The mechanical disconnection of the lithosphere from the Earth’s deep mantle by a more ductile zone in the upper mantle – the asthenosphere – suggests that the lithosphere might move independently. If that were the case then points on the surface would shift relative to the axis of rotation and the magnetic poles, irrespective of plate tectonics. So it makes sense to speak of absolute and relative motions of tectonic plates. The second relates to plates’ motions relative to each other and to the ancient position of the magnetic poles, assumed to be reasonably close to that of the past pole of rotation, yet measurable from the direction of palaeomagnetism retained in rocks on this or that tectonic plate. Plotting palaeomagnetic pole positions through time for each tectonic plate gives the impression that the poles have wandered. Such apparent polar wandering has long been a key element in judging ancient plate motions. Absolute plate motion judges the direction and speed of plates relative to supposedly fixed mantle plumes beneath volcanic hot spots, the classic case being Hawaii, over which the Pacific Plate has moved to leave a chain of extinct volcanoes that become progressively older to the west. But it turns out that between about 80 to 50 Ma there are some gross misfits using the hot-spot frame of reference. An example is the 60° bend of the Hawaiian chain to become the Emperor seamount chain that some have ascribed to hot spots shifting (see http://earth-pages.co.uk/2009/05/01/the-great-bend-of-the-pacific-ocean-floor/).
Age of Pacific Ocean floor, showing the Hawaii-Emperor seamount chain in black. (credit: Wikipedia)
Ideas have shifted dramatically since it became clear that hot spots can shift, and there has been an attempt to estimate their actual motions (Doubrovine, P.V. et al. 2012. Absolute plate motions in a reference frame defined by moving hot spots in the Pacific, Atlantic, and Indian oceans. Journal of Geophysics Research: Solid Earth, v. 117, B09101, doi:10.1029/2011JB009072). It is early days for the revised view of absolute motion of the lithosphere and estimates go back only 120 Ma. However, one outcome has been a realistic examination of whether the positions of the poles have shifted through time; a possibility that is hidden in apparent polar wander paths. Since the mid-Cretaceous it seems that a slow and hesitant, but significant polar shuffle has taken place, varying between 0.1 and 1.0° Ma-1, starting in one direction and then the movement retraced its steps to achieve the current proximity of magnetic poles to the poles of rotation.
Historical seismicity across the Sunda trench(credit: Wikipedia)
Few people will fail to remember the Indian Ocean tsunamis of 26 December 2004 because of their quarter-million death toll. The earthquake responsible for them resulted from thrusting movements on the subduction zone where part of the India-Australia plate descends beneath Sumatra. There have been some equally large but far less devastating events and many lesser earthquakes in the same region since. Some have been on the massive Wadati-Benioff zone but many, including two with magnitudes >8 in April 2012, have occurred well off the known plate boundary. Oddly, those two had strike-slip motions and were the largest such events since seismic records have been kept. Such motions where masses of lithosphere move past one another laterally can be devastating on land, yet offshore ones rarely cause tsunamis, for a simple reason: they neither lift nor drop parts of the ocean floor. So, to the world at large, both events went unreported.
To geophysicists, however, they were revealing oddities, for there is no bathymetric sign of an active sea-floor strike-slip fault. But there is a series of linear gravity anomalies running roughly N-S thought to represent transform faults that were thought to have shut down about 45 Ma ago (Delescluse, M. et al. 2012. April 2012 intra-oceanic seismicity off Sumatra boosted by the Banda-Aceh megathrust. Nature (on-line 27 September issue) doi:10.1038/nature11520). Examining the post-December 2004 seismic record of the area the authors noted a flurry of lesser events, mostly in the vicinity of the long dead fracture zones. Their analysis leads them to suggest not only that the Banda-Aceh earthquake and others along the Sumatran subduction zone reactivated the old strike-slip faults but that differences in the motion of the India-Australia plate continually stress the lithosphere. Indian continental crust is resisting subduction beneath the Himalaya thereby slowing plate movement in its wake. Ocean lithosphere north of Australia slides more easily down the subduction zone, so its northward motion is substantially faster, creating a torque in the region affected by the strike-slip motions. Ultimately, it is thought, this will split the plate into separate Indian and Australian plates.
Another surprising outcome of this complex seismic linkage in the far-east of the Indian Ocean is that the April strike-slip earthquake set the Earth ringing. For six days afterwards there was a five-fold increase in events of magnitudes greater than 5.5 more than 1500 km away, including some of around magnitude 7.0 (Polliitz, F.F. et al. 2012. The 11 April 2012 east Indian Ocean earthquake triggered large aftershocks worldwide. Nature (on-line 27 September issue) doi:10.1038/nature11504). Although distant minor shocks often follow large earthquakes, this is the first time that a swarm of magnitude 5.5 and greater has been noticed.
Recent earthquakes in the US mid-west around New Madrid Missouri. Image via Wikipedia
Almost all devastating earthquakes within living memory and the tsunamis that ensued from some of them have occurred where tectonic plates meet and move past one another either horizontally through strike-slip motion or vertically as a result of subduction. This link between real events and the central theory of global dynamics gives an impression of inherent predictability about where damaging and deadly earthquakes might happen, if not the more useful matter of when the lithosphere might rupture. Such confidence is potentially highly dangerous: the most deadly earthquake in recorded history killed at least 800 thousand people in China’s Shanxi Province in 1556 when according to a description written shortly afterwards, ‘… various misfortunes took place… In some places, the ground suddenly rose up and formed new hills, or it sank abruptly and became new valleys. In other areas, a stream burst out in an instant, or the ground broke and new gullies appeared…’. Shanxi is far from any plate boundary. A study of Chinese historic records covering the last two millennia (Liu, M. et al. 2011. 2000 years of migrating earthquakes in North China: How earthquakes in midcontinents differ from those at plate boundaries. Lithosphere, v. 3, p. 128-132) shows a pattern to the position of large intraplate events. Rather than occurring along lines as do those at plate boundaries, earthquakes ‘hopped’ from place to place without affecting the same areas twice. Liu and colleagues consider this almost random pattern to result from reactivation of interlinked faults through broad-scale and gradual tectonic loading of the crust by far off plate movements. After a short period of reactivation one fault locks so that energy build-up is eventually released by another in the plexus of crustal weaknesses.
The best studied site of such intraplate seismicity lies midway along the Mississippi valley in the mid-US, between St Louis and Memphis. In 1811 and 1812 four Magnitude 7 to 8 earthquakes struck, the most affected place being the small township of New Madrid on the banks of the great river where mud and sand spouted from numerous sediment volcanoes. No-one died there but tremors were felt over a million square kilometers, bells ringing spontaneously as far away as Boston and Toronto. It is now known that this section of the Mississippi basin lies above a graben that affects the ancient basement beneath the alluvial sediments, one of whose faults was reactivated, perhaps in an analogous way to the hypothesis about Chinese seismicity. A coauthor in Liu et al. (2011), Seth Stein of Northwestern University, Illinois, believes stress redistribution through a Mid-western fault network was responsible and other events are likely at some uncertain time in the future on this and other areas underpinned by ancient fault complexes. Indeed sporadic ‘quakes up to Magnitude 7 have affected the eastern US and Canada and the Atlantic seaboard since European settlement. But since the largest of the New Madrid quartet of earthquakes, populations have grown across the likely areas of tenuous risk and future ones could have extremely serious consequences for which it is difficult to plan by virtue of unpredictability of both place and timing: in some respects a more worrying prospect than is the case where major events are inevitable – sometime – as along the San Andreas Fault. There are few, if any, major conurbations worldwide that could be considered seismically safe if the theory of networked stress redistribution through otherwise inert parts of continental crust is borne out.
In some respects the theory is a small-scale version of the suggested mechanical linkage through all major plate boundaries that has been suggested by some to account for the clustering in time of great earthquakes – around and above Magnitude 8 – around the globe. Since 2000 great earthquakes have occurred on subduction zones beneath Sumatra, the Himalaya, the Andes, Central America, Alaska, New Guinea, the mid-Pacific, Japan and the Kurile islands, on the strike-slip system that cuts New Zealand and in the intraplate setting of the 2008 Sichuan earthquake in China. Almost all plate boundaries link up globally, but although it seems likely that stress is redistributed along boundaries, especially between adjacent segments, as documented for the great Anatolian fault system of Turkey and the Indonesian subduction zone, a mechanism that transmits stress beyond individual plates seems unlikely.
Norwegian Eclogite. Image by kevinzim via Flickr
For more than 30 years a debate has raged about the antiquity of plate tectonics: some claim it has always operated since the Earth first acquired a rigid carapace not long after a molten state following formation of the Moon; others look to the earliest occurrences of island-arc volcanism, oceanic crust thrust onto continents as ophiolite complexes, and to high-pressure, low-temperature metamorphic rocks. The earliest evidence of this kind has been cited from as far apart in time as the oldest Archaean rocks of Greenland (3.9 Ga) and the Neoproterozoic (1 Ga to 542 Ma). A key feature produced by plate interactions that can be preserved are high-P, low-T rocks formed where old, cool oceanic lithosphere is pulled by its own increasing density into the mantle at subduction zones to form eclogites and blueschists. In the accessible crust, both rock types are unstable as well as rare and can be retrogressed to different metamorphic mineral assemblages by high-temperature events at lower pressures than those at which they formed. Relics dating back to the earliest subduction may be in the mantle, but that seems inaccessible. Yet, from time to time explosive magmatism from very deep sources brings mantle-depth materials to the surface in kimberlite pipes that are most commonly found in stabilised blocks of ancient continental crust or cratons. Again there is the problem of mineral stability when solids enter different physical conditions, but there is one mineral that preserves characteristics of its deep origins – diamond. Steven Shirer and Stephen Richardson of the Carnegie Institution of Washington and the University of Cape Town have shed light on early subduction by exploiting the relative ease of dating diamonds and their capacity for preserving other minerals captured within them (Shirey, S.B. & Richardson, S.H. 2011. Start of the Wilson cycle at 3 Ga shown by diamonds from the subcontinental mantle. Science, v. 333, p. 434-436). Their study used data from over four thousand silicate inclusions in previously dated large diamonds, made almost worthless as gemstones by their contaminants. It is these inclusions that are amenable to dating, principally by the Sm-Nd method. Adrift in the mantle high temperature would result in daughter isotopes diffusing from the minerals. Once locked within diamond that isotopic loss would be stopped by the strength of the diamond structure, so building up with time to yield an age of entrapment when sampled. The collection spans five cratons in Australia, Africa, Asia and North America, and has an age spectrum from 1.0 to 3.5 Ga. Note that diamonds are not formed by subduction but grow as a result of reduction of carbonates or oxidation of methane in the mantle at depths between 125 to 175 km. In growing they may envelop fragments of their surroundings that formed by other processes.
A notable feature of the inclusions is that before 3.2 Ga only mantle peridotites (olivine and pyroxene) are trapped, whereas in diamonds younger than 3.0 Ga the inclusions are dominated by eclogite minerals (garnet and Na-, Al-rich omphacite pyroxenes). This dichotomy is paralleled by the rhenium and osmium isotope composition of sulfide mineral inclusions. To the authors these consistent features point to an absence of steep-angled subduction, characteristic of modern plate tectonics, from the Earth system before 3 Ga. But does that rule out plate tectonics in earlier times and cast doubt on structural and other evidence for it? Not entirely, because consumption of spreading oceanic lithosphere by the mantle can take place if basaltic rock is not converted to eclogite by high-P, low-T metamorphism when the consumed lithosphere is warmer than it generally is nowadays – this happens beneath a large stretch of the Central Andes where subduction is at a shallow angle. What Shirey and Richardson have conveyed is a sense that the dominant force of modern plate tectonics – slab-pull that is driven by increased density of eclogitised basalt – did not operate in the first 1.5 Ga of Earth history. Eclogite can also form, under the right physical conditions, when chunks of basaltic material (perhaps underplated magmatically to the base of continents) founder and fall into the mantle. The absence of eclogite inclusions seems also to rule out such delamination from the early Earth system. So whatever tectonic activity and mantle convection did take place upon and within the pre-3 Ga Earth it was probably simpler than modern geodynamics. The other matter is that the shift to dominant eclogite inclusions appears quite abrupt from the data, perhaps suggesting major upheavals around 3 Ga. The Archaean cratons do provide some evidence for a major transformation in the rate of growth of continental crust around 3 Ga; about 30-40 percent of modern continental material was generated in the following 500 Ma to reach a total of 60% of the current amount, the remaining 40% taking 2.5 Ga to form through modern plate tectonics
- Deep Diamonds Shed Light on Ancient Continental Movements (news.sciencemag.org)
- van Kranendonk, M.J. 2011. Onset of plate tectonics. Science, v. 333, p. 413-414.
Rio de Janeiro, a threatened city? Image by Alcindo Correa Filho via Flickr
Earthquake prediction has not had a good record, but it seems that vastly larger tectonic processes are now becoming the subject of risk analysis (Nikolaeva, K. et al. 2011. Numerical analysis of subduction initiation risk along the Atlantic American passive margins. Geology, v. 39, p. 463-466). The Swiss, Russian and Portuguese authors focus on the old (Jurassic ~170 Ma) and presumably cold oceanic lithosphere on the western flank of the Atlantic, against both the North and South American continents. Increased density with ageing imparts a potential downwards force, but that has to overcome resistance to plate failure at passive margins. The dominance of upper continental lithosphere by rheologically weak quartz tends to make it more likely to fail than more or less quartz-free oceanic lithosphere. So, if subduction at a passive continental margin is to take place, then where and when it begins depends on the nature of the abutting continental lithosphere. That on the Atlantic’s western flank varies a lot, ranging from 75-150 km thick. Consequently the temperature at the Moho, the junction between continental lithosphere and weaker asthenosphere, varies too. The loading by marginal sedimentation also plays a role, as do continent-wide forces associated with far-distant mountain ranges, such as the Western Cordillera and Andes, and the forces from opposed sea-floor spreading from the Juan de Fuca and East Pacific systems that affect the whole of western South America, most of Central America and the far NW of North America.
Analysing all pertinent forces acting along 9 lines of section through both North and South America, the authors’ focus fell on the relatively thin continental lithosphere of the Atlantic margin of South America. It is at its thinnest along the southernmost part of the margin adjacent to Brazil, where the Moho temperature reaches as high as 735°C: the weakest link in the American continental lithosphere, where there is seismicity and also indications of igneous activity. The modelling suggests that incipient deformation may begin off southern Brazil within 4 Ma to form a zone of overthrusting, eventually evolving towards failure of the ocean-continent interface and the start of proper subduction in the succeeding 20 Ma. Other stretches of the eastern Americas are deemed safe from subduction for considerably longer by virtue of their greater thickness, lower Moho temperatures and thus higher strength. It is an interesting situation because, insofar as I understand plate tectonics, extensional or compressional failure needed to generate plate boundaries must also propagate from the weak spots that first fail; plate boundaries are lines not points. If that does not happen, then the very strength of the overwhelming longer continent-ocean interface will surely prevent subduction at a single, albeit weak link.
Paper PDF at http://xa.yimg.com/kq/groups/13231164/1842350625/name/Geology-2011-Nikolaeva-463-6.pdf