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76
These structural observations provide a clear picture of early east-directed
deformation overprinted by later west-directed deformation, which is inconsistent with
it having formed solely as a result of exhumation along the NSRD.
3.3.2 Paleomagnetic constraints
In addition to the structural observations, the deformed mafic dikes provide
an opportunity to place paleomagnetic constraints on both vertical- and horizontal-axis
rotations within the mylonite zone. The mafic dikes in this predominantly east-vergent
mylonite zone are highly deformed and west dipping, which raises questions
about their emplacement history and their relationship to the evolution of the zone.
Two possibilities are: (1) the dikes were intruded vertically and then rotated top-east in
an east-vergent shear zone; or (2) the dikes were intruded into west-dipping extension
fractures that opened during a period of west-directed shear within the zone, and have
not been significantly rotated since. To differentiate between these two possibilities,
we carried out a paleomagnetic study of both dikes to determine the orientation and
magnitude of post-emplacement dike rotation.
3.3.2.1 Methods
Samples for magnetic analysis were collected from multiple sites on each of the
mafic dikes (Figure 3.3c) using a portable gasoline-powered rock drill. Where possible,
samples were drilled close to the chilled margin and in the center of the dikes to check
for magnetic homogeneity. 18 cores were collected from dike 1 and 15 from dike 2 to
allow for statistical analysis. The cores were oriented using a Brunton compass and
an orienting platform which includes a sun compass for magnetically strong samples.
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 91 |
| Contributing entity | University of Southern California |
| Repository email | cisadmin@lib.usc.edu |
| Full text | 76 These structural observations provide a clear picture of early east-directed deformation overprinted by later west-directed deformation, which is inconsistent with it having formed solely as a result of exhumation along the NSRD. 3.3.2 Paleomagnetic constraints In addition to the structural observations, the deformed mafic dikes provide an opportunity to place paleomagnetic constraints on both vertical- and horizontal-axis rotations within the mylonite zone. The mafic dikes in this predominantly east-vergent mylonite zone are highly deformed and west dipping, which raises questions about their emplacement history and their relationship to the evolution of the zone. Two possibilities are: (1) the dikes were intruded vertically and then rotated top-east in an east-vergent shear zone; or (2) the dikes were intruded into west-dipping extension fractures that opened during a period of west-directed shear within the zone, and have not been significantly rotated since. To differentiate between these two possibilities, we carried out a paleomagnetic study of both dikes to determine the orientation and magnitude of post-emplacement dike rotation. 3.3.2.1 Methods Samples for magnetic analysis were collected from multiple sites on each of the mafic dikes (Figure 3.3c) using a portable gasoline-powered rock drill. Where possible, samples were drilled close to the chilled margin and in the center of the dikes to check for magnetic homogeneity. 18 cores were collected from dike 1 and 15 from dike 2 to allow for statistical analysis. The cores were oriented using a Brunton compass and an orienting platform which includes a sun compass for magnetically strong samples. |
