FROM RIFTING TO DRIFTING - TECTONIC EVOLUTION OF THE SOUTH-ALPINE UPPER CRUST FROM THE TRIASSIC TO THE EARLY CRETACEOUS

The tectonic evolution of the South-Alpine rifted margin is discussed on the base of four palinspastic upper crustal profiles. Extension, re lated to the movements between Adria and Europe, began in the Norian a fter Variscan orogeny, Late Carboniferous to Middle Permian orogenic c ollapse and continental-scale wrenching. From the Late Triassic to the Early Liassic stretching was mostly limited to the Lombardian basin. During this time, extension was mainly controlled by four major listri c faults, symmetrically centred around the Late Carboniferous-Early Pe rmian Collio grabens. Smaller faults which also started in the Norian, were progressively de-activated during the Late Triassic. After the M iddle Liassic, faulting in the Lombardian basin gradually ceased and t he site of extension shifted westwards, i.e. towards the future site o f crustal separation. Extension was then controlled by a set of west-d ipping normal faults. Oceanic crust was formed not later than 157 Ma. The overall extension along the profile (which had a final length of 2 90 km) was 52 km which corresponds to a stretching factor of 1.22 for the whole length of the preserved margin. The strain rate is ca. 1 X 1 0(-16)/s. The sedimentation history suggests that the extension of the South-Alpine margin was a continuous process from the Norian to the M iddle Jurassic. The changes in tectonic pattern are related to progres sive hardening of the lithospheric segment undergoing slow extension.