Causes of Extensional Deformation in Front of the Corner of the South Alpine Indentor: an Experimental Study

Deformation of the Eastern European Alps in the Tertiary is a classical example of indentation tectonics. The northward motion of the South Alpine indentor caused the lateral escape of a crustal wedge confined by the sinistral ENE-striking SEMP Fault in the north and the dextral WSW-striking Pustertal Fault in the South. In front of the corner of the indentor significant E-W extension occurred and was accommodated along the N-S striking Brenner Fault. This low-angle normal fault forms the western boundary of the Tauern metamorphic dome. Experimental models scaled for density and viscosity investigated the effects of indentation obliquity and rheological stratification on the deformation patterns caused by continental indentation. The shape and orientation of the indentor were inspired by the Dolomites indenter of the Southern Alps (Italy). The results of our experimental models showed that small changes in the angle of convergence induce marked differences in the patterns of deformation. The only models whose fault patterns satisfyingly reproduced that of the Eastern Alps were characterized by NNE-directed motion of the indenter. In these models E-W extension formed in front of the leading edge of the indenter, as observed in the Eastern Alps along the Brenner extensional fault. Extensional deformation of the models maintained compatibility between the areas located on both sides of the indenter edge, which shortened at different rates and in different directions. Therefore, extension developed as a consequence of the indenter geometry during bulk shortening of the lithospheric model. Extension was not caused by gravitational instabilities, but by the kinematic and geometrical boundary conditions imposed by the indenter shape and the convergence direction. The analogy between the deformation pattern of these experiments and the one of the Tertiary Eastern Alps suggests that E-W extension along the Brenner Fault, i.e., in front of the corner of the South Alpine indentor, is not driven by a large scale tectonic phase of extension, nor by orogenic collapse, but rather by the kinematics and geometry of the indentor corner during collision.