The Salt Range, Pakistan
The Salt Range of Pakistan contain the outermost hills at the northern fringe of the collision zone formed as the Indian plate continues to drive northwards into Asia. The collision started around 50 million years ago and has formed the high Tibetan plateau and the Himalayas. But the Salt Range has only formed in the past few million years. The continued plate convergence drives on into the plains of the Punjab, forcing up sedimentary rocks that have only recently been deposited in front of the growing mountain ranges. So the process of continental collision recycles sedimentary rocks. They are uplifted and eroded from the developing mountains, deposited by rivers on the edges of the hills and then re-incorporated into the mountain belt as collision continues. The same pattern of reworking sedimentary rocks is preserved along the edge of the Variscan mountain belt which can be seen around Tenby in south Wales.
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NASA image of Pakistan's mountain ranges: © NASA
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The Salt Range gets its name from a layer of salt of Cambrian age (c 550 million years old) that lies at the base of the sedimentary succession that was deposited upon the Indian continental crust. The salt has acted as a lubricant so that, as India has collided with Asia, its sedimentary layers have detached from the underlying crust – forming folds rather in the way that a table cloth can crumple above a rigid table top beneath.