By Sid Perkins
The Himalayas, the Asian mountain range that includes some of the tallest peaks in the world, turns out to be about 15 million years younger than geologists thought.
Various dating techniques have shown that many of the sediments deposited along the Himalayas’ southern foothills are no more than 35 million years old. But in one area of northern Pakistan, an 8-kilometer-thick layer of otherwise fossil-free sediments–the so-called Balakot formation–includes several bands of sediment that contain 55-million-year-old fossils. This disparity led scientists to believe that some portions of the Himalayas were thrust up much earlier than others, says Yani Najman, a geologist at Edinburgh University in Scotland.
Najman and her colleagues analyzed 257 small grains of mica taken from samples at nine different locations within the Balakot formation’s fossil-free sediments. They found that many of the individual mica grains were between 36 million and 40 million years old. Najman says that the rocks that encased those grains must have been even younger, because sediments laid down by erosion can’t be older than their constituent grains. The researchers report their findings in the March 8 Nature.
The technique used to date the rocks would be misleading if the mica grains had ever been heated to more than 375C, Najman explains. However, other characteristics of the sediments indicate their temperature never rose above 300C.
Najman’s team also conducted new geological surveys of the Balakot formation, including analyses of rocks recently exposed by a spate of road building. In many places, the layers in the formation fold back on themselves as a result of being crumpled by the tectonic-driven crash of the Indian subcontinent into southern Asia. That explains how the 55-million-year-old fossil-bearing layers became sandwiched within what the new study demonstrates are younger sediments, Najman notes.
A revised timeline for the Himalayas’ growth could force scientists to reevaluate models of geological change, Najman says. If the mountains are 15 million years younger than previously thought, then actual rates of erosion and sedimentation have been much higher than assumed. The consequences could be quite broad, because these factors, in turn, can influence calculated rates of change in ocean and atmospheric chemistry.
