SAGE 1998-2001Integrated Magnetotellurics

Derrick Hasterok

University of Utah

Thursday, July 12, 2000

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

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

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

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?

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 %

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)

Water at great depths

How does the water get down there?

• Meteoric– ground water circulation

• Metamorphic dehydration

• Sub-crustal– mantle and magma degassing

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:

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

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)

• 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

2D Inversions 1998 to 2001 data

Distance (km)N60W N60E

2000 soundings (rotation = N60E)

RMS = 1.4462

0

35

15

Dep

th (

km)

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)

2D Inversions 1998 to 2001 data

Distance (km)

Dep

th (

km)

N45W N45E

1998 soundings (rotation = N45E)

RMS = 3.0007

0

35

15

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