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41
contained staurolite were suitable for thermobarometric work due to excessive biotite
retrogression or lack of plagioclase.
2.4.2.2 Thermobarometric approach
The petrography and geochemistry of the samples discussed suggests that rim
compositions of carefully selected M2 minerals, even if not in contact, are suitable
for thermobarometric determination. The garnet + biotite + muscovite + plagioclase
assemblage present in each of the seven samples permits the application of the well
established garnet-biotite (GARB) thermometer [Ferry and Spear, 1978] and garnet-biotite-
muscovite-plagioclase (GMBP) barometer [Ghent and Stout, 1981].
The GARB thermometer is based on the temperature-dependent exchange of
Fe2+ and Mg cations between garnet and biotite, and is defined by the reaction:
almandine + phlogopite = pyrope + annite
This reaction does not imply the growth of either garnet or biotite, but a prograde
metamorphic path produces a coupled decrease in the Fe/Mg ratio of both phases,
with a relatively more rapid decrease in garnet. A retrograde path would produce
the reverse effect, by raising the Fe/Mg ratios. This retrogressive exchange is most
likely to affect biotite and the outermost margins of garnet, and therefore its effects on
thermobarometry can be minimized with careful mineral characterization, particularly
from line traverses, which can elucidate core to rim changes in Fe and Mg content, and
from analyzing multiple biotite grains in each sample (Figure 2.8e).
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 56 |
| Contributing entity | University of Southern California |
| Repository email | cisadmin@lib.usc.edu |
| Full text | 41 contained staurolite were suitable for thermobarometric work due to excessive biotite retrogression or lack of plagioclase. 2.4.2.2 Thermobarometric approach The petrography and geochemistry of the samples discussed suggests that rim compositions of carefully selected M2 minerals, even if not in contact, are suitable for thermobarometric determination. The garnet + biotite + muscovite + plagioclase assemblage present in each of the seven samples permits the application of the well established garnet-biotite (GARB) thermometer [Ferry and Spear, 1978] and garnet-biotite- muscovite-plagioclase (GMBP) barometer [Ghent and Stout, 1981]. The GARB thermometer is based on the temperature-dependent exchange of Fe2+ and Mg cations between garnet and biotite, and is defined by the reaction: almandine + phlogopite = pyrope + annite This reaction does not imply the growth of either garnet or biotite, but a prograde metamorphic path produces a coupled decrease in the Fe/Mg ratio of both phases, with a relatively more rapid decrease in garnet. A retrograde path would produce the reverse effect, by raising the Fe/Mg ratios. This retrogressive exchange is most likely to affect biotite and the outermost margins of garnet, and therefore its effects on thermobarometry can be minimized with careful mineral characterization, particularly from line traverses, which can elucidate core to rim changes in Fe and Mg content, and from analyzing multiple biotite grains in each sample (Figure 2.8e). |
