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92
the mylonite zone of the Ruby Mountains metamorphic core complex in north-central
Nevada. This flow formed a fanning pattern inconsistent with the lineation direction
of the mylonitic fabric, implying that flow and mylonitization were unrelated. No such
contrast has been observed in the Snake Range.
(3) The mylonite zone represents a mid-crustal velocity discontinuity between
localized and distributed deformation that accumulated displacement from faults of
varying magnitude and direction over time. This discontinuity was later captured by
a moderately-dipping normal fault that soled down into it and rolled it back into a
subhorizontal orientation at the surface as a result of isostatic rebound [cf. Lee, 1995].
In this interpretation the brittle hangingwall to the NSRD represents a series of rotated
normal fault blocks, and the ductile footwall represents an exhumed middle crustal
velocity discontinuity. This model fits best with our observations in the mylonite
zone because it explains the presence of both east- and west- phases of deformation,
the greenschist-grade temperatures and associated ductile deformation, and the
disappearance of the mylonite zone in the northwestern corner of the range.
The schematic model shown in Figure 3.14 describes a possible evolution of
the northern Snake Range mylonite zone based on the data presented in this paper and
the thermochronological constraints of Lee and Sutter [1991], Lee [1995] and Miller
et al. [1999]. We propose that initial crustal extension in the early Tertiary produced
a subhorizontal mid-crustal velocity discontinuity, below the brittle-ductile transition,
that separated upper and lower crustal domains extending in different ways. Above
the transition, rocks were deformed by motion along localized zones such as brittle
faults or discrete ductile shear zones, transferring significant displacements downwards
into the velocity discontinuity, whereas below the transition, deformation occurred by
distributed crustal flow. The transition itself was marked by a zone of high-strain rocks
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 107 |
| Contributing entity | University of Southern California |
| Repository email | cisadmin@lib.usc.edu |
| Full text | 92 the mylonite zone of the Ruby Mountains metamorphic core complex in north-central Nevada. This flow formed a fanning pattern inconsistent with the lineation direction of the mylonitic fabric, implying that flow and mylonitization were unrelated. No such contrast has been observed in the Snake Range. (3) The mylonite zone represents a mid-crustal velocity discontinuity between localized and distributed deformation that accumulated displacement from faults of varying magnitude and direction over time. This discontinuity was later captured by a moderately-dipping normal fault that soled down into it and rolled it back into a subhorizontal orientation at the surface as a result of isostatic rebound [cf. Lee, 1995]. In this interpretation the brittle hangingwall to the NSRD represents a series of rotated normal fault blocks, and the ductile footwall represents an exhumed middle crustal velocity discontinuity. This model fits best with our observations in the mylonite zone because it explains the presence of both east- and west- phases of deformation, the greenschist-grade temperatures and associated ductile deformation, and the disappearance of the mylonite zone in the northwestern corner of the range. The schematic model shown in Figure 3.14 describes a possible evolution of the northern Snake Range mylonite zone based on the data presented in this paper and the thermochronological constraints of Lee and Sutter [1991], Lee [1995] and Miller et al. [1999]. We propose that initial crustal extension in the early Tertiary produced a subhorizontal mid-crustal velocity discontinuity, below the brittle-ductile transition, that separated upper and lower crustal domains extending in different ways. Above the transition, rocks were deformed by motion along localized zones such as brittle faults or discrete ductile shear zones, transferring significant displacements downwards into the velocity discontinuity, whereas below the transition, deformation occurred by distributed crustal flow. The transition itself was marked by a zone of high-strain rocks |
