Geological nature of crustal electrical conductivity anomalies
|Location||International Geological Congress,oslo 2008|
|Holding Date||03 September 2008|
Crustal electrical conductivity anomalies (CA) can be subdivided into three groups according to their localization in one of tectonic provinces: (1) rifts, (2) recent subduction zones, (3) platforms. (1) CA have been found in all (covered by EM observations) rifts at depths of 10-20 km. Taking into account high heat flow in the rifts, CA can be treated as a zone of melting, that agrees with the idea of hot mantle material uplift in rifts and supports it. (2) Subduction draws water saturated sediments in deep interior where they are slowly warmed up and pass through consequent facies of metamorphism. Amphibole facies, taking place at 400o-700oC, can be accompanied by partial melting and enhanced conductivity that have been proven by special laboratory experiments. So, the most probable explanations of CA in subduction zones are salt water and partial low temperature melt. The latter version naturally explains narrowness of some anomalies, for example the Carpathian CA. Explanation by intergranular film of carbon material is also possible. (3) In platform regions CA manifest ancient rifts, suture zones, deep-seated faults, serpentinite belts, carbon films and large accumulations of rocks with electronic conductivity (graphitic and pyrrhite- or pyrrhotine-bearing schists). Partial melting is less probable explanation for platforms, but it cannot be excluded for a regions of modern tectonic activation and CA may be one of the first manifestation of the activation. Comparison of CA (especially Kirovograd and Ladoga lake CA) with regional crustal permanent field magnetic anomalies (RPMA) reveals their spatial correlation: CA tend to run between positive and negative RPMA. Recent work by Kiss et al. "Second-order magnetic phase transition in the Earth" (GRL, 2005, 32, L24310) gives some ground for supposition that both kinds of anomalies can have partially common cause, that is Hopkinson effect - sharp enhancement of magnetic permeability of ferromagnetic material in the vicinity of Curie point. Krutikhovskaya et al. (1982) gave evidence that content of ferromagnetic components rise with depth and mean specific inductive magnetic moment of middle crust from the depth of 10 km to the temperature Curie level attains 0.1-1 A/m depending on geological history of region. Petromagnetic analysis revealed that magnetic rocks are formed in expansion zones (rifts, areas of interplate magmatism, island arcs, ets), non-magnetic ones are formed in compression zones (collision, orogenic and folding zones). If the correlation of CA and RPMA turns out to be meaningful, we get deeper insight into petrological nature of CA.