Page 116 |
Save page Remove page | Previous | 116 of 200 | Next |
|
small (250x250 max)
medium (500x500 max)
Large (1000x1000 max)
Extra Large
large ( > 500x500)
Full Resolution
All (PDF)
|
This page
All
|
101
Proterozoic to Permian miogeoclinal sequence cut by a domiform detachment fault —
the northern Snake Range décollement (NSRD) — that separates a distended brittle
upper plate from a ductilely stretched and metamorphosed lower plate (Figure 4.1).
Despite the NSRD being the center of considerable debate over the last few decades,
details of its origin, evolution, and relationship to mylonitic footwall deformation,
remain unresolved. Current estimates of initial dip vary from subhorizontal, based on a
consistent stratigraphic position [Miller et al., 1983], to 40° east, based on a progressive
westward increase in 40Ar/39Ar and fission track ages [Lee and Sutter, 1991; Lee, 1995].
Displacement estimates vary from <10 km, assuming the NSRD represents a 6–7 km-deep
subhorizontal zone of crustal brittle-ductile decoupling [Miller et al., 1983; Gans
et al., 1985], to a low-angle normal fault with ≥ 60 km of displacement [Bartley and
Wernicke, 1984]. More recently, Lee [1995] postulated that the footwall rocks were
exhumed by a rolling-hinge fault system [Buck, 1988; Wernicke and Axen, 1988] in
which initially subhorizontal footwall units were tilted to the east during motion along
a moderately-dipping fault ramp [Bartley and Wernicke, 1984; Lewis et al., 1999],
before being rolled back into a subhorizontal orientation at the surface as an isostatic
response to footwall denudation.
Mylonitic deformation in the footwall is defined by a subhorizontal mylonitic
foliation and a consistent WNW–ESE stretching lineation [Gaudemer and Tapponnier,
1987; Lee et al., 1987; Miller et al., 1983]. The mylonite zone is generally interpreted
to be a Tertiary feature related to extension and exhumation along the NSRD, but it
decreases in intensity from east to west, disappearing completely in the northwestern
part of the range [Lee et al., 1987; Miller et al., 1983]. 40Ar/39Ar cooling ages [Lee
and Sutter, 1991; Lee, 1995] denote a systematic east to west increase in the age of
mylonitization from 20 Ma to 50 Ma, suggesting that ductile deformation in the western
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 116 |
| Contributing entity | University of Southern California |
| Repository email | cisadmin@lib.usc.edu |
| Full text | 101 Proterozoic to Permian miogeoclinal sequence cut by a domiform detachment fault — the northern Snake Range décollement (NSRD) — that separates a distended brittle upper plate from a ductilely stretched and metamorphosed lower plate (Figure 4.1). Despite the NSRD being the center of considerable debate over the last few decades, details of its origin, evolution, and relationship to mylonitic footwall deformation, remain unresolved. Current estimates of initial dip vary from subhorizontal, based on a consistent stratigraphic position [Miller et al., 1983], to 40° east, based on a progressive westward increase in 40Ar/39Ar and fission track ages [Lee and Sutter, 1991; Lee, 1995]. Displacement estimates vary from <10 km, assuming the NSRD represents a 6–7 km-deep subhorizontal zone of crustal brittle-ductile decoupling [Miller et al., 1983; Gans et al., 1985], to a low-angle normal fault with ≥ 60 km of displacement [Bartley and Wernicke, 1984]. More recently, Lee [1995] postulated that the footwall rocks were exhumed by a rolling-hinge fault system [Buck, 1988; Wernicke and Axen, 1988] in which initially subhorizontal footwall units were tilted to the east during motion along a moderately-dipping fault ramp [Bartley and Wernicke, 1984; Lewis et al., 1999], before being rolled back into a subhorizontal orientation at the surface as an isostatic response to footwall denudation. Mylonitic deformation in the footwall is defined by a subhorizontal mylonitic foliation and a consistent WNW–ESE stretching lineation [Gaudemer and Tapponnier, 1987; Lee et al., 1987; Miller et al., 1983]. The mylonite zone is generally interpreted to be a Tertiary feature related to extension and exhumation along the NSRD, but it decreases in intensity from east to west, disappearing completely in the northwestern part of the range [Lee et al., 1987; Miller et al., 1983]. 40Ar/39Ar cooling ages [Lee and Sutter, 1991; Lee, 1995] denote a systematic east to west increase in the age of mylonitization from 20 Ma to 50 Ma, suggesting that ductile deformation in the western |
