The Hercynian basement of the Southern Alps west of Lago di Lugano consists of two units: the Ivrea-Verbano, which is well known for its lower crust and upper mantle rocks and as the seat of a marked gravimetric positive anomaly, and the Serie dei Laghi, an intermediate to upper crustal metasedimentary and metagranitic complex. The contact between them is a subvertical tectonic discontinuity, the Cossato-Mergozzo-Brissago Line (CMB) and Pogallo Line, recently the subject of considerable interest as the possible analogue of a low-angle normal fault (later tilted to the present upright position), such as those increasingly revealed by the deep reflection seismics in thinned continental crust. Late Hercynian igneous activity mainly took place along this lineament. The contact is however very complex. The Cossato-lower Valsesia segment is poorly exposed and has been reactivated by a younger fault (Cremosina Line). In the Valsesia-Val d'Ossola (Toce River) segment the CMB Line is expressed as a strong structural and lithological contrast as well as by a narrow belt of mylonites; in the Val Cannobina-Lago Maggiore sector the mylonites are distributed over a considerable thickness mostly within the Serie dei Laghi. They are connected with large-scale ductile folding on nearly vertical axes ("Schlingenbau"). The CMB Line is dissected by several orthogonal and oblique subvertical faults (Grottaccio, Quarna, Val Lessa, Val Pogallo faults). The presence of appinitic calcalkaline dykes and stocks of about 285 Ma, intruded along particular segments of the CMB isolated by those faults, suggests that the attitude of the main tectonic boundary was upright at the time of intrusion. The intrusion apparently occurred in a transtensional environment and induced a temperature increase in the country rocks causing a low pressure metamorphic overprint in the Val Cannobina-Lago Maggiore segment and dehydration melting in a belt 2-3 km wide between Valsesia and Val d'Ossola. The Pogallo Line cuts across the CMB Line with a sinistral horizontal slip of about 11 km and is also characterized by amphibolite facies mylonites. The absence of mylonitic appinites and consistent geologic evidence point to formation of the fault before 285 Ma. A significant Mesozoic displacement along both faults is therefore excluded, even though post-Permian reactivations are demonstrated by the presence of Rb-Sr and K-Ar mica mixed ages along the CMB Line. A new evolutionary model can be proposed to account for most of the geological, petrological and geophysical data: Early Paleozoic continental accretion, subduction and magmatic arc followed by a more external, probably oblique, Hercynian reactivation inducing important transcurrent movements along CMB and Pogallo before the Early Permian magmatic phase. © 1990.
