An international team of researchers has found the strongest
evidence yet that parts of North America and Antarctica were
connected 1.1 billion years ago, long before the supercontinent
Pangaea formed.
"I can go to the Franklin Mountains in West Texas and stand next to
what was once part of Coats Land in Antarctica," said Staci Loewy,
a geochemist at California State University, Bakersfield, who led
the study. "That's so amazing."
Loewy and her colleagues discovered that rocks collected from both
locations have the exact same composition of lead isotopes. Earlier
analyses showed the rocks to be the exact same age and have the
same chemical and geologic properties. The work, published online 5
August (ahead of print) in the September issue of the
journal Geology,
strengthens support for the so-called SWEAT hypothesis, which
posits that ancestral North America and East Antarctica were joined
in an earlier supercontinent called Rodinia.
The approximately 1.1 billion year old North American Mid-continent
Rift System extends across the continent from the Great Lakes to
Texas. Volcanic rocks associated with the rift, which appears to
represent an aborted tectonic attempt to split the ancestral North
American continent of Laurentia, are well exposed in the Keweenaw
Peninsula of the Upper Peninsula of Michigan from which they take
their name, the Keweenawan large igneous province. The rift extends
in the subsurface beneath Minnesota, Iowa, Nebraska, Kansas and
Oklahoma to the Franklin Mountains near El Paso, Texas where
related rocks are exposed. In this latest report, Loewy, Ian
Dalziel, research professor at The University of Texas at Austin,
Richard Hanson of Texas Christian University and colleagues from
several overseas institutions, find that rocks barely peeking
through the ice in Coats Land, a remote part of the Antarctic
continent south of the Atlantic Ocean basin, reflect a former
continuation of the North American rift system. Loewy began her
collaboration with Dalziel several years ago as a graduate student
at the University of Texas at Austin.
Loewy et al. use new lead (Pb) isotopic data from the
1.1-billion-year-old rocks from Coats Land, to constrain the
positions of Laurentia (ancestral North America) and Kalahari
(ancestral southern Africa) in the 1-billion-year-old
supercontinent, Rodinia. The Coats Land rocks are identical in age
to both the Keweenawan large igneous province of the North American
mid-continent rift and the contemporaneous Umkondo large igneous
province of southern Africa. Comparison of the isotopic
compositions, however, unequivocally links the Coats Land rocks
with the Keweenawan province. Together with paleomagnetic data this
suggests that the Coats Land block was a piece of Laurentia near
west Texas 1.1 billion years ago. Furthermore, the Coats Land block
collided with the Kalahari Precambrian craton of Africa during a
1-billion-year-old collision. Based on this reconstruction,
Laurentia collided with Kalahari along Antarctica's Maud mountain
belt, which would represent a continuation of the
1-billion-year-old Grenville mountain belt of eastern and southern
North America.
Thus the tiny Coats Land block of Antarctica is a 'tectonic tracer'
providing critical clues to the geographic relationships between
three of the major continents of the planet in the time interval
1.1 – 1.0 billion years ago, just prior to the opening of the
Pacific Ocean basin, the hypothesized 'Snowball Earth' glaciations,
and the rise of multi-cellular life.
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Thanks to Geological Society of America for this article.