Geology of the Himalayan Mountains


The collision between the Indian subcontinent and Eurasian continent, which started in Paleogene time and continues today, produced the Himalayas and Tibetan Plateau, spectacular modern examples of the effects of plate tectonics. Tibetan Plateau itself is a collage of microplates or continental fragments that were successively added to the Eurasian plate during the Paleozoic and Mesozoic eras. Paleomagnetic analysis indicates that these older microplates were in the southern hemisphere during the Paleozoic era. Each older fragment, like the larger Indian Plate, made a long northward journey as the intervening ocean was subducted, and was accreted to the Eurasian continent. The resulting sutures are marked by scattered occurrences of ophiolite, ocean floor material that was caught up between the crustal blocks during accretion. The collision that produced the Himalayas was only the latest, albeit climactic, episode in this long series of collisions.

Cross Section

The Indian plate continues to move northward relative to Asia about 5 cm per year. Given the great magnitudes of the blocks of the Earth's crust involved this is a remarkable rate, about twice the speed at which your fingernails grow. This movement caused the rocks along the leading edge of the Indian plate to deform and fracture. Immense slices of Indian crust were pushed southward and stacked up to produce the Himalayan orogen (above figure). Several distinct sequences of rock are recognized in the Himalayas, separated from one another and rocks of the Indian plate by northward-dipping fault zones. It is these zones that accommodated the huge amounts of crustal shortening as the rock sequences were slid one over another. From north to south, these rock packages and fault zones are the Tibetan Sedimentary Sequence (blue), South Tibetan Detachment Zone (Detachment Fault), Greater Himalayan Sequence (red and pink), Main Central Thrust (MCT) and Maha Bharat Thrust (MT), Lesser Himalayan Sequence (light blue), Main Boundary Thrust (MBT) and Main Frontal Thrust (MFT), and Gangetic Basin of the Indian subcontinent (yellow).

NASA's on-line geomorphology book describes the Himalayas and Tibetan Plateau and the associated geology of Afghanistan, Pakistan, western India. Project INDEPTH, which has done seismic profiling across Tibet and the Himalaya, has an interesting, but technical, web page with some great pictures at: http://www.geo.cornell.edu/geology/indepth/indepth.html.

Readings


Nepal & Tibet Nepal & Tibet