results in the self-exciting dynamo believed to be responsible for the geomagnetic field and its periodic reversals in polarity.  If an electrical conductor moves in a magnetic field a current is generated in it, which in turn creates a magnetic field, hence self-excitation.  The outer core’s convective motion requires a heat supply of some kind.  There are three general possibilities: the heat is left over from the Earth’s energy of accretion; it is generated from latent heat released as the solid inner core grows slowly by crystallization of iron-nickel alloy; or there is significant radioactive decay in the core.  Compared with estimates for the Earth’s overall radioactive heat production, based on the composition of the primitive meteorites (ordinary chondrites) from which it is thought to have formed, there is excess heat flowing through the surface.  This is believed to emanate from the core.  Separating the three possible heat sources is not yet possible, but it is possible to rule the generation of enough to account for excess heat flow by one of the possible mechanisms.  If the inner core has been crystallising out since the core formed around 4500 Ma ago, the latent heat being released is too small.  Much attention has focussed on a possible radioactive source, for which the unstable natural isotope of potassium (⁴⁰K) is a plausible candidate.  The sulphide phases of metal and chondritic meteorites do contain potassium, so the element has affinities for sulphur as well as its dominant tendency to enter silicate melts and minerals.  The core almost certainly contains a sizeable proportion of Fe-Ni sulphides.  One geoscientist, Marvin Herndon based in San Diego, California, reckons there is another possibility (Battersby, S. 2004.  Fire down below.  New Scientist, 7 August 2004 issue, p. 26-29); uranium.  To most geochemists, the idea is implausible, because uranium has such a strong affinity for silicates that it ought never to have entered the metallic and sulphide liquids that sank through the early Earth to form the core.  Herndon bases his idea on an alternative type of meteorite from which the Earth could have formed by accretion, enstatite chondrites.  They have lower oxygen contents than ordinary chondrites, and would have created strongly reducing conditions in the undifferentiated early Earth.  Such planetary chemistry, claims Herndon, would induce uranium to enter dense sulphide liquids and the core.  This view has not found much support, but experiments in detection of neutrinos and antineutrinos, when they are more efficient than at present, may resolve the issue of radioactivity in the core, because decay of unstable isotopes produces antineutrinos.

In EPN of January 2004, there appeared a summary of Warren Hamilton’s sceptical view of recent ideas about what happens beneath the 660 km mantle discontinuity (Geoscience consensus challenged).  It is below that level that the dominant mantle mineral, olivine (MgSiO₄), is thought to change to the more densely packed perovskite (MgSiO₃).  Encouraged by an experiment which suggests that at the pressure and temperature just above the core-mantle boundary (CMB) perovskite itself undergoes a phase change to define the D” seismic discontinuity (Murakami, M. et al. 2004.  Post-perovskite phase transition in MgSiO₃.  Science, v. 304, p. 855-858),  Edward Garnero of Arizona State University takes a very different view.  In his Science Perspectives review of the CMB region (Garnero, E.J. 2004.  A new paradigm for the Earth’s core-mantle boundary.  Science, v. 304, p. 834-836) he builds into a comprehensive, illustrated model everything that Hamilton finds dubious: whole-mantle plumes and slab descent; zones of ultra-low velocity close to the CMB; undulations on it; and massive bulges of low-velocity mantle above D”, such as that suggested to underlie the South Atlantic and southern Africa from which constellations of plumes rise.  He links this to a wealth of anisotropies which basalt-oriented geochemists have found and continue to relish.  His enthusiastic account makes fascinating reading, but makes no mention of Hamilton’s and others’ doubts about gilding the lily of only a few short years of seismic tomography.

Mesoproterozoic large igneous province and Rodinia

Flood basalt events in the Phanerozoic seem generally to have preceded the break-up of supercontinents, and many geoscientists believe that their formation is implicated in the mechanism of continental disaggregation.  So it comes as something of a surprise to learn that the assembly of most continental lithosphere to form the Rodinia supercontinent about 1100 Ma ago, which ranks in size with Pangaea, was probably accompanied by massive igneous activity (Hanson, R.E. et al. 2004.  Coeval large-scale magmatism in the Kalahari and Laurentian cratons during Rodinia assembly.  Science, v. 304, p. 1126-1129).  The Proterozoic sediments of southern Africa and once-adjacent Antarctica are intruded, wherever they occur, by basaltic sills up to hundreds of metres thick.  In a few places relics of flood basalts above the sedimentary groups have the same composition and age, around 1100 Ma.  Like Phanerozoic large igneous provinces, most of the magmatism occupied only a few million years, perhaps less than 1Ma.  The distribution of the probable feeder intrusions for the few relics of CFBs suggests that the province in the Kalahari craton formerly covered about 2 million km², so it ranks in size with most Phanerozoic LIPs.  In North America, cored by the craton of Laurentia, there occurs the Keeweenawan dyke swarm and other mainly mafic intrusions, that probably fed another veneer of CFBs.  Dating them using the same single-crystal U-Pb method reveals ages that are within error of those from southern Africa.  Combined, the two LIPs are much larger than the biggest know LIP from the Phanerozoic – the Ontong-Java Plateau that formed on the floor of the West Pacific Ocean during the Cretaceous.  So, were there two massive, but short-lived igneous events while Rodinia was assembling, or one that unites both the Kalahari and Laurentian cratons?  In many models of Rodinia, stitched together using orogenic belts that formed in the late Mesoproterozoic between1150 and 950 Ma, the Kalahari craton has been placed against Laurentia; both LIPs could be a single super-province.  However, the same authors also measured palaeomagnetic pole positions from the southern African igneous rocks.  They are different from those revealed by the Laurentian LIP, and imply considerable separation of the two continental masses at the time of igneous activity.  That suggests either separate melting events in the mantle beneath both cratons at the same time, or that both are parts of an even larger magmatic upheaval that spanned about 1/5 of a hemisphere.  Whichever turns out to be the case, this ancient large-scale mantle event bucks the Phanerozoic trend of LIPs’ presaging or accompanying continental break-up.  Maybe the rare mantle upwellings thought to generate LIPs are really random in their positioning, and “just happened” to rise beneath Pangaea and its fragments from the Devonian onwards.

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.

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.

More, or less plumes

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 http://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 Africa and the Pacific also appear to spawn lesser plumes, that in turn sometimes split

In the Lake District of Cumbria, asking older local farmers how the fells grew will often get the response that they started out as pebbles.  The justification of this seemingly implausible hypothesis is that once a field is cleared of boulders, about 20 to 30 years later new ones have appeared and the clearing has to start again.  Geologists have their own ideas.  Compressive deformation of continental crust will thicken it, and gravity acting on this low-density material will ensure that its surface rises.  Counter-intuitively, the action of erosion can cause mountains to rise as well.  Debris flushed from deep valleys lessens the load on the underlying crust, so that it continually rises to drive up the elevations of the remaining ridges and peaks.  The compressional origin of the Himalaya is hard to dispute, yet they bounced up quite quickly, long after they began to form.  Current ideas, backed up by a variety of evidence, suggests that a lump of the dense lithosphere beneath the India-Asia collision zone fell off (delaminated) and sank in the mantle.  That reduced the mass of the lithosphere beneath and the gravitational field, so that the surface rose.  The second highest mountains, the Andes, offer no such mechanism, for they are not products of compression associated with collision.  Dense Pacific Ocean lithosphere subducts beneath them and the forces involved are insufficient to raise the Andes to even half their present elevation.  Simon Lamb of the University of Oxford and Paul Davies of the University of California, Los Angeles have attempted an explanation for the anomalously high Central Andes (Lamb, S. & Davies P. 2003.  Cenozoic climate change as a possible cause for the rise of the Andes.  Nature, v. 425, p. 792-797).  Their idea is that sediments that pour into subduction-related trenches from rising arcs, to form part of the accretionary prism where lithosphere starts to go down, lubricate subduction because of the pore water in them.  If there is little sediment supply from the rising crust, then frictional forces build up along the line of the subduction zone.  That focuses the plate boundary stresses over a narrow zone, thereby giving sufficient force to drive the crust higher and higher.  Today the cold northward ocean current along western South America provides little rainfall to the Central Andes, so erosion is much slowed.  Episodic global cooling since the Mid-Eocene probably reduced erosion there several times during the Cenozoic.  So for long periods the worlds largest subduction zone would have been starved of lubricants, thereby driving up the Andes.  The mountains themselves, by forcing maritime air upwards, would also starve the rising peaks and the great Altiplano plateau of rainfall, further influencing sediment supply to the trench system.  Lamb and Davies reckon that the Andes are fortuitous results of a N-S subduction zone at a continental margin, combined with its development during a period of global cooling and tropical drying.

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.

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…

Wandering hot spots

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.

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.

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!

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!

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.

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.

Eskola’s mantled gneiss domes revisited

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 he 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.

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.

Beowulf and mapping the mantle

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.

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”!

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.

This item and others about Tectonics can be read at Earth-logs in the Tectonics archive for 2002

The detachment of lithospheric masses and their falling-off into the mantle, either by delamination of deep lithosphere beneath continents or the breaking of a subducted slab, have become popular means of explaining a variety of unusual phenomena in mountain belts.  In the Himalaya and Tibetan Plateau, such models have been evoked for the formation of odd K-rich basalts in the Eocene and Miocene, and the crustal melting that generated leucogranites around 20 Ma ago along the entire length of the Greater Himalaya.  Taking all the oddities of the Indo-Asian collision zone together does seem to support such a model (Kohn, M.J. & Parkinson, C.D. 2002.  Petrologic case for Eocene slab breakoff during the Indo-Asian collision.  Geology, v. 30, p. 591-594).  However, there is still no tangible direct evidence beneath the region.

Using seismograms for deep-Earth tomography appears to be able to resolve a range of proposed variants of tectonics, as well as the gross behaviour of the deep mantle. The site where two plates are being subducted on the west side of the North Pacific, marked by the Kamchatka peninsula, is pretty odd as well.  Although rates of subduction of both plates are high, the part of Kamchatka at one boundary no longer has active volcanoes, whereas the other does.  In fact one of the volcanoes there holds the world record for magma output.  Up to 5 Ma ago, the whole of Kamchatka was actively volcanic.  An explanation for the sudden halt to volcanism is that the dehydrating slab which provides the essential watery fluid for partial melting of the overlying mantle wedge – the source of subduction-zone magmas – broke away from the subduction zone and “fell” into the mantle 5 Ma ago.  That would have removed the source of hydrous fluid at a stroke.  Seismic tomography now seems to be capable of resolving just such a foundered slab (Levin, V. et al. 2002.  Seismic evidence for catastrophic slab loss beneath Kamchatka.  Nature, v. 418, p. 763-767).  There is no slab beneath the presently inactive volcanoes, whereas it is intact beneath the active ones.  The authors also claim that the seismic structure reveals a more recently foundered piece of lithosphere, whose rapid loss of hydrous fluid helps explain the phenomenally high magma production of the Klyuchevskoy volcano.  Such slab break-off is clearly a potential engine for enormous changes in magmatism, and the first seismic evidence for it is bound to spur a search for more examples.

Analysis of travel paths taken by many S waves that travelled beneath the African continent, largely by geophysicists at the California Institute of Technology, shows that beneath it is a large zone of anomalously low wave speeds.  Part of the zone dips down obliquely from the rough location at shallow depths of the Afar plume beneath Ethiopia/Yemen to the core-mantle boundary between the surface locations of Africa and South America.  The structure is well placed for seismic tomography, by virtue of its good match with useful earthquakes and the world-wide network of seismometers.  More advanced analysis (Ni, S et al. 2002.  Sharp sides to the African superplume.  Science, v. 296, p. 1850-1862) shows up a strangely sharp-sided part of the plume that rises from the core-mantle boundary for about 1500 km below southern Africa.  There its boundary with more normal mantle is little more than 50 km wide.  Modelling suggests that the upward flow has caught up a dense layer with possibly different chemistry, which would result in a tilt towards the direction of movement so that instead of rising vertically, the plume would have an oblique trajectory.  The tilt also fits with Africa’s north-eastwards drift (in an absolute frame of reference, relative to other hotspots) since 100 Ma ago.

Whatever its origin, a rising, hot mantle zone beneath Africa is consistent with the continent’s high overall topography, which has encouraged the lithosphere to rift.  This extension has resulted in the East African Rift, which further encouraged partial melting in the underlying mantle and the resulting volcanism.  By far the most important aspect of Africa’s recent volcanic activity has been the Eocene to Oligocene flood-basalt event of the Ethiopian Plateau and the current activity in the Afar part of the Rift.

Subduction metamorphism and earthquakes

The recently commissioned Hi-net array of 600 digital seismometers in Japan paid dividends in an unexpected way during 2001, by picking up long-lived vibrations rather than discrete seismic events Obara, K. 2002.  Nonvolcanic seep tremor associated with subduction in southwest Japan.  Science, v. 296, p. 1679-1681).  The tremors occurred in a part of Japan where there are no active volcanoes, with which protracted vibrations are usually associated.  Their epicentres define a clear zone, at about the depth of the Moho and on the Wadati-Benioff zone where the Philippine Plate is being subducted.  This region is where dehydration reactions that convert cold, wet oceanic crust to dense eclogite, the driving force for plate tectonics through slab pull, are predicted to occur by thermodynamics.  Kazushige Obara, of Japan’s National Research Institute for Earth Science and Disaster Prevention, suggests that this correlation might fit with the release and rise of hydrothermal fluids released by dehydration of the slab.  Part of his evidence is that such tremors seem not to occur where the much older (and therefore cooler) Pacific Plate is being subducted beneath the northwest of Japan.  It probably does not undergo such reactions until it has reached about 100 km depth, where temperature would be sufficient to enter the field of eclogite stability.  Detecting fluid motion at 3 times the depth of that beneath southwest Japan might emerge with more specialized procesing.

See also:  Julian, B.  2002.  Seismological detection of slab metamorphism.  Science, v. 296, p. 1625-1626.

Continental roots

Crustal shortening and thickening in collisional orogeny produces mountain belts with a root of crust beneath them.  This truism is central to isostasy, where the mass of uplifted mountains is balanced by a compensating mass of low-density root material beneath that penetrates the mantle lithosphere.  The classic story of the reduction of mountain belts to a peneplain involves continuous isostatic uplift as the topography is eroded away.  Finally, no root remains and the exposed rocks reflect in their high-grade metamorphism a steady upward passage from the root.  Later cover rests with profound unconformity upon this peneplain.  Yet this essentially simple theory does not hold in many cases, especially for older collisional orogens.  As Karen Fischer of Brown University, USA has shown (Fischer, K.M. 2002.  Waning buoyancy in the crustal roots of old mountains.  Nature, v. 417, p. 933-936), there is a crude correlation between the age of orogens and their ratio of elevation to root thickness.  The ratio decreases from around 0.15 (root about 7 times thicker than surface elevation) in active orogens to zero before 1 Ga ago, when peneplained orogens still have a substantial root.

In order for this to happen, either the roots’ buoyancy must somehow decline with age or the mantle lithosphere which it penetrates becomes too rigid to allow isostatic uplift to occur.  Resolving which has most effect depends on analysing the gravity anomalies above orogens.  It is no easy task to model the two processes, and this is what Fischer has achieved.  She finds that mantle viscosity is not responsible, and that the cause is variation in root density.  This is probably a result of slow decline in heat flow, and the resulting mineralogical equilibria in the root.  For mafic granulite roots, a change from heat flow values of 70mWm^-2 to around 40 mWm^-2 could increase their density by 100-150 kg m^-3, by an increase in the proportion of garnet, perhaps to the extent of producing eclogites at the deepest levels.  Eclogites would be seismically very similar to mantle lithosphere, so that even thicker, hidden roots may be present.  Reduction in buoyancy by this means could take as little as 20 Ma, before which the elevation to root thickness ratio has declined below that in active orogens.

One implication of this process is that orogenic collapse by lateral extension of highly elevated crust, which might lead to rapid root thinning, is not the general process that many structural geologists believe.  If it was, orogenic roots would be removed relatively quickly.  Decrease in root buoyancy is also a plausible explanation for the creation of cratons, where quite low-grade metamorphic rocks, formed at shallow crustal levels occupy vast areas of low-lying shields.

Although they are seismically precarious, the major coastal cities of the Americas and East Asia that lie close to destructive plate margins probably owe their survival to a greasy assemblage of hydrated ultramafic minerals – serpentine, talc and magnesium hydroxide (brucite).  Detailed tomographic images using the records of natural earthquakes along the subduction zone beneath western North America show a zone of exceptionally reduced S-wave speeds at the “corner” formed by the subducted slab and the base of the crust (Bostock, M.G. et al. 2002.  Inverted continental Moho and serpentinization of the forearc mantle.  Nature, v. 417, p. 536-538).  This low-speed zone coincides with the fore-arc region of the destructive margin, roughly along the coast.  Normally the Moho marks a sudden increase in wave speed in the mantle underlying the crust, but here the situation is reversed (inverted).  The best explanation is that S-wave speed slows because of an abundance of weak rock, between 35 and 60 km down.  The likely candidate is mantle peridotite that has become hydrated by fluids seeping upwards from cold, wet oceanic lithosphere as it begins to be subducted.  Low-temperature, high-pressure metamorphism of hydrothermally altered basaltic crust begins to transform it to anhydrous eclogite, so releasing masses of water vapour.  It is this fluid release that is implicated in the generation of magmas beneath volcanic arcs, because it reduces the beginning-of-melting temperature in the overriding mantle wedge.  However, such partial melting is possible only when temperature is high.  In the cooler, shallow regions of the fore arc rising watery fluids serve to convert peridotite to hydrous minerals, especially serpentine.  One outcome is the creation of anomalously low-density mantle, which bulges upwards to create fore-arc ridges at some destructive margins, even squirting serpentinite upwards in bizarre mud volcanoes.  Yet all hydrated, ultramafic minerals are natural lubricants, and would act to ease sudden rupture along the subduction zone, thereby preventing extremely high-magnitude earthquakes whose surface effects would be devastating.

See also: Zandt, G. 2002.  The slippery slope.  Nature, v. 417, p. 497-498

Variations in the density and rigidity of the mantle induce changes in the speed at which seismic waves move through it.  Mapping mantle regions with slowed and faster waves in three-dimensions is the basis for assessing temperature anomalies within the deep Earth.  It has been such tomography that has begun to test ideas about the depth from which mantle plumes rise and the fate of subducted slabs of oceanic lithosphere, and an increasingly certain model for mantle motions has evolved with improvements in the resolution of seismic analyses.  However, the P and S waves used in tomography have other properties than simply speed.  These include direction, polarization, signs of conversion of P to S waves, and even interference properties for which the birefringence observed in petrography is an analogue.

Analysing these properties reveals that there are deviations in the structure of the minerals that make up mantle rocks from random arrangement; there are anisotropies (Park, J and Levin, V.  Seismic Anisotropy: tracing plate dynamics in the mantle.  Science, v. 296, p. 485-489).  Deformation lines up minerals in such a way that the bulk rock structure affects the propagation of seismic waves in different directions – again, the way in which crystallographic anisotropy of minerals affects light passing through them is a means of visualizing what happens on vastly larger scales.  In their review, Park and Lewin describe how this novel approach is revealing aspects of convection in the upper mantle, how lithospheric plates have formed and features spatially related to accretionary boundaries in continents.

Field studies of ophiolites have shown that the dominant olivines of mantle peridotite are commonly aligned, probably as convection dragged it at right angles to the axes of lithospheric spreading.  Indeed, seismic anisotropy confirms that view with trends normal to the mid-Atlantic, Pacific and Indian Ocean spreading centres.  Destructive margins show two trends, those parallel to trenches and those in the direction of subduction, but there are complex variations depending on depth.  Once resolved into indicators of past motions, that complexity may tell volcanologists a lot about large-scale variations in magmatism.  The Hawaiian hot spot has associated vertical anisotropy, that is consistent with a disturbance of the overall flow of shallow mantle.  Several ancient orogens in continents, dating back to the Precambrian, show anisotropy in the mantle beneath them, often parallel to the orogenic trends, but occasionally more complex.  Clearly, this use of natural earthquake signals has a lot to contribute, but depends on much more complex computations than “conventional” tomography and awaits the wider distribution of software and powerful hardware.

The latest significant development from tomography based on detection of wave-speed anomalies relates to the Earth’s two major mantle plumes, beneath Africa and the Pacific Ocean (Romanowicz, B. and Gung, Y. 2002.  Superplumes from the core-mantle boundary to the lithosphere: implications for heat flux.  Science, v. 296, p. 513-516).  Both apparently persist through the transition zone of mantle wave speeds at 670 km below the surface, to become deflected laterally beneath the lithosphere.  They may well be supplying heat to the asthenosphere that could find its way to spreading ridge systems.  The lowering of viscosity in the asthenosphere as a result of this heat originally from the core-mantle boundary (some of it may be heat lost by the core) would act as a lubricant for plate motions.  In particular, it could enhance the influence of slab-pull force at subduction zones, such as those around the Pacific, thereby speeding up tectonics.  The mantle beneath the African lithosphere has probably been heated.  The huge topographic and gravitational anomaly generated by massive flood basalt eruptions in Kenya and Ethiopia may more easily have been able to convert the resulting extensional stresses into extensional deformation, thereby driving the East African Rift system above a zone of thermal lubrication.  Far more gravitationally unstable lithosphere beneath young orogens does undergo lateral collapse, but the lack of associated plumes makes it impossible for the entire lithosphere to fail through lack of such lubrication.  And when superplumes eventually wane, as perhaps have those beneath Iceland and western North America, that too would influence both plate tectonics and that on more local scales by increasing viscous drag in the asthenosphere.