Shear zone petrochronology
The questions
Geologists have spent decades debating the “room problem”: How does an igneous rock transit through the crust when there is already a rock occupying that space? My work examines the spatial and temporal relationships between magmatism and deformation to understand how the heat of magmatism weakens rocks and facilitates deformation and if flow related to deformation makes space for new igneous rocks to form through magmatism.
The study sites
One place to examine magmatism and deformation is in mid-crustal shear zones where granites have been deformed by convergent margin tectonics. Granites typically contain the minerals zircon and titanite which can be dated by U-Pb geochronology. By interpreting zircon and titanite ages within the context of the textures and chemistry of these minerals, these minerals can be used to date the timing of magmatism and deformation.
I have been working in two shear zones, the Rosy Finch shear zone in California and the western Idaho shear zone, because uplift and erosion over the past ~100 million years has exposed the mid-crustal roots of the shear zones. See below for more information about these shear zones.
The methods
cathodoluminescence imaging (zircon)
back-scattered electron imaging (titanite)
electron backscattered diffraction measurements of crystallographic misorientation (titanite)
geochemical characterization via laser ablation inductively coupled plasma mass spectrometry (LA-ICPMS; zircon and titanite)
high-precision geochronology via isotope dilution thermal ionization mass spectrometry (ID-TIMS; zircon and titanite)
Rosy Finch shear zone
The Rosy Finch shear zone (RFSZ) is interesting because it cuts through one (nearly) homogenous lithology, so we can use the RFSZ to answer questions about how strain localizes with time without the complication of lithology. Does the center of the shear zone show more strain because it deformed for longer? Or, was deformation was more intense in the center of the shear zone for the same amount of time as the shear zone margins?
Our initial zircon work in this area suggests the pluton emplaced as a series of sills over about 1.5 Myr. Our titanite ages overlap with and are slightly younger than the zircon ages, suggesting deformation followed soon after pluton emplacement. It also looks like the area of maximum solid-state deformation is offset from the locus of magma emplacement by about a kilometer—an interesting observation that I am still working on explaining!
Jamie Vincent, me, and Sarah Trevino relaxing on an outcrop of the Mono Creek Granite, Sierra Nevada mountains, California.
Me and my mini Australian shepherd in a meadow near McCall, Idaho, on our way to an outcrop.
Western Idaho shear zone
Compared to the Rosy Finch shear zone, the western Idaho shear zone (WISZ) is more complex, with many different lithologies and deformation that jumps around spatially and temporally. With this shear zone, we can see strain as a response to deformation induced by local pluton intrusion as well as in response to regional deformation. Additionally, our titanite deformation ages that the shear zone is wider than previously mapped and persisted for longer than previously known.
We have a new paper out! Our work on titanite petrochronology in the western Idaho shear zone has recently been published in Earth and Planetary Science Letters.
Harrigan, CO, Trevino, SF, Schmitz, MD, and Tikoff, B, 2024. Determining the initiation of shear zone deformation using titanite petrochronology. Earth and Planetary Science Letters. 631: 118620. https://doi.org/10.1016/j.epsl.2024.118620
The highlights:
We can distinguish between different generations of magmatic to subsolidus titanite using trace element geochemistry and microstructures.
By identifying and precisely dating recrystallized and neoblastic titanite, we can date shear zone initiation.
The western Idaho shear zone initiated between 98 and 96 Ma.
By tracking the chemistry, textures, and degree of crystallographic misorientation in different morphologies of titanite, we can distinguish between grains that were recrystallized during the onset of deformation versus grains that grew as neoblasts (new grains) while deformation was ongoing.
Manuscripts in preparation
Harrigan, CO, Trevino, SF, Schmitz, MD, Tikoff, B. Spatial-temporal patterns of strain localization in the western Idaho shear zone. (for submission to GSA Bulletin)
Harrigan, CO, Trevino, SF, Schmitz, MD, Tikoff, B. The pace of pluton emplacement and deformation in the Rosy Finch shear zone, Sierra Nevada, CA. (for submission to Tectonics)
Conference presentations on shear zone petrochronology
Harrigan, CO, Trevino, SF, Schmitz, MD, Tikoff, B., 2021. Titanite petrochronology dates shear zone initiation to between 99 and 96 Ma in the western Idaho shear zone. Geological Society of America Abstracts with Programs. 53(6): Abstract no. 127-5. (poster)
Harrigan, C, Schmitz, MD, Kahn, M, Tikoff, B., 2019. Titanite U-Pb Petrochronology Reveals ~26 Million Years of Deformation in a Mid-crustal High-temperature Shear Zone. Geological Society of America Abstracts with Programs. Geological Society of America Abstracts with Programs. 51(5): Abstract no. 217-4. (talk)
Shields, SF, Tikoff, B, Harrigan, CO, Schmitz, M. 2019. Kinematic and Textural Analysis of Titanite: Establishing a Microstructural Framework for a High-Temperature Deformation Geochronometer. American Geophysical Union Fall Meeting. 2019: Abstract T43C-0436. (poster)
Shields, SF, Tikoff, B, Harrigan, CO, Schmitz, M. 2018. Microstructural and magnetic characterization of a syn-magmatic shear zone, Sierra Nevada Batholith, California. American Geophysical Union Fall Meeting. 2018: Abstract T21G-0305. (poster)
Ostwald C, Kahn M, Schmitz MD, Tikoff B. 2017. Finding strain localization and dating the onset of deformation in the western Idaho shear zone using titanite U-Pb geochronology and crystallographic vorticity axis analysis. Geological Society of America Abstracts with Programs. 49(6): Abstract no. 386-4. (poster)
Ostwald C, Schmitz MD. 2017. Tracking the tempo of deformation in western Idaho using 105 million-year-old granite. Graduate Student Showcase, Boise State University (April 3, 2017) and Student Poster Session, Boise Section of the Society for Mining, Metallurgy, and Exploration (April 11, 2017). (poster)
Ostwald C, Schmitz MD. 2016. Zircon and Titanite U-Pb Geochronology Reveal Migrating Magmatism and Deformation in the Western Idaho Shear Zone. American Geophysical Union Fall Meeting. 2016: Abstract T53D-05. (talk)
Ostwald C, Schmitz MD. 2016. Zircon and titanite petrochronology of deformed and non-deformed Payette River tonalite north of McCall, ID; implications for the timing of high temperature deformation in the Western Idaho shear zone. Abstracts with Programs - Geological Society of America. 48(6): Abstract no. 34-3. (talk)
Next...
I want to explore how lithology influences the location and duration of strain localization and how multiple shear zones contemporaneously partition strain on the batholith scale. Idaho and California are great natural laboratories to continue this work.