Pierre Labaume

How pressure solution creep and fracturing processes interact in the upper crust to make it behave in both a brittle and viscous manner

Abstract The upper crust has been described as being dominated by brittle deformation along faults, or ductile where folds and cleavage have developed. These two mechanical behaviors are explained by two different mechanisms of deformation: (i) fracture; and (ii) fluid-enhanced deformation (e.g. pressure solution). These two mechanisms operate at two time scales: fast for brittle deformation, slow for pressure solution. Natural observations of relationships between pressure solution and fractures in sandstones, or indented pebbles, and experimental results of pressure solution with an indenter technique indicate that both mechanisms can interact: fracture development increases the kinetics of the pressure solution process, pressure solution relaxes the stress between fracturing events. A simple model of brittle–ductile deformation, applied to indented limestone pebbles, shows that cycles of slow deformation can alternate with short-time fracture.

Low‐angle crustal ramp and basin geometry in the Gulf of Lion passive margin: Oligocene‐Aquitanian Vistrenque graben, SE France

With more than 4000 m of Oligocene-Aquitanian sediments, the Vistrenque graben (SE France) is the deepest synrift depocenter of the Gulf of Lion passive margin, NW Mediterranean. Detailed analysis of industrial seismic reflection profiles and borehole data show that the steep Nimes fault, which bounds the graben to the NW, forms at depth a low-angle (25°) crustal ramp. Along-strike changes of hangingwall geometry allow us to infer along-strike changes of fault shape: A rollover structure and divergent Oligocene-Aquitanian basin fill are associated with a listric geometry of the fault in the southern part of the graben, while a pseudo-rollover and compensation graben result from a two-segments planar geometry of the fault in the northern part. Preexisting structures inherited from Mesozoic extension and Late Cretaceous-Eocene Pyrenean thickening controlled the location of the Nimes fault and the transfer zones which divide the graben into different compartments. Since both hangingwall and fault profile are well constrained, restoration techniques can be used to estimate the prerift topography. The Vistrenque graben was formed close to sealevel, but at the front of a > 1 km-high elevated area resulting from the Pyrenean orogeny. In the studied transect, the Nimes fault formed the landward (NW) boundary of the basement faulted domain of the margin. Extensional deformation was restricted to this domain during most of the rifting interval. Small amounts of extension were transmitted landward to Mesozoic cover decollement rooted in the Nimes fault, only during short episodes, probably resulting from gravitational instability during margin collapse. The Nimes low-angle crustal ramp, as well as the other crustal ramps of the margin of similar orientation, are probably newly formed extensional structures rather than reactivated Pyrenean thrusts. Their activation at a low-angle may have been allowed by crustal weakening resulting from the previous Pyrenean thickening. Upper crustal extension corresponding to the graben formation was transmitted basinward through an intracrustal detachment, or/and distributed in the lower crust across the margin. In contrast to the more stretched areas of the margin which do not display thick synrift series due to their initial high surface elevation, the Vistrenque basin fill records the whole rifting episode because of its location at the front of the Pyrenean orogen.

Syn-diagenetic evolution of shear structures in superficial nappes : an example from the northern Apennines (NW Italy)

Abstract In the superficial nappes of the Northern Apennines, meso- and microstructures in sheared mudstones and fine-grained turbidites consist mainly of networks of striated faults and calcite vein arrays. The striated faults correspond to the first generation of structures. They form two sets, respectively, synthetic and antithetic to the general shearing. Locally, a sigmoidal microfabric is associated with the synthetic faults in scaly deformation bands up to a few metres thick. Late laterally extensive calcite veins are superposed on these structures. They result from the opening of multiple releasing oversteps between shear surfaces. The ‘crack-seal’ mechanism of deformation played a major role in their formation. Structures of the first generation result from heterogeneous ductile deformation of initially poorly lithified and water-rich sediments. The late formation of the calcite veins corresponds to a transitional ductile-brittle behaviour in sufficiently lithified sediment. Evolution of structures probably depended on a strain-hardening process and was dependent on the progressive compaction and dewatering of the sediment. The gently dipping faults and veins probably formed the main pathway for the interstitial fluids, possibly overpressured, expelled from the sediment. We emphasize that the geometry of the structures described provides important shear-sense criteria for the study of superficial nappe tectonics.

Sediment dewatering and pore fluid migration along thrust faults in a foreland basin inferred from isotopic and elemental geochemical analyses (Eocene southern Pyrenees, Spain)

Abstract The lower Eocene Ainsa basin was formed during the first stages of the south-Pyrenean foreland basin evolution due to southwestward migration of imbricated thrust-folds. Isotopic and elemental geochemistry of syn-kinematic veins (calcite and celestite) and their marly host-rock, sampled in three thrust-fault zones and one footwall syncline, allows us to characterize the origin of pore fluids and the early stages of their evolution and circulation during the early deformation of the basin-fill. The isotopic composition of sulfur and the 87 Sr 86 Sr ratios of calcites and celestites from the veins in the footwall syncline show that the original fluid had the isotopic composition of Eocene seawater. The different 87 Sr 86 Sr ratio in veins from the thrust-fault zones compared with the same ratio in the marly host-rock of the footwall syncline indicates that the thrust-fault zones acted as conduits for advective fluids. The relatively high 87 Sr 86 Sr ratio in the veins related to the thrust-fault zones indicates that the fluid originated from the interaction of seawater with an external fluid coming from deeper sources or from the meteoric weathering of the emerged part of the belt. δ18O and δ13C values of calcites show that the isotopic composition of the calcite-cements in veins was controlled by the isotopic composition of the marly host sediment. Depletion of both δ18O and δ13C with respect to Eocene seawater composition, together with elemental geochemistry of calcite cements in the veins, points to burial transformations of a seawater-derived fluid to a formation water composition. The distribution of δ18O and δ13C values of the marly host-rock and calcite cements in veins of the four outcrops probably resulted from differences in the meteoric water influences. The hydrogeological regime at the toe of the submarine thrust system was dominated by tectonically-induced dewatering of the foreland basin sediments. The thrust-fault zones were the channelizing paths for migration of fluids expelled from the surrounding sediments, as well as fluids derived from more internal parts of the belt.

Early deformations at a submarine compressive front: the quaternary Catania foredeep south of Mt. Etna, Sicily, Italy

Abstract In the Catania foredeep, south of Mt. Etna, oil drilling has revealed the existence at depth of an allochthonous complex emplaced during the middle Quaternary. At the surface, outcropping middle Quaternary marine marly clays and sands display complex deformations which are partly synsedimentary and which can be correlated with the emplacement of this allochthonous complex. Most of these deformations are thought to result from gravitational gliding of slope sediment apron. This process, probably facilitated by high pore-pressure, would be in particular the cause of a semi-penetrative shear deformation with an eastward vergence in the clays. Thus, our study rules out the classical view of a shallow pre-Etnean gulf that was steadily filled in before emerging some 400,000 to 300,000 years ago. This particular example of neotectonics shows the precocity and complexity of deformations occurring in sediments deposited along the compressive margin of a foreland basin or similarly at an accretionary prism front.

Fluid Systems in Foreland Fold-and-Thrust Belts: An Overview from the Southern Pyrenees

The analysis of three different regions of the South- Pyrenean fold-and-thrust belt reveals that during the Tertiary compression the hydrological system was compartmentalised in time and space. During the early-middle Eocene, when the thrust front affected marine soft-sediments in the Ainsa basin, the thrust fault zones were dominated by formation fluids derived from Eocene marine waters trapped in the underlying Eocene marls, although influences of meteoric waters were also present. During the middle-late Eocene, when the thrust front emplaced marine rocks over continental redbeds in the eastern Catalan basin (L’Escala thrust), the thrust fault zones were dominated by meteoric fluids. These fluids flowed preferentially along these faults, draining laterally the meteoric fluids and acting as barriers hindering their flowing towards more external parts of the belt. During the Oligocene, the most external part of the fold-and-thrust belt in the eastern Catalan basin developed on top of a salt detachment horizon. The thrust front affected continental materials of late Eocene-Oligocene age. At this moment, the thrusts were conduits for meteoric fluids arriving from the surface and also for evolved meteoric fluids migrating over short distance upwards after being in contact with the underlying evaporitic beds.

Crack-seal patterns: records of uncorrelated stress release variations in crustal rocks

Abstract Statistical properties of crack-seal veins are investigated with a view to assessing stress release fluctuations in crustal rocks. Crack-seal patterns correspond to sets of successive parallel fractures that are assumed to have propagated by a subcritical crack mechanism in the presence of a reactive fluid. They represent a time-sequence record of an aseismic and anelastic process of rock deformation. The statistical characteristics of several crack-seal patterns containing several hundreds of successive cracks have been studied. Samples were collected in three different areas, gold-bearing quartz veins from Abitibi in Canada, serpentine veins from the San Andreas system in California and calcite veins from the Apennine Mountains in Italy. Digitized pictures acquired from thin sections allow accurate measurement of crack-seal growth increments. All the samples show the same statistical behaviour regardless of their geological origin. The crack-seal statistical properties are described by an exponential distribution with a characteristic length scale and do not show any spatial correlation. They differ from other fracture patterns, such as earthquake data, which exhibit power-law correlations (Gutenberg-Richter relationship). Crack-seal series represent a natural fossil record of stress release variations (less than 50 bars) in the crust that show a characteristic length scale, associated with the resistance of rock to effective tension, and no correlation in time.

Fluid migration during Eocene thrust emplacement in the south Pyrenean foreland basin (Spain): an integrated structural, mineralogical and geochemical approach

Abstract In the frontal part of the south Pyrenean Eocene thrust-fault system, syn-kinematic fluid flow during the early compressional deformation of the foreland basin marls is evidenced macroscopically by the abundance of calcite shear veins within the thrust-fault zones and folds. The geometry and distribution of the veins are indicative of the mechanisms and kinematics of fluid-deformation relationships, and give assessment of the fluid migration paths. The crack-seal mechanism of formation of the shear veins attests to the episodic nature of fault-slip and associated fluid flow in fractures. The distribution of the veins suggests that the main source of fluid was the dewatering of the overpressured, poorly permeable marls from the thrust footwalls, probably related to both (i) vertical compaction due to burial under thrust sheets and (ii) tectonic horizontal shortening. These fluids drained upwards towards the thrust-fault zones, in which they migrated laterally towards the thrust front due to the anisotropy of the fracture permeability in these zones. The geochemistry of the vein-filling minerals and their comparison with the geochemistry and mineralogy of the host marls are indicative of the fluid types, fluid origins, fluid-sediment interactions, and fluid migration paths. The δ34S and 87Sr/86Sr ratio of the host marl calcite and of the calcite and celestite in the veins away from the thrust-fault zones indicate that the original water trapped interstitially in the marls was Eocene seawater. The elemental composition (Ca, Sr, Mg, Mn, and Fe), δ18O, and δ13C of the same samples reveal a change of the pore-water composition from marine to formation water during the early burial stage. Fluid-inclusion analyses of the celestite in the veins reveal the presence of a hot, saline ascending fluid restricted to these discontinuities, where it was mixed with the local formation water. These two types of fluids drained towards the thrust-fault zones where they acquired a higher 87Sr/86Sr ratio, probably related to local fluid-sediment reactions. Indeed, dickite precipitated during cleavage formation in the most intensely strained part of the fault zones, and its formation was probably mainly controlled by stress. δ18O depletion in the calcite from the structurally highest/innermost thrust-fault zones suggests also the influence of meteoric water derived from the emerged part of the belt in these structures. The earlier fluid regime in the Ainsa basin was an intergranular (porous) flow regime (compactional flow) allowing for a pervasive isotopic, and elemental exchange of the marls prior to vein formation. With the onset of compressional deformation, channelized flow along tectonic slip surfaces became dominant.

Central Japan triple junction: a three-dimensional compression model

Abstract We present a new interpretation of the tectonics of the trench-trench-trench triple junction off central Japan, using more specifically data obtained during the French-Japanese Kaiko cruise in 1984. Data analysis and consideration on the three-dimensional geometry of the junction and its kinematics lead to the conclusion that the whole area undergoes large-scale compressional intraplate deformation. This is caused by the subduction of the Philippine Sea plate in the restricted space between the Pacific and Northern Honshu plates. We then apply the same analysis to the whole central Japan area, for which we consider the role of the interaction of the three plates at depth as a basis for tectonic reconstructions since the Miocene.

Fluid flow in cataclastic thrust fault zones in sandstones, Sub-Andean Zone, southern Bolivia

Abstract The Bolivian Sub-Andean Zone (SAZ) corresponds to a Neogene thrust system that affects an about 10-km thick Palaeozoic to Neogene siliciclastic succession. The analysis of macro and microstructures and cement distribution in thrust fault zones shows that they are sealed by quartz at depths >∼3 km, due to local silica transfer by pressure-solution/precipitation activated at temperatures >70–90 °C. At shallower depths, faults have remained open and could be preferential drains for lateral flow of carbonate-bearing fluids, as shown by the occurrence of carbonate cements in fractures and their host-sandstone. Due to decreasing burial, resulting from foothill erosion during fault activity, critically buried fault segments can be affected by non-quartz-sealed structures that post-date initial quartz-sealed structures. The integration of textural, fluid inclusion and isotopic data shows that carbonates precipitated at shallow depth (