Extension in compressional orogenic belts: The eastern Alps

Extension in the eastern Alps encompasses the following: (1) Extension accompanying simple shear crustal stacking. Subhorizontal stretching occurs along flat-lying foliations and regionally consistent stretching lineations with associated high strains. On a local scale, stretching results from extrusion of ductile sediments squeezed between rigid basement blocks basement blocks during thrust-sheet imbrication. The overall geometric effect is crustal thickening. (2) Classical extensional tectonics. Extension first occurs as a consequence of accretion and underthrusting of continental and oceanic material and represents gravitational adjustment of an unstable orogenic wedge. Extension then occurs as a consequence of terminal continental collision and is accommodated by lateral extrusion of crustal of crustal blocks. The overall geometric effect is crustal thinning. Extension is generally oriented subparallel to the strike of the orogen.