Open AccessKTB and the electrical conductivity of the crust
Author(s) -
V. Haak,
Fiona Simpson,
Karsten Bahr,
Jürgen Bigalke,
Markus Eisel,
Ulrich Harms,
G. Hirschmann,
Ernst Huenges,
H. Jödicke,
Agnes Kontny,
Jochem Kück,
G. Nover,
A Rauen,
Johannes Stoll,
John V. Walther,
Helmuth Winter,
Gernold Zulauf,
Johann Wolfgang
Publication year - 1997
Publication title -
journal of geophysical research: solid earth
Language(s) - English
Resource type - Journals
SCImago Journal Rank - 1.67
H-Index - 298
eISSN - 2156-2202
pISSN - 0148-0227
DOI - 10.1029/96jb03861
Subject(s) - geology , crust , tectonics , shearing (physics) , upper crust , shear zone , petrology , geochemistry , electrical resistivity and conductivity , electrical resistivity tomography , geophysics , seismology , geotechnical engineering , engineering , electrical engineering
The German Continental Deep Drilling Program (KTB) drilled two holes through crystalline rocks which are rich in both high‐salinity fluids and graphite accumulated along shear zones. Analyses of a large number of borehole measurements yield models for the electrical resistivity of the upper and middle crust in the vicinity of the KTB holes. High observed resistivity, of more than 10 5 Ω m in the lowermost part of the 9000 m deep main hole, in a rather “wet” crust, indicates that effective mechanisms exist to cut down connections between fluid accumulations and therefore that fluids are not the likely cause of high‐conductivity anomalies. On the other hand, graphite accumulations appear to be connected along shear lineaments over hundreds of meters or more. Structural, mineralogical, and geochemical studies suggest a tectonic model which explains the deposition of graphite as the relic and witness of a shearing process that occurred during the late Variscan (Upper Carboniferous) thrusting. This process took place while this part of the crust resided at temperatures between 240° and 380°C. Subsequent independent reverse faulting lifted this part to the Earth's surface. Our conclusion is that the KTB case indicates how high electrical conductivities in the upper crust, which originated from the middle to lower crust, are caused by graphite accumulations, rather than by fluids, and that these anomalies are related to shearing processes. Such graphite accumulations may exist elsewhere and may be of relevance in the context of present‐day midcrustal conductors.