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86
To place tighter constraints on the temperature of mylonitic deformation, and
in particular on the different phases of deformation, we carried out calcite-dolomite
thermometry on samples containing both east- and west-directed structures (samples
in Figures 3.4b and 3.5d). Analytical work was carried out on the JEOL JXA-8200
electron microprobe at UCLA. Adjacent calcite and dolomite grains were located from
1 μm resolution X-ray composition maps of Ca and Mg created with an accelerating
voltage of 15 kV and a beam current of 100 nA (Figure 3.11b,c). Grains were then
analyzed with an accelerating voltage of 15 kV and a beam diameter of 10 μm to avoid
crystal damage. Oxide weight percentages were calculated with JEOL software which
uses ZAF corrections for matrix effects, excepting CO2, which was determined from
stoichiometry. Temperatures were calculated using the calcite-dolomite geothermometer
of Anovitz and Essene [1987].
The results shown in Table 3.2 indicate that there is a ca. 80°C difference in
temperature between the two phases of deformation, with top-east shear occurring at
433 ± 33°C and top-west shear at 351 ± 20°C. This is consistent with our interpretation
that the west-directed deformation happened later, and indicates that both phases
occurred under conditions in which calcite is ductile.
3.4.1.2 Electron backscatter diffraction
Electron backscatter diffraction (EBSD) analyses were carried out on the same
two samples used for the calcite-dolomite thermometry to (1) place constraints on the
style of deformation during each phase of deformation, and (2) provide an independent
indication of the temperatures under which this deformation occurred. EBSD analyses
were carried out at UCSB using a JEOL 6300 scanning electron microscope fitted with
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 101 |
| Contributing entity | University of Southern California |
| Repository email | cisadmin@lib.usc.edu |
| Full text | 86 To place tighter constraints on the temperature of mylonitic deformation, and in particular on the different phases of deformation, we carried out calcite-dolomite thermometry on samples containing both east- and west-directed structures (samples in Figures 3.4b and 3.5d). Analytical work was carried out on the JEOL JXA-8200 electron microprobe at UCLA. Adjacent calcite and dolomite grains were located from 1 μm resolution X-ray composition maps of Ca and Mg created with an accelerating voltage of 15 kV and a beam current of 100 nA (Figure 3.11b,c). Grains were then analyzed with an accelerating voltage of 15 kV and a beam diameter of 10 μm to avoid crystal damage. Oxide weight percentages were calculated with JEOL software which uses ZAF corrections for matrix effects, excepting CO2, which was determined from stoichiometry. Temperatures were calculated using the calcite-dolomite geothermometer of Anovitz and Essene [1987]. The results shown in Table 3.2 indicate that there is a ca. 80°C difference in temperature between the two phases of deformation, with top-east shear occurring at 433 ± 33°C and top-west shear at 351 ± 20°C. This is consistent with our interpretation that the west-directed deformation happened later, and indicates that both phases occurred under conditions in which calcite is ductile. 3.4.1.2 Electron backscatter diffraction Electron backscatter diffraction (EBSD) analyses were carried out on the same two samples used for the calcite-dolomite thermometry to (1) place constraints on the style of deformation during each phase of deformation, and (2) provide an independent indication of the temperatures under which this deformation occurred. EBSD analyses were carried out at UCSB using a JEOL 6300 scanning electron microscope fitted with |
