Francis Odonne

Analogue modelling of fault reactivation: tectonic inversion and oblique remobilisation of grabens

Abstract Analogue models of sand above a silicone layer were examined to determine the effects of normal fault reactivation. Models were first subjected to extension, which lead to the formation of two linear grabens. Each model was then subjected to a second phase deformation, either parallel or oblique to the previous initial extension, and either extensional or contractional. The influence of sedimentation has been evaluated using experiments with and without sedimentation. In cases of oblique secondary deformation, all newly formed faults were parallel to the older grabens, thus they were oblique to the direction of the principal stress directions during the second deformation phase. In experiments without sedimentation, all older faults were reactivated, whereas in experiments with sedimentation, some of the older faults were not reactivated. In the case of an oblique compressional secondary deformation phase with post rift sedimentation, strain partitioning occurred between reactivated older normal faults and new reverse faults. σ 2 was vertical on reactivated normal fault planes, whereas σ 3 was vertical on reverse fault planes. In the case of oblique compressional secondary deformation phase without post rift sedimentation, no strain partitioning was observed. In this model, σ 3 was vertical on every fault plane. It is therefore concluded that sedimentation within grabens induces a variation of stress orientation and strain partitioning.

Analogue models of folds above a wrench fault

Abstract This paper describes the experimental deformation of models made with sheets of paraffin wax, simulating a bedded cover resting on a basement wrench fault. During the experiments, “en echelon” folds appear in the cover. As a result of early fault motion, folds first appear at heterogeneities in the bedding and with axes at about 45° to the trace of the wrench fault. Further fault displacement causes a bulk rotation of fold axes towards parallelism with the basement wrench fault, and a resulting curvature of fold axes at larger fault displacement. Folding affects an area which tends to quickly stabilize in width, since folding weakens the sheared cover and subsequent deformation is concentrated in it. Axial surfaces of folds are initially upright, then tend to become inclined with an external vergence, forming a fan centered on the basement wrench fault. Deeper layer-deformation, close to the basement, involves fold reorientations that are greater than in the upper layers. Therefore, down a given vertical line, there is no continuity between surface and deep structures. The geometry and orientation of folds appearing at later stages of wrenching is controlled by the geometry and orientation of already extant folds.

Restoration and balance of a folded and faulted surface by best-fitting of finite elements: principle and applications

Abstract A computer program is presented which allows us to test the restoration of a folded and faulted thin competent layer and then to balance this surface. The balance of such a surface is useful both to constrain the three-dimensional shape of the folds and the geometry of the limits of the faults. If a part of the surface is fixed the restoration can also give the finite displacement field linked to the deformation of the layer. The principle of the method is given and its accuracy is tested for the restoration of an experimentally folded sheet of paper. Finally the applicability to the restoration of natural structures is discussed.

Pluton emplacement during transpression in brittle crust: New views from analogue experiments

Analogue experiments were used to investigate pluton emplacement during transpression in brittle crust. Models consisted of silicone putty placed within slots in a rigid basement and an overburden of dry sand. The models were transpressed, causing emplacement of silicone intrusions into the overlying sand. Space was made for intrusions by upward displacements of overburden along oblique reverse shear zones, in some cases accompanied by extensional shearing above the intrusions. Silicone putty was progressively accreted to the bases of uplifted blocks, and growing intrusions rode up the shear zones, leaving space for accretion of more silicone. The resulting intrusion shapes depended largely on the geometries of shear zone systems, which in turn depended on overburden thickness. The intrusion profiles and the structural relationships between intrusions and host in the experiments are comparable to some natural examples. A magma accretion model may thus account for pluton emplacement and batholith assembly within some transpressive orogens.

Abnormal reverse faulting above a depleting reservoir

Subsurface deformation is observed during pumping of some hydrocarbon fields. Deformation features include subsidence centered on the field and subsidence-related centripetal horizontal displacements and faulting. Focal mechanisms yield reverse movements on steeply dipping faults. In our sand-silicone analogue model, the reservoir is represented by a latex balloon or by undercompacted ground sand. Deflation of the reservoir results in formation of steeply dipping reverse faults bounding a downward-opened cone. The cone moves downward to follow the reservoir contraction. Faults along the cone are straight beneath a thick reservoir cover and tend to curve upwards with decreasing cover. Our results, similar to natural structures observed around magma chambers, allow us to reinterpret Paul Segall9s numerical model of poroelastic stresses caused by changes in the distribution of pore fluids and draw a new pattern of active faults.

Influence of differential compaction above basement steps on salt tectonics in the Ligurian-Provencal Basin, northwest Mediterranean

Detailed mapping of the Ligurian-Provencal Basin (northwestern Mediterranean) indicates that salt diapirs in the deep basin are restricted to an area whose upslope boundary forms reentrants located above deep crustal transfer zones associated with the opening of the basin in Oligo -Miocene times. Because these basement faults were no longer active in Messinian and post-Messinian times, the geographic correlation between diapirs and basement faults cannot be attributed to slip along these basement faults coeval with salt tectonics. Using three physical experiments, we examine how dormant (i.e. inactive) basement steps can affect the development of the overlying salt structures during combined gravity-driven gliding and spreading. Where a basement step trends obliquely with respect to the direction of the slope and initially offsets the base salt, grabens and salt ridges form above and downdip from the basement step, in turn forming a reentrant pointing upslope. Where the basement step is buried under pre-Messinian compactable sediments, loading by Messinian and post-Messinian sediments causes differential subsidence above the step, forcing grabens and salt ridges to form above and updip of the basement step. There, too, the salt structures are confined in a triangular, reentrant-shaped area pointing upslope. The combination of these two mechanisms with passive diapir growth during Plio-Pleistocene times explains the striking geographic correlation between salt diapirs and basement structures in the Gulf of Lion. q 2003 Elsevier Science Ltd. All rights reserved.

Volume loss and deformation around conjugate fractures : comparison between a natural example and analogue experiments

Abstract In well-stratified sedimentary rocks, measurement of the bedding offsets along fractures gives a value for the relative displacement at each location. To restore the undeformed geometry of the layers, the offset along each fracture must be cancelled. Restoration of the geometry enables the location of places where shortening is concentrated to be established, namely at the fracture intersection and at fracture tips. Slow displacement during deformation is demonstrated by the growth of fibres on the fault surfaces. Pressure solution inside the layers is responsible for the internal strain with associated volume loss. This mechanism is compatible with low strain rates giving aseismic displacements along the fractures. To model rock rheology dominated by pressure solution, a viscous material (paraffin wax) has been employed in the analogue model. Volume loss in nature is represented by area changes during the analogue experiments. In the experiments, shortening is concentrated around the fractures in exactly the same geometrical positions as in nature. When the amount of displacement is different on each of the two fractures, one of the fractures is observed to offset its conjugate fracture. When this is the case, relative timing of displacement along conjugate fractures cannot be determined without ambiguity from the observation of fracture offsets.

Dependence of joint spacing on rock properties in carbonate strata

Joint density is studied in relation to petrographic and petrophysical parameters in two sedimentary carbonate formations characterized by different diagenetic histories: the Kimmeridgian limestones of the Chay Peninsula (western France) with a mean joint density of 6.37 fractures per meter (fr/m) (1.94 fr/ft), and the Bathonian limestones of the Bouye outcrop (western France) with a mean joint density of 1.9 fr/m (0.58 fr/ft). The Chay carbonates are characterized by a lower CaCO3 content, a higher average porosity, and a lower sound velocity than values recorded in the Bouye limestones. The compressive strength and Youngs modulus of the Bouye carbonates are, respectively, 10 and 3 times higher than in the Chay carbonates. A statistical analysis was used to identify relationships between joint density and carbonate rock properties. When facies variations are marked, the joint density at outcrop scale is related to the mean bed thickness, the facies descriptors, and the Youngs modulus. When textural variations are more limited, the joint density is controlled by the porosity. At the scale of a sedimentary basin, 63.8% of the variation in joint density may be accounted for by Youngs modulus and the sparite/micrite ratio. The decrease in the sparite/micrite ratio reflects an increased number of grain boundaries in the carbonate rock, which limits grain deformation and enhances joint density. The variations in Youngs modulus depend essentially on the porosity and mineralogy of the studied rocks. Any increase in CaCO3 content or decrease in porosity is associated with an increase in the elastic properties of the rock and a reduction of joint density.

The control of deformation intensity around a fault: natural and experimental examples

Abstract The deformation intensity around a wrench fault is studied through a comparison between a well-known natural fault and experimental models. Each model consists of a horizontal paraffin wax layer, containing a precut plane, which was submitted to uniaxial compression. The Meyrueis Fault, in the Jurassic formations of the Causses massif (southern French Massif Central) gives the opportunity of estimating strain around a wrench fault. The comparison of this fault with the experimental models shows a good similarity in relation to the following aspects: variation of the principal strain axes around the fault; rotation of the strain axes with time, which confirms a non-coaxial strain history; heterogeneity of strain with some domains strongly deformed and others protected; symmetry of these domains with respect to the fault; and the strong influence of kinematic boundary conditions in the direction perpendicular to the compression. These features provide new criteria for the interpretation of natural wrench faults.

Kinematic behaviour of an interface and competence contrast: analogue models with different degrees of bonding between deformable inclusions and their matrix

Abstract In a system composed of a ductile matrix with a particle included in it, the deformation of the matrix, the displacement field, and the rotation and the deformation of the block are related to the viscosity ratio between block and matrix and also to the degree of bonding between the matrix and the block. In this paper, belemnites from the Lower Lias of the Alps provide a natural example of moderately deformed objects included in a slaty matrix. They are compared with analogue models made of paraffin, in which a long block of a more competent paraffin is included. Depending on the degree of bonding between matrix and object, the same block appears to be deformable or quite rigid. With a high degree of bonding, the strain refraction observed corresponds to the viscosity ratio between the block and the matrix. When the degree of bonding is weak, the block is quite undeformed, its rotation is great and the deviation of the strain in the matrix is analogous to that observed around a fault. The kinematic conditions appear to have a greater effect on the finite strain than the viscosity ratio of the materials.