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48
Within the 2σ confidence limits, the results imply a drop in pressure of between
0.2 and 3.6 kbar (equivalent to a depth difference of 0.8 to 13.6 km) from Hampton
Creek to Hendry’s Creek over a north-south distance of only 3 km.
Temperatures are less consistent within the two groups. Of the higher-pressure
group, the two most northerly samples, from Deadman Creek and Deep Canyon, give
the highest average temperature of 652 ± 64°C, while the two Hampton Creek samples
give a lower temperature of 573 ± 58°C. It is probable that this temperature difference is
related to the proximity of Deadman Creek and Deep Canyon to Cretaceous-age granite
and pegmatite intrusions (Figure 2.2). In the lower pressure group, the most southerly
sample, from Silver Creek gives a relatively high temperature of 548 ± 69°C, whereas
the Hendry’s Creek samples to the north give a lower average temperature of 507 ±
53°C. However, this temperature difference may simply reflect the large uncertainties,
since the temperatures overlap within the confidence limits.
The ΔPT results (Figure 2.10c; Table 2.5) significantly reduce the uncertainties
on the P-T determinations, placing better constraints on the grouping of the data and
the relative differences between each of the P-T points. In Figure 10c, the ΔPT results
for six of the samples are plotted relative to the absolute P-T determination for sample
FHe269. Sample FHe269 was chosen as an anchor point because it has the smallest
absolute uncertainty. Therefore, the position of all the points can shift by the amount
given by the grey error bars on the anchor point, FHe269. The reduced uncertainties
on the ΔPT results remove the overlap in pressure between the two groups of data,
suggesting that there is a real difference between the two. A recalculation of the pressure
drop from Hampton Creek to Hendry’s Creek gives a more precise result of 0.7 to 3.1
kbar, equivalent to a depth difference of 2.6 to 11.7 km across a north-south distance
of 3 km.
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 63 |
| Contributing entity | University of Southern California |
| Repository email | cisadmin@lib.usc.edu |
| Full text | 48 Within the 2σ confidence limits, the results imply a drop in pressure of between 0.2 and 3.6 kbar (equivalent to a depth difference of 0.8 to 13.6 km) from Hampton Creek to Hendry’s Creek over a north-south distance of only 3 km. Temperatures are less consistent within the two groups. Of the higher-pressure group, the two most northerly samples, from Deadman Creek and Deep Canyon, give the highest average temperature of 652 ± 64°C, while the two Hampton Creek samples give a lower temperature of 573 ± 58°C. It is probable that this temperature difference is related to the proximity of Deadman Creek and Deep Canyon to Cretaceous-age granite and pegmatite intrusions (Figure 2.2). In the lower pressure group, the most southerly sample, from Silver Creek gives a relatively high temperature of 548 ± 69°C, whereas the Hendry’s Creek samples to the north give a lower average temperature of 507 ± 53°C. However, this temperature difference may simply reflect the large uncertainties, since the temperatures overlap within the confidence limits. The ΔPT results (Figure 2.10c; Table 2.5) significantly reduce the uncertainties on the P-T determinations, placing better constraints on the grouping of the data and the relative differences between each of the P-T points. In Figure 10c, the ΔPT results for six of the samples are plotted relative to the absolute P-T determination for sample FHe269. Sample FHe269 was chosen as an anchor point because it has the smallest absolute uncertainty. Therefore, the position of all the points can shift by the amount given by the grey error bars on the anchor point, FHe269. The reduced uncertainties on the ΔPT results remove the overlap in pressure between the two groups of data, suggesting that there is a real difference between the two. A recalculation of the pressure drop from Hampton Creek to Hendry’s Creek gives a more precise result of 0.7 to 3.1 kbar, equivalent to a depth difference of 2.6 to 11.7 km across a north-south distance of 3 km. |
