Relating quartz crystallographic preferred orientation intensity to finite strain magnitude in the Northern Snake Range metamorphic core complex, Nevada: a new tool for characterizing strain patterns in ductilely sheared rocks

nolan Blackford; Sean P. Long; Jeffrey Lee; Kyle P. Larson; Gareth Seward; Julia L. Stevens; Hadeel Al Harthi

Relating quartz crystallographic preferred orientation intensity to finite strain magnitude in the Northern Snake Range metamorphic core complex, Nevada: a new tool for characterizing strain patterns in ductilely sheared rocks

By nolan Blackford¹, Sean P. Long², Jeffrey Lee³, Kyle P. Larson⁴, Gareth Seward⁵, Julia L. Stevens², Hadeel Al Harthi²

1. University of Arizona 2. Washington State University 3. Colorado School of Mines 4. University of British Columbia, Okanagan 5. University of California, Santa Barbara

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25 Dec 2023

Abstract

Documenting the magnitude of finite strain within ductile shear zones is critical for understanding lithospheric deformation. However, pervasive recrystallization within shear zones often destroys the deformed markers from which strain can be measured. Intensity parameters calculated from quartz crystallographic preferred orientation (CPO) distributions have been interpreted as proxies for the relative strain magnitude within shear zones, but thus far have not been calibrated to absolute strain magnitude. Here, we present equations that quantify the relationship between CPO intensity parameters (cylindricity and density norm) and finite strain magnitude, which we calculate by integrating quartz CPO analyses (n=87) with strain ellipsoids from stretched detrital quartz clasts (n=49) and macro-scale ductile thinning measurements (n=7) from the footwall of the Northern Snake Range décollement (NSRD) in Nevada. The NSRD footwall exhibits a dramatic strain gradient, with lineation-parallel Rs (strain ellipticity ratio) values increasing from 5.4±1.4 to 282±122 eastward across the range. Cylindricity increases from 0.52 to 0.83 as Rs increases from 5.4 to 23.5, and increases gradually to 0.92 at Rs values between 160-404. Density norm increases from 1.68 to 2.97 as Rs increases from 5.4 to 23.5, but stays approximately constant until Rs values between 160-404. We present equations that express average finite strain as a function of average cylindricity and density norm, which provide a broadly applicable tool for estimating the 1^st-order finite strain magnitude within any shear zone from which quartz CPO intensity can be measured.

Bio

This work was supported by National Science Foundation grants EAR-2022973 awarded to Sean Long and EAR-2022979 awarded to Jeffrey Lee.

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Nolan R. Blackford, Sean P. Long, Jeffrey Lee, Kyle P. Larson, Gareth Seward, Julia L. Stevens, and Hadeel Al Harthi

Cite this work

Researchers should cite this work as follows:

nolan Blackford; Sean P. Long; Jeffrey Lee; Kyle P. Larson; Gareth Seward; Julia L. Stevens; Hadeel Al Harthi (2023), "Relating quartz crystallographic preferred orientation intensity to finite strain magnitude in the Northern Snake Range metamorphic core complex, Nevada: a new tool for characterizing strain patterns in ductilely sheared rocks," https://mygeohub.org/resources/1693.

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