- University of Cagliari, Department of Chemical and Geological Sciences, Monserrato, Italy (fabrcocco@unica.it)
Fold-and-thrust belts (FTBs) usually develop in the external zones of the orogens, between the mountain belt and the foreland basin. The structural style of FTBs varies greatly depends on the mechanical stratigraphy and the influence of inherited tectonic features. Back-thrusts are structures typical of FTBs, but they are commonly less frequent than fore-thrust. The Variscan FTB outcropping in SW Sardinia is characterized by the extensive development of back-thrusts that affect a poly-deformed and mechanically heterogeneous stratigraphic succession, suggesting a cause-and-effect relationship. We investigate the geometry and kinematics of back-thrusts and the role of structural inheritance, in order to better understand the mechanism of their progressive development.
The Variscan FTB of SW Sardinia consists of two stacked tectonic units, the Iglesiente and Arburese units, separated by a regional Variscan structure, the Arburese thrust. The Iglesiente Unit has been overthrusted by the Arburese Unit with a top-to-the-west transport direction during the collisional phase of the Variscan Orogeny, in Early Carboniferous times.
The geological setting of the Iglesiente Unit arises from a complex stratigraphic and tectonic evolution because of the superposition of the lower Cambrian extensional tectonics, the compressional Ordovician Sardic Phase and the Variscan deformation. The following superposed structures characterize the Iglesiente Unit: 1) N-trending normal faults; 2) E-trending Sardic close folds; 3) E-trending Variscan open folds, 4) N-trending Variscan inclined folds; 5) Variscan fore- and 6) back-thrusts. As the back-thrusts are the youngest structures, they develop in a non-layer cake stratigraphic succession, cutting across strata whose steepness ranges from horizontal to vertical and the strike varies from parallel to perpendicular to the thrusts.
Our findings from field surveys and cartographic and structural analysis suggest that the extensive back-thrusting development is due to the occurrence of the structural domes that acted as an inherited buttress that prevents the fore-ward propagation of deformation. The structural domes formed because of the superposed E-trending Sardic and N-trending Variscan folds and consists, at the core, of sandstones, limestone and dolostones of the lower Cambrian succession.
During their progressive emplacement, the back-thrusts cut across the Sardic folds, that have the axes perpendicular to the back-thrusts strike. Thus, back-thrusts cut across vertical strata in the limbs of the fold and sub-horizontal strata in the hinge of the fold. We infer a relationship between steepness and displacement of back-thrusts and the attitude of the strata involved. Moving from the limb to the hinge of the folds, the steepness and the displacement of the back-thrust decrease. The geometry of the thrust surface varies accordingly, taking up either a synformal shape when cut across a hinge of a synform dipping in the same dip direction of the thrust, or an antiformal shape when cut across a vertical limb perpendicular to the thrust strike. Thus, what looks like a folded back-thrust is rather an effect due to the geometric and mechanical anisotropies of the involved stratigraphic succession.
How to cite: Cocco, F. and Funedda, A.: Large-scale back-thrusting development in fold-and-thrust belts: the case study of the Variscan External Zone of Sardinia, EGU General Assembly 2025, Vienna, Austria, 27 Apr–2 May 2025, EGU25-16259, https://doi.org/10.5194/egusphere-egu25-16259, 2025.