Denali Fault tore apart ancient joining of two landmasses

New research shows that three sites spread along an approximately 620-mile portion of today’s Denali Fault were once a smaller united geologic feature indicative of the final joining of two land masses. That feature was then torn apart by millions of years of tectonic activity.

The work, led by associate professor Sean Regan at the University of Alaska Fairbanks Geophysical Institute and UAF College of Natural Science and Mathematics, is featured on the cover of the December edition of Geology, the journal of The Geological Society of America.

Regan is the research paper’s lead author. UAF co-authors include doctoral student McKenzie 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 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,” Regan said. “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.”

The research focused on formations at three locations: the Clearwater Mountains of Southcentral Alaska, the Kluane Lake region of Canada’s southwestern Yukon, and the Coast Mountains near Juneau. Previous thinking among geologists is mixed, with some suggesting the three locations formed individually.

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

Regan’s work defines one of several places where the Wrangellia Composite Terrane, an oceanic plate that originated far from its current position, accreted to the western edge of North America between 72 million and 56 million years ago.

“When you think about geologists crawling around Earth’s surface trying to understand what the heck happened, it makes some sense that they might not link things that are so far apart,” Regan said of the three sites he studied. “With different geologists working in different areas, the dots don’t really get connected until you can reconstruct deformation on the Denali Fault.”

Regan’s reconstruction focused on the three sites’ inverted metamorphism, a geological phenomenon where rocks formed under higher temperatures and pressures are found overlying rocks formed under lower temperatures and pressures. This is the reverse of the typical sequence observed in regional metamorphism, where temperature and pressure generally increase with depth.

Inverted metamorphism is a key indicator of tectonic complexity and helps geologists reconstruct the processes of crustal deformation and mountain building.

“We showed that each of these three independent inverted metamorphic belts all formed at the same time under similar conditions,” Regan said. “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.”

Regan connected the three locations by analyzing their monazite, which consists of the rare earth elements lanthanum, cerium, neodymium and sometimes yttrium. He collected monazite from the two Alaska locations and used Kluane data published earlier in the year by another scientist.

“It is just the most special little mineral,” Regan said. “It can participate in a lot of reactions, so we can use it as a way to track the mineralogical evolution of a rock.”

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

A metamorphic-plutonic belt is a region characterized by the close association of metamorphic rocks and plutonic rocks that form as a result of intense tectonic activity, typically during mountain-building processes. These belts are commonly found in areas where tectonic plates converge.

“It was amazing to me that the 1993 paper hadn’t caught more attention back in the day,” Regan said. “I had this paper hung up on my wall for the last four years, because I thought it was really ahead of its time.”