by Sara E. Pratt Monday, January 5, 2015
The cordillera of western North America is a patchwork of various landmasses, or terranes, that assembled through collision and accretion to the Laurentian Shield, leaving a complicated tectonic history for geologists to unravel.
Two of those terranes, the Alexander and Wrangellia, emplaced along western British Columbia, southwest Yukon, and eastern Alaska, have seemingly different geologies and were thought to have evolved separately. Now, a new study in Lithosphere shows that not only do they share a common tectonic history, but that part of the Alexander terrane forms part of Wrangellia’s basement.
Both terranes were intruded by the same plutons during the Late Pennsylvanian, between 307 million and 301 million years ago, which made them coeval, but their geologies are otherwise so dissimilar that previous researchers surmised they were unrelated before the Pennsylvanian and had possibly traveled great distances, separately, before accreting to Laurentia.
Now, Steve Israel of the Yukon Geological Survey and colleagues report new data from zircon uranium-lead geochronology and other geochemical analysis from Paleozoic rocks in the Yukon that show the two terranes have likely been linked since the late Devonian, about 363 million years ago, and later were part of the same ancient volcanic arc complex, called the Skolai arc.
“Although the Alexander terrane and Wrangellia are seemingly exotic to Laurentia, we show they are not exotic from one another,” the team wrote. “From at least the Late Devonian, the two can be considered as being tectonically and stratigraphically linked.”
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