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21
consistent with this interpretation. This result, combined with an observed decrease
in the intensity of crystal-plastic deformation, metamorphic recrystallization, and
metamorphic grade from the northern Snake Range west to the Schell Creek range,
led them to suggest that the entire miogeoclinal sequence had originally been tilted
to the east during a phase of Mesozoic west-directed thrusting. This sequence was
subsequently exhumed in the Tertiary along an east-dipping low-angle shear zone with
a similar trajectory to the earlier thrust fault.
Two 40Ar/39Ar thermochronology studies [Lee and Sutter, 1991; Lee, 1995]
suggested that during exhumation, footwall rocks cooled differentially from east to west
within a single structural horizon. Their thermal data indicated that the northwestern
flank of the range dropped below 300°C by 46 Ma and reached 115°C by 20 Ma,
whereas the eastern parts of the range were above 300°C until 19 Ma. Assuming that
isotherms were subhorizontal during exhumation, this led Lee [1995] to postulate that
the footwall rocks were exhumed by a rolling hinge fault system [Buck, 1988; Wernicke
and Axen, 1988] in which the 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 hangingwall denudation.
2.3 Previous footwall studies
2.3.1 Metamorphic petrology and geochronology
Miller et al. [1988] recognized three metamorphic and deformational events
in the lower plate rocks of the northern Snake Range. The first (M1), of Jurassic age,
is limited to the southernmost part of the range where Lower Cambrian quartzites
and metapelites have been intruded and contact metamorphosed by a mid-Jurassic
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 36 |
| Contributing entity | University of Southern California |
| Repository email | cisadmin@lib.usc.edu |
| Full text | 21 consistent with this interpretation. This result, combined with an observed decrease in the intensity of crystal-plastic deformation, metamorphic recrystallization, and metamorphic grade from the northern Snake Range west to the Schell Creek range, led them to suggest that the entire miogeoclinal sequence had originally been tilted to the east during a phase of Mesozoic west-directed thrusting. This sequence was subsequently exhumed in the Tertiary along an east-dipping low-angle shear zone with a similar trajectory to the earlier thrust fault. Two 40Ar/39Ar thermochronology studies [Lee and Sutter, 1991; Lee, 1995] suggested that during exhumation, footwall rocks cooled differentially from east to west within a single structural horizon. Their thermal data indicated that the northwestern flank of the range dropped below 300°C by 46 Ma and reached 115°C by 20 Ma, whereas the eastern parts of the range were above 300°C until 19 Ma. Assuming that isotherms were subhorizontal during exhumation, this led Lee [1995] to postulate that the footwall rocks were exhumed by a rolling hinge fault system [Buck, 1988; Wernicke and Axen, 1988] in which the 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 hangingwall denudation. 2.3 Previous footwall studies 2.3.1 Metamorphic petrology and geochronology Miller et al. [1988] recognized three metamorphic and deformational events in the lower plate rocks of the northern Snake Range. The first (M1), of Jurassic age, is limited to the southernmost part of the range where Lower Cambrian quartzites and metapelites have been intruded and contact metamorphosed by a mid-Jurassic |
