A finite-element model of the stress field in strike-slip basins: implications for the Permian tectonics of the Southern Alps (Italy)

Abstract

A set of finite-element models have been used to simulate pull-apart basin formation related to a releasing overstep along two right strike-slip faults. The modelling results have shown the stress field at a releasing fault overstep with respect to strike-slip, transtensional and transpressional boundary constraints. Based on the assumption that normal faults strike parallel to the maximum horizontal stress we have predicted the orientation of potential normal faults within the overstep zone. Linear elastic rheology and plane stress conditions were assumed, and four kinds of models with different overlap/separation ratios (0, 0.5, 1 and 2) between the strike-slip faults were analysed. Displacement boundary conditions were applied to the upper left-hand and lower right-hand corners of the models in order to simulate a right strike-slip movement; outward and inward displacement components were applied respectively to reproduce transtensional and transpressional conditions. All the models mostly show an extensional stress field (i.e. with at least one extensional principal stress axis). Large zones inside the overstep are affected by general extension (both principal axes are extensional), and their size increases with both an increase in the overlap/separation ratio and with changes from transpressional to transtensional conditions. The areas of maximum extension are located near the ends of the fault segments inside the overlap area. It is fairly probable that the genesis of the extensional faults that bound the pull-apart basins observed throughout the world occurs in these maximum extensional areas, and that the extensional faults are parallel to the maximum compression stress axis trajectories. The angle between the potential normal faults and the strike-slip faults is strongly dependent on the overall boundary conditions (i.e. pure strike-slip, transtension and transpression) of the model. In contrast, the elastic rock properties and the overlap/separation ratio of the faults have a minor control. The angle between the normal and strike-slip faults can vary from a few degrees in transtensional conditions to 100°–120° in transpressional conditions. Inside the overlap zone an area of subsidence occurs, the axis of which is coincident with the junction of the strike-slip fault tips, where it is also at a maximum (i.e. in the same zones of maximum extension). A comparison of the modelling results with the structural setting of the Collio and Pramollo Permo-Carboniferous pull-apart basins (Southern Alps, Italy) indicates that the Collio basin developed under strike-slip conditions, while the Pramollo basin was formed under transtensional conditions.