REGIONAL METAMORPHISM AND UPLIFT ALONG THE SOUTHERN MARGIN OF THE GOTTHARD MASSIF - RESULTS FROM THE NUFENENPASS AREA

A new hypothesis for the strong N to S increase of metamorphic conditi ons in the Nufenenpass area was made possible by identifying the Corno schuppe as a Penninic tectonic unit, possibly as a part of the sedimen tary cover of the Lebendun crystalline nappe. Detailed lithological co mparison between the Corno profile and the adjacent Ultrahelvetic Nufe nenzone showed rocks typical for the Northpenninic realm, such as Perm o-Carboniferous psammitic gneisses, much higher abundance and variety of Triassic marbles as well as conspicuous garnet-quartz-plagioclase-f elses. The lithological evidence is supported by microstructural obser vations of chemically similar strata mainly showing that garnet porphy roblasts grew post-deformational in the Lias of Nufenenzone but are st rongly rotated in corresponding Corno rocks. A metamorphic profile fro m the Gotthard massif into the Penninic Bundnerschiefer is fully consi stent with this interpretation. Rocks N from the newly defined Helveti c-Penninic tectonic boundary hold peak-conditions grouped between 465 to 490-degrees-C and 4800 to 5000 bar whereas the Penninic rocks hold 515-525-degrees-C around 6000 bar. The abrupt increase of metamorphic conditions on one hand underscores petrographic and microstructural ob servations and on the other hand documents syn- to post-peak metamorph ic motion along the Helvetic-Penninic boundary, lifting the southern u nits with respect to the northern ones. This locus of major movement i s characterized by a striking abundance of hornblende-garbenschiefer i n its close vicinity. Mineral growth-deformation relations show that h ornblende is a syn- to post-D3 porphyroblast. These rocks interestingl y hold the same peak-P-T conditions as do the Penninic rocks in the ar ea which poses new questions about timing and locus of the climax of T ertiary metamorphism in the northern Lepontine Alps. Clearly the stron g increase of metamorphic conditions in the Nufenenpass area is due to juxtaposition of higher- and lower grade rocks. There is no evidence for an anomalously high geothermal gradient which has formerly been us ed to explain the metamorphic pattern. Observed growth deformation rel ations as well as the metamorphic profile can be explained in terms of syn- to post-peak vertical block movement (RIDLEY, 1989). Further stu dies along the southern border of the Gotthard massif, e.g. in Val Pio ra, will show whether the model of major syn- to post-D3 motion can be applied in an extended regional scale.