Discussion
Most of the applied aspects and the main geometrical and structural features of the faults developed in the sand models can be highlighted and described looking at maps and sections taken from the models and from the measurements performed on them (see Bonini et al., in prep). Nevertheless, there are topics and evidence that can be thoroughly understood only by means of a 3D reconstruction and visualisation.
1) The first is to follow the trend of the faults also in depth, inside the model, and not only in map. The bending changes both in the fault trace and inclination can be followed during the evolution of the model (i.e. through the different sand-box experiments) and also in the single evolution step in a continuous way all over the width of the models. Looking at the 3D visualisation of the brittle structures, it’s evident how the faults start from the base of the apparatus (i.e. from the active LANF) and progressively develop toward the topographic surface, reaching it also in the very early stages of deformation (0.5cm), following the increase of the displacement imposed by the backstop on the LANF. This is particularly evident in the first evolution steps where newly formed faults are present on the topographic surface.
2) A detailed analysis of the kinematics and type of the faults evidenced a displacement gait that is not constant and homogeneous through the width of the models. The setting of the main faults is, in fact, hardly influenced at the tips of the faults where the strike of fault planes evidently deviates from the strike of the main buried LANF. A complexity of the faults and their displacement amounts is very evident; this could not be possible or, at least, it could be much more difficult looking only at single cross sections. Being able to build the cut-off lines along a fault plane and observe them in a continuous and 3D way allowed us to highlight how deformation is partitioned along different fault planes while extension increases. The partition emerges only thanks to 3D reconstructions that enables and facilitates the study of displacement on a single fault.
3) The study on faults and their displacements is directly related to the topographic variations observed, in particular, in the central part of the models as the imposed basal displacement increases. In an area more or less in the middle of the sand-box, an evident and asymmetric graben takes place starting from the very early stages of deformation (few millimetres). The topography variation, visible also from some 2D sections (in particular from those coming from the central sector in the most advanced steps of the models), is clearly evident and well-defined by a complete analysis of the topographic surface of the whole model. Moreover, by associating 3D model depressions on the topography and normal faults, it is easier to determine which faults hold the main responsibility for the topographic variations.
From a complete reconstruction of the surface of the models, one can determine the area where the graben takes place and also its lateral terminations. Different amounts of displacement can explain the shape of the topography. Generally topographic depression (visible also in the natural case) is due to tectonic causes and the overall structural setting of the area is driven by the presence of an active LANFs.