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Rocks in a Hard Place

The Kohistan formation, nestled in a picturesque corner of Pakistan, near the border of Afghanistan and China, is a window on the tectonic processes that created the Earth’s continents over 100 million years ago. Geologists refer to it as an “island arc”–a place within an ocean formed during a geodynamical process called “subduction” (i.e., sinking of an oceanic plate into the mantle), during which magma from the Earth’s mantle comes to the surface to create new landforms. The Kohistan arc, known as the “Jijal complex,” is one of only two such well-preserved sites on Earth. It is constituted at its base by a roughly 3.5 km-thick pile of magnesium-rich ultramafic rocks, overlaid by nearly 3 km of mafic rocks, which are less rich in magnesium.1 To determine whether the ultramafic and mafic units come from the same source of magma, Dhuime and his colleagues2 from the University of Montpellier, ran numerous isotope tests on Jijal rocks, creating, in effect, a family tree for the rocks. They showed that the ultramafic and mafic minerals were not in fact twins, but originated millions of years apart.3
Their model proposes a multi-stage process that started with the onset of subduction, when the ancient Neo-Tethyan ocean plate slid beneath the oceanic part of the Karakoram plate about 117 million years ago. Soon, the upper levels of the sinking plate began to dehydrate. The rising fluids triggered the melting of the “mantle wedge” (the mantle area between the subducting plate and the overriding plate) and the emission of the first magmas. As the subducting plate sank further, the arc progressively grew in height and volume. Complex processes, including the fusion of the arc of the base and the recycling of residual material into the overlying mantle, marked the last stage which modified the global composition of the arc crust to closely resemble the continental crust.   
“The Kohistan island arc is very young from a geological standpoint, and the process it illustrates can only explain the formation of landforms for the last 500 million years or so,” Dhuime explains. He is currently looking even further back in time, to describe how these processes would have occurred
4 billion years ago.
Mark Reynolds

Notes :

1. Mafic rocks refer to a group of dark-colored minerals, composed chiefly of magnesium and iron, that occur in igneous rocks. Ultramafic rocks are lighter and are believed to make up most of the Earth's mantle.
2. Laboratoire Géosciences Montpellier (CNRS / Université Montpellier-II). Dhuime currently works in the Dept. of Earth Sciences, University of Bristol, England.
3. B. Dhuime, et al., “Multistage evolution of the Jijal ultramafic-mafic complex (Kohistan, N Pakistan): Implications for building the roots of island arcs,” Earth Planet. Sci. Lett. 261: 179-200.2007.

Contacts :

Bruno Dhuime,University of Bristol, England.


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