No abstract

[1] In this paper we present the results of the analysis of fault spacing from a population of faults confined to a 4.5 m thick mechanical layer. We demonstrate the control of a discrete layer on the specific geometry of a so-called ''domino-style'' or ''bookshelf'' fault population. The fault population shows a logarithmic-normal frequency distribution of fault spacing, with a minimum value of spacing S* ($0.25 layer thickness), revealing a nearly regular spacing distribution between the ''long faults'' (i.e., length greater than height), which are confined within the layer. We also observe an upper limit of fault linkage at relay ramp close to the minimum value of spacing S*, after which free overlapping between faults having the same dip direction is allowed. On the basis of field observations, we simulate the quasi-static displacement-related Coulomb shear stress perturbation of faults of various aspect ratios (length/height). The models show that on faults that increase in aspect ratio with a constant height (as expected for the confined faults), the horizontal extent of the local stress reduction tends to localize at a constant distance from the fault surface close to S*. For the studied case, the correspondence between the models and the field data suggests that the limited extent of the stress reduction around the confined faults controls fault spacing and fault ability to link at relay ramps. Both field data collection from different scales and modeling suggest that fault spacing in confined fault populations is linearly related to the mechanical layer thickness. We therefore highlight the importance of the thickness of layers confining faults in the evaluation of interaction, linkage and propagation of active fault segments over a broad range of scales.

Deformation bands are common subseismic structures in porous sandstones that vary with respect to deformation mechanisms, geometries and distribution. The amount of cataclasis involved largely determines how they impact fluid flow, and cataclasis is generally promoted by coarse grain size, good sorting, high porosity and overburden (usually >500–1000 m). Most bands involve a combination of shear and compaction, and a distinction can be made between those where shear displacement greatly exceeds compaction (compactional shear bands or CSB), where the two are of similar magnitude (shear-enhanced compaction bands or SECB), and pure compaction bands (PCB). The latter two only occur in the contractional regime, are characterized by high (70–100°) dihedral angles (SECB) or perpendicularity (PCB) to σ1 (the maximum principal stress) and are restricted to layers with very high porosity. Contraction generally tends to produce populations of well-distributed deformation bands, whereas in the extensional regime the majority of bands are clustered around faults. Deformation bands also favour highly porous parts of a reservoir, which may result in a homogenization of the overall reservoir permeability and enhance sweep during hydrocarbon production. A number of intrinsic and external variables must therefore be considered when assessing the influence of deformation bands on reservoir performance.

No abstract

No abstract

The way faults control upward fluid flow in nonmagmatic hydrothermal systems in extensional context is still unclear. In the Eastern Pyrénées, an alignment of twenty-nine hot springs (29 ∘ C to 73 ∘ C), along the normal Têt fault, offers the opportunity to study this process. Using an integrated multiscale geological approach including mapping, remote sensing, and macro-and microscopic analyses of fault zones, we show that emergence is always located in crystalline rocks at gneiss-metasediments contacts, mostly in the Têt fault footwall. The hot springs distribution is related to high topographic reliefs, which are associated with fault throw and segmentation. In more detail, emergence localizes either (1) in brittle fault damage zones at the intersection between the Têt fault and subsidiary faults or (2) in ductile faults where dissolution cavities are observed along foliations, allowing juxtaposition of metasediments. Using these observations and 2D simple numerical simulation, we propose a hydrogeological model of upward hydrothermal flow. Meteoric fluids, infiltrated at high elevation in the fault footwall relief, get warmer at depth because of the geothermal gradient. Topography-related hydraulic gradient and buoyancy forces cause hot fluid rise along permeability anisotropies associated with lithological juxtapositions, fracture, and fault zone compositions.

No abstract

(U‐Th)/He ages on apatite obtained in the vicinity of the Têt fault hydrothermal system show a large variability. In the inner damage zone adjacent to the fault core, where fluid flows are concentrated, AHe ages display a large scatter (3–41 Ma) and apatite ageing. Samples from the outer damage zone show young ages with less dispersion (0.9–21.1 Ma) and apatite rejuvenation. Outside the damage zone, ages are consistent with the regional exhumation history between 20 and 12 Ma. The important age dispersion found in the damage zone is interpreted as the result of 4He mobility during fluid infiltration. Our results show that thermochronological data close to a fault should be interpreted with caution, but may offer a new tool for geothermal exploration.