D2 asymmetric folds and their vergence meaning in the Montagnola Senese metamorphic rocks (inner northern Apennines, central Italy)

Abstract

This study describes asymmetric folds whose vergence is derived from refolding of a pre-existing inclined foliation. The study area is in the Montagnola Senese area where Mesozoic rocks in the greenschist facies crop out. These rocks were affected by deformation during the collisional stage (late Oligocene–early Miocene) of the Northern Apennines (D1 event), and during the post-collisional extensional tectonics (D2 event) that affected the inner zone of the Northern Apennines since the early–middle Miocene. During the D1 event, SW dipping axial plane schistosity and non-cylindrical folds indicative of a highly heterogeneous strain developed. During the second event westward verging folds developed. These folds are characterised by thick steep limbs. The S2 foliation, which is well developed in narrow and localised zones, is mainly a pressure solution cleavage with NE plunging stylolitic teeth. Micro- and meso-structural observations are used to discuss the relation between cleavage distribution and strain during the D2 event. The evolution of D2 folds was mainly controlled by a dissolution process and by a component of partitioned shear-strain. In the relatively ‘high’ strain domains, deformation took place by the combined effects of volume loss and shear displacement. In low strain domains, deformation took place by a veining and flattening process. In this way, the pre-existing foliation can be deformed in folds with vergence opposite to the sense of shear.