Category: Tectonics

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.

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.