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 this past Saturday: The deadly earthquake, centered in Nepal, which had an estimated death toll of nearly 4,000 people as of Monday evening.



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.





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 (1). 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 (2). 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 (3). 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 (4). 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. [Originally produced for "The Structure of Mountain Ranges," by Peter Molnar, in Scientific American, July 1986; Illustration by Ian Worpole]





SHINGLING EFFECTS occur when tectonic plates collide and create thrust faults. Such shingling—the result of the India-Asia plate collision—has occurred in the Himalaya. Faults of a second type are found near the crest of the Himalaya, dipping northward below the Tibetan Plateau. Constituting what is known as the South Tibetan fault system, these faults share geometric similarities with the thrust faults, but rocks slip along this system in the opposite direction. This fault system may also mark the top of the fluid lower crustal channel below Tibet. New evidence suggests that northward slip along the South Tibetan fault system and simultaneous southward slip along the southern faults permit the southward extrusion of this channel toward the Himalayan range front. (Tan regions are moving north. Purple and gray regions are moving south.) [Originally produced for "Climate and the Evolution of Mountains," By Kip Hodges, in Scientific American, August 2006; Graphic by Jen Christiansen; Source: “Southward Extrusion of Tibetan Crust and its Effect on Himalayan Tectonics," By K. V. Hodges, J. M. Hurtado and K. X. Whipple, in Tectonics, VOL . 20, NO. 6 , pages 799–809; 2001].



PRINCIPAL TECTONIC FEATURES that are thought to be associated with continuing northward push of the India plate againstthe Eurasia plate have been plotted by the authors, partly on the basis of the analysis of ERTS photographs and partly on the basis of studies of major earthquakes (colored dots), which reveal how the crust has moved along faults. The straight lines without arrowheads through dots indicate thrust faults. The double-headed arrows indicate normal faults. The pairs of anti parallel arrows indicate movement along strikeslip faults. The areas in color appear to be zones of recent uplift resulting from crustal shortening. The overall impression is that the large Eurasian land mass that lies to the west of 70 degrees east longitude has remained more or less undeformed as China bas been pushed to east. [Originally produced for "The Collision between India and Eurasia," By Peter Molnar and Paul Tapponnier, Scientific American, April 1977; Graphic by Andrew Tomko].