Temperature dependence of the electrical conductivity of granite and
quartz-monzonite from south Bulgaria: geodynamic inferences

Stefan Shanov,*+ Yotzo Yanev* and Marcela Lastovickova**

* Geological Institute, Bulgarian Academy of Sciences, 1113 Sofia, Bulgaria

** Geophysical Institute of Czech Academy of Sciences, 14131 Praha, Czech Republic

+ Corresponding author: E-mail: shanov@geology.bas.bg

(Received 16 November 1999; accepted 28 January 2000)

Abstract: The temperature dependence of the electrical conductivity for granite and quartz-monzonite specimens from South Bulgaria has been studied, using equipment of the Geophysical Institute of Czech Academy of Sciences, which enables the automatic control of the experiment. Comparisons between electrical conductivity curves measurements under laboratory and the real local deep geothermal conditions have been made in order to evaluate the influence of the mineral transitions on the formation of geophysical layers in the Earth's crust. The abrupt decrease in electrical conductivity at 200o-330C correlates with the beginning of the transition magnetite-maghemite. The maximum of electrical conductivity in the temperature interval 750-820C in granite coincides with the transition of b -quartz to b 2-tridymite. All these mineral transitions are accompanied by volume changes. The probable fluctuations of the heat flow in the Earth's crust in South Bulgaria, near the critical phase transition zones, are the cause of the volume changes in the rocks, the consecutive increase in the stress and the restoration of the tectonic equilibrium by weak earthquakes sequences. The hypothesis for sinking of the tectonic block is not acceptable because the recent vertical movements are positive - 1 to 2 mm/year. What remains is the hypothesis of progressing of the thermal flow from the deep parts of the Earth's crust and upper mantle towards the most shallow layers of the crust. The area of the most active influence of the heat flow from the asthenosphere uplifting is exactly the zone of the Maritza fault, from where the studied specimens originate.

Key Words: Electrical Conductivity, Temperature Dependence, Granite, Monzonite, Phase Transitions, Earthquakes.