TILTING OF THE BERGELL PLUTON AND CENTRAL LEPONTINE AREA - COMBINED EVIDENCE FROM PALEOMAGNETIC, STRUCTURAL AND PETROLOGICAL DATA

Paleomagnetic data from two undeformed sites in the steep N-dipping to nalite rail of the Oligocene Bergell pluton indicate a mean natural re manent magnetization (NRM) with declination D = 303.5 degrees and incl ination I = 38 degrees. Restoration of the measured NRM direction to t he NRM direction of stable Europe in the Oligocene is achieved by thre e different rotations corresponding to three distinct post-crystalliza tion deformation events. 1) 25 degrees dextral rotation around a verti cal axis in order to restore the sinistral block rotation suffered by each segment of the tonalite tail during dextral shearing along Insubr ic Riedel faults. 2) 20 degrees dextral rotation (looking westward) ar ound an horizontal E-W striking axis in order to restore the rotation suffered by the tonalite tail during backthrusting along the Insubric Line. 3) 11 degrees sinistral rotation (looking northward) around an h orizontal N-S striking axis in order to restore the effect of eastward tilting of the Bergell pluton. The similar inclinations of the Oligoc ene NRM of stable Europe and that of the Bergell tonalite after rotati on of only 20 degrees around an E-W horizontal axis indicate that the Bergell tonalite tail was in a steep orientation at the time it passed through its solidus. Moreover, the concordant contact of the tonalite rail with the enclosing rocks of the Southern Steep Belt of the Centr al Alps implies that the Southern Steep Belt already existed when the tonalite tail reached its solidus. It is suggested that vertical uplif t of the western Lepontine induced eastward tilting of the Central Lep ontine area. This rotation around a N-S striking horizontal axis expla ins deviation of paleomagnetic directions (Heller 1980) from the orien tation of stable Europe in the Oligocene.