Foliation relationships and structural facing vs. vergence determinations in refolded low-grade metamorphic rocks: an example from the Tuscan Metamorphic ‘Basement’ (Northern Apennines, Italy)

Abstract

Three tectonic foliations and their angular relation with bedding allow the determination of facing vs. vergence and fold interference pattern in polyphase low-grade metamorphic terrains, composed of Palaeozoic and Triassic turbiditic rocks cropping out in the inner part of the Northern Apennines (Mt Leoni area). The D 1 deformational event (Late Oligocene–Early Miocene) is typified by southeast-verging folds (F 1) and related tectonic foliation (S 1) with a HP–LT mineralogical assemblage, developed during the emplacement of the Northern Apennines tectonic units. The D 2 deformational event (Early–Middle Miocene) caused F 2 east-verging folds and related tectonic foliation (S 2) during greenschist facies metamorphism. The D 3 deformational event (Middle Miocene–?) formed F 3 gentle upright folds with a foliation (S 3) developed only in the fold closures, and never accompanied by blastesis. Foliation angular relationships, as well as their intersections with the bedding, allowed us to define the facing, vergence and interference pattern of map-scale folds. Where a penetrative S 2 crenulation-cleavage affected the metapelitic tops of turbiditic strata, S 1 structural facing could be misinterpreted at the outcrop scale, simulating a northwest-vergence of F 1 folds. This is due to a false dihedral angle between S 0 and S 1 occurring in the metapelites due to the rotation of S 1 in the hinges of the overprinting crenulation-cleavage domains (S 2).