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62
The fact that this late Cretaceous structure is not evident at the surface today
suggests that the northern Snake Range footwall underwent differential exhumation
during Tertiary extension. This can be explained by significant ductile stretching and
thinning of the crust during exhumation [Miller et al., 1983 suggested 450–500%
extension based on upper plate brittle normal faulting], which would have flattened
the fold structure into a subhorizontal surface. If this is the case, then there should
be some duplication of stratigraphy where the two limbs overlapped. A true test of
this hypothesis, therefore, would be to drill down through the stratigraphy in Hendry’s
Creek, where the presence of a duplicated and overturned McCoy Creek group layer
would point towards preservation of the lower hinge at depth.
2.6 Conclusions
Thermobarometric results from metapelitic units within the northern Snake
Range footwall yield P-T estimates of 6–8 kbar and 500–650°C, equivalent to burial
depths of 23 to 30 km. In the WNW–ESE direction of footwall transport, there is no
change in burial depth, though there is a significant difference of ca. 2.4 kbar (ca. 9.1
km) across a north-south distance of ca. 3 km. We propose that this dramatic drop in
burial depth reflects the presence of a Mesozoic contractional structure, probably a
large-scale recumbent fold, which has since been obscured by stretching and thinning
during extensional exhumation. The lack of a burial gradient in the direction of
footwall transport indicates that footwall rocks, which are today subhorizontal in the
direction of fault transport, were also subhorizontal at depth in the late Cretaceous.
This suggests that the footwall was not tilted about the normal to the fault transport
direction during exhumation, precluding the low-angle shear zone models of Bartley
and Wernicke [1984] and Lewis et al. [1999]. The depth from which the footwall has
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 77 |
| Contributing entity | University of Southern California |
| Repository email | cisadmin@lib.usc.edu |
| Full text | 62 The fact that this late Cretaceous structure is not evident at the surface today suggests that the northern Snake Range footwall underwent differential exhumation during Tertiary extension. This can be explained by significant ductile stretching and thinning of the crust during exhumation [Miller et al., 1983 suggested 450–500% extension based on upper plate brittle normal faulting], which would have flattened the fold structure into a subhorizontal surface. If this is the case, then there should be some duplication of stratigraphy where the two limbs overlapped. A true test of this hypothesis, therefore, would be to drill down through the stratigraphy in Hendry’s Creek, where the presence of a duplicated and overturned McCoy Creek group layer would point towards preservation of the lower hinge at depth. 2.6 Conclusions Thermobarometric results from metapelitic units within the northern Snake Range footwall yield P-T estimates of 6–8 kbar and 500–650°C, equivalent to burial depths of 23 to 30 km. In the WNW–ESE direction of footwall transport, there is no change in burial depth, though there is a significant difference of ca. 2.4 kbar (ca. 9.1 km) across a north-south distance of ca. 3 km. We propose that this dramatic drop in burial depth reflects the presence of a Mesozoic contractional structure, probably a large-scale recumbent fold, which has since been obscured by stretching and thinning during extensional exhumation. The lack of a burial gradient in the direction of footwall transport indicates that footwall rocks, which are today subhorizontal in the direction of fault transport, were also subhorizontal at depth in the late Cretaceous. This suggests that the footwall was not tilted about the normal to the fault transport direction during exhumation, precluding the low-angle shear zone models of Bartley and Wernicke [1984] and Lewis et al. [1999]. The depth from which the footwall has |
