Electrical conductivity of the earth's crust and upper mantle - MTNet

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134 GERHARD SCHWARZ 0,01 1 10 2 ' r 10 -2 1 10 2 10 4 10 6 10 8 .... .. ! . . . . . , ! ! e ~ .' ..... .-- > ~ [J~m] Basis-Sedlment -Mohorovi~i~-Pl~ohe Verteilung des epez~Wider~tande8 mlt der Tlefe z [kmJ. Fig. 1. Ancient view on electrical resistivity versus depth (Angenheister, 1962): Due to the decreasing fluid-filled pore volume with depth in the sedimentary cover, electrical resistivity increases. In the crystalline basement solid state resistivity should be controlled by the non-electrolytic temperature dependent conductance (thick line). Resistivity of ocean water (Meer) is given for comparison. affecting the ability to resolve the vertical and lateral extent of conductive zones. Superposed conductive structures at depth are hard to resolve by magnetotellurics, e.g., a conducting layer beneath another conducting layer is difficult to detect. Interpretation of coherent noisy data is also problematic (e.g., Fontes et al., 1988). Geomagnetic deep soundings as well as other geophysical methods suffer from lack of resolution if the station spacing versus depth of the investigated structure is too large, and in fact calculated models are not unique (Figure 2). On the other hand geomagnetic deep soundings may detect the center of an induction anomaly in the lateral direction better than any other EM-method, due to the vanishing vertical component of the induced magnetic field. Recent discussions on electrical conductivity studies include Hjelt (1988), Chave and Booker (1987), Roberts (1986a), Haak and HuRon (1986), Schmucker (1985b), Haak (1985). In this discussion, I focus on reported electrical structures in conti- nental regions that have further geophysical or geological information available, excluding shallow low conductive structures, e.g., in geothermal areas. I also discuss the efforts to interpret low electrical resistivities, e.g., the role of fluids, volatiles and graphite in the crust and upper mantle. Conductive Structures of Regional Extent Regional studies can be divided into two generic classes: those carried out in tectonically active areas, e.g., subduction or rifting zones, and those concerning stable, passive regions, e.g., cratons. In reviewing electrical conductivity of the J
O 2 tO0 200 4O0 20 BASIN&RANGE RIO GRANDE GREAT PLAINS RIFT ZONE CONDUCTIVITY OF CRUST AND UPPER MANTLE 135 lllllllllll[llmllHII/llllllfllhImllllllIIIIIIIIIIIIIIIIllIIH [[LH]I I-9,~m ~ 10-2~m [~30-60D, m E~-~1100-200nm~3OO-4Oonm~ IOOOg=m SWIFT (1967) (a) BASIN & RANGE RIO GRANDE RIFT ZONE GREAT PLAINS ::::::::::::::::::::: 200 :i:?:!:i:i:i:i:i:!:i:!:i:i:i:i:i:i:?:!:!:!:i: rrTr]]110-25 P.m 400 [- r"S"~ 40 ,Q.m E~ 70 - 150.qm ~200 ~m ~Z50-~OOnm E~SSO-1000~m WEIDELT (1975) (c) !iiiiiii!i!!liii!ilt Iiiili!ii!lii!iI E zoo i{iii~ii~{ii~i~i.~.~;~{iiii~{i~ii,~iiiii~ii{~ 400 r ~ ~o.., zzz~o-~o.., r~-~,oo-,~.,. E~r~145-17~5 ~lm ~ I95-235,0.m ~2~ 290 gm SCHMUCKER & JANKOWSKI (1972) (b) HERMANCE (1977) 0 x 5 -r t-- 50 n la.I "" I00 I I I 9 I t 0 400 800 0 400 800 ' DISTANCE (KM) DISTANCE (KM} LORDSBURG RIO GRANDE CARLSBAD / ",FTZONE / iiii!iii!iiiiiiiiiii!iiiiiiiiiiiiiiiiiiii!iiiiiiii!iiiiiiiiiiiiii!iiiiiiii ~ .~. 4 f/.m ~5am , c=~30am'~loOOam 0 I00 200 DISTANCE (KM) LTIH j KESHET & HERMANCE (1986) .o. I E / TUCSON ~ t~ ~94"~G'4e" ~/~ .. coc : ;-- ~" t MEX)CO (e) (f) (d) i 05 9 i oo, ~OLIORAO0 " KANSAS l -] .......... - SANTA FE ~4"4~ ~ k ~ )RROI '~ i § HOP LEAISE M GA=f SWE ," :' s..s t L PASO VAN HORN ~ IEXA$ lOS* loo. Fig. 2. Magnetovariational methods and their limited resolution of resistivity structures: Five different models (a-e) for the Rio Grande rift zone and adjacent areas (f) along latitude 32 ~ N equally explaining the .data (after Keshet and Hermance, 1986). Note that the scale used in Figure 2e is different from the scale in Figure 2a-d, and vertical scale is partly exaggerated earth, there is no way to give adequate coverage to all papers published in the last few years. Only some of the studies described are discussed, concentrating on common geophysical features or causes. However, the reader will find an extended list of references at the end. If some papers have escaped my attention I apologize and assure the authors that the omissions are inadvertent. Electrical conductivity
Page 1: ELECTRICAL CONDUCTIVITY OF THE EART
Page 5 and 6: CONDUCTIVITY OF CRUST AND UPPER MAN
Page 7 and 8: N S THE KNO 8OR HEL LIS I'~R i I I
Page 11 and 12: \A + + \A A '$ + "//I~ + + 9 ", /1
Page 17 and 18: 5 o_ log Resistivity 1 2 3 4 5 ohm
Page 29: CONDUCTIVITY OF CRUST AND UPPER MAN

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