SAGE 19982001 Integrated Magnetotellurics

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SAGE 1998-2001Integrated Magnetotellurics

• Derrick Hasterok
• University of Utah
• Thursday, July 12, 2000

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Topics for Discussion

• Mid-crustal conductor (MCC)
• physical properties
• possible causes
• implications on reology
• nature of MCC from integrated 1998 to 2001 SAGE
  MT data
• Results from 2000 work
• Integrated results 1998-2001
• Deep electrical structure beneath the Santo
• Domingo and Española basins

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Mid-Crustal Conductor

• What is the MCC?
• A widespread (world-wide?) conductive layer at
  great depth
• May correspond to the brittle-ductile transition
  zone or an isotherm (350º - 650º C)
• May correspond to similar depth as seismic
  reflectors

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Physical Properties and Depths

• Resistivities of MCC are lower under active
  tectonic regions and occur at shallower depths
• Most dry rocks expected at great depth have high
  resistivities gt103 ?-m

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Mid-Crustal Conductor

• Possible causes
• Magma (probably not cause in Rio Grande rift)
• Hot mineralized (saline) water (perhaps)
• Graphite, Ilmenite, Pyhrrotite, Pyrite and other
  conductive solid phase minerals
• Must be interconnected. How do you get
  interconnectivity?
• Dihedral angle (What is this?)
• What is the porosity necessary?

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Interconnecting Fluid (porosity)

• Porosity is determined by Archies Law
• rrock a rmat f-m
• r resistivity
• f porosity
• m cementation factor
• approximation
• m 1 for a thin film
• f 1.4 rmat/rrock

• Porosity of fluid (rrock 10 ?)
• magma
• rmat 0.5 ?-m
• f 7
• hot saline
• rmat 0.01 ?-m
• f 0.14
• graphite
• rmat 0.5 ?-m
• f 1.4x10-5

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Interconnecting Fluid(dihedral angle)

• What is the dihedral angle (?)?
• the angle of intersection between the rock grains
  and fluid contacts
• governed by type of fluid and solids
• for interconnectivity 60? ? ? ? 0? (for most
  fluids)

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Water at great depths

• How does the water get down there?

• Meteoric
• ground water circulation
• Metamorphic dehydration
• Sub-crustal
• mantle and magma degassing

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Water at great depth (cont.)

• More discussion on water

• Water depth corresponds to brittle-ductile
  transition zone
• can move laterally very rapidly
• pore geometry prevents rapid assent of water
• Water must be in P-T equilibrium with retrograde
  metamorphism

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Graphite

• Where does the graphite come from?
• The graphite comes from reduction of CO2
• Could be result of P-T conditions (i.e. MCC is
  result of P-T isotherm)
• Where does the CO2 originate?
• CO2 is present in magmas and the mantle and
  produced during some metamorphic reactions

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1998 and 2000 Integrated Model
2-D Inversion of 1998 and 2000 MT Soundings (TE
and TM)
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Depth (km)
15
35
Distance (km)
N45W
S45E
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2D Inversions of 98-01 MT Data

• Stations differentially rotated (polar plots at
  long period)
• 1998 - N45E
• 1999 - 2001 - N50W
• 2000 - N60E
• Rotations roughly correspond to gravity strike on
  west side of line.
• Station s0102 not included because of possible 3D
  effects (i.e. Cerrillos Hills)
• Station s9902 not used because of bad data

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2D Inversions 1998 to 2001 data
2000 soundings (rotation N60E)
0
15
Depth (km)
35
Distance (km)
N60W
N60E
RMS 1.4462
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2D Inversions 1998 to 2001 data
1999 and 2001 soundings (rotation N50W)
0
15
Depth (km)
35
Distance (km)
N50E
N50W
RMS 3.3235
15
2D Inversions 1998 to 2001 data
1998 soundings (rotation N45E)
0
15
Depth (km)
35
Distance (km)
N45W
N45E
RMS 3.0007
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Conclusions

• SAGE 1998 to 2001 MT data
• Mid-Crustal Conductor
• Depth of MCC decreases from west to east
• Resistivity of MCC increases from west to east
• Cause
• hot saline water?
• graphite?
• not melt
• Move off active rift on east side of profile