Subduction and continental collision in the Himalaya

The Indian subcontinent after it separated from Madagascar in the Late Cretaceous to move northwards to its destined collision with Eurasia and the formation of the Himalaya. (Credit: Frame from an animation ©Christopher Scotese)

During the Early Cretaceous (~140 Ma ago) India, Madagascar, Antarctica and Australia parted company with Africa after 400 Ma of unity as components of the Gondwana supercontinent. By 120 Ma Antarctica and Australia split from India and Madagascar, and the Indian Ocean began to form. India moved northwards , leaving Madagascar in its wake after about 70 Ma ago. By 50 Ma the subcontinent began to collide with Eurasia, its northward motion driving before it crustal materials that eventually formed the Himalaya. This highly complex process is wonderfully documented in an animation made in 2015 by Christopher Scotese, Emeritus Professor in the Department of Earth and Environmental Sciences, Northwestern University, USA. At the start of its journey India moved northwards at a slow rate of about 5 cm per year. After 80 Ma it speeded up dramatically to 15 cm per year, about twice as fast as any modern continental drift and a pace that lasted for over 30 Ma until collision began. How could that, in a geological sense, sudden and sustained acceleration have been induced? It would have required a change in the slab-pull force that is the primary driver of plate tectonics, suggesting an increase in the amount of subduction in the Tethys Ocean that formerly lay between India and Eurasia, probably at two, now hidden destructive plate margins.

A group of geoscientists from Canada, the US and Pakistan has documented that collision in terms of the record of metamorphism experienced beneath the Himalaya as slab after slab of once near-surface rocks were driven beneath the rising orogen (Soret, M. et al. 2021. How Himalayan collision stems from subduction. Geology, v. 49, p. 894-898; DOI: 10.1130/G48803.1). The Western Himalaya has trapped a deformed and tilted magmatic rock sequence of an island arc – the Kohistan Arc – between  the Eurasian plate and a zone of crustal thickening and shortening that was thrust southward over the ancient metamorphic basement of India itself. That crust was mantled by a variety of younger sediments deposited on the Tethyan continental shelf of the northern Indian plate which became involved in the process of crustal thickening. The Kohistan Arc probably formed above one of the destructive margins that consumed the oceanic lithosphere of the now vanished Tethys Ocean. Two distinct types of rock make up the slabs stacked-up by thrusting.

The uppermost, which also forms the highest part of the Western Himalaya in the form of Nanga Parbat (at 8,126 metres the world’s ninth highest mountain) comprises rocks thought to represent Tethyan oceanic lithosphere subducted perhaps at the second destructive margin. Their mineral assemblages, especially those of eclogites, indicate that they have been metamorphosed under pressures corresponding to depths of up to 100 km, but at low temperatures along a geothermal gradient of about 7°C km-1, i.e. in a low heat-flow environment. These ultra-high pressure (UHP) metamorphic rocks formed at the start of the India-Eurasia collision. The sequence of sedimentary slabs now overridden by the UHP slab were metamorphosed at around the same time, but under very different conditions. Their burial reached only about 35 km – the normal thickness of the continental crust – and a temperature of about 600°C on a 30°C km-1 geothermal gradient. Detailed mineralogy of the UHP slab reveals that as it was driven over the metasediments it evolved to the same geothermal conditions.

Matthew Soret and his colleagues explain how this marked metamorphic duality may have arisen in rocks that are now part of the same huge thrust complex. Their results are consistent with slicing together of oceanic lithosphere in a subduction zone to form a tectonic wedge of UHP mineral assemblages at the same time as continental shelf sediments were metamorphosed under more normal geothermal conditions. This was happening just as India came into contact with Eurasia. When crustal thickening began in earnest through the inter-slicing of the two assemblages, pressure on the UHP rocks fell rapidly as a result of their being thrust over the dominantly metasedimentary shelf sequence. It also moved into a zone of normal heat flow, first heating up equally quickly and then following a path of decreasing pressure and temperature as erosion pared away the newly thickened crust. Both assemblages now became part of the same metamorphic regime. In this way a subduction system evolved to become incorporated in an orogenic zone as two continents collided; a complex process that finds parallels in other orogens such as the Alps.

Plate tectonics and the Cambrian Explosion

A rough-and-ready way of assessing the rate at which silicic magmatic activity has varied through time is to separate out grains of zircon that have accumulated in sedimentary rocks of different ages. Zircon is readily datable using the U-Pb method, if you have access to mass spectrometry. While some of the zircons will date from much older continental crust that was exposed while the sediments originated, sometimes there are grains that formed only a few million years before the sediments accumulated. Those are likely to have crystallized from silica-rich volcanic rocks above subduction zones where ocean-floor has been driven beneath continental crust; i.e. at continental volcanic arcs. Such young zircons therefore help assess the tectonic conditions close to sedimentary basins. The potential of detrital zircon geochronology was first suggested to me by Dr M.V.N. Murthy of the Geological Survey of India in 1978, long before anyone could aspire to mass zircon dating. M.V.N. had by then amassed kilograms of zircon grains from every imaginable source in India, and may have been the first geologist to realise their potential. It has become a lot quicker and cheaper in the last two decades, thanks to methods of dating single zircon grains both precisely and accurately and M.V.N.’s prescient suggestion has been borne out globally.

Optical microscope photograph; the length of t...
A detrital zircon grain about 0.25 mm long. (Photo credit: Wikipedia)

Results for the late Precambrian to early Palaeozoic have recently been compiled (McKenzie, N.R. et al. 2014. Plate tectonic influences on Neoproterozoic-early Paleozoic climate and animal evolution. Geology, online publication doi:10.1130/G34962.1). One of the striking correlations is between the abundance of ‘young’ zircons relative to Cambrian sedimentary deposition and the pace of diversification of animal faunas during the Cambrian.  During the Cambrian Period there may have been far more continental-margin arc volcanism than in the preceding late Neoproterozoic or later in the early Palaeozoic. That would match with evidence for the Cambrian atmosphere having reached the greatest CO2 concentration of Phanerozoic times and the fact that the Gondwana supercontinent (comprising the present southern continents plus India) was assembled at that time by collision of several Precambrian continental masses. Global temperatures must have been rising.

Reconstruction of Earth 550 Ma ago showing the...
Earth at abround the start of the Cambrian showing the cratons that collided to form Gondwana (Photo credit: Wikipedia)

The rapid emergence of all the major animal groups by the middle Cambrian – the Cambrian Explosion – took place during and despite climatic warming. Environmental stress, perhaps increased calcium and bicarbonate ions in sea water as a result of acid conditions, may have forced animals to develop means of getting both ions out of their cells to form carbonate skeletons: the Cambrian Explosion really marks the first appearance of shelly faunas and a good chance of fossilisation. Yet at the peak of volcanically-induced warming faunal diversity, especially of reef-building animals, fell-off dramatically to create what some palaeobiologsts have termed the Cambrian ‘dead interval’. Marine life really took-off in a big way during the Ordovician while temperatures were falling globally; so much so that the close of the Ordovician was marked by the first major glaciation focused on Gondwana. The zircon record indicates that continental-arc volcanism also declined during the Ordovician, and maybe the Cambrian silicic volcanics were chemically weathered during that Period to remove carbon-dioxide from the atmosphere, along with renewed reef building to bury carbonate fossils.

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