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35
grew during the lower temperature M3 event. This relationship is also illustrated by
the reddish-brown coloration of matrix biotites, compared with the greener-coloured
retrogressive biotites. Therefore, although thermobarometry should usually be carried
out on minerals in mutual contact, that is not the case for this sample.
(3) Sample FHe269 (Hendry’s Creek)
Unlike the two previous amphibolite-grade samples, this sample contains a
greenschist-grade mineral assemblage. Although chlorite is present in garnet pressure
shadows (Figure 2.9a), suggestive of later M3 growth, it also coexists with biotite in the
matrix, suggesting growth together in stable equilibrium (Figure 2.9d). A comparison
between chlorite in the garnet pressure shadows and chlorite in the matrix (Figure
2.9e) reveals a bimodal composition distribution, with pressure shadow chlorites
showing a lower Al and Fe/Mg content than the matrix chlorites. Al content is known
to increase with increasing temperature [e.g. Cathelineau and Nieva, 1985], and so
this compositional distinction suggests that the matrix chlorites may have grown under
higher temperature conditions, and thus earlier, than the pressure shadow chlorites.
Although there is also a difference in Fe/Mg content, Fe/Mg is poorly correlated with
temperature, and this difference may reflect changes in the composition of the reacting
volume between M2 and M3 metamorphism.
The garnet X-ray map and traverse in Figure 2.9b and c indicate a normal
prograde zonation profile, suggesting little or no high-temperature re-equilibration, and
thus formation under relatively lower temperatures than samples FDC62 and FHa270.
Neither this sample, nor the other Hendry’s Creek sample, FHe9, contains staurolite.
There is staurolite within the same unit in the middle reaches of Hendry’s Creek, but
this probably reflects exposure of the deeper levels of the unit. None of the samples that
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 50 |
| Contributing entity | University of Southern California |
| Repository email | cisadmin@lib.usc.edu |
| Full text | 35 grew during the lower temperature M3 event. This relationship is also illustrated by the reddish-brown coloration of matrix biotites, compared with the greener-coloured retrogressive biotites. Therefore, although thermobarometry should usually be carried out on minerals in mutual contact, that is not the case for this sample. (3) Sample FHe269 (Hendry’s Creek) Unlike the two previous amphibolite-grade samples, this sample contains a greenschist-grade mineral assemblage. Although chlorite is present in garnet pressure shadows (Figure 2.9a), suggestive of later M3 growth, it also coexists with biotite in the matrix, suggesting growth together in stable equilibrium (Figure 2.9d). A comparison between chlorite in the garnet pressure shadows and chlorite in the matrix (Figure 2.9e) reveals a bimodal composition distribution, with pressure shadow chlorites showing a lower Al and Fe/Mg content than the matrix chlorites. Al content is known to increase with increasing temperature [e.g. Cathelineau and Nieva, 1985], and so this compositional distinction suggests that the matrix chlorites may have grown under higher temperature conditions, and thus earlier, than the pressure shadow chlorites. Although there is also a difference in Fe/Mg content, Fe/Mg is poorly correlated with temperature, and this difference may reflect changes in the composition of the reacting volume between M2 and M3 metamorphism. The garnet X-ray map and traverse in Figure 2.9b and c indicate a normal prograde zonation profile, suggesting little or no high-temperature re-equilibration, and thus formation under relatively lower temperatures than samples FDC62 and FHa270. Neither this sample, nor the other Hendry’s Creek sample, FHe9, contains staurolite. There is staurolite within the same unit in the middle reaches of Hendry’s Creek, but this probably reflects exposure of the deeper levels of the unit. None of the samples that |
