Normal faults, layering and elastic properties of rocks
Résumé
We study mesoscale normal faults cutting alternating limestone and clay-rich layers in several localities in the South-Eastern Mesozoic sedimentary basin (France). The displacement gradients, defined as the displacement variation per unit length along fault profile, and the mean fault dips are correlated to the structural and petrophysical properties of the host rock, including the carbonate content, the stiffness, the layering pattern and the maximum burial depth. Analysis of the fault dips indicates that the faults propagate (downward or upward) from an initial fracture from one unit to another rather than through connections of fractures that nucleated in different units. The fault dips are consistent with shear, mixed mode and tensile failures in limestone units. They are consistent with shear failure or are abnormally low in the clay-rich units. Among the studied attributes, in limestone units the failure mode is related to the contrast of the Young's modulus between the limestone and the clay-rich layers. A high contrast promotes tensile failure, whereas a low contrast promotes mixed mode or shear failures. In clay-rich layers, the dip is related to the layering pattern and abnormally low dips are promoted in thin clay-rich layers surrounded by thick limestone units. Specific displacement gradients characterize each lithology of the layered section. It ranges from 0.06 to 0.2 in the clay-rich units and it increases with Young's modulus. The analyses of the fault dips and the displacement gradients have implications in term of local stress. Both analyses converge and they can be related to a variation in stress controlled by the variation in stiffness through the layering.