Author: Lori Stiles
Publication: UANews.org
Date: September 22, 2003
URL: http://uanews.org/cgi-bin/WebObjects/UANews.woa/1/wa/MainStoryDetails?ArticleID=7831
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.org
"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."