Page 100 |
Save page Remove page | Previous | 100 of 200 | Next |
|
small (250x250 max)
medium (500x500 max)
Large (1000x1000 max)
Extra Large
large ( > 500x500)
Full Resolution
All (PDF)
|
This page
All
|
85
A similar attempt at reconstructing dike 1 proved unsuccessful as the
paleomagnetic results were unstable. This is most likely due to the higher degree of
alteration and chloritization in dike 1, possibly a function of its smaller grain size.
3.4 Temperature and timing of deformation
Constraining the temperature and timing of the different phases of mylonitic
deformation is crucial to understanding how the mylonite zone formed. Here we
combine new calcite-dolomite thermometry and electron backscatter diffraction data
with published 40Ar/39Ar and fission track thermochronology to build a coherent picture
of mylonite zone evolution.
3.4.1 Temperature
3.4.1.1 Calcite-dolomite thermometry
The coexistence of quartz and dolomite in calcareous samples from Marble
Wash (Figure 3.11) suggests that mylonitic deformation occurred at temperatures <
500°C [Winter, 2001]. This temperature range is consistent with observations of the
mylonitic fabric in metapelitic and quartzite units in the southern part of the range,
where growth of chlorite and muscovite suggest that deformation occurred under
greenschist grade conditions, while crystal-plastic quartz and brittle feldspar textures
suggest deformation temperatures of 300–450°C [Lee et al., 1987; Miller et al., 1988].
The preservation of this assemblage does however suggest that the Cambrian carbonate
rocks of Marble Wash did not reach amphibolite-facies conditions during Cretaceous
peak metamorphism, in contrast to the more deeply exhumed Proterozoic rocks in the
southern part of the range [Lewis et al., 1999; Cooper et al., in prep].
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 100 |
| Contributing entity | University of Southern California |
| Repository email | cisadmin@lib.usc.edu |
| Full text | 85 A similar attempt at reconstructing dike 1 proved unsuccessful as the paleomagnetic results were unstable. This is most likely due to the higher degree of alteration and chloritization in dike 1, possibly a function of its smaller grain size. 3.4 Temperature and timing of deformation Constraining the temperature and timing of the different phases of mylonitic deformation is crucial to understanding how the mylonite zone formed. Here we combine new calcite-dolomite thermometry and electron backscatter diffraction data with published 40Ar/39Ar and fission track thermochronology to build a coherent picture of mylonite zone evolution. 3.4.1 Temperature 3.4.1.1 Calcite-dolomite thermometry The coexistence of quartz and dolomite in calcareous samples from Marble Wash (Figure 3.11) suggests that mylonitic deformation occurred at temperatures < 500°C [Winter, 2001]. This temperature range is consistent with observations of the mylonitic fabric in metapelitic and quartzite units in the southern part of the range, where growth of chlorite and muscovite suggest that deformation occurred under greenschist grade conditions, while crystal-plastic quartz and brittle feldspar textures suggest deformation temperatures of 300–450°C [Lee et al., 1987; Miller et al., 1988]. The preservation of this assemblage does however suggest that the Cambrian carbonate rocks of Marble Wash did not reach amphibolite-facies conditions during Cretaceous peak metamorphism, in contrast to the more deeply exhumed Proterozoic rocks in the southern part of the range [Lewis et al., 1999; Cooper et al., in prep]. |
