SAGE 1998 2001 Integrated Magnetotellurics Derrick Hasterok University

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: 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. 4 x 10 -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 0 (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. ) 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

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

1998 and 2000 Integrated Model 2 -D Inversion of 1998 and 2000 MT Soundings (TE and TM) Depth (km) 0 15 35 N 45 W Distance (km) S 45 E

2 D Inversions of 98 -01 MT Data • Stations differentially rotated (polar plots at long period) – 1998 - N 45 E – 1999 - 2001 - N 50 W – 2000 - N 60 E • Rotations roughly correspond to gravity strike on west side of line. • Station s 0102 not included because of possible 3 D effects (i. e. Cerrillos Hills) • Station s 9902 not used because of bad data

2 D Inversions 1998 to 2001 data 2000 soundings (rotation = N 60 E) Depth (km) 0 15 35 N 60 W Distance (km) N 60 E RMS = 1. 4462

2 D Inversions 1998 to 2001 data 1999 and 2001 soundings (rotation = N 50 W) Depth (km) 0 15 35 N 50 E Distance (km) N 50 W RMS = 3. 3235

2 D Inversions 1998 to 2001 data 1998 soundings (rotation = N 45 E) Depth (km) 0 15 35 N 45 W Distance (km) N 45 E RMS = 3. 0007

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