The Indian plate’s days as a Cretaceous boy-racer

The power of plate tectonics lies in its simplicity: the Earth’s surface is a planetary jigsaw puzzle made up of a couple of dozen rigid, interlocking pieces, and most geological activity on the planet, past and present, is a result of the interactions between these pieces as they either move apart, or bump and grind together. However, understanding that the plates move, and being able to measure their past and present motions, doesn’t mean that we completely understand why they move the way that they do. For example, most plates drift across the face of the planet at roughly the same speed that your fingernails grow: perhaps five centimetres a year. But sometimes they can move much faster, as palaeomagnetic results from India demonstrate [1]. 80 million years ago, as India rifted away from Madagascar during the break-up of Pangaea, it was located about 40 degrees south; 50-55 million years ago – barely a geological eye-blink later – it was almost at the equator, crashing headlong into Asia to create the Himalayas.

Indianreconstructions.png

This corresponds to a total drift of about 4500 kilometres in only 15-20 million years, or almost 20 centimetres a year; in contrast, during the same time interval Africa, Antarctica and Australia, India’s Pangaean neighbours, were drifting around at less than 5 cm/yr. Why the large disparity? New geophysical data, published last week in Nature [2], suggest that the explanation for the Indian plate’s unseemly haste may lie in its surprisingly streamlined underbelly.


‘Plate’ is not a synonym for ‘crust'; the upper part of the mantle, known as the lithosphere, is also cold and strong enough to act rigidly over geological timescales. Thus, the base of a plate is not defined by a change in composition, but by a change in mechanical properties: it is the point at which mantle rocks become warm and weak enough to deform plastically, which marks the transition into the convecting asthenosphere. Think of ice covering a pond in winter – it’s still water, but its reduced temperature has made it strong enough to support your weight, unlike the stuff it covers (don’t think that this means that the asthenosphere is fluid in the ‘molten lava’ sense, though – it is still many orders of magnitude more viscous than the stickiest treacle).
Because the boundary between lithosphere and asthenosphere is controlled by temperature, and because older usually corresponds to colder, plate thickness generally increases with age, with the thickest lithosphere of all found beneath the ancient centres of the continents. Here, the upper mantle has had billions of years to cool down, so the lithosphere can extend 250 km or more below the surface. But it doesn’t always, and this is what Prakash Kumar of the Indian National Geophysical Research Institute and his colleagues were investigating using some rather clever seismological trickery. For reasons that are still not completely understood, the boundary between the lithosphere and asthenosphere (LAB) is fairly abrupt, so has strong effects on seismic waves passing through it. One such effect is the creation of converted phases; some of the energy in a transverse S-wave can be converted to compressional P-waves as it passes across the boundary, or vice-versa. In the figure below, an S-wave generated by a distant earthquake encounters the base of the lithosphere, and is split in two, with a secondary P-wave being generated alongside the original S-wave. Both of these waves leave the boundary at precisely the same time, but in any given medium, a P-wave will move faster than an S-wave. Therefore, if the lithosphere is thin, the converted P-wave will arrive at the surface only a little before its parent S-wave does; but if the lithosphere is thick, then the P-wave has more time to pull ahead and the time difference will be much larger.

Swavereceiver.png

What Kumar and his colleagues have found that the difference in arrival times for converted and parent waves generated beneath Southern Africa, Antarctica, and Australia is usually between 20 and 30 seconds, which is consistent with very thick lithosphere – almost 300 km in the case of South Africa. However, the time differences recorded by stations in India are only about 10 seconds; this suggests that the lithosphere beneath the subcontinent is a mere 100 km thick, much thinner than you might expect for such old crust. The authors suggest that it was not always thus: diamond-bearing kimberlites found on the Indian craton are strong evidence that the lithosphere beneath was once comparable in thickness to that found beneath the other cratons surveyed (kimberlites are only formed by the melting of continental lithosphere at depths of more than 150 km), and that it has actually got thinner with age. The break-up of southern Pangea appears to have coincided with the impact of several mantle plumes between 180 and million years, and it is possible that the upwelling plumes provided a pulse of heat which ‘reset’ the lower parts of the Indian lithosphere back to asthenosphere, creating a much thinner tectonic plate after rifting was complete.

lithospheretop.jpg

Here, then, is a possible explanation for India’s speedy geological past. Most plates incorporate a chunk of old continent with a thick lithospheric root projecting down into the asthenosphere, generating drag and slowing plate motions. The apparent removal of India’s lithospheric root would have considerably reduced the drag on the bottom of the plate, potentially allowing much faster movement.

platedrag.png

It’s an interesting idea, but I’m not sure that it’s the whole story. Of all the forces driving plate motion, the drag exerted by subducting slabs as they descend back into the lower mantle is thought to be particularly important. The maps at the top of this post indicate that in the Late Cretaceous, the closure of the Tethys Sea was accomplished by subduction along the entire northern edge of the Indian plate; in contrast, the Africa and Antarctic plates were mainly rimmed by spreading ridges. Plume-induced streamlining may have helped the Indian plate to leave its tectonic rivals in the dust, but a subduction-induced turbo-boost probably didn’t hurt, either.
[1] Klootwijk, CT et al. (1992). Geology 20, p395-98.
[2] Kumar, P et al. (2007). Nature 449, p894-97.

Categories: geology, geophysics, Mesozoic, paper reviews, tectonics

Comments (7)

  1. NJ says:

    India is the catamaran of continents!
    Interesting that the eruption of the Deccan traps about half-way through the journey did not seem to slow it down; I would have guessed that the loading of the basalt would have begun to create a new continental keel with a resultant decrease in velocity.
    Or might this be something that hasn’t been teased out of the data yet? A higher speed start from 80-65 mya followed by a slowing from 65-50 mya?

  2. chezjake says:

    Thanks for this excellent and well illustrated post. I know relatively little geology, but plate tectonics has always fascinated me. This post has expanded both my knowledge and my curiosity.
    Is it correct to guess that the increased velocity of the Indian plate also helps explain the degree of mountain building seen in the Himalayas?

  3. Divalent says:

    Well, what chezjake said… thanks for the interesting and educational post. A bit of an open-loop spew follows:
    A question: occasionally (frequently?) continents will split apart opening a new sea. Does the “opposite” ever occur? Where an intact and relatively stable continent begins to compact itself to form a mountain chain mid continent? (I.e., the type of event that, had it occured in oceanic crust, would have formed a subduction zone.)
    I guess the issue I have in mind is that you often hear it stated that India “collided” with Asia to form the Himalayas, and the image that comes to my mind is of a moving car smashing into parked car: the cars buckle, and eventually the motion comes to a halt as the crumpling of the cars and the displacement of the parked car absorb all the energy of the moving car. Is the formation of the himalayas an analogous process that will eventually halt teh motion of India, or will that process continue for as long as the underlying driving process (mantle convection) continues? Does the mountain forming process retard the mantle conduction? Does mountain building require a collision? (see my initial question). (Does India have an actual momentum that is driving the process (I know V is small, but hell, M is huge!)).
    Okay, I’ll stop now. :)
    Cheers,
    divalent

  4. Andrew Dodds says:

    Divalent –
    The answer is no – momentum is actually tiny – Someone calculated that if just a million people pushed all together, they could stop the African plate, if just momentum was taken into account.
    Remember that the forces driving the collision – a descending slab and a ‘pushing’ ridge – don’t go away just because the continents hit. The himalayan collision will only stop when the subduction zone jumps back to the edge of India.
    NJ –
    Actually, the arrival of a plume creates a ‘hill’ if you like, which the plate can slide away from, so you actually speed things up (A mere few km of basalt dosen’t do much to a plate).
    So it looks like thinned/delaminated? lithosphere, extra ‘push’ from a plume, and perhaps old tethys ocean crust all acting together.

  5. tom says:

    I love Indians stories.

  6. Chris Rowan says:

    Mountain belts like the Himalayas are only formed in plate collision zones, but within a plate you can get regions of broad uplift above mantle hotspots (because warm rock is more buoyant it will cause uplift). Examples include the Colorado Plateau (where the Grand Canyon is).
    It seems clear that whatever forces that acted to drive the Indian plate so speedily northward are still in operation, because India is still moving into Asia at 5cm a year even with buoyant continental crust now clogging up the plate boundary. This is one of the reasons why the Himalayas are so high – the topography of any mountain range is a balance between uplift, erosion and gravitational collapse, and the uplift component is extremely large in this case.

  7. NJ says:

    Andrew:
    I realize that the plate would have been buoyed up by the initial pulse of the plume (and so would like have ‘sped’ downhill), but after it came off the plume, there would have been some isostatic depression due to the loading. After all, you get crustal depression from a few km of sediment or ice.
    So maybe an arm-wave model is this: high velocity movement before the plume, an acceleration down the backside of the plume, and a slight slow down prior to impact?