It turns out that the Denali fault tore apart the connection between two ancient continents

Three sites that stretch along a roughly 620-mile stretch of what is now the Denali Fault were once small, unified geological features that marked the eventual joining of two landmasses, according to a new study. The landscape was then torn apart by millions of years of tectonic activity.

The study, led by Associate Professor Sean Regan of the University of Alaska Fairbanks Geophysical Institute and UAF College of Natural Sciences and Mathematics, is featured on the cover of the December issue of Geology.

Regan is the lead author of the research paper. UAF co-authors include doctoral student Mackenzie Miller, recent master’s graduate Sean Marble, and research assistant professor Florian Hofmann. Other co-authors are from St. Lawrence University, South Dakota School of Mines and Technology, and the University of California, Santa Barbara.

“Our understanding of lithospheric growth, or plate growth, along the western margin of North America is becoming clearer, and a large part of that has to do with the reconstruction of strike-slip faults like the Denali fault,” Regan said. he said. “We are beginning to recognize the key features involved in stitching, or suturing, the once-distant landmasses to the North American plate.”

The study focused on geological formations from three locations: the Clearwater Mountains in south-central Alaska, the Kluan Lake region in southwestern Yukon Territory, Canada, and the Coast Range near Juneau. Previous ideas among geologists are mixed, with some suggesting that the three sites formed independently.

Regan’s historical reconstruction of the Denali Fault’s 300 miles of horizontal movement over millions of years revealed that three locations formed a terminal suture zone at the same time. The terminal suture zone represents the final integration of tectonic plates or crustal fragments into larger masses.

Regan’s research shows that between 72 million and 56 million years ago, several regions of the Langelian complex, an oceanic plate that originated far from its current location and was attached to the western edge of North America, are located. Defining one of the locations.

“Given that geologists are crawling around the surface trying to understand what the hell happened, it’s no wonder they might not connect things that far apart,” Regan said. He talked about the three ruins he investigated. “Different geologists are working in different regions, so the dots won’t really connect until we can reconstruct the deformation of the Denali Fault.”

Regan’s reconstruction focused on the reverse metamorphism of the three sites. This is a geological phenomenon in which rocks formed under high temperature and high pressure overlap rocks that were formed under low temperature and high pressure. This is the reverse of the typical order observed in regional metamorphism, where temperature and pressure generally increase with depth.

Reverse metamorphism is an important indicator of tectonic complexity and helps geologists reconstruct tectonic and mountain-building processes.

“We showed that these three independent reverse metamorphic belts all formed at the same time under similar conditions,” Regan said. “And they all occupy a very similar structural environment. Not only are they the same age, but they all behave in a similar way. They age downward, structurally. ”

Regan connected the three locations by analyzing monazite, which is composed of the rare earth elements lanthanum, cerium, neodymium, and in some cases yttrium. He collected monazite from two locations in Alaska and used data from Kluan that another scientist published earlier this year.

“It’s just the most special little mineral,” Regan said. “Because it can be involved in many reactions, it can be used as a way to track the mineralogical evolution of rocks.”

Regan began his quest after reading a 1993 paper published in Geology by researchers at the University of Alberta and the University of British Columbia. That paper claimed similarities to the Denali fault region later studied by Regan, but only went so far as to label them as a single metamorphic plutonic belt.

Metamorphic plutonic zones are regions characterized by a close association of metamorphic and plutonic rocks, usually formed as a result of intense tectonic activity during mountain building processes. These belts are often found in areas where tectonic plates meet.

“It was surprising to me that the 1993 paper didn’t get more attention at the time,” Regan said. “I’ve had this paper on my wall for the past four years because I thought it was really ahead of its time.”