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47
each other out. This leaves just the uncertainties associated with analytical precision
and compositional heterogeneity, which are specific to each rock.
Therefore, in order to place precise constraints on the differences in pressure
and temperature between each of the samples, we applied the ΔPT method of Worley
and Powell [2000], which minimizes the contribution of systematic uncertainties
stemming from a-x models and thermodynamic data. ΔPT results were calculated
with THERMOCALC v. 3.31 allowing only for uncertainties in mineral heterogeneity.
Sample FHe269, which gave the most precisely constrained absolute P-T result,
was chosen as an anchor point, while the other six samples were calculated as P-T
differences relative to this sample (Table 2.5). These P-T differences were then plotted
in pressure-temperature space with respect to the absolute P-T conditions of sample
FHe269 (Figure 2.10c). Therefore, the position of all seven P-T points can shift by the
amount of the absolute uncertainties on the anchor point, FHe269 (indicated by grey
error bars in Figure 2.10c), but the relative differences between samples remain fixed.
2.4.2.3 Absolute P-T and ΔPT results
The absolute P-T results in Figure 2.10b and Table 2.4 form two groups,
distinguished by both pressure and temperature. Samples from Deadman Creek, Deep
Canyon, and Hampton Creek give an average pressure of 8.0 ± 0.9 kbar, equivalent
to a burial depth of ca. 30 km. This is in close agreement with the 8.1 ± 0.7 kbar
that Lewis et al. [1999] constrained for Hampton Creek (Figure 2.2). To the south of
Hampton Creek, samples from Hendry’s Creek and Silver Creek give a significantly
lower average pressure of 6.0 ± 0.9 kbar.
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 62 |
| Contributing entity | University of Southern California |
| Repository email | cisadmin@lib.usc.edu |
| Full text | 47 each other out. This leaves just the uncertainties associated with analytical precision and compositional heterogeneity, which are specific to each rock. Therefore, in order to place precise constraints on the differences in pressure and temperature between each of the samples, we applied the ΔPT method of Worley and Powell [2000], which minimizes the contribution of systematic uncertainties stemming from a-x models and thermodynamic data. ΔPT results were calculated with THERMOCALC v. 3.31 allowing only for uncertainties in mineral heterogeneity. Sample FHe269, which gave the most precisely constrained absolute P-T result, was chosen as an anchor point, while the other six samples were calculated as P-T differences relative to this sample (Table 2.5). These P-T differences were then plotted in pressure-temperature space with respect to the absolute P-T conditions of sample FHe269 (Figure 2.10c). Therefore, the position of all seven P-T points can shift by the amount of the absolute uncertainties on the anchor point, FHe269 (indicated by grey error bars in Figure 2.10c), but the relative differences between samples remain fixed. 2.4.2.3 Absolute P-T and ΔPT results The absolute P-T results in Figure 2.10b and Table 2.4 form two groups, distinguished by both pressure and temperature. Samples from Deadman Creek, Deep Canyon, and Hampton Creek give an average pressure of 8.0 ± 0.9 kbar, equivalent to a burial depth of ca. 30 km. This is in close agreement with the 8.1 ± 0.7 kbar that Lewis et al. [1999] constrained for Hampton Creek (Figure 2.2). To the south of Hampton Creek, samples from Hendry’s Creek and Silver Creek give a significantly lower average pressure of 6.0 ± 0.9 kbar. |
