STRUCTURE AND DEFORMATIONAL HISTORY OF THE INFRAHELVETIC FLYSCH UNITS, GLARUS ALPS, EASTERN SWITZERLAND

Structural mapping of the Infrahelvetic flysch units has revealed a co herency within the Sardona and Blattengrat units. An important late th rust, the Orglen thrust, has been traced into the Sardona unit. Two fo ld phases have been recognised: 'similar', asymmetrical, N-NNW-vergent folds with wavelengths of 500 to 1500 m, which plunge gently to the E /ENE or W/WSW and are associated with an axial planar cleavage, plus a second, overprinting N-S series of open folds. Kinematic data reveal that penetrative compressive deformation associated with N- to NNW-dir ected movement on the Glarus Overthrust was followed by E-W striking d extral faulting apparently only affecting the Infrahelvetic units. Ove rprinting by later deformational phases has largely obscured penetrati ve structures associated with the earliest phase of deformation, in wh ich the embryonic North Alpine Foreland Basin was telescoped onto the European (Helvetic) foreland. However, the structural contacts between the Sardona, Blattengrat and North Helvetic Flysch units can be resto red to southward-dipping thrusts, suggesting that these early structur es formed under horizontal compression. A thrust wedge of Penninic and Austroalpine nappes migrating towards the foreland might have provide d the necessary horizontal force to detach these units and translate t hem northwards. The Infrahelvetic flysch units were subsequently burie d to depths of 8-12 km within the nappe pile.