Geologists
have long thought that the Appalachian Mountains, which run 2,500 kilometers
from Canada to Alabama, were formed by three separate mountain-building events
over some 300 million years. New research is suggesting, however, that the Central
Appalachian Mountains likely formed through only one prolonged event.
The folded and faulted rocks along the
Potomac River in Virginia indicate extreme metamorphism and complex geology.
New research indicates that conventional thinking about the timing of the building
of the Appalachian Mountains may be incorrect. Image copyright Gary P. Fleming.
All three mountain-building, or orogenic, events that were believed to have
shaped the entire Appalachian chain occurred during the Paleozoic period. According
to conventional wisdom, the Appalachians first started forming some 480 million
years ago, as continents and land masses collided. The mountains then eroded
away when the area was covered with a vast sea, only to rise again 400 to 350
million years ago, when plates recollided, with more erosion to follow. Around
280 to 265 million years ago, when Pangea began to form, the mountains again
arose, and as blocks of continental crust rode over one another, the area’s
rocks underwent intense metamorphism, folding and faulting.
Although this orogenic story has been around for decades, many scientists who
have studied the region knew there was much more to learn. “From the 1800s
to the early 1960s, the rocks of the piedmont to the Blue Ridge Mountains were
pretty much called one unit, as they were too complicated to sort out,”
says Scott Southworth, a geologist with the U.S. Geological Survey in Reston,
Va. Since the 1960s, scientists have tried to differentiate one formation from
another, but the extreme folds, untraceable faults and severely metamorphosed
rocks have made it difficult, he says. Hoping to figure out the complicated
heating and cooling history of the eastern portion of the Appalachians, Southworth,
Michael Kunk and colleagues examined rocks along the Potomac River from western
Virginia to Washington, D.C., as they reported in the September GSA Bulletin.
They mapped the rocks and faults, and Kunk took samples back to a lab in Denver
for isotopic dating.
The researchers analyzed 24 rock samples primarily using argon age-spectrum
dating. The isotopic studies enabled Southworth and colleagues to differentiate
rock formations within the Potomac terrane — a belt of folded and faulted
rocks stretching from Pennsylvania to central Virginia — even ones that
looked exactly the same. They also could now separate out rocks that underwent
anomalously high-temperature and high-pressure periods. The results suggest
that instead of three separate events that equally affected the mountain chain,
this part of the Appalachians began growing in the Ordovician 480 million years
ago, with periods of activity until 300 million years ago at least, Southworth
says.
Furthermore, although “all three orogenic events can be demonstrated somewhere
along the mountain chain,” they appear not to have occurred everywhere,
he says. During the last 300-plus million years, significant deformation and
metamorphism occurred, both on a smaller regional scale and on the larger scale.
“I think they’re quite right to point out that the history is quite
complex … and their research is a good contribution,” says George
Fisher, a geologist at Johns Hopkins University in Maryland, who started this
discussion in 1963. And, he says, the basic age for the Appalachians, especially
in the mid-Atlantic, is likely Ordovician with continuous deformation through
the Triassic. On the other hand, he says, “I’m not sure that tectonic
schemes” developed for one region of the Appalachians apply to another.
Nonetheless, Fisher says, Southworth and colleagues “have presented important
new information that will help in creating a coherent regional picture.”
Further isotopic dating, he says, especially using lead-uranium isotopes, would
help to pin down more exactly the history of this complex region.
Megan Sever
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