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Zuni Mountains Tell a Story of Complex Uplift

figure
Photomicrograph of apatite grains used for (U-Th)/He analysis.
(click for a larger version)
Jacob Thacker
figure
View of the Zuni Mountains in northwestern New Mexico from Mt. Sedgwick.
(click for a larger version)
Jacob Thacker

Zuni Mountains, NM
— June 28, 2021

Rocks in the Zuni Mountains record two distinct intervals of uplift over the last 80 million years preserved as cooling signatures in apatite mineral grains, according to a new paper published in Tectonics by New Mexico Bureau of Geology Field Geologist Dr. Jacob Thacker and Senior Geophysicist/Field Geologist Dr. Shari Kelley. The initial phase occurred during the Laramide orogeny, a deformation event that affected a wide swath of the western U.S. from 90 to 40 million years ago. The second uplift phase occurred at or after 20 million years ago, maybe as recently as 10 million years ago, and may be the result of convection in the mantle.

“Fundamentally, knowing when something happened is extremely important for knowing why it happened,” said Thacker. “The Zuni Mountains work constrains past cooling events in west-central New Mexico, and we can use that information to interpret what those cooling events signify.”

The research, conducted as part of Thacker’s doctoral studies at the University of New Mexico and co-authored by Dr. Karl Karlstrom, used two methods for examining the cooling history of apatite mineral grains, fission-track thermochronology and (U-Th)/He (“uranium-thorium-helium”) thermochronology. Both methods record when the mineral grains moved from the high temperatures located at depth below the surface through cooling windows as they were uncovered by uplift and erosion of overlying rock layers.

These dates, combined with a stratigraphic column compiled from previous work and inverse and forward modeling of cooling histories, reconstructed the tectonic history of the Zuni Mountains.

The Laramide orogeny, which formed many of the Rocky Mountains ranges from Montana to New Mexico, occurred due to compressive forces transmitted more than 1,000 kilometers into the continent from the plate margin to the west. This deformation was due to subduction of the Farallon oceanic plate under the North American plate in a “flat-slab” configuration, meaning the plate stayed fairly shallow beneath North America during subduction, rather than sinking down at a steep angle into the mantle.

There are two prevalent models proposed for this far inboard deformation: endloading at the plate margin with possible scraping off of the mantle lithosphere beneath the continent, or basal traction between the overriding and subducting plates. Both models could possibly include weakening of the North American plate through hydration, allowing deformation to occur farther inboard. Researchers also debate whether deformation occurred simultaneously or if there was a sweep of deformation that moved from west to east through time.

The key to addressing all of these questions is dating. The Zuni Mountains are uniquely placed on the southeastern Colorado Plateau, and between established cooling histories at Grand Canyon and in the Southern Rocky Mountains in north-central New Mexico.

Data interpreted here suggest that tectonism in the Zuni Mountains started approximately 80 million years ago, which appears to be younger than the start of Laramide tectonism to the west and older than Laramide tectonism to the east.

“Depending what Laramide camp you are in, the results of this work either challenge or confirm previous models for Laramide tectonism,” explained Thacker. “The west-to-east sweep model appears to fit, and that is very important for deducing what plate tectonic process actually caused the Laramide orogeny.”

Inverse and forward modeling was used to determine “cooling paths” that fit the thermochronology data. The models indicate that two separate cooling events created the modern topography seen today.

“The younger cooling event that occurred after the Laramide orogeny is really interesting in terms of its implications for the more recent history of the southwestern United States,” explained Thacker. Cooling from this time period found elsewhere on the Colorado Plateau has been variably interpreted as either regional uplift associated with convection in the mantle, or a cooling down of the crust following expansive volcanism in the Mogollon-Datil volcanic province located to the south that occurred from about 36 to 24 million years ago. The timing of the young cooling in the Zuni Mountains does not agree with the volcanic interpretation, and instead fits better with regional uplift of the Colorado Plateau.

This younger cooling event was only recorded by samples from the northeast flank of the range. As the range crest uplifted during the Laramide orogeny, this flank was tilted and buried, initially preventing it from rising and cooling to the same extent as other parts of the range. Then the second regional uplift event finally raised these rocks to the surface.

The Zuni Mountains will continue to be a focus for Thacker, and for the Bureau of Geology in general. The stakeholders who guide the Geologic Mapping group have selected the western part of the range as a mapping target due to interest in groundwater resources and hazards in relation to the population center of Gallup. Thacker is also individually pursuing a research project examining faults in the mountains to understand how the range uplifted and tilted as it formed.

“I am really looking forward to mapping there starting this summer,” said Thacker. “I am very fortunate to have worked and to continue to work in the Zuni Mountains – it is a geologically and historically interesting area.”