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91
lower hinge, which creates a top to the east sense of shear, and second through an upper
hinge, with an opposite, top to the west sense of shear (Figure 3.13). The problem with
this model is that the west-directed structures would have formed near the surface,
and hence most probably in the brittle field, but this is inconsistent with the calculated
temperatures and observed ductile style of west-directed deformation.
(2) The mylonite zone represents counterflow of the deep crust toward an
extensional breakaway zone, driven by a pressure gradient between the breakaway and
the surrounding terrain [e.g. Block and Royden, 1990; McKenzie et al., 2000; McKenzie
and Jackson, 2002; Wernicke, 1992]. This type of large-scale flow of the lower crust
probably requires temperatures higher than the 350–430°C determined in Marble Wash,
however. It would also predict an east-directed shear sense, and therefore does not
account for the observed west-directed shear. MacCready et al. [1997] described a late
Eocene-Oligocene flow pattern in the migmatitic infrastructure immediately beneath
West East
Upper hinge:
West-directed
structures overprint
older east-directed
structures
Lower hinge:
East-directed
structures
Figure 3.13 Schematic illustration to show how east- and west-directed structures could have
formed as the footwall passed through two hinges with opposing senses of displacement during
Tertiary exhumation. East-directed structures form early during passage through the lower
hinge, and are subsequently overprinted by west-directed structures as the footwall passes
through the upper hinge. Modified from Axen et al. [1995].
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 106 |
| Contributing entity | University of Southern California |
| Repository email | cisadmin@lib.usc.edu |
| Full text | 91 lower hinge, which creates a top to the east sense of shear, and second through an upper hinge, with an opposite, top to the west sense of shear (Figure 3.13). The problem with this model is that the west-directed structures would have formed near the surface, and hence most probably in the brittle field, but this is inconsistent with the calculated temperatures and observed ductile style of west-directed deformation. (2) The mylonite zone represents counterflow of the deep crust toward an extensional breakaway zone, driven by a pressure gradient between the breakaway and the surrounding terrain [e.g. Block and Royden, 1990; McKenzie et al., 2000; McKenzie and Jackson, 2002; Wernicke, 1992]. This type of large-scale flow of the lower crust probably requires temperatures higher than the 350–430°C determined in Marble Wash, however. It would also predict an east-directed shear sense, and therefore does not account for the observed west-directed shear. MacCready et al. [1997] described a late Eocene-Oligocene flow pattern in the migmatitic infrastructure immediately beneath West East Upper hinge: West-directed structures overprint older east-directed structures Lower hinge: East-directed structures Figure 3.13 Schematic illustration to show how east- and west-directed structures could have formed as the footwall passed through two hinges with opposing senses of displacement during Tertiary exhumation. East-directed structures form early during passage through the lower hinge, and are subsequently overprinted by west-directed structures as the footwall passes through the upper hinge. Modified from Axen et al. [1995]. |
