STRUCTURE OF THE NORTHERN MAGGIA AND LEBENDUN NAPPES, CENTRAL ALPS, SWITZERLAND

In the Basodino-Cristallina-Campolungo area, five deformation phases a re distinguishable on the basis of overprinting criteria observable ov er a broad range of scales, from regional to microscopic. The first ph ase, related to initial thrust and nappe development, is preserved as recumbent tight to isoclinal folds with cores of pre-Triassic basement surrounded by a discontinuous envelope of dolomitic marbles and quart zites. During the second phase, the nappe pile was refolded into major recumbent isoclinal folds. These folds show a strong penetrative axia l planar schistosity and a marked elongation lineation, which is paral lel to small scale fold hinges. Several major D-2 folds can be followe d over long distances (> 50 km), and were mapped by earlier workers as distinct fold nappes (e.g. the ''Antigorio nappe'', whose antiformal core is a D-2 fold). The regional pattern of Mesozoic-cored synforms ( e.g. Teggiolo, Campolungo, Piora and Molare) and intervening basement- cored antiforms is mainly due to large-scale D-2 folds. The third phas e structures developed obliquely to the trend of earlier structures an d to the Penninic zone as a whole. Third phase folds are more open and have a characteristic chevron or corrugated style, with much lower li mb to hinge ratios than folds of the first two deformation phases. A n ew crenulation cleavage is variably developed parallel to the axial pl anes of Ds folds, particularly in more micaceous lithologies. In the n orthern Maggia area, amphibolite facies metamorphic conditions were re ached after D-2 and maintained through D-3. Superposition of second an d third phase folds resulted in Types 1, 2 and 3 interference patterns on all scales. Third phase folds have much greater regional importanc e than has been previously realised: the main structures related to th is deformation phase are the Campo Tencia synform and the Maggia Steep Zone. Fourth phase folds represent the backfolds of the Northern Stee p Zone, which locally reactivated and modified existing third phase st ructures (e.g. the Basodino fold). The ''northern steep zone'' represe nts the steep to overturned northern limb of a broad, regional fourth phase synform (from west to east known as the Berisal, Basodino and Ch iera synforms) with a nearly horizontal fold axis and shallow to moder ately NW-dipping axial plane. The regional interference between D-3 an d D-4 folds resulted in the overall double dome structure of the Lepon tine Alps (i.e. the Simplon and Ticino culminations) as marked by the regional attitude of the main schistosity (i.e. S-2). Fifth phase stru ctures are coaxial with F-4 and locally developed within the steep zon e as kinklike folds with nearly horizontal axial planes. They reflect a late vertical shortening. most likely related to orogenic collapse d uring continued Alpine convergence. Possibly coeval late minor fault, joint and quartz vein development demonstrates late stretching paralle l to the Alpine trend (i.e. SW-NE:, consistent with the Neogene develo pment of the low-angle normal Simplon and Brenner Fault Zones.