New studies reveal that three distinct locations along an approximately 620-mile segment of what is now the Denali Fault were originally part of a smaller, interconnected geological structure, which was a sign of the final merging of two landmasses. Over millions of years, this feature was fragmented due to tectonic activity.
New studies reveal that three distinct locations along an approximately 620-mile segment of what is now the Denali Fault were originally part of a smaller, interconnected geological structure, which was a sign of the final merging of two landmasses. Over millions of years, this feature was fragmented due to tectonic activity.
This research, led by Associate Professor Sean Regan from the University of Alaska Fairbanks Geophysical Institute and the UAF College of Natural Science and Mathematics, is highlighted on the cover of the December issue of *Geology,* the journal of The Geological Society of America.
Regan serves as the primary author of the research paper. Co-authors from UAF include doctoral student McKenzie Miller, recent master’s graduate Sean Marble, and research assistant professor Florian Hofmann. Other co-authors hail from St. Lawrence University, South Dakota School of Mines and Technology, and the University of California, Santa Barbara.
“Our insight into the growth of the lithosphere, or plate development, along North America’s western boundary is becoming clearer, largely due to the rejuvenated study of strike-slip faults like the Denali Fault,” Regan noted. “We are beginning to identify the key features involved in the merging of once-disparate land masses into the North American plate.”
The research concentrated on formations located in three areas: the Clearwater Mountains in Southcentral Alaska, the Kluane Lake area in southwestern Yukon, Canada, and the Coast Mountains near Juneau. Geologists previously held mixed views, with some speculating that these places developed separately.
Regan’s historical analysis revealed that these sites experienced 300 miles of horizontal displacement along the Denali Fault over millions of years, indicating that they once formed a terminal suture zone, which is the final amalgamation of tectonic plates or pieces of the Earth’s crust into a larger whole.
His study identifies one of several locations where the Wrangellia Composite Terrane, an oceanic plate that was originally located far from its current site, merged with North America’s western edge between 72 million and 56 million years ago.
“When considering geologists exploring the Earth’s surface to understand its history, it’s understandable why they might not connect sites that are geographically distant,” Regan explained regarding the studied locations. “With different geologists analyzing separate regions, it isn’t until there’s a reconstruction of the Denali Fault’s deformation that the pieces start to connect.”
His reconstruction focused on the inverted metamorphism present at the three sites, a geological process where rocks that formed in high-temperature and high-pressure conditions lie above rocks that formed in lower temperatures and pressures, reversing the typical pattern seen in regional metamorphism, where temperature and pressure usually rise with depth.
Inverted metamorphism serves as a significant marker of tectonic intricacy and aids geologists in piecing together the history of crustal deformation and mountain formation.
“We demonstrated that each of these three separate inverted metamorphic belts emerged simultaneously under similar conditions,” Regan stated. “All three occupy a comparable structural setting. Not only are they of the same age, but they also exhibit similar behaviors, decreasing in age from top to bottom.”
Regan unified the three locations through an analysis of their monazite, which includes rare earth elements like lanthanum, cerium, neodymium, and occasionally yttrium. He collected samples from the two Alaskan locations and utilized previously published Kluane data from earlier this year.
“Monazite is truly a remarkable mineral,” Regan remarked. “It can participate in various reactions, allowing us to trace the mineralogical progression of a rock.”
Regan’s curiosity was piqued after reading a 1993 paper from researchers at the University of Alberta and the University of British Columbia, published in *Geology.* That paper posited that there were similarities in the Denali Fault region that Regan later investigated but merely referred to that area as a single metamorphic-plutonic belt.
A metamorphic-plutonic belt is characterized by a close association of metamorphic and plutonic rocks, resulting from intense tectonic forces, typically during mountain-building events. These belts are frequently found at converging tectonic plate boundaries.
“I was surprised that the 1993 paper didn’t garner more attention at the time,” Regan noted. “I’ve had that paper displayed on my wall for the past four years because I find it to be a really forward-thinking piece of work.”
