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65
CHAPTER 3:
Opposing senses of shear in a sub-detachment mylonite zone:
implications for core complex mechanics
Abstract
Global studies of metamorphic core complexes and low-angle detachment faults
have highlighted a fundamental problem: since detachments excise crustal section, the
relationship between their mylonitic footwall and brittle hangingwall rocks is often
unclear. Mylonites can either reflect ductile deformation related to exhumation along
the detachment fault, or they can be a more general feature of the extending middle
crust that has been “captured” by the detachment. In the first case we would expect the
kinematics of the mylonite zone to mirror the sense of movement on the detachment; in
the second case both the direction and sense of shear in the mylonites could vary.
The northern Snake Range décollement (NSRD) is a classic Basin and Range
detachment fault with a well-documented top-east sense of displacement. Here we
present structural, paleomagnetic, geochronological and geothermometric evidence to
suggest that the NSRD mylonite zone experienced at least two phases of deformation,
inconsistent with movement along a single fault. We, therefore, propose that footwall
mylonites represent a pre-existing high-strain middle-crustal discontinuity that separated
localized deformation above from distributed crustal flow below (Localized-Distributed
Transition — LDT). The mylonites were subsequently captured by a moderately-dipping
brittle NSRD that soled down to the middle crust and exhumed them around
a rolling hinge into a subhorizontal orientation at the surface. In this interpretation
the brittle hangingwall represents a series of rotated normal fault blocks, whereas the
ductile footwall represents one or more exhumed LDTs.
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 80 |
| Contributing entity | University of Southern California |
| Repository email | cisadmin@lib.usc.edu |
| Full text | 65 CHAPTER 3: Opposing senses of shear in a sub-detachment mylonite zone: implications for core complex mechanics Abstract Global studies of metamorphic core complexes and low-angle detachment faults have highlighted a fundamental problem: since detachments excise crustal section, the relationship between their mylonitic footwall and brittle hangingwall rocks is often unclear. Mylonites can either reflect ductile deformation related to exhumation along the detachment fault, or they can be a more general feature of the extending middle crust that has been “captured” by the detachment. In the first case we would expect the kinematics of the mylonite zone to mirror the sense of movement on the detachment; in the second case both the direction and sense of shear in the mylonites could vary. The northern Snake Range décollement (NSRD) is a classic Basin and Range detachment fault with a well-documented top-east sense of displacement. Here we present structural, paleomagnetic, geochronological and geothermometric evidence to suggest that the NSRD mylonite zone experienced at least two phases of deformation, inconsistent with movement along a single fault. We, therefore, propose that footwall mylonites represent a pre-existing high-strain middle-crustal discontinuity that separated localized deformation above from distributed crustal flow below (Localized-Distributed Transition — LDT). The mylonites were subsequently captured by a moderately-dipping brittle NSRD that soled down to the middle crust and exhumed them around a rolling hinge into a subhorizontal orientation at the surface. In this interpretation the brittle hangingwall represents a series of rotated normal fault blocks, whereas the ductile footwall represents one or more exhumed LDTs. |
