The problem of the deep structure of the Southern Alps: 3-D material balance considerations and regional consequences

Abstract

The Lombardic Southern Alps are a typical thin-skinned frontal fold- and thrust belt. Although this recent reassessment is widely accepted, figures for shortening and schemes for transfer of shortening at their eastern end in the Giudicarie vary. Exact section balancing is not feasible because of insufficient information, but approximate shortening is on an order probably exceeding 100 km, with about 80–100 km attributable to the Middle to Late Miocene “Milan belt”. Acceptance of this quantitative conclusion has farreaching consequences. Transfer of the Milan belt shortening in the east follows the lateral edge of the allochthon which is sinistrally transpressive, the Giudicarie line, with its sedimentary décollement zone, the Giudicarie belt. The pre-existing Adamello intrusions and the Jorio-Tonale line are part of the thrust masses and consequently allochthonous. Transfer of shortening across the Insubric line into the Tauern antiform and associated structures of Neo-Alpine compression in the Eastern Alps resulted in the sinistral dissection of the Insubric line at the Giudicarie line. Superposition of this compression on Pannonian stretching east of and transtensive motion at the Brenner line west of the Tauern produced a confusing structural pattern whose quantitative kinematics have not been worked out. Another sinistral, probably compressive transfer zone is postulated at the western end of the Milan belt. Retrodeformation leads simultaneously to both a straightening of the Insubric line and the connection of the Canavese line with the Villalvernia-Varzi-Levanto line to define the boundary of the Adriatic block at the beginning of Neo-Alpine deformation.