[1] 3D numerical modeling has been used to investigate how the variations of mechanical properties in sedimentary layered sections affect the development of normal faults. We calculated the distribution of the Coulomb stress to assess the proximity of the layers to failure through an elastic layered section. The simulation of various combinations of rock properties allowed us to compare the effect of the stiffness and strength contrasts, which promote or inhibit faulting in the stiff layer, respectively. For rock systems showing little variation in strength, nucleation of the fault occurs in the stiff layer (e.g., limestones or sandstones), whereas it occurs in the compliant layer (e.g., clay-rich rocks) if the stiff layer has a high cohesion. Considering a mean strength profile of the carbonate sequences, nucleation occurs in limestones if the ratio of Young's moduli between the limestone and clay-rich rock is greater than 2; otherwise, clay-rich layers fail first. We also showed that nucleation is promoted in sandstones or limestones if these layers are thinner than the clayey layers. In a second set of simulation, using a slip on a fault, we examined the conditions needed to overcome the restriction of the fault propagation. Our results suggest that the lateral propagation of the fault, within a layer, produces increasingly favorable conditions for vertical propagation. A maximum aspect ratio of width to height of 13 is predicted for faults in limestone-clay sequences, and this maximum aspect ratio is expected to decrease as the contrast in the rock properties decreases.Citation: Roche, V., C. Homberg, and M. Rocher (2013), Fault nucleation, restriction, and aspect ratio in layered sections: Quantification of the strength and stiffness roles using numerical modeling,

The ability to generate deep flow in massive crystalline rocks is governed by the interconnectivity of the fracture network and its permeability, which in turn is largely dependent on the in-situ stress field. The increase of stress with depth reduces fracture aperture, leading to a decrease in rock mass permeability. The frequency of natural fractures also decreases with depth, resulting in less connectivity. The permeability of crystalline rocks is typically reduced to about 10 −17 -10 −15 m 2 at targeted depths for Enhanced Geothermal Systems (EGS) applications, i.e., > 3 km. Therefore, fluid injection methods are required to hydraulically fracture the rock and increase its permeability.

Mechanical heterogeneity of a sedimentary sequence exerts a primary control on the geometry of fault zones and the proportion of offset accommodated by folding. The Wildensbuch Fault Zone in the Swiss Molasse Basin, with a maximum throw of 40 m, intersects a Mesozoic section containing a thick (120 m) clay-dominated unit (Opalinus Clay) over- and underlain by more competent limestone units. Interpretation of a 3D seismic reflection survey indicates that the fault zone formed by upward propagation of an east–west-trending basement structure, through the Mesozoic section, in response to NE–SW Miocene extension. This configuration formed an array of left-stepping normal fault segments above and below the Opalinus Clay. In cross-section a broad monoclinal fold is observed in the Opalinus Clay. Folding, however, is not ubiquitous and occurs in the Opalinus Clay where fault segments above and below are oblique to one another; where they are parallel the fault passes through the Opalinus Clay with little folding. These observations demonstrate that, even in strongly heterogeneous sequences, here a four-fold difference in both Young's modulus and cohesion between layers, the occurrence of folding may depend on the local relationship between fault geometry and applied stress field rather than rheological properties alone.

The distribution of displacement along faults is a key parameter in various areas of geology such as earthquake studies, threedimensional strain restoration, fault growth, and reservoir and seal strata relations in hydrocarbon systems. It is essential therefore to understand how local conditions govern displacement distribution. We analyse dip-parallel displacement profiles of normal faults cutting five alternating limestone and shale layers and we discuss their evolution, from their nucleation to their restriction by lithological interfaces or bed-parallel faults in clays, or to their further propagation through several layers.Local displacement gradients control the shape of displacement profiles and are highly variable over the course of fault history. Accordingly, the Dmax-L relation is nonlinear. Bed-parallel faults prove stronger restrictors than lithological interfaces and the correlation of the local gradient with lithology during restriction and propagation indicates that knowledge of these gradients is required if we are to understand how faults develop in multilayer systems.Evolution of the fault displacement profile • V. Roche et al.