Anna Travé

QUANTIFYING THE KINEMATICS OF DETACHMENT FOLDS USING THREE-DIMENSIONAL GEOMETRY : APPLICATION TO THE MEDIANO ANTICLINE (PYRENEES, SPAIN)

The kinematics of detachment folds have been described by three different models: (1) hinge migration is responsible for fold amplification, (2) fold amplification is due to limb rotation, and (3) both hinge migration and limb rotation cause fold amplification. A numerical method is proposed to determine which of these mechanisms is responsible for the formation of natural detachment folds. This procedure consists of measuring and plotting geometric data collected from cross sections constructed across the termination of a fold where shortening dies out laterally, or in an area with a lateral shortening gradient. Assuming that observed spatial variations in fold geometry reflect temporal geometric evolution, the procedure allows determination of equations that govern the kinematics of the particular detachment fold analyzed. To validate the results obtained from the application of this technique to natural examples, they must be contrasted with other indicators of fold-amplification mechanisms such as microstructures, mesostructures, and syntectonic sediment patterns. This analysis is applied to an asymmetric growth fold, the Mediano anticline in the Southern Pyrenees of Spain, and shows that it grew due to limb rotation and minor hinge migration. These data, coupled with analysis of the growth stratal patterns using reverse and forward modeling techniques, are used to derive deformation rates and to display the kinematics of this fold.

Layer parallel shortening in salt-detached folds: constraint on cross-section restoration

Abstract Cleavage-fissility perpendicular to bedding is a common feature in the external part of fold-and-thrust belts. Three techniques were used to determine the internal distortion in the frontal Southern Pyrenees: the analysis of strain markers such as burrows and rain drops, the measurement of fissility, and the measurement of anisotropy of magnetic susceptibility (AMS). The comparison of the three techniques showed a good fit although they differ in sensitivity to penetrative strain variations in the range of deformation values explored in the study case. On the regional scale, the values of layer parallel shortening (LPS) derived from the markers analysis are very constant and account for 16–23% of shortening. These values are two to three times larger than the shortening values calculated from the restoration of the macroscopic scale structures and indicate a good decoupling above the Cardona salt Formation. This study permitted an accurate restoration of the low-amplitude el Guix detached anticline.

Fluid history related to the Alpine compression at the margin of the south-Pyrenean Foreland basin: the El Guix anticline

The El Guix anticline is the southernmost structure of the south-Pyrenean fold-and-thrust belt (NE Spain). Compressional activity in the area represents the latest stages of Alpine compressional tectonics and aVects the Upper Eocene‐Oligocene fluvio-lacustrine deposits overlying an evaporite sequence. The evolution pattern of the fractures containing calcite cement consists of three stages of microfractures which reflect the evolution of the structure and the relationships between fluids and thrust development. In each fracture, deformation started with a network of discontinuous microfractures which acted as traps for local meteoric fluids (stage 1). During the second stage, dilatant thrust faults serving as the conduit for both evolved meteoric ascending and local meteoric fluids were developed. Migration of fluids through the thrusts was multiepisodic. After the precipitation of cements in microfracture stage 2, the thrusts were practically occluded by calcite cements, acting as fluid barriers and dividing the structure into diVerent hydrological compartments. The last stage of microfracture (stage 3) is attributed to a younger phase, where the circulation of fluids concerned only meteoric fluids and the previously formed fractures acted as barriers to fluid circulation. Meteoric fluids, with high Fe/Mn and Fe/Mg ratios, without Na and Sr, enriched with 13C and with low 87Sr/86Sr with respect to their host rock were widely distributed in the structure throughout all its evolution, within a relatively open palaeohydrological system. Evolved meteoric fluids, with lower Fe/Mn and Fe/Mg ratios, with Na and Sr, depleted in 13C and with high 87Sr/86Sr with respect to their host rock were only present during thrust faults development (stage 2) within a relatively closed palaeohydrological system. The underlying evaporites acted as the lower boundary of the aquifer. Comparison with older thrust fronts of the same system reveals that the Pyrenean fold-and-thrust belt and its deformed southern foreland basin were compartmentalised hydrologically in time and space. During the early Eocene, when the thrust front aVected soft-sediment in the Ainsa basin, the thrust faults were dominated by a medium scale fluid flow. The fluids in the basin were basically formation fluids derived from Eocene marine waters trapped in the underlying Eocene marls, although influences of meteoric waters were also present. During the middle Eocene, coeval with the Gavarnie thrust emplacement, the thrust fault was dominated by a medium scale fluid flow. The fluid was basically a hypersaline Sr-rich brine stored within Triassic redbeds. No evidence of a significant input of either surface or metamorphic fluids during thrusting was found. During the same period, in the crystalline basement of the central Pyrenees the thrust faults were dominated by a large scale fluid flow mainly derived from the underlying silicate rocks

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.

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.

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.

Influence of fault rock foliation on fault zone permeability: The case of deeply buried arkosic sandstones (Gres d'Annot, southeastern France)

We describe the structure, microstructure, and petrophysical properties of fault rocks from two normal fault zones formed in low-porosity turbiditic arkosic sandstones, in deep diagenesis conditions similar to those of deeply buried reservoirs. These fault rocks are characterized by a foliated fabric and quartz-calcite sealed veins, which formation resulted from the combination of the (1) pressure solution of quartz, (2) intense fracturing sealed by quartz and calcite cements, and (3) neoformation of synkinematic white micas derived from the alteration of feldspars and chlorite. Fluid inclusion microthermometry in quartz and calcite cements demonstrates fault activity at temperatures of 195C to 268C. Permeability measurements on plugs oriented parallel with the principal axes of the finite strain ellipsoid show that the Y axis (parallel with the foliation and veins) is the direction of highest permeability in the foliated sandstone (10–2 md for Y against 10–3 md for X, Z, and the protolith, measured at a confining pressure of 20 bars). Microstructural observations document the localization of the preferential fluid path between the phyllosilicate particles forming the foliation. Hence, the direction of highest permeability in these fault rocks would be parallel with the fault and subhorizontal, that is, perpendicular to the slickenlines representing the local slip direction on the fault surface. We suggest that a similar relationship between kinematic markers and fault rock permeability anisotropy may be found in other fault zone types (reverse or strike-slip) affecting feldspar-rich lithologies in deep diagenesis conditions.

Activation of stylolites as conduits for overpressured fluid flow in dolomitized platform carbonates

Abstract This study investigates the Late Aptian–earliest Albian platform carbonates of the Benicàssim area (Maestrat Basin, Spain) in order to assess the relationship between bed-parallel stylolites and the flow of diagenetic fluids during dolomitization and subsequent hydrothermal alteration. Dolostones and burial dolomite and calcite cements were studied by a combination of field geology and standard petrographic and isotope analysis. Field data indicate that dolostones are closely associated with seismic-scale synsedimentary faults, preferentially replace grain-dominated facies and typically show wavy dolomitizing fronts that mostly correspond to bed-parallel stylolites. The dolostones are corroded and contain bed-parallel pores that are filled with hydrothermal saddle dolomite and blocky calcite cements. This late calcite cement frequently engulfs clasts of the host dolostones, suggesting that hydraulic brecciation likely associated with overpressured fluid occurred. Results indicate that stylolites play a key role in the distribution of dolostones and subsequent hydrothermal mineralization. During the replacement stage, stylolites acted as baffles for the dolomitzing fluids controlling lateral fluid flow and resulting in the stratabound dolostone distribution. During the post-dolomitization stage, stylolites became preferred pathways for overpressured hydrothermal corrosive and mineralizing fluids that likely came from the underlying basement, and increased bed-parallel stylolitic porosity and probably also permeability.

Geofluid behaviour in successive extensional and compressional events: a case study from the southwestern end of the Vallès-Penedès Fault (Catalan Coastal Ranges, NE Spain)

The structural position of the Upper Jurassic–Lower Cretaceous carbonates located in the central part of the Catalan Coastal Ranges corresponds to the southwestern end of the Vallès-Penedès Fault. This fault was reactivated at different times during successive extensional and compressional events and several generations of fractures and cementations were formed. Based on petrological and geochemical analyses of this cementation an evolution of the fluids related to the different tectonic stages can be deduced. (1) During the Mesozoic extension, the parent fluids resulted either from a mixing of trapped Upper Jurassic–Lower Cretaceous seawater and meteoric water, or from buffered meteoric waters. (2) Related to the Paleogene compression, the fluids came from the percolation of meteoric waters indicating shallow-depth deformation. (3) During the transitional phase between Paleogene compression and Neogene extension, a karstic dissolution took place and the porosities were infilled by different generations of sediments and cements deposited from meteoric fluids. (4) During the Neogene extension several episodes of meteoric percolations and fracturing processes occurred. The Neogene extensional faults used the earlier karstic system to develop and, later, during the late post-rift stage, a new karstic system occurred, covering the walls of open fractures with speleothems.