Abstract

In the Basòdino-Cristallina-Campolungo area, five deformation phases are distinguishable on the basis of overprinting criteria observable over a broad range of scales, from regional to microscopic. The first phase, related to initial thrust and nappe development, is preserved as recumbent tight to isoclinal folds with cores of pre-Triassic basement surrounded by a discontinuousenvelope of dolomitic marbles and quartzites.

During the second phase, the nappe pile was refolded into major recumbent isoclinal folds. These folds show a strong penetrative axial planar schistosity and a marked elongation lineation, which is parallel to small scale fold hinges. Several major D2 folds can be followed over long distances (> 50 km), and were mapped by earlier workers as distinct fold nappes (e.g. the "Antigorio nappe", whoseantiformal core is a D2 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 D2 folds.

The third phase structures developed obliquely to the trend of earlier structuresand to the Penninic zone as a whole. Third phase folds are more open and have a characteristic chevron or corrugated style, with much lower limb to hinge ratios than folds of the first two deformation phases. A new crenulation cleavage is variably developed parallel to the axial planes of D3 folds, particularly in more micaceous lithologies. In the northern Maggia area, amphibolite facies metamorphic conditions were reached after D2 and maintained through D3. Superposition of second and third phase folds resulted in Types 1, 2 and 3 interference patterns on all scales. Third phase folds have much greater regional importance than has been previously realised: the main structures related to this deformation phase are the Campo Tencia synform and the Maggia Steep Zone.

Fourth phase folds represent the backfolds of the Northern Steep Zone, which locally reactivated and modified existing third phase structures (e.g. the Basòdino fold). The "northern steep zone" represents the steep to overturned northern limb of a broad, regional fourth phase synform (from west to east known as the Berisal, Basòdino and Chièra synforms) with a nearly horizontal fold axis and shallow to moderately NW-dipping axial plane. The regional interference between D3 and D4 folds resulted in the overall double dome structure of the Lepontine Alps (i.e. the Simplon and Ticino culminations) as marked by the regional attitude of the main schistosity (i.e. S2 ).

Fifth phase structures are coaxial with F4 and locally developed within the steep zone as kink-like folds with nearly horizontal axial planes. They reflect a late vertical shortening, most likely related to orogenic collapse during continued Alpine convergence. Possibly coeval late minor fault, joint and quartz vein development demonstrates late stretching parallel to the Alpine trend (i.e. SW-NE), consistent with the Neogene development of the low-angle normal Simplon and Brenner Fault Zones.