This work reports structural and anisotropy of magnetic susceptibility (AMS) measurements from an Upper Miocene décollement level exposed in the central Apennine fold-and-thrust belt of Italy. The décollement level is located at the boundary between the Mesozoic-Cenozoic carbonate sequence and the overlying Miocene-Pliocene syn-orogenic siliciclastic sediments and consists of a thick shear zone, mainly affecting detritic limestones, dominated by pressure solution cleavage oblique to the bedding.
In the study area, the décollement level was active during the early stages of evolution of the fold-and-thrust belt, when deeper décollement levels were active in more internal zones, and accounted for the bulk top-to-foreland translation of the siliciclastic portion of the multilayer. The increasing deformation led to the progressive foreland migration of deeper décollement levels (and of thrusts emanating from these levels) that were active in the more internal sectors. Consequently, the Mesozoic-Cenozoic carbonate sequence was folded and faulted and the studied décollement became inactive and was incorporated into the fold-and-thrust belt. Pressure solution cleavage within the décollement became a passive marker that underwent rotations associated with later deformational phases. Results indicate that pressure solution cleavage registers two phases of deformation: (1) during thrusting and folding, when beds were folded and (2) at a later stage, when cleavage was rotated, together with thrusts and folds, about vertical axes. This second stage is interpreted as having been induced by the oblique inversion of a Liassic normal fault, during the Miocene compression. Copyright © 2010 John Wiley & Sons, Ltd.