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134
5.4.2 Application to the northern Snake Range
A preliminary study of quartz microstructures from four Prospect Mountain
Quartzite samples collected along an east-west traverse of the northern Snake Range
footwall (Figure 5.9) suggests that deformation is highly variable and, therefore, could be
related to more than one phase of deformation. Sample FFM281 from the western side of
the range shows a classic Hirth and Tullis [1992] Regime 2 microstructure (Figure 5.9a,
cf. Figure 5.8c). Quartz exhibits undulatory extinction, grain bulging, subgrains, and a
core and mantle structure. Meanwhile, feldspar exhibits brittle behavior, suggesting a
fairly low deformation temperature. The top-to-the-east sense of shear is highlighted
by a deformed mica grain. Dynamically recrystallized grain size ≈ 4 μm. Moving
east, sample FMS284 shows a dramatically different microstructure with no obvious
deformation fabric (Figure 5.9b). Some quartz grains contain deformation lamellae, but
otherwise the rock shows little evidence for deformation. This could, perhaps, reflect a
short-lived, high-stress event that did not last long enough to significantly deform the
rock. Dynamically recrystallized grain size ≈ 23 μm, which is considerably larger than
sample FFM281. Further east still, sample FTa290 has a much smaller grain size than
FFM281 and looks more strongly deformed, similar again to the Regime 2 (Figure 5.9c,
cf. Figure 5.8c). Quartz exhibits numerous subgrains and growth of new grains at grain
boundaries, creating a core and mantle structure. Dynamically recrystallized grain size
≈ 10 μm. Sample FQu262, which derives from the eastern side of the range, shows a
dramatically different microstructure to the previous three. The sample has a very strong
c-axis fabric and the quartz has experienced 100% dynamic recrystallization, indicative
of Regime 3 deformation (Figure 5.9d, cf. Figure 5.8d). Dynamically recrystallized
grain size ≈ 9 μm.
Object Description
| Title | Structural and thermobarometric constraints on the exhumation of the northern Snake Range metamorphic core complex, Nevada |
| Author | Cooper, Frances Jacqueline |
| Author email | fcooper@usc.edu; fcooper@usc.edu |
| Degree | Doctor of Philosophy |
| Document type | Dissertation |
| Degree program | Geological Sciences |
| School | College of Letters, Arts and Sciences |
| Date defended/completed | 2008-08-27 |
| Date submitted | 2008 |
| Restricted until | Unrestricted |
| Date published | 2008-10-22 |
| Advisor (committee chair) | Platt, John P. |
| Advisor (committee member) |
Davis, Gregory A. Morrison, Jean Platzman, Ellen Thompson, Mark E. |
| Abstract | Observations from areas of large-scale continental extension, including the Basin and Range Province in western North America, have revealed the presence of regionally subhorizontal normal faults that appear to have exhumed rocks from mid- to lower-crustal levels. These detachment faults separate upper plate rocks extended on arrays of high-angle brittle normal faults from lower plate rocks exhibiting ductile mylonitic stretching and medium- to high-grade metamorphism. The origin and evolution of these detachments has been a matter of debate for decades, and yet a number of issues remain unresolved: (1) the dip of the faults when they were initiated and were active; (2) their penetration depth into the crust; (3) their role in exhuming high-grade metamorphic rocks; and (4) the origin and significance of the mylonitic deformation in their footwalls.; I explored these issues in the footwall to a classic detachment fault -- the northern Snake Range décollement (NSRD) in eastern Nevada -- using a combination of structural geology, geothermobarometry, paleomagnetism, isotope geochronology, and electron backscatter diffraction (EBSD) analysis. Garnet-biotite-muscovite-plagioclase thermobarometry suggests that the footwall to the NSRD experienced late Cretaceous peak metamorphic conditions of 6–8 kbar and 500–650°C, equivalent to a burial depth of ≤ 30 km. Calcite-dolomite thermometry indicates that Tertiary mylonitic deformation occurred under lower temperature conditions of 350–430°C, equivalent to mid-crustal levels. Structural, paleomagnetic, and EBSD data demonstrate that mylonites experienced two phases of shear (top-east and top-west), inconsistent with movement along a single throughgoing normal fault.; I conclude that exhumation of the northern Snake Range footwall was a two-step process. Initial ductile stretching and thinning of the crust exhumed footwall rocks to the middle crust beneath a discontinuity, referred to as the localized-distributed transition (LDT), that separated extension along brittle normal faults above from localized ductile shear zones below. Mylonites formed along the LDT were subsequently captured by a moderately-dipping NSRD that soled into the middle crust. The NSRD, therefore, appears to be a late-stage brittle normal fault that was responsible for only about half the total exhumation of the footwall, and is not directly related to the mylonitic deformation. |
| Keyword | continental extension; extensional tectonics; Basin and Range province; Cordillera; metamorphism; mylonite zone |
| Geographic subject | tectonic features: Snake Range décollement |
| Geographic subject (state) | Nevada |
| Geographic subject (country) | USA |
| Language | English |
| Part of collection | University of Southern California dissertations and theses |
| Publisher (of the original version) | University of Southern California |
| Place of publication (of the original version) | Los Angeles, California |
| Publisher (of the digital version) | University of Southern California. Libraries |
| Provenance | Electronically uploaded by the author |
| Type | texts |
| Legacy record ID | usctheses-m1695 |
| Contributing entity | University of Southern California |
| Rights | Cooper, Frances Jacqueline |
| Repository name | Libraries, University of Southern California |
| Repository address | Los Angeles, California |
| Repository email | cisadmin@lib.usc.edu |
| Filename | etd-Cooper-2458 |
| Archival file | uscthesesreloadpub_Volume40/etd-Cooper-2458.pdf |
Description
| Title | Page 149 |
| Contributing entity | University of Southern California |
| Repository email | cisadmin@lib.usc.edu |
| Full text | 134 5.4.2 Application to the northern Snake Range A preliminary study of quartz microstructures from four Prospect Mountain Quartzite samples collected along an east-west traverse of the northern Snake Range footwall (Figure 5.9) suggests that deformation is highly variable and, therefore, could be related to more than one phase of deformation. Sample FFM281 from the western side of the range shows a classic Hirth and Tullis [1992] Regime 2 microstructure (Figure 5.9a, cf. Figure 5.8c). Quartz exhibits undulatory extinction, grain bulging, subgrains, and a core and mantle structure. Meanwhile, feldspar exhibits brittle behavior, suggesting a fairly low deformation temperature. The top-to-the-east sense of shear is highlighted by a deformed mica grain. Dynamically recrystallized grain size ≈ 4 μm. Moving east, sample FMS284 shows a dramatically different microstructure with no obvious deformation fabric (Figure 5.9b). Some quartz grains contain deformation lamellae, but otherwise the rock shows little evidence for deformation. This could, perhaps, reflect a short-lived, high-stress event that did not last long enough to significantly deform the rock. Dynamically recrystallized grain size ≈ 23 μm, which is considerably larger than sample FFM281. Further east still, sample FTa290 has a much smaller grain size than FFM281 and looks more strongly deformed, similar again to the Regime 2 (Figure 5.9c, cf. Figure 5.8c). Quartz exhibits numerous subgrains and growth of new grains at grain boundaries, creating a core and mantle structure. Dynamically recrystallized grain size ≈ 10 μm. Sample FQu262, which derives from the eastern side of the range, shows a dramatically different microstructure to the previous three. The sample has a very strong c-axis fabric and the quartz has experienced 100% dynamic recrystallization, indicative of Regime 3 deformation (Figure 5.9d, cf. Figure 5.8d). Dynamically recrystallized grain size ≈ 9 μm. |
