SHALLOW AND DEEP ROTATIONS IN THE MIOCENE ALPS

Mapping and the construction of balanced cross-sections reveal dextral rotation in addition to N-S contraction during the late middle to lat e Miocene Jura phase both at the northern and the southern front of th e Central Alps (Jura and Lombardic Southern Alps, respectively). These rotations may be ascribed to one and the same shallow domain or subli d, rotated clockwise around a pivot somewhere at the southwestern end of the Aar massif. In a crude first approximation model the sublid may be considered rigid, although the data indicate differential rotation s and longitudinal stretching within the lid. The eastern boundary of the sublid crosses the Alps at the Giudicarie-Brenner line, which, acc ording to very recent data, in the Jura phase formed a kinematically l inked system, whereas from Innsbruck to the northwest it is postulated to join, as a diffuse or accommodation zone, the eastern tip of the J ura. In the southwest, the Lombardic thrusts enter into the interior o f the Western Alps. From there to the southwestern end of the Jura the lid boundary is tentatively chosen arbitrarily to pass somewhere near the northern end of the Belledonne Massif. A similar sublid, though l ess well substantiated, may comprise the Tauern and the Venetian South ern Alps. The rotational components are superimposed on elements of in ternal strain, particularly transverse contraction and longitudinal st retching due to simple dextral shear and lateral escape. The model of dextrally rotating sublids pulls together a number of hitherto unexpla ined structures and opens new perspectives on such recalcitrant proble ms as the eastward disappearance of the Jura and the Adige embayment i n the Southern Alps. Dextral rotation of such shallow sublids may be c onsidered to constitute ''mode 1'' of the dextral component at the Eur opa-Adria plate boundary. There is also a ''mode 2'' that affects its deep parts. At the surface, its manifestation is a dextral belt of en echelon folds and pull-apart domains that crosses the sublids diagonal ly. It consists of the Massifs line in the west and the Neo-Pustertal line in the east. They approximately join in the Brenner area, where t hey cross and interfere with the sublid boundaries. The combined Massi fs-Pustertal line describes an arcuate partial northern boundary of th e Adriatic plate with an apparent center of sinistral rotation in cent ral Italy. This sinistral rotation may be responsible for the contract ional features in the southern part of the western Alps which cannot b e accounted for by SE-NW Adria-Europa convergence, It could have cause d the development of the fold and thrust belts of Sisteron, Castellane and Nice. These arcs, in turn, suggest the action of more or less ind ependent shallow sublids also in this part of the Alps.