Along the northwestern coast of North America, a curling crack in Earth’s crust tells the story of cataclysmic tectonic activity that ripped through the region millions of years ago. Due to this massive shake-up, geological features that were once formed together in the same zone are now separated by some 997 kilometers (620 miles).
The split-up occurred along the Denali Fault, a 2,011-kilometer (1,250-mile) long fault that carves along the southern tip of the Alaska Peninsula, across the bottom of the state, and down into the southwestern Yukon Territory of Canada.
In a new study, geoscientists at the University of Alaska Fairbanks took a close look at features along the fault and found evidence that three sites – the Clearwater Mountains in Southcentral Alaska, Kluane Lake in Yukon, and the Coast Mountains near Juneau – were once part of a unified geologic feature.
Researchers had previously speculated whether the three locations formed individually, but the new research concluded they were once all part of a zone formed during the final clashing of land masses that formed North America.
This formation most likely occurred when the Wrangellia Composite Terrane, an oceanic plate, gradually stitched into the western edge of North America between 72 million and 56 million years ago.
A 1993 paper had previously alluded to this idea, but this latest effort puts more evidence in its favor by analyzing samples of monazite – a mineral containing the rare earth elements lanthanum, cerium, neodymium, and sometimes yttrium – from the sites.
The geology at those three sites showed clear evidence of inverted metamorphism, a topsy-turvy geological phenomenon where rocks formed under higher temperatures and pressures are found overlying rocks formed under lower temperatures and pressures (typically, you’d expect to find the opposite).
“We showed that each of these three independent inverted metamorphic belts all formed at the same time under similar conditions. And all occupy a very similar structural setting. Not only are they the same age, they all behaved in a similar fashion. They decrease in age, structurally, downward,” Sean Regan, lead study author and associate professor at the University of Alaska Fairbanks Geophysical Institute, said in a statement.
“Our understanding of lithospheric growth, or plate growth, along the western margin in North America is becoming clearer, and a big part of that is related to reconstruction of strike-slip faults such as the Denali Fault. We’re starting to recognize those primary features involved in the stitching, or the suturing, of once-distant land masses to the North American plate,” added Regan.
The new study is published in the journal Geology.
