Leaving Gondwana
For most of the last five hundred million years, the land that is now Pakistan lay along the northern edge of Gondwana, the great southern supercontinent, facing a wide ocean called Tethys. Around 130 million years ago the Indian plate rifted away from Gondwana and began to move north — and it moved fast, one of the quickest plate migrations known, crossing thousands of kilometres of ocean over the following tens of millions of years.
As India advanced, the Tethyan ocean floor ahead of it was consumed, sinking back into the mantle along subduction zones at the southern margin of Asia.
An island arc in the way
That subduction did not just destroy ocean crust; it built new rock. Above the descending slab, chains of volcanoes grew — island arcs sitting out in the Tethys. One of them, the Kohistan arc, now forms a great block of volcanic and deep crustal rock in northern Pakistan. As the ocean finally closed, this arc was caught between the converging plates and welded into the mountain belt, a fossil arc stranded far from any sea.
The collision, around fifty million years ago
India met Asia head-on roughly fifty-five to fifty million years ago. The remaining ocean was squeezed out of existence; its sediments were scraped up, stacked and thrust over one another; and the crust thickened enormously as one continent drove beneath the other. Marine limestones that had formed on quiet Tethyan shelves were folded, faulted and lifted thousands of metres — which is why shallow-sea fossils can be found high in the mountains today.
The joins between the old plates survive as suture zones. In Pakistan the boundary between the Indian plate and the Kohistan arc is marked by the Main Mantle Thrust, and the join between the arc and the Asian (Karakoram) block by the Main Karakoram Thrust.
Stacking the Himalaya
North of the collision, the crust responded by breaking into a series of great south-directed thrust faults, each carrying older rocks over younger ones. From north to south these step down through the range — the Main Central Thrust, the Main Boundary Thrust and, at the mountain front, the Main Frontal Thrust — progressively younger structures that carry the deformation out into the plains. This thrust stacking is what physically builds a mountain belt: it doubles and redoubles the crust, piling it high.
At the plate’s northwestern corner, the collision is so intense that it has bent the whole grain of the ranges into tight knots, or syntaxes — the Nanga Parbat massif being the most dramatic, where some of the fastest rock uplift on the planet is still happening.
The foreland fills
Every rising mountain sheds sediment. As the ranges grew, their rivers dumped an immense wedge of debris into the foreland basin to the south — the Siwalik molasse — burying the older record under kilometres of fresh sand and mud and creating, in the process, the traps and reservoirs that hold much of the country’s oil and gas.
A collision still in progress
India is still pushing north today, at centimetres a year. The country’s frequent earthquakes, its growing ranges and its actively deforming margins are the present-day pulse of the same event that organises its entire stratigraphy. The rock record of Pakistan is, from bottom to top, a near-continuous account of an ocean closing and a mountain belt being born.
See the units raised by this collision in the formations index, or place them in deep time on the time scale.