sage 1998-2001 integrated magnetotellurics derrick hasterok university of utah thursday, july 12,...
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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 >103 -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 Archie’s Law:
rock = a mat -m
– = resistivity
– = porosity
– m = cementation factor
• approximation– m = 1 for a thin film
– = 1.4 mat/rock
• Porosity of fluid(rock = 10 )
– magma• mat = 0.5 -m
• = 7 %
– hot saline• mat = 0.01 -m
• = 0.14 %
– graphite• mat = 0.5 -m
• = 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 (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.)
• 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
More discussion on water:
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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
Distance (km)
2-D Inversion of 1998 and 2000 MT Soundings (TE and TM)
N45W S45E
0
35
15
Dep
th (
km)
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• 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
2D Inversions of 98-01 MT Data
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2D Inversions 1998 to 2001 data
Distance (km)N60W N60E
2000 soundings (rotation = N60E)
RMS = 1.4462
0
35
15
Dep
th (
km)
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2D Inversions 1998 to 2001 data
Distance (km)N50E N50W
1999 and 2001 soundings (rotation = N50W)
RMS = 3.3235
0
35
15
Dep
th (
km)
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2D Inversions 1998 to 2001 data
Distance (km)
Dep
th (
km)
N45W N45E
1998 soundings (rotation = N45E)
RMS = 3.0007
0
35
15
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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