Tuesday, September 11, 2018

Before The Himalaya: Story Of A Late Cretaceous Subduction Zone

My article on the geology of the upper catchment of the Brahmaputra River in southern Tibet has been published in The Wire Sciences. This region is technically called a suture zone. It contains the remnants of the Tethys Ocean floor made up of basaltic oceanic crust and overlying deep sea sediments. These rocks were deformed and uplifted during the India Asia collision.

I focus on a paper on the Jiachala Formation by Hanpu Fu and colleagues published in a recent issue of Science China Earth Sciences. They use a technique known as detrital zircon geochronology to resolve a long standing problem about the age of that sedimentary deposit. In my article I explain how this technique works. I also elaborate on the broader story these deposits tell us about plate tectonics and the beginnings of the Himalaya.

An excerpt:

In the Cretaceous Period, the Indian plate, which had been moving northwards since the breakup of Gondwanaland, was approaching the Asian continent. The southern edge of the Asian continent was lighter continental crust, whereas the leading part of the Indian plate was denser oceanic crust. As a result, in the zone where the the two plates converged, the denser Indian plate slid below the Asian plate, forming a subduction zone.

As the Indian lithosphere sunk deeper into the mantle, it heated up and released water trapped in sediments and hydrated oceanic crust. This water penetrated the overlying Asian plate, lowering the melting point of its rocks and triggering magma generation. This buoyant magma rose through the Asian continental crust. Some of it reached the surface, resulting in extensive volcanism. The rest solidified in the subsurface, forming giant bodies of granite known as batholiths.

Such terrains have the grandiose name of magmatic arcs. The town of Leh and the surrounding settlements in the Indian region of Ladakh are situated partly on a magmatic arc.

Two sedimentary basins since developed south of this arc. Immediately adjacent to the magmatic arc was the forearc basin. A deeper depression, known as the trench, formed further away on the Indian oceanic lithosphere, at the junction where the Indian plate had slid under the Asian plate. Both were receiving sediments derived from the erosion of the Asian continent.

During subduction, slices of the Indian plate were scraped off and thrust to the surface. Such fault-bounded piles of sediment and oceanic crust are called accretionary wedges, and they, along with a chain of oceanic volcanoes  that formed to the west in the region between Ladakh and Kohistan, would have been the first island ranges formed in the Tethys Ocean between India and Asia.

 Read the complete article here.

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