More than 140 million years ago, India was part of an immense
supercontinent called Gondwana, which covered much of the Southern
Hemisphere. Around 120 million years ago, what is now India broke off
and started slowly migrating north, at about 5 centimeters per year.
Then, about 80 million years ago, the continent suddenly sped up, racing
north at about 15 centimeters per year -- about twice as fast as the
fastest modern tectonic drift. The continent collided with Eurasia about
50 million years ago, giving rise to the Himalayas.
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| Himalayas |
For years, scientists have struggled to explain how India could have
drifted northward so quickly. Now geologists at MIT have offered up an
answer: India was pulled northward by the combination of two subduction
zones -- regions in the Earth's mantle where the edge of one tectonic
plate sinks under another plate. As one plate sinks, it pulls along any
connected landmasses. The geologists reasoned that two such sinking
plates would provide twice the pulling power, doubling India's drift
velocity.
The team found relics of what may have been two subduction zones by
sampling and dating rocks from the Himalayan region. They then developed
a model for a double subduction system, and determined that India's
ancient drift velocity could have depended on two factors within the
system: the width of the subducting plates, and the distance between
them. If the plates are relatively narrow and far apart, they would
likely cause India to drift at a faster rate.
The group incorporated the measurements they obtained from the
Himalayas into their new model, and found that a double subduction
system may indeed have driven India to drift at high speed toward
Eurasia some 80 million years ago.
"In earth science, it's hard to be completely sure of anything," says
Leigh Royden, a professor of geology and geophysics in MIT's Department
of Earth, Atmospheric and Planetary Sciences. "But there are so many
pieces of evidence that all fit together here that we're pretty
convinced."
Royden and colleagues including Oliver Jagoutz, an associate
professor of earth, atmospheric, and planetary sciences at MIT, and
others at the University of Southern California have published their
results this week in the journal Nature Geoscience.
What drives drift?
Based on the geologic record, India's migration appears to have
started about 120 million years ago, when Gondwana began to break apart.
India was sent adrift across what was then the Tethys Ocean -- an
immense body of water that separated Gondwana from Eurasia. India
drifted along at an unremarkable 40 millimeters per year until about 80
million years ago, when it suddenly sped up to 150 millimeters per year.
India kept up this velocity for another 30 million years before hitting
the brakes -- just when the continent collided with Eurasia.
"When you look at simulations of Gondwana breaking up, the plates
kind of start to move, and then India comes slowly off of Antarctica,
and suddenly it just zooms across -- it's very dramatic," Royden says.
In 2011, scientists believed they had identified the driving force
behind India's fast drift: a plume of magma that welled up from the
Earth's mantle. According to their hypothesis, the plume created a
volcanic jet of material underneath India, which the subcontinent could
effectively "surf" at high speed.
However, when others modeled this scenario, they found that any
volcanic activity would have lasted, at most, for 5 million years -- not
nearly enough time to account for India's 30 million years of
high-velocity drift.
Squeezing honey
Instead, Royden and Jagoutz believe that India's fast drift may be
explained by the subduction of two plates: the tectonic plate carrying
India and a second plate in the middle of the Tethys Ocean.
In 2013, the team, along with 30 students, trekked through the
Himalayas, where they collected rocks and took paleomagnetic
measurements to determine where the rocks originally formed. From the
data, the researchers determined that about 80 million years ago, an arc
of volcanoes formed near the equator, which was then in the middle of
the Tethys Ocean.
A volcanic arc is typically a sign of a subduction zone, and the
group identified a second volcanic arc south of the first, near where
India first began to break away from Gondwana. The data suggested that
there may have been two subducting plates: a northern oceanic plate, and
a southern tectonic plate that carried India.
Back at MIT, Royden and Jagoutz developed a model of double
subduction involving a northern and a southern plate. They calculated
how the plates would move as each subducted, or sank into the Earth's
mantle. As plates sink, they squeeze material out between their edges.
The more material that can be squeezed out, the faster a plate can
migrate. The team calculated that plates that are relatively narrow and
far apart can squeeze more material out, resulting in faster drift.
"Imagine it's easier to squeeze honey through a wide tube, versus a
very narrow tube," Royden says. "It's exactly the same phenomenon."
Royden and Jagoutz's measurements from the Himalayas showed that the
northern oceanic plate remained extremely wide, spanning nearly
one-third of the Earth's circumference. However, the southern plate
carrying India underwent a radical change: About 80 million years ago, a
collision with Africa cut that plate down to 3,000 kilometers -- right
around the time India started to speed up.
The team believes the diminished plate allowed more material to
escape between the two plates. Based on the dimensions of the plates,
the researchers calculated that India would have sped up from 50 to 150
millimeters per year. While others have calculated similar rates for
India's drift, this is the first evidence that double subduction acted
as the continent's driving force.
"It's a lucky coincidence of events," says Jagoutz, who sees the
results as a starting point for a new set of questions. "There were a
lot of changes going on in that time period, including climate, that may
be explained by this phenomenon. So we have a few ideas we want to look
at in the future."
This Story is taken from Science Daily
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