STRUCTURAL-ANALYSIS OF THE SCHAMS NAPPES AND ADJACENT TECTONIC UNITS - IMPLICATIONS FOR THE OROGENIC EVOLUTION OF THE PENNINIC ZONE IN EASTERN SWITZERLAND

The low-grade Mesozoic and possibly Tertiary metasediments of the Midd le Penninic Schams cover nappes have undergone a polyphase Alpine defo rmation history, which is superimposed on a complicated Jurassic paleo tectonic evolution. During the early stages of a first deformation eve nt (D1), the Schams sediments were detached from their crystalline sub stratum, which is most probably represented by parts of the present-da y Tambo and/or Suretta basement nappes. Continued thrusting and isocli nal folding during D1 caused the stacking of Middle and North Penninic nappes. For the overall shear deformation during D1 a top to the NW t o NNW movement direction can be inferred. Radiometric dating of D1 syn tectonically grown fine-grained white mica of the Schams nappes yields Eocene to Oligocene ages (45-30 Ma). From these data a Paleogene age is concluded for D1. It is associated with the stacking of the Pennini c nappe pile during the final stages of collision between the European and Apulian/Adriatic plate. A second phase of deformation (D2) during Oligocene and/or Neogene times severely modified the D1 nappe pile. D uring this phase part of the D1 nappe pile is inverted and backthruste d above the subhorizontal axial trace of a large-scale F2 fold (Niemet -Beverin fold). The flat-lying Suretta and Tambo basement nappes are s ituated in the core of this large-scale N-closing fold. Shear-sense cr iteria associated with D2 show dominantly top to the SE directed movem ents in tectonic units on the upper limb of the Niemet-Beverin fold. D uring intense deformation on this upper limb, F2 fold axes were reorie nted toward the approximately SE-NW to SSE-NNW trending transport dire ction. The second phase deformation is interpreted to be due to a diff erential subhorizontal, NW to NNW-directed tectonic transport within v iscous Penninic units below a rigid orogenic lid, consisting of previo usly emplaced and cooled South Penninic and Austroalpine units. A vert ical transport velocity profile is proposed with decreasing NW to NNW- directed velocities near the boundary between viscous Penninic units a nd the overlying orogenic lid. In this case, the top to the SE directe d movements only imply relative displacements within an overall NW to NNW-directed movement regime. The youngest deformation phase (D3) is p robably of Miocene age and consists of NW-vergent folds. It not only a ffects the Penninic basement and cover nappes, but also the basal thru st of the Austroalpine nappes.