Earliest direct evidence of plate motions

There are two ways that we recognise the movement of tectonic plates. Since the latter half of the Mesozoic Era, following break up of the Pangaea supercontinent, it bests manifests itself in the magnetic ‘stripes’ on the ocean floor. They result from alternating polarisation of the geomagnetic field as new oceanic lithosphere is generated at constructive plate boundaries to drive sea-floor spreading. The oldest remaining stripes date back to the early Jurassic. For earlier times geologists have to turn to the continental crust.  Lavas and some sedimentary rocks undergo magnetisation at the time of their formation and retained that imprint. Such remanent, palaeomagnetism reveals the original latitude at which it was imprinted, together with the subsequent rotation of a drifting continent relative to an assumed N to S axis joining the opposed magnetic poles. The apparent ‘wandering’ of the pole through time when successive ancient pole positions of different ages are plotted in relation to the present position of a continent is a good guide to its history of drifting as a result of plate tectonics. Comparing the polar-wander paths of two continents allows the time when they were formerly united to be estimated. So palaeomagnetic pole data makes it possible to reconstruct not just Pangaea but a whole series of earlier supercontinents, ancient magnetic data being supplemented by other geological evidence such as reconnecting the trends on different continents of ancient mountain belts.

Apparent polar wander paths for two continents for a period when they were united then split and were separated by sea-floor spreading, eventually to collide and reunite

The further back in time the fewer palaeomagnetic pole positions have been estimated, and the more uncertain are the apparent polar wander paths and the more complex each continent’s accumulated geological history. One of the reasons for such uncertainty is that episodes of metamorphism can reset a rock’s remanent magnetisation, hundreds of million years after it originally formed. Thus, the harder it becomes to be certain about early supercontinents that have been suggested, of which there are quite a few. The earliest that has been proposed is Vaalbara, albeit on grounds of geological similarity, that supposedly united the Kaapvaal and Pilbara Cratons of southern Africa and Western Australia, respectively. Its duration is suggested to have been between 3.6 to 2.8 Ga (billion years ago). The oldest supercontinents with sound palaeomagnetic records date from the end of the Archaean Eon (2.5 Ga). It is the lack or uncertainty of earlier palaeomagnetic evidence that makes the start of plate tectonics the subject of so much debate.

However, geophysicists continually strive to improve the detection of ancient magnetisation, and advances have been made recently to unravel original magnetisation signals from those that have been superimposed later. The fruits of these developments are borne out by a study of a sequence of mafic lavas from the Pilbara Craton that formed about 3.2 Ga ago (Brenner, A.R. et al. 2020. Paleomagnetic evidence for modern-like plate motion velocities at 3.2 Ga. Science Advances, v. 6, article eaaz8670; DOI: 10.1126/sciadv.aaz8670). Alec Brenner and colleagues from several US universities measured palaeomagnetism in more than 200 diamond drill cores from two localities in this sequence and combined their data with others from the Pilbara to cover a roughly 600 Ma period between 3.35 to 2.77 Ga. The palaeopoles form a polar wander path that spans roughly 50 degrees of palaeolatitude. From this they have been able to estimate, in considerable detail, the rate at which the Pilbara Craton had moved in Mesoarchaean. In the first 170 Ma the average horizontal motion was about 2.5 cm per year, falling rapidly to 0.4 cm per year over the following 410 Ma. The earlier speed is comparable with the average of modern plate motions. Data from the later period suggests relative stagnation. Motions over the entire ~600 Ma could be due to episodic operation of plate tectonics on the global scale, or a local slowing in the rate of plate growth.

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