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