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113
CHAPTER 5:
Future work
5.1 Introduction
The work presented in this thesis places new constraints on both the Mesozoic
burial and Tertiary exhumation of the northern Snake Range footwall. However, in
both cases, the data is restricted by limitations in the applied techniques (such as a lack
of suitable mineral assemblages for thermobarometry), or the absence of mylonitic
structures with, for example, good rheological contrasts or datable markers. However,
there are a number of methods that would provide additional information, but have
been beyond the scope of this thesis. Here, therefore, I outline three projects that could
build on the work presented in this thesis, and which I hope to pursue in the future.
Firstly, in order to expand on the thermobarometry results, I propose a
comparative study of peak metamorphic temperatures across the northern Snake Range,
southern Snake Range and Schell Creek Range. Peak temperatures would be derived
from the crystallinity of carbonaceous material in metasedimentary units, which has
the advantage of being applicable to both metapelites and carbonates, and does not
require a specific mineral assemblage.
Secondly, I propose to investigate the P-T evolution of the northern Snake Range
footwall during exhumation using a combination of chlorite-phengite thermobarometry
and 40Ar/39Ar thermochronology. This would place important constraints on the
geometry of the footwall during exhumation, and thus on the two-step model proposed
in this thesis.
Thirdly, I propose a range-wide study of footwall mylonites in order to
determine whether the mylonitic deformation we see in Marble Wash is different
from the deformation seen further west. In particular, I would like to investigate
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 128 |
| Contributing entity | University of Southern California |
| Repository email | cisadmin@lib.usc.edu |
| Full text | 113 CHAPTER 5: Future work 5.1 Introduction The work presented in this thesis places new constraints on both the Mesozoic burial and Tertiary exhumation of the northern Snake Range footwall. However, in both cases, the data is restricted by limitations in the applied techniques (such as a lack of suitable mineral assemblages for thermobarometry), or the absence of mylonitic structures with, for example, good rheological contrasts or datable markers. However, there are a number of methods that would provide additional information, but have been beyond the scope of this thesis. Here, therefore, I outline three projects that could build on the work presented in this thesis, and which I hope to pursue in the future. Firstly, in order to expand on the thermobarometry results, I propose a comparative study of peak metamorphic temperatures across the northern Snake Range, southern Snake Range and Schell Creek Range. Peak temperatures would be derived from the crystallinity of carbonaceous material in metasedimentary units, which has the advantage of being applicable to both metapelites and carbonates, and does not require a specific mineral assemblage. Secondly, I propose to investigate the P-T evolution of the northern Snake Range footwall during exhumation using a combination of chlorite-phengite thermobarometry and 40Ar/39Ar thermochronology. This would place important constraints on the geometry of the footwall during exhumation, and thus on the two-step model proposed in this thesis. Thirdly, I propose a range-wide study of footwall mylonites in order to determine whether the mylonitic deformation we see in Marble Wash is different from the deformation seen further west. In particular, I would like to investigate |
