GNGTS 2013 - Atti del 32° Convegno Nazionale

The crust in Italy from seismic prospecting and additional investigating techniques R. Nicolich DIA, University of Trieste, Italy The near-vertical reflection seismic method was the most appropriate exploration tool utilized in the crustal exploration of Italy and neighbouring Seas with the CROP (CROsta Profonda) project and with similar programs. The method, subsequent to the improvements in data acquisition and processing with increased dynamic range of digital data and more powerful processing software, provides signal easily interpreted in geological terms. To complete the information needed for major geological synthesis a combination of different geophysical investigating techniques were required and added. Significant the wide-angle refraction/reflection method (WAR/R), which yield propagation velocities at depth with greater accuracy. Both reflection and refraction techniques, separately used, supply results which are different in nature though complementary. Examples of the current interpretation and the open debates about the structure and geodynamics of the crust of the Italian area will be presented with regard to: first, the definition of the seismic nature of the Moho discontinuity, in terms of its position, topography, smoothness and continuity; second, the lower crust contribution in the complexities of the collision mechanisms; third, the presence of decoupling levels within the subducting lithosphere or the intracrustal ones, related to the Neogenic evolution, an insight into the processes that built the geological structures of the upper crust; fourth, what to do and future improvements to crustal exploration. Active seismic prospecting can give major information in the first 50-60 km of the lithosphere, with limited data quality in the collision zones where the utilization of the receiver functions analysis can help in the precise indication of the positions in the crust-mantle transitions of the colliding plates. The tectonic melange may dominate the thickened crust after collision, making difficult to isolate the different bodies because the physical differences are small. Larger depths are investigated by earthquake hypocentre distribution or by tomographic analysis of teleseismic events, outside of my presentation. Of interest, the acquisition of wide-angle reflection fans, successfully employed for imaging the complex geometries of the M-interface on collision belts once information was available about an approximate position at depth of the target. The study of the crust started in Italy with controlled source refraction profiles in the Western Alps in 1956, in replay to requests for international collaboration among the confining countries. Analog recorders were used, and the spacing between stations, the number of the acquired profiles, were related to the availability of instruments and operators (students) from different participating countries. Reliable results were obtained only on the gross changes in crustal thickness and on the main velocity structures, but the data quality increased as years go following the technological evolution (from the initial coffins to the Coca-Cola cans of ALP-2000). The deep seismic soundings covered the whole Italian Peninsula with profiles integrated with other pertinent geophysical methods. This activity was concluded with the long N-S transect of the European GeoTraverse (EGT) program in 1986 (Ansorge et al. , 1992). We returned to the Alpine chain with reflection seismics updated technologies in joint international multidisciplinary programs, exploring Western Alps in 1985-1986, first with a French-Italian joint venture (CROP-ECORS), followed by a co-operation with the Swiss NRP20 project in the Central Alps. The Eastern Alps were explored from 1999-2000 by partner institutions of Italy, Austria and Germany acquiring data along the TRANSALP profile. In the mean time the CROP venture programmed acquisitions across the Apenninic chain; Sicily was explored in 2009, but with different funding following the CROP-11 solution. XIII GNGTS 2013 L ectio M agistralis