The Siena Graben: combined interpretation of DES and MT soundings

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S. Giammetti
D. Patella
A. Siniscalchi
A. Tramacere


The Siena Graben study area is located in Tuscany, Central Italy. The local geological structures were extensively studied in the frame of the Italian National Research Council (CNR) and the Commission of the European Communities (CEC) geothermal projects. Axial dipole-dipole geoelectrical soundings (DES) were also performed. Recently we carried out 13 broadband magnetotelluric (MT) soundings, most of which were located very close to the station sites of the mentioned DES. For six of them we made a DES-MT combined interpretation in order to put in evidence resistivity frequency dispersion effects. Indeed, four sites showed DES-MT anomalous responses, in the frequency range 1 ¸ 102 Hz, which can be properly explained as due to dispersion effects in shallow layers. Three of these anomalous sounding sites are located on the neoautochthonous clayey formation, while the fourth is located on the eastern boundary of the graben over an extensive outcrop of the «Macigno» complex. The dispersion-affected soundings are all located in the northern part of the graben, while there is no evidence of such effects in the southern part. This circumstance, together with the estimate of very low time constants of the fitting Cole-Cole dispersion model, can be tentatively explained as due to local and shallow lithological effects (clay-like membrane polarization) rather than to deep geothermal effects (sulphide-like electrode polarization). Moreover, the MT soundings delineated a conductive zone in the upper crust below the resistive geoelectrical basement, located in the northern part of the graben, which appears at present difficult to interpret. Furthermore, the combined analysis of the DES and MT soundings in the same sites has allowed us to resolve one of the most intriguing ambiguities concerning the determination of the depth to the graben basement. The result is a remarkable reduction of the depth to the basement top, especially in the northern part of the graben. Finally, the easternmost MT soundings, located on the resistive outcrops of the massive calcareous formations of the basement and of the «Macigno» complex, permitted us to investigate almost all the lithospheric slab, thanks to the locally greater skin depth penetration. The lithosphere-asthenosphere transition is very well delineated, whereas the crust-mantle transition seems not to have an evident electrical signature.

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Giammetti, S., Patella, D., Siniscalchi, A. and Tramacere, A. (1996) “The Siena Graben: combined interpretation of DES and MT soundings”, Annals of Geophysics, 39(1). doi: 10.4401/ag-3961.

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