Magnetotelluric profiling along the crop-04 section in the Southern Apennines

Abstract

We show the results of a magnetotelluric (MT) survey along a SW-NE profile from the Tyrrhenian to the Adriatic sea close to the CROP-04 transect, orthogonal to the main structural axis of Southern Apennines, integrated by a NW-SE profile placed on the Apulia foreland. The NW-SE profile displayed a nearly 1D apparent resistivity pattern, which was used to constrain the 2D modelling along the orthogonal profile. To improve the 2D model across the main SW-NE profile, a combined MT-gravity analysis was carried out until mutual consistency was reached. Starting from the southwestern side of the SW-NE profile, the MT cross-section allows the shallow Tertiary basinal units, with resistivity less than 100 m, to be differentiated from the rootless nappes of the Apennine thrust, with resistivity less than 1000 m and thickness around 7 km, which include the Western carbonate platform and Lagonegro basin sequence. Proceeding downward in the southwestern portion of the MT SW-NE cross-section, a layer, about 1 km thick and with resistivity in the range 3-30 m, is found between the Apennine thrust and a lower stratum with resistivity less than 10000 m, extending down to the bottom of the model placed at 15 km of depth, which is assumed to correspond to the Apulia carbonate platform. As the conductivity of the thin layer is consistent with the presence of high fluid concentrations, the thin layer is assumed to indicate a major decollement level. The whole SW-NE MT cross-section delineates the large-scale trend of the top of the Apulia carbonate platform. Its SW-bending beneath the Bradano Trough and marked deepening down to 5 km beneath the Ofanto basin conforms to the results from the CROP-04 seismic profile. A body with resistivity greater than 10000 m, found beneath the Apulia platform at a depth from about 6 km beneath the Apulia foreland likely down to more than 15 km beneath the Ofanto basin, is interpreted as the crystalline basement, which appears separated from the overlying Apulia platform by a conductive layer ascribed to Permo-Triassic clastic sequences. A further important feature of the resistivity model across the SW-NE profile is the presence of two laterally bounded vertical bodies with resistivity less than 3 m, cutting the whole Apulia platform and part of the Apennine thrust, beneath the Sele graben and Ofanto basin, respectively. Accounting for geochemical, geothermal and active and passive seismological data, such low resistivity regions are interpreted as highly fractured zones characterized by melt intrusions and/or fluid circulation. Referring to the MT model across the NW-SE secondary profile, a quite simple structure is observed crossing the foreland-foredeep areas. A slow bending of the Apulia platform and a thickening of the much less resistive deposits of the Bradano Trough are observed proceeding northwestwards. A remarkable result is that the Apulia platform does not show any important variation of thickness along the entire NW-SE section