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33
to have experienced two phases of growth. The core is marked by flat Fe, Mg, Ca, Mn
and Fe/(Fe+Mg) profiles, perhaps indicating the loss of original growth zoning through
high-temperature diffusional re-equilibration [Spear, 1993; Woodsworth, 1977]. The
rims of the garnet, on the other hand, show a more normal, though still rather flat,
prograde zonation profile. At the very outer margins of the garnet (c. 50 μm) there is an
inflection in chemical compositions, particularly Fe and Mg, indicating re-equilibration
by retrogressive reactions [Grant and Weiblen, 1971; Spear, 1993]. Inclusion trails
within the garnet mirror the chemical profile: the core of the garnet is rich with aligned
inclusions of quartz and opaque oxides that cross the garnet at a high angle to the
external mylonitic foliation. The rim is relatively inclusion free, though a few inclusions
indicate a slight reorientation of the internal fabric (Figure 2.7a,b).
(2) Sample FHa270 (Hampton Creek)
This sample was collected from a locality close to that of sample 3 in Lewis et al.
[1999] (Figure 2.2), providing an opportunity to directly compare our thermobarometric
results with those of the earlier study.
The sample is more heavily retrogressed than FDC62, making it even more
important to thoroughly characterize both the textural and geochemical relationships
between minerals. Figure 2.8a illustrates the typical textural context for thermobarometric
analysis. Garnet and plagioclase are present as large, rigid porphyroblasts that the later
M3 mylonitic fabric wraps around. The garnet is heavily fractured and X-ray mapping
shows that it has been eroded on two sides, removing the outer part of its zonation profile
(Figure 2.8b). A detailed analytical traverse reveals a relatively flat prograde zonation
profile, similar to sample FDC62, though there is no evidence for more than one phase
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 48 |
| Contributing entity | University of Southern California |
| Repository email | cisadmin@lib.usc.edu |
| Full text | 33 to have experienced two phases of growth. The core is marked by flat Fe, Mg, Ca, Mn and Fe/(Fe+Mg) profiles, perhaps indicating the loss of original growth zoning through high-temperature diffusional re-equilibration [Spear, 1993; Woodsworth, 1977]. The rims of the garnet, on the other hand, show a more normal, though still rather flat, prograde zonation profile. At the very outer margins of the garnet (c. 50 μm) there is an inflection in chemical compositions, particularly Fe and Mg, indicating re-equilibration by retrogressive reactions [Grant and Weiblen, 1971; Spear, 1993]. Inclusion trails within the garnet mirror the chemical profile: the core of the garnet is rich with aligned inclusions of quartz and opaque oxides that cross the garnet at a high angle to the external mylonitic foliation. The rim is relatively inclusion free, though a few inclusions indicate a slight reorientation of the internal fabric (Figure 2.7a,b). (2) Sample FHa270 (Hampton Creek) This sample was collected from a locality close to that of sample 3 in Lewis et al. [1999] (Figure 2.2), providing an opportunity to directly compare our thermobarometric results with those of the earlier study. The sample is more heavily retrogressed than FDC62, making it even more important to thoroughly characterize both the textural and geochemical relationships between minerals. Figure 2.8a illustrates the typical textural context for thermobarometric analysis. Garnet and plagioclase are present as large, rigid porphyroblasts that the later M3 mylonitic fabric wraps around. The garnet is heavily fractured and X-ray mapping shows that it has been eroded on two sides, removing the outer part of its zonation profile (Figure 2.8b). A detailed analytical traverse reveals a relatively flat prograde zonation profile, similar to sample FDC62, though there is no evidence for more than one phase |
