The Role of Mechanical Anisotropy in Controlling Fault Trajectories within Multilayered Carbonate and Clay-rich Rocks

The mechanical stratigraphy of multilayered carbonates and clay-rich rocks may have a major influence in petroleum system, since marly horizons potentially act as top seals. Faults affected these heterogeneous lithology show complex geometries. We here investigate, through structural field analysis and laboratory rock deformation experiments, the role of fractures and faults in exhumed mechanically multilayered rocks in order to better constrain the deformation mechanisms characterizing heterogeneous rocks. This study aims at evaluating 1) the fault zone evolution from incipient failure to mature faults, with increasing displacement up to tens of metres; 2) the role of the mechanical properties of multilayered rocks in fault initiation and evolution. Within mechanical multilayers, faults show staircase trajectory characterized by steeper portions in calcareous competent layers and flatter portions in incompetent marly layers. Mechanical data further provide an independent constrain for the high θ angles observed in the outcrop in correspondence of marly layers. With the progressive accumulation of displacement, this refracted trajectory tends to develop dilational jogs at first, then to straighten their trajectory and develop wider fault zones. Moreover, the overall asymmetry of the structure, due to the stress orientation, points out the important role played by the anisotropy in controlling fault geometry.