Evolution, formation mechanism and kinematics of a contractional shallow shear zone within sedimentary rocks of the Northern Apennines (Italy)

Abstract

Integration of a meso- and microstructural study with illite-cristallinity, X-ray and SEM analyses allow us to describe the fabric, kinematics, evolution and formation mechanism of a shallow level shear zone, developed during thrusting within the Tuscan Nappe, the deepest non metamorphic tectonic unit of the Northern Apennines. This shear zone has been described for the Mt. Aquilaia area, located west of the Mt. Amiata geothermal area. Shearing took place during the Late Oligocene-Early Miocene and accompanied the superimposition of Middle Jurassic carbonate rocks (Marne a Posidonomya Fm.) on the Eocene-Oligocene pelagic sequence (Scaglia Toscana Fm.). The shear zone, less then 10 m in thickness, consists of strongly deformed rocks which can be subdivided in three shear zone domains characterised by different structures indicating top-to-the-east shearing. The A domain (about 1 m thick) is characterised by an highly damaged zone composed of a clayey gouge with centimetre and decimetre limestone clasts dispersed within the clayey “matrix”. The B domain (max 4 m thick) consists of detached and overturned decimetre folds, a pervasive tectonic foliation, reverse faults and s-c structures which affected the Jurassic calcareous rocks. The C-domain (max 2 m thick) consists of s-c structures. The shear zone developed through two uninterrupted stages of a single episode of deformation: the first stage produced the highly damaged zone within the Scaglia Toscana clayey rocks (A-domain) and the folding within the Marne a Posidonomya calcareous rocks (B-domain); the second stage concentrated on the A-domain, caused the detachment of the folds and the development of reverse faults within the B-domain, and produced the development of S-C structures within the C-domain. During shearing, strain was partitioned. Strain partitioning was mainly controlled by different processes, such as: (a) lithological contrast, (b) lithological anisotropy and (c) fluid-assisted deformation and related metamorphic reactions. Pressure solution and solution transfer are the mechanisms which produced the S1 foliation. A very low-grade metamorphism affected only the very strongly deformed pelitic dominantly rocks. The metamorphism produced the development of new clay minerals (illite-illite/smectite), only developed within the cleavage domains of the folded rocks. This has been interpreted as mainly due to deformation heating, coupled with fluid circulation associated with cleavage development.