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99
stretching. Footwall rocks commonly exhibit an upward increase in ductile strain,
culminating in a zone of mylonitic deformation directly beneath the detachment. This
mylonitic deformation could, therefore, form as a direct result of shear along the deeper
portions of the detachment fault in the ductile crust. However, in some examples, such
as the Whipple Mountains core complex in California, the mylonitic deformation and
brittle detachment fault separate in the up-dip direction, implying that the mylonite zone
is a pre-existing feature that has been captured and exhumed by a later fault [Davis,
1988; Lister and Davis, 1989]. These observations have led to considerable debate
about the role these faults play in exhuming footwall rocks, and in particular, how
mylonitic footwall deformation formed, and how it relates to the brittle detachment.
In some areas detachments appear to be implicated in exhumation of rocks from
depths of as much as 60 km, and perhaps much more [southern Norway: Andersen,
1998; the Aegean: Ring et al., 2001], but most models of detachment faults in Basin
and Range core complexes regard them as exhuming rocks from just below the brittle-ductile
transition, by implication at a depth of around 15 km [Miller et al., 1983;
Davis, 1988; John and Foster, 1993]. Thermobarometry suggests, however, that peak
pressures in footwall rocks reached 8–10 kbar in the Funeral Mountains and the East
Humboldt Range [Hodges and Walker, 1990; Hodges and Walker, 1992; McGrew et
al., 2000], and 8 kbar in the northern Snake Range [Lewis et al., 1999; Cooper et
al., in prep.], equivalent to depths of 30–35 km. Peak metamorphic conditions in the
Basin and Range appear to have been reached in Late Cretaceous time [Anderson et
al., 1988; Miller and Gans, 1989; Cooper et al., in prep.], 40–50 million years before
the initiation of the detachments seen at the surface today. Therefore, what role did the
detachments play in the exhumation of these deep-seated rocks, and on what timescale
did exhumation occur?
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 114 |
| Contributing entity | University of Southern California |
| Repository email | cisadmin@lib.usc.edu |
| Full text | 99 stretching. Footwall rocks commonly exhibit an upward increase in ductile strain, culminating in a zone of mylonitic deformation directly beneath the detachment. This mylonitic deformation could, therefore, form as a direct result of shear along the deeper portions of the detachment fault in the ductile crust. However, in some examples, such as the Whipple Mountains core complex in California, the mylonitic deformation and brittle detachment fault separate in the up-dip direction, implying that the mylonite zone is a pre-existing feature that has been captured and exhumed by a later fault [Davis, 1988; Lister and Davis, 1989]. These observations have led to considerable debate about the role these faults play in exhuming footwall rocks, and in particular, how mylonitic footwall deformation formed, and how it relates to the brittle detachment. In some areas detachments appear to be implicated in exhumation of rocks from depths of as much as 60 km, and perhaps much more [southern Norway: Andersen, 1998; the Aegean: Ring et al., 2001], but most models of detachment faults in Basin and Range core complexes regard them as exhuming rocks from just below the brittle-ductile transition, by implication at a depth of around 15 km [Miller et al., 1983; Davis, 1988; John and Foster, 1993]. Thermobarometry suggests, however, that peak pressures in footwall rocks reached 8–10 kbar in the Funeral Mountains and the East Humboldt Range [Hodges and Walker, 1990; Hodges and Walker, 1992; McGrew et al., 2000], and 8 kbar in the northern Snake Range [Lewis et al., 1999; Cooper et al., in prep.], equivalent to depths of 30–35 km. Peak metamorphic conditions in the Basin and Range appear to have been reached in Late Cretaceous time [Anderson et al., 1988; Miller and Gans, 1989; Cooper et al., in prep.], 40–50 million years before the initiation of the detachments seen at the surface today. Therefore, what role did the detachments play in the exhumation of these deep-seated rocks, and on what timescale did exhumation occur? |
