Anisotropy of Magnetic Susceptibility in the Brevard Shear Zone, North Carolina

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Anisotropy of Magnetic Susceptibility in the
Brevard Shear Zone, North Carolina
JoAnn Gage Bryn Mawr College Geology Department
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Project Goals

• Determine relationship between observed strain
  gradient in the Brevard zone and the Anisotropy
  of Magnetic Susceptibility ellipsoid
• Assess the potential for using AMS as a proxy
  for finite strain in shear zones

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Outline

• Rock Magnetism
• Anisotropy of Magnetic Susceptibility
• Shear Zones
• The Brevard Zone
• Analytical Methods
• Results/Conclusions

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Rock Magnetism
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Magnetic Susceptibility
Random orientation of magnetic domains
Applied Magnetic Field
Magnetic domains line up in an applied field
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Types of Magnetism
Magnetization remains when applied field is
removed.
Magnetization disappears when applied field is
removed.
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Anisotropy of Magnetic Susceptibility(AMS)

• The variation of magnetic susceptibility with
  orientation

Ellipsoid Shape
Prolate
Oblate
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How Is AMS Useful?

• Rocks have a primary magnetic fabric that is a
  result of their formation
• If that rock undergoes deformation the primary
  magnetic fabric is overprinted by a new or
  secondary magnetic fabric as a result of the
  strain

Undeformed State Primary Fabric
Deformed State Secondary Fabric
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Shear Zones

• Shear zones are zones of highly strained rocks
  that form at deeper crustal levels of a fault

Upper Crust
Brittle
Ductile
Lower Crust
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Brevard Shear Zone
ROSMAN
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Brevard Zone and Deformation
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Sample Locations
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Sample Collection and Preparation
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AnalyticalMethods
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Compositional Analysis

• Bulk Mineralogy

• X-Ray Diffraction

• Thin Section point counts

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Compositional Analysis

• Ferromagnetic Mineralogy
• Isothermal Remanent Magnetization Acquisition

Applied Field
1.0
0.9
0.8
0.7
0.6
BZ-06
BZ-07
Normalized Intensity (A/m)
0.5
BZ-21
0.4
0.3
0.2
0.1
0.0
0
500
1000
1500
2000
2500
3000
Applied Field (mT)
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Compositional Analysis

• Ferromagnetic Mineralogy
• 3-Axis Thermal Demagnetization of IRM

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Compositional Analysis

• Ferromagnetic Mineralogy
• Temperature Dependent Susceptibility

BZ-36
2.5
2
1.5
Normalizied Susceptibility
1
0.5
0
-200
-100
0
100
200
300
400
500
600
700
800
Temperature (degrees Celcius)
-0.5
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AMS Results
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AMS ResultsAll Data
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Filtering Criteria
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AMS ResultsFiltered Data
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AMS ResultsEllipsoid Shape
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Conclusions

• Deformation in the Brevard Zone produces both
  mineralogic change and grain size reduction
• Detailed mineralogic information at the site
  level is needed to properly correlate the AMS
  ellipsoid with the deformation gradient across
  the Brevard Zone
• Degree of anisotropy of AMS ellipsoid is
  correlative to the observed strain gradient in
  the Brevard zone

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Acknowledgements

• Dr. Arlo Weil, my advisor
• Dorothy Nepper Marshall fund and geology
  department Watson fund, for funding my research
• Dean Karen Tidmarsh, Professor Robert Dostal, and
  Professor Richard Hamilton, faculty advisors of
  the Marshall Program
• Kirsten Hawkins, Emily Jensen, Katherine Johnson,
  Kirbi Krisfalusi, Cristina Nistor, Risa Rice,
  Chelsea Rosenthal, and Shuba Sunder, my fellow
  Marshall Scholars