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Mechanical control of a lithological alternation on normal fault morphology, growth and reactivation

¹ Université Paris XI, UMR 7072, Bât. 504, 91405 Orsay cedex, France. soliva{at}mines.unr.edu; Antonio.Benedicto{at}geol.u-psud.fr; vergely{at}geol.u-psud.fr
² Total E & P, Pau, France. thierry.rives{at}total.com

This paper presents an analysis of the control of lithological variation on normal fault morphology, growth and reactivation. We study a normal fault population contained within an inter-bedded sequence of marly-limestones and clay rich layers. The analysis of cross sectional and bedding plane exposure of faults reveals that the plastic clay layers act as barriers to vertical fault propagation. Only the long vertically restricted normal faults (i.e. confined between two clay layers) are later reactivated and show extensional-shear mode of deformation. The likelihood of reactivation of the faults was probably favoured by the small plastic strength of the clay rich layers. We discuss the extensional-shear mode in terms of structural context, reactivation and rock rigidity.

Displacement profile analysis of only isolated non-reactivated faults allows us to distinguish the faults mechanically influenced by the rheological discontinuities from those that are contained within the same lithological unit. Using both cross-sectional observations and displacement-length data of the fault population we estimate the average aspect ratio (length/height ~ 2) of the faults contained within the same lithological unit. A 3-D displacement-length scaling law that integrates post yield fracture mechanics (PYFM) and the principal fault dimensions (length and height) reveals the importance of the low rigidity of the marly-limestone on the displacement of the faults contained into a same lithological unit. A comparison of our displacement-length data with those compiled from the literature suggests that the displacement-length variability is strongly related to the rock mechanical properties and contrasts in layered rocks.

The bulk of our analysis, based on field observations and theory, shows that: (i) fault shape, (ii) fault ability to be reactivated, (iii) shear mode, and (iv) displacement-length values are strongly sensitive to the lithological contrasts, and are therefore dependent on the fault dimension relative to the thicknesses of the sedimentary bodies. Therefore, regardless the variety of fault initiation processes, our analysis confirms that both fault morphology and fault growth are not self similar in heterogeneous layered rocks from centimetre to kilometre scale.

Key Words: Normal fault • Layered rocks • Scaling law • Mechanical properties • Reactivation • Extensional-shear