POLYPHASE ALPINE DEFORMATION AND METAMORP HISM IN THE NORTHERN CIMA-LUNGA UNIT, CENTRAL ALPS (SWITZERLAND)

The Cima Lunga unit correlates tectonically with the middle Penninic A dula nappe: It consists mainly of pelitic and semipelitic gneisses, wh ich contain a suite of basic and ultrabasic lenses. Some of these lens es contain mineral parageneses of an Eocene (45 to 40 Ma) high-pressur e subduction metamorphism. No high-pressure relies have as yet been fo und in the surrounding gneisses. In all lithologies four phases of Alp ine deformation (D-1 to D-4) have been recognized. The structural rela tionships indicate that these lithologies must have been juxtaposed pr e- or at least syn-D-1. The first phase D-1 is characterized by strong isoclinal folding, an axial plane foliation S-1 and a stretching line ation L(1) (ca. NNE-SSW) and can be related to the nappe stacking of t he Cima Lunga unit over the underlying Simano nappe. Mineral inclusion s in garnet-porphyroclasts in pelitic gneisses indicate a prograde, si ngle-phase garnet growth under amphibolite facies conditions (6-8 kbar ; 600-660 degrees C). The second phase D-2 produced a series of flat-l ying tight to isoclinal megafolds, an axial plane schistosity S-2, and a NNW-SSE oriented penetrative stretching lineation L(2). D-2 fold ax es are parallel to L(2) Shear bands, rotated porphyroclasts and asymme tric foliation boudinage indicate a top-to-the-SSE sense of shear. D-2 , probably in combination with D-1, leads to strong boudinage of the m afic and ultramafic lenses in two directions (E-W and NNW-SSE). The am phibolite facies meso-Alpine regional metamorphism reached its peak sy n- to post-D-2. The third phase D-3 developed more open folds with ste ep NW-SE-striking axial planes dipping to the SW and took place under amphibolite facies conditions. A fourth phase D, produced a very local crenulation in mica-rich schists. The growth of chlorite in pressure shadows of garnet-porphyroclasts indicates greenschist facies conditio ns. All four structural phases are also common to both the over- and u nderlying nappes of the Cima Lunga unit, however Eocene high-pressure metamorphism is unique to the Adula-Cima Lunga nappe system. Therefore the high-pressure metamorphism is considered to be pre-D-1, whereas t he amphibolite facies meso-Alpine regional metamorphism is common to a ll nappes and is syn- D-2 to D-3. The geometry of the Cima Lunga unit largely represents a fold interference pattern as the result of the tw o almost coaxial deformation phases D-1 and D-2. D-2 produced a large fold closing to W. This explains why the Cima Lunga unit does not appe ar W of Val Verzasca. The boundary between the Simano nappe and the no rthern Cima Lunga unit is located in a horizon of carbonatic rocks, ma fic and ultramafic lenses and appears to be folded by these deformatio ns, too.