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17
2.2 Geologic setting
The northern Snake Range of east-central Nevada is a 50 km-long, north-south-
trending range that forms part of a 2,500 km-long NW-SE-trending belt of
metamorphic core complexes in the hinterland of the Sevier fold and thrust belt (Figure
2.1). The range is a classic example of a normal-fault bounded metamorphic core
complex comprising a low-angle domiform detachment fault — the northern Snake
Range décollement (NSRD) — that separates a distended brittle upper plate from a
ductilely stretched lower plate. The stratigraphy of the northern Snake Range and
adjacent ranges is characterized by a relatively uniform 15 km-thick late Proterozoic to
Permian miogeoclinal shelf sequence surrounded by Miocene to Quaternary sediment-filled
extensional basins [Hose and Blake, 1976; Stewart and Poole, 1974; Miller et
al., 1989; Miller et al., 1990]. In the northern Snake Range, the upper plate comprises
pervasively-faulted Paleozoic limestones, shales, quartzites and dolomites, while the
footwall is dominated by ductilely-deformed and metamorphosed Upper Precambrian
to Lower Cambrian quartzites and schists intruded by Jurassic and Cretaceous granitic
plutons and Tertiary dikes (Figures 2.2 and 2.3).
The NSRD maintains a fairly consistent stratigraphic position at the top of
the Lower Cambrian Pioche Shale (Figure 2.2), and there is little to no stratigraphic
omission across it, with the oldest rocks in the hangingwall being approximately the
same age as the youngest rocks in the footwall. This led a number of authors to suggest
that the detachment initiated as a subhorizontal zone of mid-crustal decoupling within
the miogeoclinal sequence that accommodated <10 km of displacement between brittle
deformation above and ductile coaxial deformation below [Gans and Miller, 1983;
Gans et al., 1985; Lee et al., 1987; Lee and Sutter, 1991; Miller and Gans, 1989; Miller
et al., 1983; Miller et al., 1988]. This model precludes any major structural duplication
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 32 |
| Contributing entity | University of Southern California |
| Repository email | cisadmin@lib.usc.edu |
| Full text | 17 2.2 Geologic setting The northern Snake Range of east-central Nevada is a 50 km-long, north-south- trending range that forms part of a 2,500 km-long NW-SE-trending belt of metamorphic core complexes in the hinterland of the Sevier fold and thrust belt (Figure 2.1). The range is a classic example of a normal-fault bounded metamorphic core complex comprising a low-angle domiform detachment fault — the northern Snake Range décollement (NSRD) — that separates a distended brittle upper plate from a ductilely stretched lower plate. The stratigraphy of the northern Snake Range and adjacent ranges is characterized by a relatively uniform 15 km-thick late Proterozoic to Permian miogeoclinal shelf sequence surrounded by Miocene to Quaternary sediment-filled extensional basins [Hose and Blake, 1976; Stewart and Poole, 1974; Miller et al., 1989; Miller et al., 1990]. In the northern Snake Range, the upper plate comprises pervasively-faulted Paleozoic limestones, shales, quartzites and dolomites, while the footwall is dominated by ductilely-deformed and metamorphosed Upper Precambrian to Lower Cambrian quartzites and schists intruded by Jurassic and Cretaceous granitic plutons and Tertiary dikes (Figures 2.2 and 2.3). The NSRD maintains a fairly consistent stratigraphic position at the top of the Lower Cambrian Pioche Shale (Figure 2.2), and there is little to no stratigraphic omission across it, with the oldest rocks in the hangingwall being approximately the same age as the youngest rocks in the footwall. This led a number of authors to suggest that the detachment initiated as a subhorizontal zone of mid-crustal decoupling within the miogeoclinal sequence that accommodated <10 km of displacement between brittle deformation above and ductile coaxial deformation below [Gans and Miller, 1983; Gans et al., 1985; Lee et al., 1987; Lee and Sutter, 1991; Miller and Gans, 1989; Miller et al., 1983; Miller et al., 1988]. This model precludes any major structural duplication |
