The Himalaya: mountains made from mountains

Good building stones get reused. Sometimes the only traces of very old buildings are their stones, built into more modern ones. It’s the same with rocks and mountain belts. Stone that now forms parts of the Himalaya was once part of a now-vanished mountain range.

The Himalaya were formed by the collision between the Indian and Asian plates. For 50 million years, the Indian plate has been pushed down into the Himalayas where it is squashed, mangled and changed by heat and pressure. Working out the details of this process of mountain building has taken decades of careful study. Modern isotopic techniques are now so powerful that researchers studying Himalayan rocks can peer through beyond the effects of the recent mountain building to see traces of older events.

A recent open access paper by Catherine Mottram, Tom Argles and others looks at rocks in the Sikkim Himalaya, around the Main Central Thrust (MCT). As you can guess from the name (and the Use Of Capitals) this is an important structure; it can be traced over 1000km across the Himalaya and separates two distinct packages of rock known as the Lesser and Greater Himalayan Series.

Figure 2c. Cross section of MCT in the Sikkim Himalaya

Figure 2c. Cross section of MCT in the Sikkim Himalaya

As the rocks of the Indian plate were stuffed into the moutain belt, much of the movement of rock was along near-flat faults, known as thrusts. These stack up layers of rock, shortening and thickening the crust. Thrusts near the surface may be a single fault plane, but at greater depths rocks flow rather than snap and a thick thrust zone of deformed rocks is formed. This makes drawing a line on a map and calling it the Main Central Thrust rather difficult. Should the line be placed where the rock types change, or where they are most deformed, or where there is a break in metamorphism? Each approach has its advocates.

Our authors took an isotopic approach, measuring Neodymium isotopes for the whole rock and Uranium-Lead in useful crystals called Zircon. Their analysis shows that the two packages of rock separated by the MCT can be distinguished using isotopes. The actual boundary is not sharp: they prove interlayering of the two rock packages within the thrust zone, rather than a single boundary. This is not surprising given that thrusting is a gradual process and thrust surfaces are not flat.  Deformation seems to have started at the boundary between the Lesser and Greater Himalaya and gradually moved down over time.

The patterns of isotope measurements that can be used to distinguish between the Greater and Lesser Himalayan Series also tell us about what happened before India met Asia.

The zircons whose isotopes were measured are of two types, detrital and igneous. The first are grains that were eroded from old rocks and settled into a sedimentary basin. The second crystallised from molten rock: their ages record significant events. Together these sets of dates give a view of a long and complicated pre-Himalayan history.

Our authors attempt to reconstruct the leading edge of the Indian plate, as it might have looked before it crashed into Asia.

Figure 10.

Figure 10. “Schematic illustration showing the pre-Himalayan architecture of the Sikkim rocks, during the mid-Palaeozoic. The Lesser Himalayan Sequence lithologies were once separated from the Greater Himalayan Sequence rocks by a Neoproterozoic rift. The Bhimpedian orogeny was responsible for closing the rift and thickened the Greater Himalayan Sequence, causing metamorphism and intrusion of granites. The failed closed rift may represent a weak structure later exploited by the Main Central Thrust. Lithologies are the same as in the legend in Figures 1 and 2.”

The Greater Himalayan Sequence had already been heated and deformed in the roots of a mountain belt long before the Himalayas existed. This a relatively common situation. Polyorogenic rocks such as these1 need to be treated with care, otherwise we might mix up events separated by millions of years. A single garnet crystal may contain different areas that formed in totally separate mountain building events

One of the detrital zircon grains dated in this study was 3,600,000,000 years old. We can only guess how many cycles of erosion and burial, how many splittings and couplings of continents this mineral has ‘seen’. As it was buried and heated once again maybe, like the bowl of petunias in The Hitchhiker’s Guide to the Galaxy it thought to itself: “Oh no, not again”.

References

Mottram C.M., Argles T.W., Harris N.B.W., Parrish R.R., Horstwood M.S.A., Warren C.J. & Gupta S. (2014). Tectonic interleaving along the Main Central Thrust, Sikkim Himalaya, Journal of the Geological Society, 171 (2) 255-268. DOI:

Argles T.W., Prince C.I., Foster G.L. & Vance D. (1999). New garnets for old? Cautionary tales from young mountain belts, Earth and Planetary Science Letters, 172 (3-4) 301-309. DOI:

Notes:
1. Forgive the jargon, but isn’t it a lovely word?
Categories: geochemistry, Himalaya, metamorphism, mountains, open access, tectonics

Comments (11)

  1. C says:

    “Polyorogenic” is this a neologism or did it already exist? Btw I guess it doesnt make a difference at which orogen u take a closer look at, also in the bavarian variscan orogens a zircon was found dated at about 3,X Ga

  2. Metageologist says:

    Polyorogenic is not a word I made up, much as I’d have liked to.
    You’re right that all mountain belts tend to have older histories in them. Rocks affected by the Variscan orogeny (that you mentioned) are found within the Alps, where exactly the same issues of multiple histories in the same mineral grain can be found.

  3. C says:

    Btw if I see it right this thrust is also a suture without any typical ophiolitic material?

  4. 13078829 says:

    This is a very interesting part o historical geology to me as an undergraduate geology student.

    This reminds me of the plate tectonic theory whereby crustal thickening and mountain folding processes occur as a results of convergent plate boundaries.

    As unconformity is a surface between two layers of strata, is the MCT acting as an unconformity as it’s separating two packages of indian crust as you said and it’s important to know the age of the rocks separated by the MCT?

    • Metageologist says:

      The formation of the Himalayas is exactly what you describe from plate tectonic theory.
      An unconformity is a surface within or at the base of sedimentary sequence – where sediment starts being deposited. The MCT is a different kind of structure, forming by rock deformation, rather than sedimentary processes.

      In polyorogenic terranes, where there is later deformation, it can very hard to distinguish between a thrust an an unconfirmity as the evidence has been destroyed. For example in the Dalradian.

  5. unnikrishnan says:

    a wonderful blog with lots of knowledge. What are the exact definition for the terms HFF, MCT , HFT, MBT and how they formed ?

  6. Metageologist says:

    Those terms are all abbreviations for names of thrusts. The rocks in the Himalaya are made up of different packages of rock separated by faults/zones of shearing. The names are just names and are written out in the figure above.

    I’ve written a lot more about mountain building and how these structures form. Try starting here.

    For a better discussion of the actual geology, with some good cross-sections, try here.

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