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31
thermobarometry on the basis of their location, M2 metamorphic mineral assemblage,
and relatively low degree of M3 retrogression.
All samples collected from the Pioche Shale proved to have an inadequate
mineral assemblage for thermobarometric determinations due to a lack of plagioclase
and the presence of high-spessartine garnets, which can affect Fe-Mg mixing, rendering
them unsuitable for garnet-biotite thermometry [Williams and Grambling, 1990].
The mineral content of the Osceola Argillite varied considerably between
samples. In Hendry’s Creek, the unit contained a greenschist-grade assemblage of
garnet + chlorite + biotite + kyanite. However, the combination of heavily retrogressed
biotites and a lack of plagioclase made thermobarometry impossible. To the north,
samples from Deadman Creek contained a far more promising assemblage of garnet +
biotite + muscovite + plagioclase + staurolite, appropriate for both GARB thermometry
and GMBP barometry.
McCoy Creek Group unit 2 proved to be the most reliable and widespread
metapelitic unit, with a consistent M2 mineral assemblage of garnet + biotite + muscovite
+ plagioclase ± staurolite, again appropriate for both GARB thermometry and GMBP
barometry. Therefore, samples were primarily selected from this unit. In total, seven
samples (six McCoy Creek Group unit 2 and one Osceola Argillite) were found to have
an M2 mineral assemblage that had resisted M3 retrogression, and were thus suitable
for thermobarometric calculations. In this section we present new thermobarometric
data from these seven samples, collected from five of the main drainages in the range:
Deadman Creek, Deep Canyon, Hampton Creek, Hendry’s Creek, and Silver Creek
(Figure 2.2).
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 46 |
| Contributing entity | University of Southern California |
| Repository email | cisadmin@lib.usc.edu |
| Full text | 31 thermobarometry on the basis of their location, M2 metamorphic mineral assemblage, and relatively low degree of M3 retrogression. All samples collected from the Pioche Shale proved to have an inadequate mineral assemblage for thermobarometric determinations due to a lack of plagioclase and the presence of high-spessartine garnets, which can affect Fe-Mg mixing, rendering them unsuitable for garnet-biotite thermometry [Williams and Grambling, 1990]. The mineral content of the Osceola Argillite varied considerably between samples. In Hendry’s Creek, the unit contained a greenschist-grade assemblage of garnet + chlorite + biotite + kyanite. However, the combination of heavily retrogressed biotites and a lack of plagioclase made thermobarometry impossible. To the north, samples from Deadman Creek contained a far more promising assemblage of garnet + biotite + muscovite + plagioclase + staurolite, appropriate for both GARB thermometry and GMBP barometry. McCoy Creek Group unit 2 proved to be the most reliable and widespread metapelitic unit, with a consistent M2 mineral assemblage of garnet + biotite + muscovite + plagioclase ± staurolite, again appropriate for both GARB thermometry and GMBP barometry. Therefore, samples were primarily selected from this unit. In total, seven samples (six McCoy Creek Group unit 2 and one Osceola Argillite) were found to have an M2 mineral assemblage that had resisted M3 retrogression, and were thus suitable for thermobarometric calculations. In this section we present new thermobarometric data from these seven samples, collected from five of the main drainages in the range: Deadman Creek, Deep Canyon, Hampton Creek, Hendry’s Creek, and Silver Creek (Figure 2.2). |
