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133
is used as a calibration for recrystallized grain size piezometry. The classification
comprises three dislocation creep regimes in which different mechanisms of dynamic
recrystallization operate to produce distinctive microstructures (Figure 5.8). From
Regime 1 to Regime 3, a progressive increase in temperature, decrease in strain rate, or
addition of water, changes the mechanism of dynamic recrystallization deformation. In
Regime 1, bulge nucleation and grain boundary migration result in patchy undulatory
extinction and diffuse grain boundaries (Figure 5.8b). In Regime 2, subgrain rotation
recrystallization produces sweeping extinction, optically visible subgrains, deformation
lamellae, and a core and mantle structure (Figure 5.8c). In Regime 3, grain boundary
migration creates new grains at the expense of the old deformed grains, producing
complete recrystallization of the original microstructure (Figure 5.8d).
Recrystallized grain size can be determined very precisely through electron
backscatter diffraction analysis (EBSD) using a scanning electron microscope. EBSD
analyses the diffraction patterns of individual grains and uses them to determine both
the crystal structure and orientation of each grain. The EBSD method is advantageous
because it can produce high-resolution maps of deformed grains very quickly. The
output data also allows easy plotting of the sizes, shapes, and orientations of individual
grains, as well as statistics on grain populations, intragrain lattice misorientation and
intergrain misorientation.
Crystallographic preferred orientations (CPOs) can also be easily derived from
this data for any chosen domain or population of grains, providing information about
slip mechanisms, which are a function of temperature. CPO data can also be used in
combination with measurements of rotated porphyroclasts for vorticity analysis.
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 148 |
| Contributing entity | University of Southern California |
| Repository email | cisadmin@lib.usc.edu |
| Full text | 133 is used as a calibration for recrystallized grain size piezometry. The classification comprises three dislocation creep regimes in which different mechanisms of dynamic recrystallization operate to produce distinctive microstructures (Figure 5.8). From Regime 1 to Regime 3, a progressive increase in temperature, decrease in strain rate, or addition of water, changes the mechanism of dynamic recrystallization deformation. In Regime 1, bulge nucleation and grain boundary migration result in patchy undulatory extinction and diffuse grain boundaries (Figure 5.8b). In Regime 2, subgrain rotation recrystallization produces sweeping extinction, optically visible subgrains, deformation lamellae, and a core and mantle structure (Figure 5.8c). In Regime 3, grain boundary migration creates new grains at the expense of the old deformed grains, producing complete recrystallization of the original microstructure (Figure 5.8d). Recrystallized grain size can be determined very precisely through electron backscatter diffraction analysis (EBSD) using a scanning electron microscope. EBSD analyses the diffraction patterns of individual grains and uses them to determine both the crystal structure and orientation of each grain. The EBSD method is advantageous because it can produce high-resolution maps of deformed grains very quickly. The output data also allows easy plotting of the sizes, shapes, and orientations of individual grains, as well as statistics on grain populations, intragrain lattice misorientation and intergrain misorientation. Crystallographic preferred orientations (CPOs) can also be easily derived from this data for any chosen domain or population of grains, providing information about slip mechanisms, which are a function of temperature. CPO data can also be used in combination with measurements of rotated porphyroclasts for vorticity analysis. |
