3D structure of subsurface thrusts in the eastern Jaca Basin, southern Pyrenees

This paper presents a new model of the subsurface structure of the eastern Jaca flexural basin of the west-central southern Pyrenees, by means of subsurface structural maps and four new balanced cross-sections. The study is based on the interpretation of a set of publicly available seismic reflection profiles tied to deep exploration well logs, which constitute a unique database in the southern Pyrenees associated to the gas discovery of the Serrablo field. Investigation of the deep basin structure highlights strong mechanical-stratigraphic contrasts between basement, a competent Upper Cretaceous-Eocene carbonate sequence in the deep basin and a weak infill of Eocene to lower Miocene synorogenicclastic deposits. These contrasts promote the occurrence of various décollement levels and a decoupled style of deformation between intervals of different competence. A contour map for the top of basement reveals a complex structure with lateral variations of the number of thrusts and the displacement on these and local transverse elements.Between the Gavarnie thrust at the southern edge of the Axial Zone and the Guarga thrust at the leading edge of the basement thrust system, three main other basement thrusts are defined below the north-eastern Jaca Basin, from South to North the Fiscal, Yésero and Broto thrusts. In the Meso-Cenozoic sedimentary cover, two low-angle thrusts are mappedin the subsurface across the Upper Cretaceous-Eocene carbonates: i) the deep Oturia thrust, connected upsection to the emerging Oturia thrust known at the surface, and ii) the deep Jaca thrust, drilled by the Serrablo wells, and connected to the emerging Jaca thrust and Yebra de Basa anticline through a zone of disharmonic deformation. The deducedsubsurface geometrical relationships are consistent with the connection of the Gavarnie and Broto basement thrusts to the Priabonian-Rupelian Oturia and Jaca thrusts while the younger Yésero, Fiscal and Guarga basement thrusts emerge at the South Pyrenean thrust front of the Sierras Exteriores, active until the early Miocene. This study highlights the complex structural pattern that characterizes the deep structure of the South Pyrenean basin and the role of disharmonic deformation that challenges the resolution of the deeper thrust system without the help of seismic profiles

Secuencia de exhumación de las unidades cabalgantes de zócalo de los Pirineos centro-occidentales a partir del análisis de huellas de fisión en apatito

Compilation of new and published apatite fission track data along a transect of the west-central Pyrenees shows that exhumation across the Partial Annealing Zone (~120–60°C) started during the mid Eocene in the North-Pyrenean Zone and migrated southward to reach the southern edge of the Axial Zone during the early Miocene. An early Miocene stage of exhumation is also detected in the northern part of the Axial Zone, indicating a late pop-up thrust reactivation of the Axial ZoneLa compilación de datos nuevos y pre-existentes de huellas de fisión en apatito a lo largo de un perfil de los Pirineos centro-occidentales muestra que la exhumación a través de la Partial Annealing Zone (~120–60°C) comenzó en el Eoceno medio en la Zona Norpirenaica, y fue migrando hacia el sur hasta alcanzar el borde meridional de la Zona Axial en el Mioceno inferior. Al norte de la Zona Axial se registra así mismo una marcada exhumación durante el Mioceno inferior, indicando una reactivación cabalgante en pop-up de la Zona Axia

Alpine Ductile Deformation of the Upper Iberian Collided Margin (Eaux-Chaudes Massif, West-Central Pyrenean Hinterland, France)

Altres ajuts: acords transformatius de la UABThe Eaux-Chaudes massif provides keys to unravel the deep-seated deformation of the Iberian rifted margin during the Alpine orogeny in the Pyrenees. The massif conforms to an inlier of upper Cretaceous carbonate rocks within the Paleozoic basement of the western Axial Zone, originally deposited in the upper margin shelf before the Cenozoic collision. New geological mapping and cross-section construction lead to the description of the lateral structural variation from a km-scale fold nappe in the west to a ductile, imbricate fold-thrust fan in the east. The transition from a Variscan pluton to Devonian metasediments underlying the autochthonous Cretaceous induced this structural change. Recumbent folding, which involved upper Paleozoic rocks, was facilitated by a lower detachment in Silurian slates and an upper detachment in an overlying Keuper shale and evaporite thrust sheet. Remnants of this allochthonous sheet form shale and ophite bodies pinched within the upper Cretaceous carbonates, conforming unusual tertiary welds. Ductile shear in the overturned limb of the Eaux-Chaudes fold nappe imparted strong mylonitic foliation in carbonate rocks, often accompanied by N-S stretching lineation and top-to-the-south kinematic indicators. The burial of the massif by basement-involved thrust sheets and the Keuper sheet, along with their Mesozoic-Cenozoic cover, account for ductile deformation conditions and a structural style not reported hitherto for the Alpine Pyrenees. A hypothesis for the tectonic restoration of this part of the Pyrenean hinterland is finally proposed

Quantification of mass transfers and mineralogical transformations in a thrust fault (Monte Perdido thrust unit, southern Pyrenees, Spain)

In fold-and-thrust belts, shortening is mainly accommodated by thrust faults which are preferential zones for recrystallisation and mass transfer. This study focuses on a detachment fault related to the emplacement of the Monte Perdido thrust unit in the southern Pyrenees. The studied fault zone consists of a 10 m thick intensively foliated phyllonite developed within the Millaris marls, of Eocene age. The lithological homogeneity of the hanging wall and footwall allows us to compare the Millaris marls outside the fault zone with the highly deformed marls located in the fault zone and to quantify the chemical, mineralogical and volumetric changes related to deformation processes along the fault. The Millaris marls are composed of detrital quartz, illite, chlorite, minor albite and pyrite, in a micritic calcite matrix. In the fault zone, the cleavage planes are marked by clay minerals and calcite ± chlorite veins attest to fluid–mineral interactions during deformation. The mineral proportions in all samples from both the fault zone and Millaris marls have been quantified by two methods: (1) X-ray diffraction and Rietveld refinement, and (2) bulk chemical analyses as well as microprobe analyses to calculate modal composition. The excellent agreement between the results of these two methods allows us to estimate mineralogical variations using a modification of the Gresens' equation. During fault activation, up to 45 wt% of calcite was lost while the amounts of quartz and chlorite remained unchanged. Illite content remained constant to slightly enriched. The mineralogical variations were coupled with a significant volume loss (up to 45%) mostly due to the dissolution of micritic calcite grains. Deformation was accompanied by pressure solution and phyllosilicates recrystallisation. These processes accommodated slip along the fault. They required fluids as catalyst, but they did not necessitate major chemical transfers

Formation of chlorite during thrust fault reactivation. Record of fluid origin and P-T conditions in the Monte Perdido thrust fault (southern Pyrenees)

The chemical and isotopic compositions of clay minerals such as illite and chlorite are commonly used to quantify diagenetic and low-grade metamorphic conditions, an approach that is also used in the present study of the Monte Perdido thrust fault from the South Pyrenean fold-and-thrust belt. The Monte Perdido thrust fault is a shallow thrust juxtaposing upper Cretaceous-Paleocene platform carbonates and Lower Eocene marls and turbidites from the Jaca basin. The core zone of the fault, about 6m thick, consists of intensely deformed clay-bearing rocks bounded by major shear surfaces. Illite and chlorite are the main hydrous minerals in the fault zone. Illite is oriented along cleavage planes while chlorite formed along shear veins (<50μm in thickness). Authigenic chlorite provides essential information about the origin of fluids and their temperature. δ18O and δD values of newly formed chlorite support equilibration with sedimentary interstitial water, directly derived from the local hanging wall and footwall during deformation. Given the absence of large-scale fluid flow, the mineralization observed in the thrust faults records the P-T conditions of thrust activity. Temperatures of chlorite formation of about 240°C are obtained via two independent methods: chlorite compositional thermometers and oxygen isotope fractionation between cogenetic chlorite and quartz. Burial depth conditions of 7km are determined for the Monte Perdido thrust reactivation, coupling calculated temperature and fluid inclusion isochores. The present study demonstrates that both isotopic and thermodynamic methods applied to clay minerals formed in thrust fault are useful to help constrain diagenetic and low-grade metamorphic condition

Basement – Cover decoupling and progressive exhumation of metamorphic sediments at hot rifted margin. Insights from the Northeastern Pyrenean analog

International audienceWe compile field data collected along the eastern part of the North Pyrenean Zone (NPZ) to point to a tectonic evolution under peculiar thermal conditions applying to the basin sediments in relation with the opening of the Cretaceous Pyrenean rift. Based on this compilation, we show that when thinning of the continental crust increased , isotherms moved closer to the surface with the result that the brittle-ductile transition propagated upward and reached sediments deposited at the early stage of the basin opening. During the continental breakup, the pre-rift Mesozoic cover was efficiently decoupled from the Paleozoic basement along the Triassic evaporite level and underwent drastic ductile thinning and boudinage. We suggest that the upper Albian and upper Creta-ceous flysches acted as a blanket allowing temperature increase in the mobile pre-rift cover. Finally, we show that continuous spreading of the basin floor triggered the exhumation of the metamorphic, ductily sheared pre-rift cover, thus contributing to the progressive thinning of the sedimentary pile. In a second step, we investigate the detailed geological records of such a hot regime evolution along a reference-section of the eastern NPZ. We propose a balanced restoration from the Mouthoumet basement massif (north) to the Boucheville Albian basin (south). This section shows a north to south increase in the HT Pyrenean imprint from almost no metamorphic recrystallization to more than 600 °C in the pre-and syn-rift sediments. From this reconstruction, we propose a scenario of tectonic thinning involving the exhumation of the pre-rift cover by the activation of various detachment surfaces at different levels in the sedimentary pile. In a third step, examination of the architecture of current distal passive margin domains provides confident comparison between the Pyrenean case and modern analogs. Finally, we propose a general evolutionary model for the pre-rift sequence of the Northeastern Pyrenean rifted margin

Tectono-thermal history of an exhumed thrust-sheet-top basin : an example from the south Pyrenean thrust belt

This paper presents a new balanced structural cross-section of the Jaca thrust-sheet-top basin of the southern Pyrenees combined with paleo-thermometry and apatite fission track (AFT) thermochronology data. The cross-section, based on field data and interpretation of industrial seismic reflection profiles, allows refinement of previous interpretations of the south-directed thrust system, involving the identification of new thrust faults, and of the kinematic relationships between basement and cover thrusts from the middle Eocene to the early Miocene. AFT analysis shows a southward decrease in the level of fission track resetting, from totally reset Paleozoic rocks and lower Eocene turbidites (indicative of heating to Tmax>~120°C), to partially reset middle Eocene turbidites and no/very weak resetting in the upper Eocene-lower Oligocene molasse (Tmax<~60°C). AFT results indicate a late Oligocene-early Miocene cooling event throughout the Axial Zone and Jaca Basin. Paleo-maximum temperatures determined by vitrinite reflectance measurements and Raman spectroscopy of carbonaceous material reach up to ~240°C at the base of the turbidite succession. Inverse modelling of AFT and vitrinite reflectance data with the QTQt software for key samples show compatibility between vitrinite-derived Tmax and the AFT reset level for most of the samples. However, they also suggest that the highest temperatures determined in the lowermost turbidites correspond to a thermal anomaly rather than burial heating, possibly due to fluid circulation during thrust activity. From these results, we propose a new sequential restoration of the south Pyrenean thrust system propagation and related basin evolution

A novel experimental approach for studying spontaneous imbibition processes with alkaline solutions

Spontaneous imbibition processes can play an important role in oil production. It can be enhanced or influenced by wettability changes generated by properly designed chemicals or by the natural surfactants resulting from reactive crude oils in the presence of alkaline solutions. The reaction of basic salts with some components of oil can, indeed, lead to the formation of natural soaps that reduces the interfacial tension between oil and brine. The latter scenario is studied herein on samples and oil from the St Ulrich oil field in the Vienna basin. To that end, spontaneous imbibition experiments were performed with two brines differing by the absence or presence of alkali. We first present a general novel technique to monitor saturation changes on small rock samples for the purpose of assessing the efficiency of a given recovery process. Samples of only 15 mm in diameter and 20 mm in length and set at irreducible saturation were fully immersed in the solution of interest, and the evolution of the samples’ saturation with time was monitored thanks to a dedicated NMR technique involving the quantification of the sole oil phase present within the sample. A fully-3D imbibition configuration was adopted, involving counter-current flows through all faces of the sample. The experimental method is fast for two reasons: (i) the kinetics of capillary imbibition process is proportional to the square of sample size, i.e. very rapid if accurate measurements can be acquired on tiny samples, (ii) the present 3D situation also involves faster kinetics than the 1D configuration often used. The NMR technique was crucial to achieve such conditions that cannot be satisfied with conventional volumetric methods. The kinetics of oil desaturation during spontaneous imbibition is interpreted with the help of an analytical 3D diffusion model. For the alkaline solution, the diffusion coefficient is reduced by a factor of only two compared to the non-alkaline brine, although the interfacial tension between the oil and the imbibing solution is reduced by a factor of 10. Hence, a wettability change to a more water wet state has to be assumed when the alkaline solution replaces the non-alkaline solution in the imbibition process. However, no significant impact on the final saturation was observed

Rotational basement kinematics deduced from remagnetized cover rocks (Internal Sierras, southwestern Pyrenees)

Peer Reviewedhttp://deepblue.lib.umich.edu/bitstream/2027.42/95604/1/tect1852.pd