Pogallo Line, south Alps, northern Italy: an intermediate crustal level, low-angle normal fault?

The Pogallo Line is a major crustal shear zone which separates crystalline middle amphibolite-granulite facies rocks of the Ivrea zone from lower-middle amphibolite facies rocks of the Strona-Ceneri zone. U/Pb, Rb/Sr and K/Ar whole-rock and mineral ages for samples from this region indicate that the two zones pursued different time-T paths after Caledonian peak metamorphism. Cooling curves for the two terrains finally converge in the late Triassic-early Jurassic, a time of large-scale crustal extension in the south Alps that anticipated the opening of the western Tethys. The Pogallo Line may represent a low-angle normal fault that developed at intermediate crustal levels during this period of extension and attained its present subvertical orientation during Alpine deformation. (Authors' abstract)-C.N.