Neglected basement ductile deformation in balanced-section restoration: an example from the Central Southern Alps (Northern Italy)

In the construction of balanced sections through thick-skinned belts, basement bodies are frequently assumed to be rigid and internally undeformed (with the exception of regional scale fault-bend-folding) and only backstripping of thrust faults is performed during their retrodeformation, possibly leading to underestimations of the regional shortening. This simplifying assumption is generally made because reliable information on internal strain in basement is lacking and the sedimentary cover of the internal parts of thrust belts has been removed by erosion. In a basement exposure in the eastern Orobic Alps, Alpine-age ductile structures (mainly chevron to sub- isoclinal folds rarely associated with cleavage formation) were recognized on the basis of overprinting criteria, folding style and fabric orientation and their areal distribution was determined by foliation trace mapping. Although unevenly distributed in different lithologies, ductile deformation is considerable (average shortening is 47%). Neglecting such internal strain during construction of balanced sections leads, for this area, to an underestimation of at least 10%, but could be more than 40% if the observed basement shortening is extrapolated to other basement-involved thrust bodies across the entire width of the deformed belt. It is concluded that detailed structural studies of the basement, for example those using a foliation trace mapping technique, are necessary to define the internal strain of basement bodies so that the shortening of thick-skinned belts can be more accurately calculated.