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77
50 μm 50 μm
(a) (b)
Chl
Plag
Chl
Hbl
Plag
Chl
Hbl
Figure 3.7 Photomicrographs of (a) dike 1 and (b) dike 2 in plane polarized light, illustrating
the crystalline igneous texture of the samples.
Standard cylindrical cores of 2.5 cm diameter and 10 cm length were later cut into 2.25
cm length samples.
The dikes are basaltic, with a primary assemblage of fine- to medium-grained
hornblende + biotite + plagioclase ± quartz, overprinted by later greenschist
metamorphism and associated growth of chlorite (Figure 3.7). Samples collected
for paleomagnetic analysis have an undeformed crystalline igneous texture and
correspondingly low anisotropies of magnetic susceptibility (AMS) of ~1.01–1.02%
(Table 3.1).
Samples were analyzed using a 2G cryogenic magnetometer at Occidental
College, Los Angeles. The stable characteristic components of remanent magnetization
(ChRM) were isolated using stepwise thermal and alternating field (AF) demagnetization
procedures. During the thermal demagnetization procedure each sample was typically
demagnetized with 50°C steps below 500°C and 25°C steps until the intensity of
magnetization fell below the measuring range of the magnetometer. Stepwise AF
demagnetization was carried out with 2.5 mT steps from 0 to 5 mT; 5 mT steps from 5
to 40 mT; and 10 mT steps from 40 to 140 mT (Figure 3.8).
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 92 |
| Contributing entity | University of Southern California |
| Repository email | cisadmin@lib.usc.edu |
| Full text | 77 50 μm 50 μm (a) (b) Chl Plag Chl Hbl Plag Chl Hbl Figure 3.7 Photomicrographs of (a) dike 1 and (b) dike 2 in plane polarized light, illustrating the crystalline igneous texture of the samples. Standard cylindrical cores of 2.5 cm diameter and 10 cm length were later cut into 2.25 cm length samples. The dikes are basaltic, with a primary assemblage of fine- to medium-grained hornblende + biotite + plagioclase ± quartz, overprinted by later greenschist metamorphism and associated growth of chlorite (Figure 3.7). Samples collected for paleomagnetic analysis have an undeformed crystalline igneous texture and correspondingly low anisotropies of magnetic susceptibility (AMS) of ~1.01–1.02% (Table 3.1). Samples were analyzed using a 2G cryogenic magnetometer at Occidental College, Los Angeles. The stable characteristic components of remanent magnetization (ChRM) were isolated using stepwise thermal and alternating field (AF) demagnetization procedures. During the thermal demagnetization procedure each sample was typically demagnetized with 50°C steps below 500°C and 25°C steps until the intensity of magnetization fell below the measuring range of the magnetometer. Stepwise AF demagnetization was carried out with 2.5 mT steps from 0 to 5 mT; 5 mT steps from 5 to 40 mT; and 10 mT steps from 40 to 140 mT (Figure 3.8). |
