The world's highest and most spectacular mountains, the Himalaya of Nepal, India, and Bhutan, are built on the foundations of a much older mountain system, University of Arizona geoscientists have discovered.
They have dated rocks that show Earth's mightiest range is predated by ancestral mountains that existed in the same area between 450 million and 500 million years ago, long before India began plowing northward into Asia 55 million years ago.
Their findings not only revise ideas on the region's tectonic history, they offer new insight on connections between uplift of the Himalaya during the past 55 million years and simultaneous global shifts in seawater chemistry and climate.
"We conclude that the modern Himalaya Mountains are built on the foundations of an ancient mountain range that may have been of similar dimensions," said UA geosciences Professor George Gehrels, who used state-of-the-art radioisotope techniques to date rock formations in the Himalayan thrust belt.
Gehrels, UA geosciences Professor Peter G. DeCelles, UA doctoral candidate Aaron Martin, UA master's degree graduate Tank Ojha, UA undergraduate geosciences major Guy Pinhassi, and geology Professor Bishal Upreti of Tribhuvan University in Kathmandu, Nepal, have collaborated in field expeditions in rugged areas of Nepal for the past several years. They report on their research in the September issue of GSA Today, a scientific journal of the Geological Society of America, online at http://www.geosociety.orgTank Ojha (left), a UA master's degree student who now runs a geo-trekking company in Kathmandu, and Tribhuvan University geology Professor Bishal Upreti here debate the origin of boulder-borne schist from the high Himalaya.
"Our model is based on observations that, between 450 and 500 million years ago, rocks in the Himalaya were pushed down to great depth and metamorphosed," Gehrels said.
The buried rocks became so hot under great pressure that they melted, producing large granite bodies. The metamorphic schists and granite bodies contained garnets and zircon crystals that Gehrels dated to around 500 million years using uranium-lead radioisotope techniques.
These deep-level rocks were brought back up to the surface by processes of faulting, uplift, and erosion soon after burial, their observations suggest. The processes of uplift and faulting formed mountains, which eroded and produced huge volumes of sediment.
The scientists studied conglomerates and sandstones found in these "ancestral Himalaya" sediments in many different areas of the present-day range. Their main area of research, in the Annapurna range of Nepal, is a 5-day walk from the end of the nearest road.
They hired porters to carry camp gear and field equipment. Because most samples weighed around 5 kilograms (11 pounds) and were collected many miles from the nearest road, the researchers processed their samples in the field, crushing granite samples by hand and extracting garnets and zircon crystals by the panning-for-gold method.
The Himalaya is the best place on the planet for studying what happens when Earth's continents collide, Gehrels noted.
Earth's surface is covered by a series of tectonic plates. Heat from deep within the Earth drives convection currents that move the plates in different directions. India rides on a plate that steadily advances north a couple of centimeters a year, about as fast as your fingernails grow. During the past 55 million years, this action has uplifted Earth's tallest mountains, capped by 29,000-foot-plus Mount Everest.
"The birth of the Himalaya is indeed this great story of rocks being shoved down and being brought to the surface, while huge amounts of erosion take place. But we now think that much of the burial, uplift, and erosion happened between 450 million and 500 million years ago," Gehrels said. "The ancestral Himalaya Mountains appear to also have formed in a regime of continental collision, with the Indian continent being shoved beneath another landmass."
However, WHICH landmass is not yet known, he said.
"According to our model, this collisional event began with a small range forming at around 508 million years ago. The faulting, burial of rocks, formation of granite bodies, and uplift then propagated toward India through time, with the mountain range growing in width and perhaps elevation," Gehrels said.
By about 450 million years ago, as the forces of mountain building waned, erosion leveled the topography down to the deep-level metamorphic rocks, generating enormous amounts of sediment. Subsequently, the ancestral Himalaya Mountains disappeared and the region eventually subsided below sea level as the landmass was rifted away from India's northern margin, Gehrels said.
"The region remained buried below marine sediments until India collided with southern Asia around 55 million years ago and the modern Himalaya Mountains began to form," he added. More research is needed to determine the relative proportions of faulting, burial, metamorphism, generation of granites, uplift and erosion that occurred during these two phases of mountain-building, he said.
The revised geologic history also challenges Earth scientists to rethink ideas on global climate change and the global shift in seawater chemistry of about 55 million years ago.
Global climate began to cool around 55 million years ago, and scientists theorize that this may have been driven by weathering reactions in the Himalaya that remove carbon dioxide from the atmosphere, decreasing the greenhouse effect and cooling Earth.
At about the same time, Earth's oceans changed chemically, a possible result of vast quantities of Himalayan sediments carried by great rivers into the sea.
"Maybe the Himalayas have played such an important role in shaping modern climate and seawater chemistry because rocks exposed in the mountain belt were buried, metamorphosed, and uplifted during an earlier phase of mountain building," Gehrels said. "This multistage history may be key to understanding the genetic linkages between mountain building, climate change, and seawater chemistry."
Photos on UA website: http://uanews.org
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