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Woods Hole Oceanographic Institution

Dr Rob. L. Evans

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Publications
»Electrical Lithosphere Beneath the Kaapvaal Craton
»Gulf Of Mexico Gas Seep
»MELT MT Results
»MELT Area Off-Axis Structure
»Wrightsville Beach Geophysics and Hydrology
»Karst Formation off North Carolina
»Review of Shallow Offshore EM Work
»EPR MMR Experiment
»Towed EM System
»Offshore MT and Subduction Systems
»Shallow Porosity Structure on the Continental Shelf
»Oceanic and Continental Mantle Resistivity
»New Jersey EM Survey
»Eel River EM Survey
»Impact of groundwater on EM data
»Electrical structure of Slave Craton
»Report of Shoreline Change Workshop


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Rob. L. Evans1, Alan G. Jones2, Xavier Garcia3, Mark Muller2, Mark Hamilton2,a, Shane Evans4,b, C.J.S Fourie5,c, Jessica Spratt2,d, Susan Webb6, Hielke Jelsma4, Dave Hutchins7

  1. Dept of Geology and Geophysics, Woods Hole Oceanographic Institution, Woods Hole, MA 02543
  2. Dublin Institute for Advanced Studies (DIAS), 5 Merrion Square, Dublin 2, Ireland.
  3. Barcelona Center for Subsurface Imaging, Unitat de Tecnologia Marina, CSIC, Pg. Maritim de la Barceloneta 37-49, 08003 Barcelona (Spain).
  4. De Beers Group Services, Private Bag X01, Southdale 2135, South Africa
  5. Council for Geoscience, 280 Pretoria Street, Silverton, Pretoria 0001, South Africa
  6. School of Geosciences, University of Witwatersrand, Wits, South Africa.
  7. Geological Survey of Namibia, 1 Aviation Road, Windhoek, Namibia

a. Now at EMGS, Stiklestadveien 1, N-7041 Trondheim, Norway
b. Now at Moombarriga Geoscience, Box 1184, West Perth WA 6872, Australia
c. Now at  Environmental, Water and Earth Science Department, Tshwane University of Technology, Pretoria, South Africa,
d. Now at Natural Resources Canada, Ottawa, Canada.

 

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Electrical Lithosphere beneath the Kaapvaal craton, southern Africa

, J. Geophys. Res., 2011

A regional-scale magnetotelluric (MT) experiment across the southern African Kaapvaal craton and surrounding terranes, called SAMTEX (Southern African MagnetoTelluric Experiment), has revealed complex structure in the lithospheric mantle. Large variations in maximum resistivity at depths to 200-250 km relate directly to age and tectonic provenance of surface structures. Within the central portions of the Kaapvaal craton are regions of resistive lithosphere about 230 km thick, in agreement with estimates from xenolith thermo-barometry and seismic surface wave tomography, but thinner than inferred from seismic body wave tomography. The MT data are unable to discriminate between a completely dry or a slightly “damp” (a few hundred of ppm water) structure within the transitional region at the base of the lithosphere. However, the structure of the uppermost ~150km of lithosphere is consistent with enhanced, but still low, conductivities reported for hydrous olivine and Opx at levels of water reported for Kaapvaal xenoliths. The electrical lithosphere around the Kimberley and Premier diamond mines is thinner than the maximum craton thickness found between Kimberley and Johannesburg/Pretoria.

            The mantle beneath the Bushveld Complex is highly conducting at depths around 60 km. Possible explanations for these high conductivities include graphite or sulphide and/or iron metals associated with the Bushveld magmatic event. We suggest that one of these conductive phases (most likely melt related sulphides) could electrically connect iron-rich garnets in a garnet rich eclogitic composition associated with a relict subduction slab.


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