Walter D. Mooney, Ph.D.US Geological SurveyMenlo Park, California USAmooney@usgs.gov

Lecture #10:Geoelectrical Studies of Lithospheric Structure

IPRCC and SinoProbe Short Course: Lithospheric Evolution through Time April 8-12, 2011

Acknowledgement:Acknowledgement:

Geoelectric studies Geoelectric studies

of the lithosphereof the lithosphereThis lecture is by Prof. Alan G. JonesDublin Institute for Advanced Studies

Lecture presented as part of the Short Course: Integrated Studies of Lithospheric Evolution: A Global Perspectiveby Dr. Walter Mooney (USGS), 22-26 November, 2010

Electro-magnetic (EM) Electro-magnetic (EM) methods (1)...methods (1)...

• give information about...give information about...– structuresstructures– presence of fluids and/or presence of fluids and/or

conducting metasedimentsconducting metasediments– dimensionality (2D or 3D)dimensionality (2D or 3D)– strike direction and its depth strike direction and its depth

dependencedependence

EM methods EM methods (2)...(2)...

• sense a physical parameter sense a physical parameter (electrical conductivity) that (electrical conductivity) that varies by over varies by over EIGHTEIGHT orders orders of magnitudeof magnitude

• sense its lateral and vertical sense its lateral and vertical variationsvariations

• with natural sources, with natural sources, penetration to all depths is penetration to all depths is assured (but with decreasing assured (but with decreasing resolution)resolution)

Skin Depth….

Penetration to all depths is assured with MT - just need to record sufficientlylong periods

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

Conduction Conduction mechanisms in the mechanisms in the mantlemantle

ionic conduction due to movement of ionic conduction due to movement of mobile charged ionsmobile charged ions1) partial melt1) partial melt2) H2) H++

electronic conduction due to electronic conduction due to movement of electronsmovement of electrons3) hydrous mineral phase (e.g. phlogopite)3) hydrous mineral phase (e.g. phlogopite)4) carbon on grain boundary films4) carbon on grain boundary films

Electronic conduction -graphite: AB

Partial melt…

Partial melt connects Partial melt connects efficiently and efficiently and increasesincreaseselectrical conductivityelectrical conductivityby orders ofby orders ofMagnitudeMagnitude

HH220-saline mix0-saline mix

Electronic conduction -Electronic conduction -sulphides: THO (NACP)sulphides: THO (NACP)

Why MT? Sensitivity to partial Why MT? Sensitivity to partial meltmelt

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?Why use electromagnetism?Image the base of the sub-continental lithospheric Image the base of the sub-continental lithospheric mantlemantleDiamonds Diamonds only exist in only exist in thick cold thick cold cratonic cratonic rootsroots

Kimberlites Kimberlites passing passing through through such roots such roots bring bring diamonds updiamonds up

What can MT resolve?What can MT resolve?

Electrical Electrical comparedcomparedwith Seismic with Seismic asthenosphereasthenosphere

Excellent spatial Excellent spatial correlationcorrelationbetween presence of between presence of upperuppermantle low velocity zone mantle low velocity zone andandregion of high region of high conductivityconductivity

(Alekseyev et al. 1977)(Alekseyev et al. 1977)

Slave craton: EM studies• 1996: Profile 1 MT survey1996: Profile 1 MT survey

• 1998,99 & 2000: Winter road surveys1998,99 & 2000: Winter road surveys

• 1998/99 & 99/2000: Slave lakes1998/99 & 99/2000: Slave lakes

• 2000: TGI survey

Source: Mooney et al., 1998

Depth to

Moho

Tectonic AgesTectonic Ages

Archean Proterozoic Phanerozoic

3Ga 2Ga 1Ga 0Ga

SNORCLECorridor 1

SNORCLE: Rae - TibbitFlat reflection Moho beneath Anton complex at ~12 s TWTT (~36 km)Refraction Moho at 35-36 kmTeleseismic Moho at ~36 km

YKRae Tibbit

Slave craton:Rae-YK-Tibbitprofile

RaeRaeYellowknifeYellowknife

Site 106Site 106

Tibbit LakeTibbit Lake

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

Slave location map

1996 sites (dots)1996 sites (dots)1998-2000 winter 1998-2000 winter road sites (squares)road sites (squares)Lake sites (stars)Lake sites (stars)TGI sites (triangles)TGI sites (triangles)

Significant Significant kimberlite pipeskimberlite pipes(yellow diamonds)(yellow diamonds)

Going to show Going to show responses from two responses from two sites (red circles)sites (red circles)

2-D 2-D resistivity resistivity model of model of Slave Slave craton:craton:conductor conductor at shallow at shallow depths (80 - depths (80 - 120 km) in 120 km) in SCLM SCLM spatially spatially correlated correlated with Lac de with Lac de Gras (LdG) Gras (LdG) kimberlite kimberlite fieldfield

2-D 2-D resistivity resistivity model of model of Slave Slave craton:craton:conductor conductor at shallow at shallow depths (80 - depths (80 - 120 km) in 120 km) in SCLM SCLM spatially spatially correlated correlated with Lac de with Lac de Gras (LdG) Gras (LdG) kimberlite kimberlite fieldfield

Interpretation in terms of carbonWe We knowknow that the that the region is anomalous region is anomalous in its carbon content in its carbon content in the mantle.in the mantle.

Is there carbon in Is there carbon in graphite form graphite form concentrated at 80-concentrated at 80-100 km depth 100 km depth beneath Lac de beneath Lac de Gras?Gras?

SASE/KSASASE/KSASouthern African (or Kaapvaal)SeismicExperiment

2 year deployment at central (dark blue) stations

1 year only at other stations

Natural laboratory: Southern AfricaNatural laboratory: Southern AfricaTectonic 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 coverageSAMTEX cf. SASE coverageFour phases of SAMTEX covers South Africa and southern Botswana as SASE, but also covers northern Botswana and Namibia (terra incognita)

1-D responses1-D responsesResistivity is independent of rotation angle a one-dimensional layered Earth is a valid model

2-D model: Main result – variation in LAB2-D model: Main result – variation in LAB

SS NN100100

200200

300300

Lithospheric thickness varies along the profile, with the thickest part from just south of Kimberley -> north of Pretoria

??

Fishwick’s 2009 regional Fishwick’s 2009 regional modelsmodels

2009: 8200 paths2009: 8200 paths

2-stage surface wave 2-stage surface wave inversion methodinversion method1) fundamental + first four 1) fundamental + first four higher models: period range higher models: period range 50-120 seconds50-120 seconds

2) 1.5 degree splines2) 1.5 degree splines

Includes data from GFZ Includes data from GFZ stations in NW Namibiastations in NW Namibia

New velocity model VsF1.5d at 100 kmNew velocity model VsF1.5d at 100 km

Low velocity “notch” in southern extension of Angola Craton

Comparison of Comparison of velocity model velocity model and resistivity and resistivity imageimage

SAMTEX

Fishwick

Three views of the cratonic lithosphere:1. Seismic2. Thermal3. Electrical

Thermal Lithosphere Thickness (Depth to 1300 °C)

Source: Artemieva and Mooney, 2000