Folding of unconformable sequences in the Argentine Precordillera. Strain compatibility problems and resulting accommodation structures

The superposition of several orogenic episodes in the Argentine Precordillera with a similar shortening direction makes this region an ideal setting for the study of the reactivation of structures. The angular unconformity surfaces generated in these events record the reactivations of previous folds when a new orogenic episode is superimposed. Furthermore, the unconformity surfaces provide a reference framework to analyze the deformation compatibility problems that arise when two unconformable successions jointly deform. Reactivation of pre-unconformity folds was a common process, evidenced by the modification of the original unconformity angles, as a result of simple shear parallel to the layers on the folds limbs truncated by unconformities. The values of the angular shear are consistent with the degree of rotation of the fold limbs and the lithological nature of the formations involved. Furthermore, the modification of the unconformity angles implies length changes in the unconformity surface, which leads to strain compatibility problems between both sides of the unconformity, converting it into a detachment surface. Accommodation structures such as imbricated thrusts rooted into unconformity surfaces of the Precordillera are compared with similar structures described in other mountain ranges. Other evidence of fold tightening is the widespread occurrence of flexural slip faults cutting the unconformity surfaces. In an active mountain range like the Precordillera, the most common neo-tectonic structures are flexural slip faults cutting the unconformable Quaternary alluvial fans (current wedge-top deposits), which show the tightening of the folds at the present time. In fact, some of them have been active during historical earthquakes.