The Teton Fault
The start of a series of articles focused on the evolution of Yellowstone, the Tetons, and the surrounding geology.
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Above photo from from
10/10 would recommend his writings and photo gallery, they are excellent.I recently found another great geology-focused Substack by
, who recently published an article I found fascinating.
The Tetons and Yellowstone National Parks are places I love deeply and have the majority of my childhood memories associated with. I grew up around southwestern Idaho and western Montana and frequented those parks often with grandparents and cousins.
I went to school in western Montana where we would discuss Yellowstone fairly often.
That said, I felt I had a decent grasp of at least the region's first-order formation and the mountains' physical processes.
All that went out the window after reading Richard's recent post.
Here is what I learned and am still trying to figure out.
The normal fault that formed the Tetons - the Teton Fault - is estimated to have 2.5 - 3.5 kilometers (1.5 - 2.2 miles) of vertical displacement over the last 2 million years.
This blew my mind. The Tetons are incredible mountains that I could have sworn were tens of millions of years old, formed from the tectonic collision off the western coast of the United States. Boy, was I wrong.
(Granted, the rocks that make up the Tetons are millions of years old, but the mountain uplift is very young.)
The leading hypothesis for this dramatic faulting is due to a bulge formed by the Yellowstone Hotspot.
Upcoming Series
Based on the fact that I realized how little I really know about the geology, deformation, and tectonic history of the Yellowstone/Teton region, I decided to narrow my focus to this region for the next few weeks.
Richard suggested several papers and a special journal edition that focuses on these settings, and I want to stay here for several articles.
I've gotten wrapped up in papers here, so I want to share all of it with you.
Each week, I'll pick one or two papers I've been reading, summarize them, and highlight some of the things I find most fascinating.
I hope you enjoy reading the upcoming articles as much as I've liked reading and learning more about some of my favorite places on the planet.
Mechanisms of Deformation
The paper of focus this week is a 1994 article by John Byrd, Robert Smith, and John Geissman published in the Journal of Geophysical Research.
This paper aims to use geological and geophysical data to evaluate the structural evolution of the still-active Teton normal fault.
A normal fault is a fault formed from crustal extension, where one block slides down a fault relative to the adjacent block. The block that slides down is called the "hanging wall", and the block that moves (relatively) upward is called the "footwall".
Estimates for the inception of the Teton fault range from 13 million years ago to as recent as 2 million years ago.
Deposits of volcanic ash and debris from the Yellowstone eruption 2 million years ago overlying the rocks of the Teton Range have been tilted to the west. Significant tilting suggests that much of the uplift of the Teton Range along the Teton fault has accumulated in only the past 2 million years.
The Teton fault follows the base of the Teton Mountains for 70 kilometers (43 miles). The southern extent of the Teton Fault is bounded by 60-200 million-year-old faults, suggesting that previous faults influence how far the Teton Fault could propagate.
The geology of the Teton Range is primarily Precambrian metamorphic rocks and large igneous plutons (bodies of solidified magma similar to the Yosemite granitic domes) which originally began to uplift due to crustal shortening between 65 and 85 million years ago. Basement rocks that make up the Tetons have been uplifted ~1.5 kilometers (1 mile) during those times and an additional 2 kilometers (1.2 miles) of uplift occurred in the last 30 million years.
Seismicity and Seismic Hazard
The region surrounding the Tetons is seismically quiet overall, with the few earthquakes that do happen occurring below magnitude 3. However, significant earthquakes have happened along the Teton fault as part of the creation of the mountain peaks, and are likely to happen again.
Recent paleoseismic studies have found offsets of 2.8 and 1.3 m (9.1 and 4.3 feet) associated with ruptures dating between ~8000 and 7000-4800 years ago. Vertical offsets at the surface of this size are consistent with earthquakes with a magnitude of ~7.0, suggesting that the Teton fault has the capacity to generate large earthquakes.
Below is an image of the Teton Fault scarp, with a vertical offset of ~23 meters (75 feet), showing multiple ruptures along the fault in the last 20,000 years.
Models of earthquakes in this region and comparisons to other historic earthquakes lead to estimates that 10-50 magnitude 7 earthquakes have happened along the Teton fault in the last 25,000-75,000 years.
Further evidence of significant deformation in glacial deposits from the end of the last ice age 10,000-30,000 years ago also suggests most of the displacement along the Teton fault has occurred in very recent millennia, and most likely within the last two million years.
Next Week
More tectonics surrounding Yellowstone caused by the uplift from the hotspot bulge.
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My Three Favorite Posts of the Week
References
John O. D. Byrd; Robert B. Smith; John W. Geissman. (1994). The Teton fault, Wyoming: Topographic signature, neotectonics, and mechanisms of deformation. , 99(B10), 20095–20122. doi:10.1029/94jb00281
"more people would be interested in geology if they had a good teacher."
Like Nick Zentner. He has a Youtube channel.
Caveat: He might only be interesting if you live in the Pacific Northwest.
Love the Teton range, only been there a few times. The most dramatic view I ever had of them was actually not on the valley side. On the Idaho side, there's a hike up Table Mountain that, at the very peak, offers a wild view of the grand tetons from the backside. Truly something. I did not know much about them geologically, so this is really neat and I'll be following.
I guess you missed an opportunity to say "It's Your Fault" or something like that.