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.
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.
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.
Despite the increased precision of radiometric dating and the steady accumulation of ages when segments of continental crust first formed, two nagging oddities refuse to go away. There seem to have been spurts in continental growth, rather than a steady build up over time. Odder still, some areas have more crust of a certain age range than seems feasible. The problem is a fundamental one, because the Earth generates radiogenic heat continually, though the amount has declined as the heat-producing isotopes of uranium, thorium and potassium decay. Earth scientists assume that most geothermal energy exits to space through the process of sea-floor spreading. Hot, new oceanic crust is invaded by seawater, thereby losing heat through hydrothermal activity. The dominantly felsic magmas that build continental crust originate through partial melting processes where old, cold ocean floor descends at subduction zones. Although some heat escapes through volcanism associated with mantle plumes, most researchers reckon that it is unlikely that this loss has ever come close to the quiet cooling at mid-ocean ridges, except possibly during the Archaean. Averaged out, subduction and continent formation ought to keep pace with sea-floor spreading, though slowly declining over time. There are those who focus on massive mantle turnovers in the form of superplumes that build large volcanic plateaux on land and on the sea floor, suggesting that their subduction generates greater volumes of crust than usual. The main problem is that such plateaux are unlikely to be subducted.
The evidence for periods of accelerated continental growth comes from restricted regions, albeit very large. Examples are 1900 to 1650 Ma crust in North America, Greenland and Europe, and 800 to 550 Ma crust in NE Africa, whose volumes are equivalent to between 1 and 10 times the present global rate of crust production at volcanic arcs. Jonathan Patchett and Clement Chase of the University of Arizona offer a solution to the conundrums (Patchett, P.J. and Chase, C.G. 2002. Role of transform continental margins in major crustal growth episodes. Geology, v. 30, p. 39-42). They show that strike-slip movement at modern subduction zones gives a 16% probability of more than 400 km transport of new continental crust parallel to the margins of existing continents. Such motions are likely to concentrate continental growth where such terranes become docked together. Such relative plate motions stem from particular configurations of spreading axes and the margins of old continents, and can therefore vary – some periods may have been dominated by head-on subduction, others by a greater amount of oblique relative movements. By bundling together new continental material generated in magmatic arcs, the second would give the appearance of extraordinary rates of crust formation in some areas. If the transform faults that channelled such lateral movements became obscure – and early strike-slip motions in ancient terranes are not easy to find or to quantify – the special natures of terrane dockyards could go unnoticed. Patchett and Chase note that the seeming pandemonium of 800-550 Ma crustal growth in NE Africa and Arabia has a counterpart in an age gap in the record of the northern continents, and cite several other examples.
While variations in strike-slip motions of terranes helps to resolve the apparent episodicity of continental growth, there is another line of approach. Not all modern subduction zones generate voluminous magmas, even where plate motions are head to head. The Andes has two huge segments where active subduction is unaccompanied by volcanism, and the angle of subduction is unusually shallow. Low-angled subduction is likely where warmer than usual oceanic lithosphere enters a subduction zone, which is what might happen to segments blanketed by young ocean-plateau lavas formed by mantle plumes. Constant sea-floor spreading need not necessarily result in constant rates of magmagenesis at destructive plate margins.
The Institute for Frontier Research on Earth Evolution (IFREE) involves 100 Japanese researchers focusing on central aspects of tectonic evolution over the last 200 Ma. These include Pangaea break-up, mid-Cretaceous global warming and Eocene plate reorganization. One particularly interesting study begun using initial funding of US$12 million is the Bonin-Mariana subduction zone. Details at http://www.jamstec.go.jp/jamstec-j/IFREE.
Since Pentti Eskola’s recognition in 1949 that many Precambrian granitic rocks form domes surrounded by cusp-like synclines of supracrustal rocks, such mantled gneiss domes have been found in most cratons. Probably the best example characterizes the 3.5 Ga Pilbara province of the West Australian Shield. How they formed has long been a vexed topic, the most popular views being as a result of low-density basement rising through denser cover that contains abundant volcanic rocks, or as a result of regional-scale fold interference. Precise dating of the Pilbara granitic rocks and greenstones shows a common age range, with some older greenstones, The age data suggest that the dome and cusp structure is a product of the co-evolution of both, probably from a primary oceanic-like crust of mafic composition (Zegers, T.E. and van Keken, P.E. 2001. Middle Archean continent formation by crustal delamination. Geology, v. 29, p. 1083-1086).
Archaean rocks of broadly granitic composition (dominantly tonalites, trondhjemites and granodiorites, or TTG) have geochemical features setting them apart from post-Archaean varieties. Rather than signifying their origin by supra-subduction melting of the mantle wedge with fractional crystallization and crustal assimilation in the lower crust (the dominant crust-forming process in post-Archaean times), all Archaean TTG seem to have formed by partial melting of a garnet-rich mafic source. One of several possibilities is that their source was eclogite. Based on the peculiar regional structure of the Pilbara and its dominance of the whole crust, as shown by maps of gravitational potential and magnetic field strength, Zegers and van Keken revisit earlier ideas of dominantly vertical tectonics that underlay early crust formation. They suggest that efficient cooling by hydrothermal circulation allowed thick mafic crust (similar in some respects to that formed in the Mesozoic beneath ocean plateaux) to enter the field of eclogite stability at its base to form a layer denser than ultramafic mantle. Once sufficiently thick, this layer would begin to founder, or delaminate, to be replaced by hot mantle. Rebound of the remaining crust would set in motion rapid crustal uplift and extension, together with decompression melting of rising mantle (to form high-magnesium basalts high in the crustal sequence)and melting induced in the remaining mafic crust (to generate TTG magmas). Indeed, the kimberlites that puncture other Archaean cratons carry abundant eclogite xenoliths from mantle depths. Seemingly well-documented, this tectonic model does not explain all Archaean crust formation, for other cratons, such as that of west Greenland, are more readily accounted for by seemingly familiar subduction-zone processes.
Anyone who has watched a watchmaker at work may well have felt a tinge of panic at the sheer tininess of the screws, sprockets and gears, and awe at the near-superhuman patience and concentration involved in such micro-engineering. Developments in geodesy based on the Global Positioning System of navigational satellites push towards such aching precision. The fixed stations of the International GPS Service (IGS) measure geographic position and topographic elevation to within less than a millimetre. Corrections for known plate motions and Earth tides reveal motions that must be due to other forces.
Ultimately, the forces shaping the Earth’s surface are gravitational, and thus reduce to shifts of mass within and upon our planet. By far the most rapid movements of matter are those involving the atmosphere and the water vapour that it carries. Through variations in atmospheric density and the mass of water residing in soil moisture and snow cover, which arises from varying precipitation, surface load changes on an annual cycle. Meteorological and remote sensing estimates of these loads allow geophysicists to model the elastic response of the surface to the seasons. Why they have done this is not abundantly clear to me, but starting position is essential to astronavigation, hence similar attention to the Chandler Wobble (see Atmosphere linked to Earth’s rotation, Earth Pages, September 2000). Anyhow, the records suggest an annual mass transfer from hemisphere to hemisphere of around 1013 tonnes, which is sufficient to cause elastic deformation within the scope of GPS measurements. Geomaticians from the universities of Nevada and Newcastle upon Tyne (Blewitt, G. et al. 2001. A new global mode of Earth deformation: seasonal cycle detected. Science, v. 294, p. 2342-2345) have been able to chart the actual motions over the period from 1996 to mid-2001.
Performing the necessary computations on the weekly data from 66 IGS stations, and fitting curves to the results, Blewitt et al. present convincingly repetitive cycles in the motions towards the intersection of the Greenwich meridian and the Equator, towards the North Pole, and perpendicular to the surface. These tie very well to the theoretical model. Interestingly, they were able to model the shifts of displacement globally, and in series of maps show that the positions of maximum displacement shift along a path linking the continents. That is not surprising in itself, for the oceans respond by changes in water level, and only exposed continental lithosphere is likely to flex. The poles sink by around 3 mm each winter, and the Equator swings towards the winter pole by 1.5 mm. Results tally extremely well with estimates of seasonal mass shifts and theory. The surprises include an anomaly in vertical displacement in 1996-7 preceding the 1997-8 El Niño event, probably due to changes in Pacific sea level driven by winds and anomalous monsoon precipitation.
In the previous issue of Earth Pages was a resumé of a paper in Science that discusses the lateral tectonic motions that result from India’s collision with Eurasia (Continental tectonics of eastern Eurasia December 2001 Earth Pages)). The compliment to that appeared in the 22 November 2001 issue of Science (Tapponnier et al. 2001. Oblique stepwise rise and growth of the Tibet Plateau. Science, v. 294, p. 1671-1677). How and when the India-Eurasia collision zone achieved its pattern of huge elevated masses is partly an issue of tectonics, but they bear on any climatic effect that changed elevations might have had on climate, both regionally in the case of the South Asian monsoon circulation, and globally (one view is that Tibet’s deflection of atmospheric circulation may have been an important trigger for the onset of northern hemisphere glacial conditions).
Many geologists have considered the whole lithosphere of the region to have behaved in a ductile manner during collision, so that shortening and thickening were distributed more or less evenly. They ascribe the uniform height (>5 000 metres) to gravitational rebound when part of the thickened lithosphere detached and fell into the mantle, around the mid-Miocene. Erosion being unable to keep pace with uplift, the Tibetan Plateau is then thought to have become unstable and started to collapse laterally. That is seen by many as an explanation for clear evidence of E-W extension from both numerous N-S rift systems and extensional first motions on Tibetan earthquakes. However, this vast area is clearly subdivided into several major blocks by large strike-slip systems. The prevailing notion is that these faults are effects of “soft” collisional tectonics. Tapponier et al. assemble detailed evidence in relation to these faults and the blocks that they bound. They support tectonic evolution which has been controlled by coherent blocks of lithosphere, a process which was episodic rather than continuous, and accompanied by decoupling of crust and mantle lithosphere.
The linchpin of their model is the diachronous calc-alkaline magmatism of the region during the Tertiary, which becomes younger towards the north. As well as the principal site of northward subduction of Indian lithosphere beneath the Zangbo Suture, they propose that this magmatism was related to southward subduction that migrated northwards, and is hidden by thickened crust. The huge strike-slip systems are, to Tapponier et al., nothing less that oblique suture zones. The crustal blocks that they separate are, according to their model, large thrust wedges founded on major crustal detachments that accomplished most of the shortening. The process did not involve destruction of oceanic basins, but subduction of sub-continental mantle lithosphere, when crust and mantle became detached. Each successive subduction-accretion episode added its own increment to surface uplift, there probably having been three major steps in creating the highest average topography on Earth.
North America, particularly its west coast, is the best studied natural laboratory for active tectonics. Nonetheless, the downturn in Earth Science funding in the USA has threatened an ambitious project aimed at consolidating knowledge of plate interactions there. Nature (15 November 2001, p. 241) reports that the EarthScope initiative now has strong backing from the US National Academy of Sciences.
EarthScope has 4 elements: a mobile grid of seismometers; an observatory to monitor movement of plates below the NW Pacific Ocean; a programme aimed at drilling into the San Andreas Fault System; an interferometric radar satellite that will accurately measure ground movements in relation to tectonic and volcanic features. The total cost is around $400 million, shared equally between NASA and the National Science Foundation, if the funding proposal wins acceptance.
Information from: http://www.earthscope.org
Interferometric radar remote sensing provides high precision information on Earth motions associated with earthquakes (Radar analysis of Turkish earthquake, Earth Pages August 2001), but depends on “before and after” imaging. Continental tectonics is not just the outcome of occasional large movements on major faults, but of strains that continually occur throughout the lithosphere. Global positioning satellites provide means of precise location, particular when operated in differential mode, in which field-station signals are matched to those at fixed, geodetically precise base stations. Precisions to within centimetres or better are now commonplace at low cost. Structural geologists have been using GPS receivers for over a decade to check on the annual rates of plate motion across major structures such as the Alpine Fault of New Zealand and spreading centres such as that exposed on land in Iceland. In the 19 October issue of Science, such geodetic analysis of tectonics leaped by an order of magnitude.
The jewel in the crown of continental tectonics is eastern Eurasia, where the active collision of the Indian sub-continent with Asia drives a huge array of very large faults that separate rigid blocks and others, such as the Tibetan Plateau, that are deforming en masse. The spreading power of the Carlsberg and Central Indian Ridges is dissipated in motion of continental crust spanning 30° of latitude and 60° of longitude. Chinese scientists and their collaborators from the US universities of Alaska and Colorado have measure GPS positions at 354 stations throughout China, every one or two years for the last decade. Their analysis of the interim results (Wang, Q. et al. 2001. Present-day crustal deformation in China constrained by global positioning system measurements. Science, v. 294, p. 574-577) helps confirm or modify ideas about crustal motions that stemmed from seismic first-motion studies and regional field evidence. More than a third of the tectonic power accounts for crustal shortening within the Tibetan Plateau. While the western part of the huge system involves consistent motion towards the north-north-east, driving into Eurasia’s hinterland, the “free-edge” of eastern China and Indo-China seems to encourage the escape tectonics first proposed by Molnar and Tapponier. That involves a massive clockwise rotation around the East Himalayan Syntaxis, which takes up a great deal of motion. Whereas Molnar and Tapponnier proposed the shoving of south-eastern China oceanwards by the “escape” of Tibet, Wang et al’s measurements reveal that its motion to the east is only between one third and a quarter that of the adjacent east Tibetan Plateau. The lack of any sign that Tibetan crust is overriding that of south-east China, or that the latter is being shortened, may suggest that escape is funnelled around the East Himalayan Syntaxis into Burma and South-East Asia.
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