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72
oriented hinge lines that commonly show east vergence. Dikes in these high-strain
zones form east-asymmetric boudins (Figure 3.4e,f), and folds are asymmetric and
highly attenuated, with short limbs that roll over into geometries resembling delta-type
porphyroclasts with a top-east sense of shear. All these structures are cut by 1 cm to
10 m scale west-dipping, west-directed shear bands that deform the dikes and form
top-west asymmetric porphyroclasts within the main fabric. Figures 3.5a and c show
west-directed shear bands that warp the earlier mylonitic fabric. In Figure 3.5c, the
shear bands have formed along the margin of one of the mafic dikes, suggesting that, in
this instance, the dike localized the deformation. Figures 3.5b and d illustrate this west-directed
deformation at the microscale. Occasional grains of metamorphic muscovite
show a top-west sense of shear (Figure 3.5b). Within the shear bands, the shape fabric
defined by dynamically recrystallized calcite forms a sigmoidal pattern that indicates a
top-west shear sense (Figure 3.5d).
The shear bands shown in Figure 3.5 transfer displacement westward onto
earlier detachment-parallel east-directed shear zones, creating complicated structures
that represent east- and west-directed shear (e.g., Figure 3.6). Figure 3.6a shows a
dike that was boudinaged and then folded over to the west. The upper limb of this fold
has been greatly thinned, suggesting that it has experienced considerable west-directed
shear strain. Figure 3.6b illustrates the complex deformation within this top limb, which
comprises a small folded boudin and two asymmetric boudins with opposing senses
of shear. Figure 3.6c shows the complicated fabric relationships that formed where
east- and west-directed shear were combined. Here, both the mylonitic fabric and the
closely-spaced east-directed shear bands are cut and offset by a later west-directed
shear band.
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 87 |
| Contributing entity | University of Southern California |
| Repository email | cisadmin@lib.usc.edu |
| Full text | 72 oriented hinge lines that commonly show east vergence. Dikes in these high-strain zones form east-asymmetric boudins (Figure 3.4e,f), and folds are asymmetric and highly attenuated, with short limbs that roll over into geometries resembling delta-type porphyroclasts with a top-east sense of shear. All these structures are cut by 1 cm to 10 m scale west-dipping, west-directed shear bands that deform the dikes and form top-west asymmetric porphyroclasts within the main fabric. Figures 3.5a and c show west-directed shear bands that warp the earlier mylonitic fabric. In Figure 3.5c, the shear bands have formed along the margin of one of the mafic dikes, suggesting that, in this instance, the dike localized the deformation. Figures 3.5b and d illustrate this west-directed deformation at the microscale. Occasional grains of metamorphic muscovite show a top-west sense of shear (Figure 3.5b). Within the shear bands, the shape fabric defined by dynamically recrystallized calcite forms a sigmoidal pattern that indicates a top-west shear sense (Figure 3.5d). The shear bands shown in Figure 3.5 transfer displacement westward onto earlier detachment-parallel east-directed shear zones, creating complicated structures that represent east- and west-directed shear (e.g., Figure 3.6). Figure 3.6a shows a dike that was boudinaged and then folded over to the west. The upper limb of this fold has been greatly thinned, suggesting that it has experienced considerable west-directed shear strain. Figure 3.6b illustrates the complex deformation within this top limb, which comprises a small folded boudin and two asymmetric boudins with opposing senses of shear. Figure 3.6c shows the complicated fabric relationships that formed where east- and west-directed shear were combined. Here, both the mylonitic fabric and the closely-spaced east-directed shear bands are cut and offset by a later west-directed shear band. |
