Analogue modeling of positive inversion tectonics along differently oriented pre-thrusting normal faults: An application to the Central-Northern Apennines of Italy

Inversion tectonics represent a key process in many orogens worldwide. The related mechanisms of fault reactivation and the effects of an articulated preshortening setting on thrust and fold development are still challenging questions. Modes and geometries of inversion have been the object of several analogue models. In this work, we analyzed the influence of an articulated high-angle preexisting discontinuity in the development of thrusts using sandbox modeling. The model geometry is based on the architecture of the major faults in the Central-Northern Apennines of Italy, where differently oriented Mesozoic-Cenozoic inherited extensional structures are clearly detectable and display contrasting styles of positive inversion tectonics. Quartz-sand is the analogue material adopted to model Mesozoic-Cenozoic sedimentary successions, and glass microbeads represent preexisting fault rocks. The geometry of the segmented preexisting structure is composed of two segments with the same dip (similar to 60 degrees): one oblique and another orthogonal to the shortening direction. Our results show that different styles of positive inversion tectonics can coexist and that the obliquity angle between inherited structures and the shortening direction is a leading factor controlling the degree of inversion: The oblique segment of the discontinuity exhibits a complete reactivation, whereas along the orthogonal segment, shortcut is the prevalent mechanism. The oblique element, moreover, represents a cross-strike discontinuity that guides the localization and curved geometry of the thrusts, compartmentalizing the deformation. Our findings can be applied to foldand-thrust belts characterized by the presence of cross-strike discontinuities.