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103
part of the range is likely to be Eocene or older. Mylonitic deformation is predominantly
top-to-the-east [Gaudemer and Tapponnier, 1987], but there is some evidence for an
earlier coaxial history [Lee et al., 1987], and recent observations by Cooper et al. [in
review] present evidence for two phases of deformation: top-east overprinted by later
top-west.
4.3 Footwall geothermobarometry
If detachments originated as inclined normal faults penetrating the middle
or lower crust, and acted to exhume rock from those levels, then there should be a
clear-cut gradient in the original depth of burial of footwall rocks measured in the
direction of motion. If, on the other hand, detachments originated as subhorizontal
discontinuities in the ductile crust, then there should be no discernable gradient, even
if they are subsequently exhumed by a rolling-hinge mechanism.
A thermobarometric study by Cooper et al. [in prep.] of seven footwall samples
collected from metapelites within the McCoy Creek Group (Figure 4.1) yielded P-T
results of 6–8 kbar and 500–650°C, equivalent to burial depths of 23 to 30 km (Figure
4.2a). In the WNW–ESE direction of footwall transport, there is no discernable change
in pressure, and hence burial depth, but there is a significant difference of ca. 2.4 kbar
(ca. 9.1 km) across a north-south distance of ca. 3 km (Figure 4.2b).
The north-south variation in burial depth revealed by the thermobarometric
results suggests the presence of a Mesozoic contractional structure in the footwall
[probably a large recumbent fold: Cooper et al. [in prep.] that has been obscured or
eliminated by later deformation. The presence of footwall rocks at the surface today that
initially had different burial depths indicates that the footwall underwent differential
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 118 |
| Contributing entity | University of Southern California |
| Repository email | cisadmin@lib.usc.edu |
| Full text | 103 part of the range is likely to be Eocene or older. Mylonitic deformation is predominantly top-to-the-east [Gaudemer and Tapponnier, 1987], but there is some evidence for an earlier coaxial history [Lee et al., 1987], and recent observations by Cooper et al. [in review] present evidence for two phases of deformation: top-east overprinted by later top-west. 4.3 Footwall geothermobarometry If detachments originated as inclined normal faults penetrating the middle or lower crust, and acted to exhume rock from those levels, then there should be a clear-cut gradient in the original depth of burial of footwall rocks measured in the direction of motion. If, on the other hand, detachments originated as subhorizontal discontinuities in the ductile crust, then there should be no discernable gradient, even if they are subsequently exhumed by a rolling-hinge mechanism. A thermobarometric study by Cooper et al. [in prep.] of seven footwall samples collected from metapelites within the McCoy Creek Group (Figure 4.1) yielded P-T results of 6–8 kbar and 500–650°C, equivalent to burial depths of 23 to 30 km (Figure 4.2a). In the WNW–ESE direction of footwall transport, there is no discernable change in pressure, and hence burial depth, but there is a significant difference of ca. 2.4 kbar (ca. 9.1 km) across a north-south distance of ca. 3 km (Figure 4.2b). The north-south variation in burial depth revealed by the thermobarometric results suggests the presence of a Mesozoic contractional structure in the footwall [probably a large recumbent fold: Cooper et al. [in prep.] that has been obscured or eliminated by later deformation. The presence of footwall rocks at the surface today that initially had different burial depths indicates that the footwall underwent differential |
