Geology of Earthquake Rupture - Greenland

--> Now an NSF REU: See <--

Geology of Earthquake Rupture - Greenland

--> Now an NSF REU: See <--

My newest project examines how earthquake ruptures propagate through anisotropic granulite-grade gneisses at a field site in western Greenland. The work involves mapping the distribution of pseudotachylyte, a rock appearing as thin veins along a fault plane that formed by frictional melting during an earthquake rupture (see black rocks in my hand at right). We map at a scale of meters to 10's of kilometers which has now defined the world's longest-known pseudotachylyte system (>34 km). This work is motivated by an interest in comparing the geometry of deep earthquake rupture at the Ikertôq shear zone in Greenland – first described by Grocott (1981) – to the Homestake shear zone in Colorado.

Ten Concord students have travelled to the field site since 2013. The work has been funded by the American Chemical Society (ACS PRF) and is now funded as an NSF REU with expeditions planned for 2021 and 2022 for 8 students/year (NSF Polar Programs #1950842). Initial research results are in preparation for publication.

Above: Complex pseudotachylyte veins (below orange notebook in photo) at the margin of the Greenland ice cap. Below: Hand samples of thick pseudotachylyte veins at the Ikertoq shear zone.

Geologic Record of Oscillating Seismic and Interseismic Faulting – Homestake and Slide Lake shear zones, Colorado

In Colorado, I have collaborated with Concord students and other researchers on a long-term project to map and understand the origin and evolution of a massive, 1.4-billion-year-old system of pseudotachylytes and coeval mylonites. These rocks uniquely record oscillations in the earthquake cycle: Slow interseismic creep interrupted by pulses of rapid earthquake mainshocks and aftershocks to relieve accumulating stress. The fault system is more than 25 km long and developed within the Earth's middle crust (~15 km deep).

I originally encountered the Homestake pseudotachylytes during field work for my Ph.D. dissertation in the early 1990's. Since then, I have been working to expand the known distribution of the system. This has been quite a time-consuming challenge, because pseudotachylytes are thin and hard to recognize. They can superficially appear similar to dark foliation in the host rock, or be obscured by lichen. Field mapping at the multi-kilometer scale requires very close examination of every outcrop, on hands and knees with a hand lens. For every day of work spent documenting the presence of pseudotachylyte along a particular series of outcrops, many more days are spent documenting none.

Recognition of pseudotachylyte at the nearby Slide Lake shear zone in 2014 was even more challenging. Some of the pseudotachylytes are overprinted by mylonite and all of them are metamorphosed making them appear as medium-grained, cryptic dark streaks partially transposed into the older foliation of amphibolite-grade gneisses. Surprisingly, they are even less recognizable in thin section because the normal opaque, cryptocrystalline matrix shown by most other pseudotachylytes has been replaced by aligned biotite crystals.

The Homestake shear zone has been a staple of Concord's geology field camp, and nearly 100 Concord students have travelled here since 1999 for class and individual undergraduate research projects. The site has also been visited by dozens of international scientists during several field trips I have led for the Geological Society of America and other professional organizations. The work has been funded by the National Science Foundation EAR #0635894 and DUE #1525590.

This work has led to numerous publications that appear in Rocky Mountain Geology, Tectonophysics, Journal of Structural Geology, and the Geological Society of America field guide series: Allen et al. (2002), Allen (2004), Allen (2005), Shaw and Allen (2007), Lee et al., (2012), Allen and Shaw (2013).

Above: Classic paired pseudotachylyte vein at Homestake. Below: Slide Lake and Homestake Peak (13,219 ft).

Long-Term Fault Reactivation and Seismicity – Grizzly Creek Shear Zone and White River Uplift, Colorado

The Grizzly Creek shear zone is a newly recognized fault zone that I discovered quite by accident in 1996. While leading the University of Kentucky geology field camp, I made a lunch stop along the Colorado River at the I-70 Grizzly Creek rest area in Glenwood Canyon. While casually eating a typical field camp sandwich of some sort or another, I noticed that the boulder upon which I was seated was full of pseudotachylyte. But where from? Back in those days, almost no one knew how to recognize it in the field, so it is not surprising that it did not show up on any of the published maps. After exhaustive searching later in the summer, I found a 1-km-wide pseudotachylyte –mylonite system in the tributary canyons of Grizzly Creek and No Name Creek. I suppose I could have named it the No Name Shear Zone in a fit of irony, but I went with Grizzly since it sounds tougher and is more reflective of the steep canyon climbs necessary to access it. Not to mention the intense summer heat.

Our work has shown that the shear zone has a long history of reactivation. The pseudotachylytes and coeval mylonites formed at 1.4 Ga. Much later, part of the shear zone was reactivated as a brittle fault at about 70 Ma to form the southern margin of the Laramide White River uplift. This later reactivation was complex, since the anomalously east-west-trending Grizzly Creek shear zone / brittle fault cross-cuts an older north-south reverse fault and monocline (bottom photo at right). It is possible that the cross-cutting structures may represent Laramide strain partitioning.

Since 2005, mapping in Glenwood Canyon and along the southern margin of the White River uplift has become an essential part of Concord's summer geology field course. It has also been the subject of several undergraduate research projects. Research at this site has been funded by the American Chemical Society (ACF PRF), NSF, and the USGS EDMAP program.

Some of the results have been published as a geologic map for the Colorado Geological Survey (Kirkham et al., 2008). Other results appear in the journal Rocky Mountain Geology, and in special publications of the Geological Society of America and the Geological Society, London: Allen and Shaw (2008); Allen and Shaw (2011); Jones et al. (2013). The results of additional work on Laramide reactivation of the shear zone are in preparation.

Above: Steep canyon exposures in Grizzly Creek. Below: North-south Laramide monocline later cut by the Grizzly Creek fault

Footwall Deformation at the Leading Edge of the Appalachian Fold-Thrust Belt – West Virginia-Virginia

Since I arrived at Concord in 1998, I have used the local geology for field trips and short class-based field projects in all of my courses. New road cuts since 2010 have allowed me to further incorporate the Appalachian thrust belt into our geology field camp and initiate new research on coupled fault-related folding and fold-related faulting. How's that for a mouthful? We have been finding that the two processes are linked as part of a reinforcing feedback system.

Results of this work will be published as time permits. Past work on subtle structures near the margin of the Appalachian plateau have been published in Southeastern Geology and as a geologic map for the West Virginia Geologic and Economic Survey (Allen, 1993a, 1993b; Matchen et al., 2011).

Overturned limb of footwall syncline of the St. Clair thrust.

Geoscience Education – Impact of Undergraduate Research on Student Learning

I was hired at Concord in 1998 to serve as a chair for the Department of Physical Sciences and to initiate a new degree program in Environmental Geosciences. As I was the only geology faculty member for the first 6 years, I soon integrated most of my research into my classes in what is now known as a CURE (course-based undergraduate research experience). However, the projects students worked on tended to transcend each individual class. I would have them continue to build on work they started during an earlier semester, most of which was built upon samples collected at my summer geology field camp and research site at the Homestake shear zone.

I have continued this way of teaching since, and in 2015 I received an NSF grant from the Improving Undergraduate STEM Education program (IUSE, DUE #1525590) to study how this multi-semester, curriculum-based undergraduate research experience (MS-CURE) impacted student learning. In collaboration with Dr. Kuehn and an education researcher at Virginia Tech, our results show that students have increases in personal and professional gains that are equivalent to more labor-intensive, one-on-one mentored undergraduate research. In this model, all of our students have access to an extended research opportunity - not just a select few.

Results of this work have been published in GSA Today (Allen et al., 2020) and Scholarship and Practice of Undergraduate Research (SPUR) (Allen et al., 2020).

Above: Field work in the Sawatch Range, Colorado during the 2017 field camp. Samples and field data are used by subsequent student cohorts for their MS-CURE. Below: Amygdules in pseudotachylyte from Greenland imaged on Concord's electron microprobe. These became an extension of a student MS-CURE project.