1. SAGE 1998-2001 Integrated Magnetotellurics Derrick Hasterok University of Utah Thursday, July 12, 2000

2. 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

3. 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

4. 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 >10 3  -m

5. 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?

6. 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 %

7. 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)

8. Water at great depths How does the water get down there? Meteoric –ground water circulation Metamorphic dehydration Sub-crustal –mantle and magma degassing

9. 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:

10. Graphite Where does the graphite come from? –The graphite comes from reduction of CO 2 –Could be result of P-T conditions (i.e. MCC is result of P-T isotherm) Where does the CO 2 originate? –CO 2 is present in magmas and the mantle and produced during some metamorphic reactions

11. 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)

12. 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

13. 2D Inversions 1998 to 2001 data Distance (km)N60W N60E 2000 soundings (rotation = N60E) RMS = 1.4462 0 35 15 Depth (km)

14. 2D Inversions 1998 to 2001 data Distance (km)N50EN50W 1999 and 2001 soundings (rotation = N50W) RMS = 3.3235 0 35 15 Depth (km)

15. 2D Inversions 1998 to 2001 data Distance (km) Depth (km) N45W N45E 1998 soundings (rotation = N45E) RMS = 3.0007 0 35 15

16. 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