Wednesday, 23 July 2014

Forging Sulaiman Range

Eighty-million years ago, Asia was a very different place. Dinosaurs were the dominant life form. Humans and primates would not evolve for tens of millions of years. The Himalayan Mountains did not exist.

Meanwhile, to the south, the Indian plate was drifting north toward Asia at a rate of about 20 centimeters (8 inches) per year. By 60 million years ago, it had edged close enough to start a slow-motion collision of titanic proportions. Island arcs along India’s leading edge were pushed into Eurasia first, causing the Indian plate to slow down by about half. After another 30 million years, the thicker crust of India proper was smashing into Eurasia at about four centimeters per year. To this day, India still pushes and grinds northward.

The dramatic folding and thickening of continental crust along this line of collision pushed up the arc of mountains known as the Himalayas and Karakoram. These ranges are home to the world’s tallest mountains, but there is another—often overlooked—side to the India-Eurasian collision. While the northern edge of the Indian plate crashed head-on into Eurasia, the western edge sideswiped the Afghan block to the west, pushing up the north-south trending Sulaiman Mountains in what is now Pakistan and Afghanistan.

While not as high as the Himalayas or Karakoram, the Sulaiman range boast some of the most complex tectonic structures in the world. As India moved northward, it began to rotate in a counter-clockwise direction, wrenching the northwestern part of the Indian plate backwards into part of the Eurasian plate. The countervailing forces put the rocks of the Sulaiman range in a unique compressional vice, causing many of its faults to curve and stretch in convoluted ways.

The Advanced Spaceborne Thermal Emission and Reflection Radiometer (ASTER) instrument on NASA’s Terra satellite captured a scene on April 24, 2011, that highlights some of this tectonic complexity. For instance, notice the broad curvature in the southwest-northeast trending folds in the long image below. Note that the combination of visible and near-infrared light used to make this false-color image vegetation appears bright red. The false-color view also helps highlight some of the differences between rock types.

The false-color images above and below it show a closer view of two interesting areas within the larger scene. The top image shows an imbricate fan to the east of Murgha Kibzai. The fan is comprised of a series of parallel, closely-spaced slices of rock that are bounded by faults on either side.

Light-colored rock is limestone; darker-colored rock is sandstone. The presence of this feature is a sign that rock layers near the surface have become disconnected from the underlying rock below and are moving independently it. See the map published in Geomorphology from Space for a detailed look at the location and orientation of the faults.

The lower image shows a plunging syncline east of Zhob, Pakistan. A syncline is a type of fold that often forms in a when plates are pushing together to build mountains. The youngest layers of a syncline are at the center. The rocks making up this syncline are part of the Loralai formation, a series of sedimentary rocks that formed off the northern coast of India in the Jurassic period. Just to the west of this scene, there are dark rocks called ophiolite, which are bits of ocean crust uplifted during the collision. The presence of ophiolite is a clue that separate continental plates were sutured together in this area.


NASA Earth Observatory image by Robert Simmon and Adam Voiland, with data courtesy of NASA/GSFC/METI/ERSDAC/JAROS, and the U.S./Japan ASTER Science Team. Caption by Adam Voiland.
Instrument(s): 
Terra - ASTER - NASA

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