Drainage evolution in active mountain belts: extrapolation backwards from present-day Himalayan river patterns

An understanding of how drainage patterns respond to active tectonics can provide insight into past deformational events within mountain belts. The Himalayan arc mountain belt is taken as an example because it is still strongly active. It also is old enough (55 Ma) and of sufficient extent that aspects of ongoing drainage modification in the outer Subhimalayan and Lower Himalayan zones can be compared with those internal parts of the mountain belt where drainage patterns have become fixed by gorge formation.

The drainage patterns of the outer lithotectonic zones have produced 0–8 km of exhumation over the last 15 Ma and provide many examples of deflection and gorge erosion during young episodes of thrusting. In contrast, the drainage systems of the Higher Himalayan zone have been evolving for longer, having produced 8–25 km of exhumation over the 55 Ma since the Himalaya began to rise. Generalized river gradients vary strongly in different parts of the belt corresponding to different amounts of rock uplift and this strongly influences the behaviour of the rivers.