Walter D. Mooney, Ph.D. US Geological Survey Menlo Park, California USA Lecture #10: Geoelectrical Studies of Lithospheric Structure IPRCC and SinoProbe Short Course: Lithospheric Evolution through Time April 8-12, 2011

Acknowledgement: Geoelectric studies of the lithosphere This lecture is by Prof. Alan G. Jones Dublin Institute for Advanced Studies Lecture presented as part of the Short Course: Integrated Studies of Lithospheric Evolution: A Global Perspective by Dr. Walter Mooney (USGS), November, 2010

Electro-magnetic (EM) methods (1)... give information about...give information about... –structures –presence of fluids and/or conducting metasediments –dimensionality (2D or 3D) –strike direction and its depth dependence

EM methods (2)... sense a physical parameter (electrical conductivity) that varies by over EIGHT orders of magnitudesense a physical parameter (electrical conductivity) that varies by over EIGHT orders of magnitude sense its lateral and vertical variationssense its lateral and vertical variations with natural sources, penetration to all depths is assured (but with decreasing resolution)with natural sources, penetration to all depths is assured (but with decreasing resolution)

Skin Depth…. Penetration to all depths is assured with MT - just need to record sufficiently long periods

Parameter measured: Parameter measured: electrical conductivity (1/resistivity) Range...

Conduction mechanisms in the mantle ionic conduction due to movement of mobile charged ions 1) partial melt 2) H + electronic conduction due to movement of electrons 3) hydrous mineral phase (e.g. phlogopite) 4) carbon on grain boundary films

Electronic conduction - graphite: AB

Partial melt… Partial melt connects efficiently and increases electrical conductivity by orders of Magnitude H 2 0-saline mix

Electronic conduction - sulphides: THO (NACP)

Why MT? Sensitivity to partial melt Resistivity decreases by orders of magnitude at onset of partial melt High sensitivity to e.g. base of lithosphere (LAB) Partial melt of silicate rocks (dry pyroxene granulite) Partzsch et al. (2000)

Why use electromagnetism? Image the base of the sub-continental lithospheric mantle Diamonds only exist in thick cold cratonic roots Kimberlites passing through such roots bring diamonds up

What can MT resolve?

Electrical compared with Seismic asthenosphere Excellent spatial correlation between presence of upper mantle low velocity zone and region of high conductivity (Alekseyev et al. 1977)

Slave craton: EM studies 1996: Profile 1 MT survey 1996: Profile 1 MT survey 1998,99 & 2000: Winter road surveys 1998,99 & 2000: Winter road surveys 1998/99 & 99/2000: Slave lakes 1998/99 & 99/2000: Slave lakes 2000: TGI survey

Source: Mooney et al., 1998 Depth to Moho

Tectonic Ages ArcheanProterozoicPhanerozoic 3Ga2Ga1Ga0Ga

SNORCLE Corridor 1

SNORCLE: Rae - Tibbit Flat reflection Moho beneath Anton complex at ~12 s TWTT (~36 km) Refraction Moho at km Teleseismic Moho at ~36 km YKRaeTibbit

Slave craton: Rae-YK-Tibbit profile Rae Yellowknife Site 106 Tibbit Lake

Site 106 1D Models with discontinuity in Occam at 36 km

Slave location map 1996 sites (dots) winter road sites (squares) Lake sites (stars) TGI sites (triangles) Significant kimberlite pipes (yellow diamonds) Going to show responses from two sites (red circles)

2-D resistivity model of Slave craton: conductor at shallow depths ( km) in SCLM spatially correlated with Lac de Gras (LdG) kimberlite field

Interpretation in terms of carbon We know that the region is anomalous in its carbon content in the mantle. Is there carbon in graphite form concentrated at km depth beneath Lac de Gras?

SASE/KSA Southern African (or Kaapvaal) Seismic Experiment 2 year deployment at central (dark blue) stations 1 year only at other stations

Natural laboratory: Southern Africa Tectonic map from Dr. Sue Webb (Wits) Based on exposed geology in South Africa and Zimbabwe, but based on magnetic map in Namibia and Botswana where there is thick cover

SAMTEX cf. SASE coverage Four phases of SAMTEX covers South Africa and southern Botswana as SASE, but also covers northern Botswana and Namibia (terra incognita)

1-D responses Resistivity is independent of rotation angle  a one- dimensional layered Earth is a valid model

2-D model: Main result – variation in LAB S N Lithospheric thickness varies along the profile, with the thickest part from just south of Kimberley -> north of Pretoria ?

Fishwick’s 2009 regional models 2009: 8200 paths 2-stage surface wave inversion method 1) fundamental + first four higher models: period range seconds 2) 1.5 degree splines Includes data from GFZ stations in NW Namibia

New velocity model VsF1.5d at 100 km Low velocity “notch” in southern extension of Angola Craton

Comparison of velocity model and resistivity image SAMTEX   Fishwick

Three views of the cratonic lithosphere: 1. Seismic 2. Thermal 3. Electrical

Thermal Lithosphere Thickness (Depth to 1300 °C) Source: Artemieva and Mooney, 2000