A stable and 40Ar/39Ar isotope study of a major thrust in the Helvetic nappes (Swiss Alps) - Evidence for fluid-flow and constraints on nappe kinematics

Stable isotope and Ar-40/Ar-39 measurements,were made on samples associated with a major tectonic discontinuity in the Helvetic Alps, the basal thrust of the Diablerets nappe (external zone of the Alpine Belt) in order to determine both the importance of fluids in this thrust zone and the timing of thrusting. A systematic decrease in the delta(18)O values (up to 6 parts per thousand) of calcite, quartz, and white mica exists within a 10- to 70-m-wide zone over a distance of 37 km along the thrust, and they become more pronounced toward the root of the nappe. A similar decrease in the delta(13)C values of calcite is observed only in the deepest sections (up to 3 parts per thousand). The delta D-SMOW (SMOW = standard mean ocean water) values of white mica are -54 parts per thousand +/- 8 parts per thousand (n = 22) and are independent of the distance from the thrust. These variations are interpreted to reflect syntectonic solution reprecipitation during fluid passage along the thrust. The calculated delta(18)O and delta D values (versus SMOW) for the fluid in equilibrium with the analyzed minerals is 12 parts per thousand to 16 parts per thousand and -30 parts per thousand to +5 parts per thousand, respectively, for assumed temperatures of 250 to 450 degrees C. The isotopic and structural data are consistent with fluids derived from the deep-seated roots of the Helvetic nappes where large volumes of Mesozoic sediments were metamorphosed to the amphibolite facies, It is suggested that connate and metamorphic waters, overpressured by rapid tectonic burial in a subductive system escaped by upward infiltration along moderately dipping pathways until they reached the main shear zone at the base of the moving pile, where they were channeled toward the surface, This model also explains the mechanism by which large amounts of fluid were removed from the Mesozoic sediments during Alpine metamorphism. White mica Ar-49/Ar-39 ages vary from 27 Ma far from the Diablerets thrust to 15 Ma along the thrust. An older component is observed in micas far from the thrust, interpreted as a detrital signature, and indicates that regional metamorphic temperatures were less than about 350 degrees C. The;plateau and near plateau ages nearest the thrust are consistent with either neocrystallization of white mica or argon loss by recrystallization during thrusting, which may have been enhanced in the zones of highest fluid flow. The 15 Ma Ar-40/Ar-39 age plateau measured on white mica sampled exactly on the thrust surface dates the end of both fluid flow and tectonic transport.

Oligo-Miocene extensional tectonics and fluid flow across the Northern Snake Range detachment system, Nevada

The Northern Snake Range (Nevada) represents a spectacular example of a metamorphic core complex and exposes a complete section from the mylonitic footwall into the hanging wall of a fossil detachment system. Paired geochronological and stable isotopic data of mylonitic quartzite within the detachment footwall reveal that ductile deformation and infiltration of meteoric fluids occurred between 27 and 23 Ma. Ar-40/Ar-39 ages display complex recrystallization-cooling relationships but decrease systematically from 26.9 +/- 0.2 Ma at the top to 21.3 +/- 0.2 Ma at the bottom of footwall mylonite. Hydrogen isotope (delta D) values in white mica are very low (-150 to -145 %) within the top 80-90 m of detachment footwall, in contrast to values obtained from the deeper part of the section where values range from -77 to -64 %, suggesting that time-integrated interaction between rock and meteoric fluid was restricted to the uppermost part of the mylonitic footwall. Pervasive mica-water hydrogen isotope exchange is difficult to reconcile with models of Ar-40 loss during mylonitization solely by volume diffusion. Rather, we interpret the Ar-40/Ar-39 ages of white mica with low-delta D values to date syn-mylonitic hydrogen and argon isotope exchange, and we conclude that the hydrothermal system of the Northern Snake Range was active during late Oligocene (27-23 Ma) and has been exhumed by the combined effects of ductile strain, extensional detachment faulting, and erosion.

Fluid flow regulates stromal cell organization and CCL21 expression in a tissue-engineered lymph node microenvironment.

In the paracortex of the lymph node (LN), T zone fibroblastic reticular cells (TRCs) orchestrate an immune response by guiding lymphocyte migration both physically, by creating three-dimensional (3D) cell networks, and chemically, by secreting the chemokines CCL19 and CCL21 that direct interactions between CCR7-expressing cells, including mature dendritic cells and naive T cells. TRCs also enwrap matrix-based conduits that transport fluid from the subcapsular sinus to high endothelial venules, and fluid flow through the draining LN rapidly increases upon tissue injury or inflammation. To determine whether fluid flow affects TRC organization or function within a 3D network, we regenerated the 3D LN T zone stromal network by culturing murine TRC clones within a macroporous polyurethane scaffold containing type I collagen and Matrigel and applying slow interstitial flow (1-23 microm/min). We show that the 3D environment and slow interstitial flow are important regulators of TRC morphology, organization, and CCL21 secretion. Without flow, CCL21 expression could not be detected. Furthermore, when flow through the LN was blocked in mice in vivo, CCL21 gene expression was down-regulated within 2 h. These results highlight the importance of lymph flow as a homeostatic regulator of constitutive TRC activity and introduce the concept that increased lymph flow may act as an early inflammatory cue to enhance CCL21 expression by TRCs, thereby ensuring efficient immune cell trafficking, lymph sampling, and immune response induction.

Fluid flow through the sedimentary cover in northern Switzerland recorded by calcite-celestite veins (Oftringen borehole, Olten)

Abundant veins filled by calcite, celestite and pyrite were found in the core of a 719 m deep borehole drilled in Oftringen near Olten, located in the north-western Molasse basin, close to the thrust of the Folded Jura. Host rocks are calcareous marl, argillaceous limestone and limestone of the Dogger and Malm. The delta O-18 values of vein calcite are lower than in host rock carbonate and, together with microthermometric data from fluid inclusions in vein calcite, indicate precipitation from a seawater-dominated fluid at average temperatures of 56-68A degrees C. Such temperatures were reached at the time of maximum burial of the sedimentary pile in the late Miocene. The depth profile of delta C-13 and Sr-87/Sr-86 values and Sr content of both whole-rock carbonate and vein calcite show marked trends towards negative delta C-13, high Sr-87/Sr-86, and low Sr content in the uppermost 50-150 m of the Jurassic profile (upper Oxfordian). The Sr-87/Sr-86 of vein minerals is generally higher than that of host rock carbonate, up to very high values corresponding to Burdigalian seawater (Upper Marine Molasse, Miocene), which represents the last marine incursion in the region. No evidence for internally derived radiogenic Sr (clay minerals) has been found and so an external source is required. S and O isotope composition of vein celestite and pyrite can be explained by bacterial reduction of Miocene seawater sulphate. The available data set suggests the vein mineralization precipitated from descending Burdigalian seawater and not from a fluid originating in the underlying Triassic evaporites.

Numerical analysis of wave-induced fluid flow effects on seismic data: Application to monitoring of CO2 storage at the Sleipner Field

In this work we analyze how patchy distributions of CO2 and brine within sand reservoirs may lead to significant attenuation and velocity dispersion effects, which in turn may have a profound impact on surface seismic data. The ultimate goal of this paper is to contribute to the understanding of these processes within the framework of the seismic monitoring of CO2 sequestration, a key strategy to mitigate global warming. We first carry out a Monte Carlo analysis to study the statistical behavior of attenuation and velocity dispersion of compressional waves traveling through rocks with properties similar to those at the Utsira Sand, Sleipner field, containing quasi-fractal patchy distributions of CO2 and brine. These results show that the mean patch size and CO2 saturation play key roles in the observed wave-induced fluid flow effects. The latter can be remarkably important when CO2 concentrations are low and mean patch sizes are relatively large. To analyze these effects on the corresponding surface seismic data, we perform numerical simulations of wave propagation considering reservoir models and CO2 accumulation patterns similar to the CO2 injection site in the Sleipner field. These numerical experiments suggest that wave-induced fluid flow effects may produce changes in the reservoir's seismic response, modifying significantly the main seismic attributes usually employed in the characterization of these environments. Consequently, the determination of the nature of the fluid distributions as well as the proper modeling of the seismic data constitute important aspects that should not be ignored in the seismic monitoring of CO2 sequestration problems.

Measurements of seismic attenuation and transient fluid pressure in partially saturated Berea sandstone: Evidence of fluid flow on the mesoscopic scale

A novel laboratory technique is proposed to investigate wave-induced fluid flow on the mesoscopic scale as a mechanism for seismic attenuation in partially saturated rocks. This technique combines measurements of seismic attenuation in the frequency range from 1 to 100?Hz with measurements of transient fluid pressure as a response of a step stress applied on top of the sample. We used a Berea sandstone sample partially saturated with water. The laboratory results suggest that wave-induced fluid flow on the mesoscopic scale is dominant in partially saturated samples. A 3-D numerical model representing the sample was used to verify the experimental results. Biot's equations of consolidation were solved with the finite-element method. Wave-induced fluid flow on the mesoscopic scale was the only attenuation mechanism accounted for in the numerical solution. The numerically calculated transient fluid pressure reproduced the laboratory data. Moreover, the numerically calculated attenuation, superposed to the frequency-independent matrix anelasticity, reproduced the attenuation measured in the laboratory in the partially saturated sample. This experimental?numerical fit demonstrates that wave-induced fluid flow on the mesoscopic scale and matrix anelasticity are the dominant mechanisms for seismic attenuation in partially saturated Berea sandstone.

Do seismic waves and fluid flow sense the same permeability?

We study the discrepancy between the effective flow permeability and the effective seismic permeability, that is, the effective permeability controlling seismic attenuation due to wave-induced fluid flow, in 2D rock samples having mesoscopic heterogeneities and in the presence of strong permeability fluctuations. In order to do so, we employ a numerical oscillatory compressibility test to determine attenuation and velocity dispersion due to wave-induced fluid flow in these kinds of media and compare the responses with those obtained by replacing the heterogeneous permeability field by constant values, including the average permeability as well as the effective flow permeability of the sample. The latter is estimated in a separate upscaling procedure by solving the steady-state flow equation in the rock sample under study. Numerical experiments let us verify that attenuation levels are less significant and the attenuation peak gets broader in the presence of such strong permeability fluctuations. Moreover, we observe that for very low frequencies the effective seismic permeability is similar to the effective flow permeability, while for very high frequencies it approaches the arithmetic average of the permeability field.

Seismic attenuation due to wave-induced fluid flow at microscopic and macroscopic scales

Wave-induced fluid flow at microscopic and mesoscopic scales arguably constitutes the major cause of intrinsic seismic attenuation throughout the exploration seismic and sonic frequency ranges. The quantitative analysis of these phenomena is, however, complicated by the fact that the governing physical processes may be dependent. The reason for this is that the presence of microscopic heterogeneities, such as micro-cracks or broken grain contacts, causes the stiffness of the so-called modified dry frame to be complex-valued and frequency-dependent, which in turn may affect the viscoelastic behaviour in response to fluid flow at mesoscopic scales. In this work, we propose a simple but effective procedure to estimate the seismic attenuation and velocity dispersion behaviour associated with wave-induced fluid flow due to both microscopic and mesoscopic heterogeneities and discuss the results obtained for a range of pertinent scenarios.

Volcanic arcs fed by rapid pulsed fluid flow through subducting slabs

At subduction zones, oceanic lithosphere that has interacted with sea water is returned to the mantle, heats up during descent and releases fluids by devolatilization of hydrous minerals. Models for the formation of magmas feeding volcanoes above subduction zones require largescale transport of these fluids into overlying mantle wedges(1-3). Fluid flow also seems to be linked to seismicity in subducting slabs. However, the spatial and temporal scales of this fluid flow remain largely unknown, with suggested timescales ranging from tens to tens of thousands of years(3-5). Here we use the Li-Ca-Sr isotope systems to consider fluid sources and quantitatively constrain the duration of subduction-zone fluid release at similar to 70 km depth within subducting oceanic lithosphere, now exhumed in the Chinese Tianshan Mountains. Using lithium-diffusion modelling, we find that the wall-rock porosity adjacent to the flowpath of the fluids increased ten times above the background level. We show that fluids released by devolatilization travelled through the slab along major conduits in pulses with durations of about similar to 200 years. Thus, although the overall slab dehydration process is continuous over millions of years and over a wide range of pressures and temperatures, we conclude that the fluids produced by dehydration in subducting slabs are mobilized in short-lived, channelized fluid-flow events.

Development of fluid conduits in the auriferous shear zones of the Hutti Gold Mine, India: evidence for spatially and temporally heterogeneous fluid flow

The gold mineralization of the Hutti Mine is hosted by nine parallel, N - S trending, steeply dipping, 2 - 10 m wide shear zones, that transect Archaean amphibolites. The shear zones were formed after peak metamorphism during retrograde ductile D, shearing in the lower amphibolite facies. They were reactivated in the lower to mid greenschist facies by brittle-ductile D-3 shearing and intense quartz veining. The development of a S-2-S-3 crenulation cleavage facilitates the discrimination between the two deformation events and contemporaneous alteration and gold mineralization. Ductile D, shearing is associated with a pervasively developed distal chlorite - sed cite alteration assemblage in the outer parts of the shear zones and the proximal biotite-plagioclase alteration in the center of the shear zones. D3 is characterized by development of the inner chlorite-K-feldspar alteration, which forms a centimeter-scale alteration halo surrounding the laminated quartz veins and replaces earlier biotite along S-3. The average size of the laminated vein systems is 30-50 m along strike as well as down-dip and 2-6 m in width. Mass balance calculations suggest strong metasomatic changes for the proximal biotite-plagioclase alteration yielding mass and volume increase of ca. 16% and 12%, respectively. The calculated mass and volume changes of the distal chlorite-sericite alteration (ca. 11%, ca. 8%) are lower. The decrease in 6180 values of the whole rock from around 7.5 parts per thousand for the host rocks to 6-7 parts per thousand for the distal chlorite-sericite and the proximal biotite-plagioclase alteration and around 5 parts per thousand for the inner chlorite-K-feldspar alteration suggests hydrothermal alteration during two-stage deformation and fluid flow. The ductile D-2 deformation in the lower amphibolite facies has provided grain scale porosities by microfracturing. The pervasive, steady-state fluid flow resulted in a disseminated style of gold-sulfide mineralization and a penetrative alteration of the host rocks. Alternating ductile and brittle D3 deformation during lower to mid greenschist facies conditions followed the fault-valve process. Ductile creep in the shear zones resulted in a low permeability environment leading to fluid pressure build-up. Strongly episodic fluid advection and mass transfer was controlled by repeated seismic fracturing during the formation of laminated quartz(-gold) veins. The limitation of quartz veins to the extent of earlier shear zones indicate the importance of preexisting anisotropies for fault-valve action and economic gold mineralization. (C) 2003 Elsevier B.V. All rights reserved.

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 6 m 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 (< 50 mu m in thickness). Authigenic chlorite provides essential information about the origin of fluids and their temperature. delta O-18 and delta D 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 240A degrees C are obtained via two independent methods: chlorite compositional thermometers and oxygen isotope fractionation between cogenetic chlorite and quartz. Burial depth conditions of 7 km 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 conditions.

Numerical simulation of ambient seismic wavefield modification caused by pore-fluid effects in an oil reservoir

We have modeled numerically the seismic response of a poroelastic inclusion with properties applicable to an oil reservoir that interacts with an ambient wavefield. The model includes wave-induced fluid flow caused by pressure differences between mesoscopic-scale (i.e., in the order of centimeters to meters) heterogeneities. We used a viscoelastic approximation on the macroscopic scale to implement the attenuation and dispersion resulting from this mesoscopic-scale theory in numerical simulations of wave propagation on the kilometer scale. This upscaling method includes finite-element modeling of wave-induced fluid flow to determine effective seismic properties of the poroelastic media, such as attenuation of P- and S-waves. The fitted, equivalent, viscoelastic behavior is implemented in finite-difference wave propagation simulations. With this two-stage process, we model numerically the quasi-poroelastic wave-propagation on the kilometer scale and study the impact of fluid properties and fluid saturation on the modeled seismic amplitudes. In particular, we addressed the question of whether poroelastic effects within an oil reservoir may be a plausible explanation for low-frequency ambient wavefield modifications observed at oil fields in recent years. Our results indicate that ambient wavefield modification is expected to occur for oil reservoirs exhibiting high attenuation. Whether or not such modifications can be detected in surface recordings, however, will depend on acquisition design and noise mitigation processing as well as site-specific conditions, such as the geologic complexity of the subsurface, the nature of the ambient wavefield, and the amount of surface noise.

Fluid evolution at the Variscan front in the vicinity of the Aachen thrust

Quartz-carbonate-chlorite veins were studied in borehole samples of the RWTH-1 well in Aachen. Veins formed in Devonian rocks in the footwall of the Aachen thrust during Variscan deformation and associated fluid flow. Primary fluid inclusions indicate subsolvus unmixing of a homogenous H(2)O-CO(2)-CH(4)-(N(2))-Na-(K)-Cl fluid into a H(2)O-Na-(K)-Cl solution and a vapour-rich CO(2)-(H(2)O, CH(4), N(2)) fluid. The aqueous end-member composition resembles that of metamorphic fluids of the Variscan front zone with salinities ranging from 4 to 7% NaCl equiv. and maximum homogenisation temperatures of close to 400A degrees C. Pressure estimates indicate a burial depth between 4,500 and 8,000 m at geothermal gradients between 50 and 75A degrees C/26 MPa, but pressure decrease to sublithostatic conditions is also indicated, probably as a consequence of fracture opening during episodic seismic activity. A second fluid system, mainly preserved in pseudo-secondary and secondary fluid inclusions, is characterised by fluid temperatures between 200 and 250A degrees C and salinities of < 5% NaCl equiv. Bulk stable isotope analyses of fluids released from vein quartz, calcite, and dolomite by decrepitation yielded delta D(H2O) values from -89 to -113 aEuro degrees, delta(13)C(CH4) from -26.9 to -28.9aEuro degrees (VPDB) and delta(13)C(CO2) from -12.8 to -23.3aEuro degrees (VPDB). The low delta D and delta(13)C range of the fluids is considered to be due to interaction with cracked hydrocarbons. The second fluid influx caused partial isotope exchange and disequilibrium. It is envisaged that an initial short lived flux of hot metamorphic fluids expelled from the epizonal metamorphic domains of the Stavelot-Venn massif. The metamorphic fluid was focused along major thrust faults of the Variscan front zone such as the Aachen thrust. A second fluid influx was introduced from formation waters in the footwall of the Aachen thrust as a consequence of progressive deformation. Mixing of the cooler and lower salinity formation water with the hot metamorphic fluid during episodic fluid trapping resulted in an evolving range of physicochemical fluid inclusion characteristics.