QUATERNARY REACTIVATION OF FLEXURAL-SLIP FOLDS BY DIAPIRIC ACTIVITY -EXAMPLE FROM THE WESTERN EBRO BASIN (SPAIN)

In the western sector of the Ebro Basin two types of structures deform the Quaternary terraces and pediments developed by the Ebro River and its tributaries: (1) folds up to 10 km long in the lower levels of fl uvial terraces and (2) normal listric faults that produce tilting and rollover anticlines of the Quaternary deposits. Both types of structur es are linked to the geometrical and lithological features of the Tert iary beds that underlie the Quaternary deposits. Quaternary folds, who se axes are parallel to the strike of the Tertiary beds, are the resul t of reactivation of Tertiary large-scale (60 km long) folds due to di apirism of their gypsum cores, where the gypsum units reach a maximum thickness of 3 000 m. This reactivation produced flexural-slip in some beds on the limbs of the folds, bringing about the folding of the ove rlying Quaternary terraces. The mechanism of Quaternary folding involv es layer-parallel shear in alternating Tertiary units and folding link ed to detachments and reverse layer-parallel faults. Normal listric fa ults are widespread throughout the studied area. They are partly paral lel to low dipping Tertiary beds and are the result of a NNE - SSW tec tonic extension, compatible with minor structures and focal mechanisms of recent earthquakes. The relationship between the two kinds of Quat ernary structures indicates that diapirism of the gypsum cores of the anticlines was activated by extensional tectonics.