By Josh Fischman / Source: ScientificAmerican

July1986 himalaya collision

Between 55 million and 40 million years ago, the northern edge of what is now India began to slam into the giant slab of Earth’s crust that today carries Nepal and Tibet. This ancient collision had a terrible after-effect: The deadly earthquake, centered in Nepal, which had an estimated death toll of nearly 4,000 people.

India bulled its way under Nepal those many millions of years ago, shoving the northern land skyward. That move began to create the towering Himalaya, including Mt. Everest. The collision is still going on, as India moves several centimeters north each year, and this has created an unstable fissure in the planet’s crust, known as the Himalayan frontal thrust fault. This boundary zone, shown below, continues to release enormous earthquakes. Saturday’s magnitude 7.8 disaster appears to overlap a segment that released a 8.1 magnitude quake in 1934, according to Susan Hough, a seismologist with the U.S. Geological Survey in Pasadena, California. That quake killed an estimated 10,700 people. Here are illustrations that show, first, how the initial collision occured, then how the thrust fault is continuing to fracture the crust in the area, and finally where the frontal thrust fault lies in relation to other cracks in this very quake-prone zone.

Aug2006 himilaya thrust

HIMALAYAS WERE FORMED when the Indian lithospheric plate drifted northward and collided with the Eurasian plate. The collision is shown here in simplified, vertically exaggerated diagrams. Some 60 million years ago the oceanic lithosphere at the leading edge of the Indian plate was being subducted under southern Tibet. Magma rising above the Indian plate erupted from volcanoes and formed granite intrusions. Sediments and oceanic crust scraped off the descending plate piled up in an accretionary wedge, which created a forearc basin that trapped sediments eroded from Tibet. Sometime between 55 and 40 million years ago the two landmasses collided. Presumably the Indian crust was too buoyant to plunge far under Tibet; as a result a new fault, the Main Central Thrust, broke through the Indian crust. Subsequently motion continued along the fault. A slice of Indian crust, topped by Paleozoic and Mesozoic sediments that had been deposited on the continental shelf, was thrust up onto the oncoming subcontinent. The accretionary wedge and the forearc sediments were thrust northward onto Tibet. (Much of this material has since been eroded away. About 20 to 10 million years ago the Main Central Thrust became inactive. Since then India has slid northward along a second fault, the Main Boundary Fault. A second slice of crust has been thrust up onto the subcontinent, lifting up the first slice. The two uplifted slices make up the bulk of the Himalayas; many of the peaks are capped by Paleozoic sediments. The Indian plate bends slightly under the weight of the mountains, and the resulting trough, now filled with sediments, can be detected under the Ganges plain.

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