The Valtellina trough - geophysical investigations.

Three high resolution seismic surveys were carried out, in November 2000 and April 2001, across Valtellina in order to reconstruct the geometries of the sediments filling the valleys, their seismo- stratigraphy and the rock basement depth. The seismic profiles (Fig. 1) were located on the valley floor, where alluvial fan or landslide deposits most likely overlay the bedrock surface. Two profiles are located north of the Insubric Line (Bormio and Sondalo) and one southward (Teglio). To calibrate the seismic results, two boreholes were cored within the Sondalo and Teglio seismic profiles. The Bormio site, at 1160 m a.s.l., is located just 1 km south of the triple convergence of the Adda, Viola, and Frodolfo rivers, where previous borehole data indicate more than 100 m of sediments without reaching the bedrock. The uppermost valley fill is composed of sandy - gravel alluvial deposits of the Adda river with local spots of finer material and peat. The Sondalo site is located within an enlargement of the valley, at 870 m a.s.l. The uppermost valley fill is composed of sandy - gravel alluvial deposits of the Adda river, although silty-clay lacustrine deposits were reported by Guglielmin and Orombelli (2001) only 200-300 m westward of the line. Finally, the Teglio line is located south of the Insubric Line at 360 m a.s.l. The uppermost valley fill is composed of sandy - gravel alluvial deposits of the Adda River. In all three sites both sides of the valley are characterized by deep-seated gravitational slope deformations (DSGSD). The three seismic profiles were shot to sample the subsurface to a depth of about 800 m beneath the valley floor.