# Presentation: 485299

## SAGE 1998-2001Integrated 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

- Depth (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

- 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

- Depth (km)

## 2D Inversions 1998 to 2001 data

- Distance (km)

- N50E

- N50W

- 1999 and 2001 soundings (rotation = N50W)

- RMS = 3.3235

- 0

- 35

- 15

- Depth (km)

## 2D Inversions 1998 to 2001 data

- Distance (km)

- Depth (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