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Fluid flow patterns during Pyrenean thrusting
Journal of Geochemical Exploration 69–70 (2000) 539–543 www.elsevier.nl/locate/jgeoexp
Fluid flow patterns during Pyrenean thrusting A.M. McCaig a,*, J. Tritlla 1,b, D.A. Banks 2,a b
a School of Earth Sciences, The University of Leeds, Leeds LS2 9JT, UK Instituto de Geologia, Universidad Nacional Autonoma de Mexico, Ciudad Universitaria, 14510 Mexico, DF, Mexico
Abstract Geochemical data for fluid circulation patterns during thrusting in the Pyrenees are reviewed. New halogen data from fluid inclusions suggests that brines responsible for metasomatic alteration in shear zones, and expelled along the Gavarnie Thrust, were of evaporitic origin. These brines mixed with more dilute formation waters in a topographically driven flow system at higher levels. The brines probably formed in the Triassic and were also involved in the formation of Mesozoic Pb–Zn deposits. Dense fluids are hard to completely expel from upper crustal rocks, and recycling of such fluids through several metasomatic events is probably a common process. 䉷 2000 Elsevier Science B.V. All rights reserved. Keywords: halogens; fluid inclusions; thrust; shear zones; brine; Pyrenees
1. Introduction A number of important questions can be posed regarding fluid flow during orogenesis, for example: What are the driving forces for fluid flow in mountain belts, and do different driving forces operate in distinct fluid flow regimes? What are the origins of fluid reservoirs in mountain belts, and do they retain distinctive chemical signatures during orogenic events? What is the permeability structure of mountain belts, and how do lithology and deformation combine to determine fluid flow paths? In this abstract we review almost two decades of work on Alpine fluid flow in various parts of the Pyrenean thrust belt, and present new crush-leach halogen analyses from fluid
* Corresponding author. Fax: ⫹ 113-2335-259. E-mail addresses: [email protected] (A.M. McCaig), [email protected] (J. Tritlla), [email protected] (D.A. Banks). 1 Fax: ⫹ 525-6224-317. 2 Fax: ⫹ 113-2335-259.
inclusions that enable the large-scale patterns of fluid flow to be constrained.
2. The Pyrenees The Pyrenees were formed by limited convergence between Iberia and France during the early Tertiary. The geometry of the chain has been well constrained by seismic reflection profiles (Roure et al., 1989), and consists of a doubly verging thrust belt with the larger movements on southward-verging thrusts. Many of these thrusts appear to be reactivated normal faults formed during Mesozoic crustal thinning. The belt is conventionally divided into three main zones. The north Pyrenean Zone contains northward-verging structures and is bounded to the south by the North Pyrenean Fault, which was the locus of strike–slip movement and metamorphism in the middle Tertiary. The Axial Zone consists of uplifted Palaeozoic metamorphic rocks cut by late Hercynian granodiorites. The South Pyrenean Zone consists of southwardverging thrust sheets of Mesozoic carbonates and
0375-6742/00/$ - see front matter 䉷 2000 Elsevier Science B.V. All rights reserved. PII: S0375-674 2(00)00060-1
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A.M. McCaig et al. / Journal of Geochemical Exploration 69–70 (2000) 539–543
Fig. 1. Cross-section through the Gavarnie Nappe showing hypothetical geometry above present erosion level and inferred fluid flow patterns. Pic Long and La Gle`re are shear zones cutting the Ne´ouvielle granodiorite. E is approximate present-day peak top erosion level. GT is Gavarnie Thrust; MPT is Mont Perdu Thrust. Stippled areas are Triassic redbeds originally deposited in fault-bounded basins. K is Cretaceous carbonates unconformable on Triassic redbeds and Variscan basement rocks (V). Upper Triassic rocks including evaporites are only present in the footwall of the Gavarnie Thrust, beneath the Cretaceous, at the extreme southern end of the section.
Tertiary foreland basin sediments. This paper concentrates on the Axial Zone and the northern part of the South Pyrenean Zone where Axial Zone basement rocks are thrust over Mesozoic sediments. Fig. 1 shows a schematic cross-section through this part of the Pyrenees showing the locations of the localities described below. 3. Fluid flow regimes in the Pyrenees Three distinct fluid flow regimes can be distinguished related to the Alpine structure of the Pyrenees: 1. In the northern Axial Zone, steep conjugate reverse shear zones cut granodiorites and high grade metamorphic massifs. In the Ne´ouvielle granodiorite (Fig. 1), fluid flow under greenschist facies conditions has been dated as Alpine (Wayne and McCaig, 1998), and resulted in redistribution of Sr isotopes. Fluid inclusions in syntectonic quartz veins contain hypersaline calcic brines (15–35% dissolved salts) with a wide range of homogenisation temperatures (Henderson and McCaig, 1996), interpreted to reflect large pressure fluctuations during formation of the veins. There is little evidence for major fluxes of fluid in the Ne´ouvielle shear zones. In contrast, in similar shear zones cutting the Aston Massif, further east in the Axial Zone, large upward fluxes of fluids with low d 18O
ratios (⬍⫹4‰) were inferred by McCaig et al. (1990) on the basis of metasomatic reactions and oxygen isotope data. 2. On the southern margin of the Axial Zone, the Gavarnie Thrust places Silurian and Devonian phyllites onto Triassic redbeds unconformably overlain by Cretaceous carbonates. At the Pic de Port Vieux (Fig. 1), a small culmination occurs beneath the Gavarnie Thrust with imbrication of Cretaceous, Triassic and basement rocks. Abundant syntectonic quartz veins within this culmination contain hypersaline (16–23% dissolved salts) calcic brines (Banks et al., 1991). A Pb and Sr isotope study of the fluids has revealed systematic variations in isotopic ratios related to position in the culmination. These are interpreted to indicate sucking of fluid into the growing culmination from both hangingwall and footwall lithologies (Banks et al., 1991; McCaig et al., 2000). 3. Southwards from the Pic de Port Vieux, carbonate mylonites beneath the Gavarnie Thrust are systematically enriched in 87Sr compared with undeformed carbonates. McCaig et al. (1995) interpreted this to reflect southward expulsion of brines, stored in Triassic redbeds and underlying fractured basement, along the thrust. The carbonate mylonites do not show the same Pb isotope signature as the fluid inclusions and do not appear to be affected by fluid derived from the hangingwall of
A.M. McCaig et al. / Journal of Geochemical Exploration 69–70 (2000) 539–543
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Fig. 2. Halogen data from crush-leach fluid inclusion analyses. Seawater evaporation curve is from Fontes and Matray (1993). Cierco data from Johnson et al. (1996).
the thrust. McCaig et al. (2000) suggest that an early smooth slip stage with expulsion of fluid was followed by dilatant growth of the culmination with sucking of fluid into the thrust zone (Fig. 1). Permeability in the mylonite zone was probably dominated by transient fractures, although grainscale mass-transport was important in homogenising isotopic signatures (Knipe and McCaig, 1994). 4. Carbonate-dominated thrust sheets in the South Pyrenean Zone also show evidence for channe-
lised flow along faults and into vein arrays (Rye and Bradbury, 1988), although isotopic evidence can be interpreted either in terms of downward movement of high-level formation waters or upward movement of metamorphic dehydration waters. At higher levels and to the south, upward movement of fluid appears to have been channelised into thrust faults cutting Tertiary marls in the foreland basin (Trave´ et al., 1998), although there is no evidence for generation of fluid outside the sedimentary pile.
Fig. 3. Plot showing change in trend of seawater evaporation curve (Fontes and Matray, 1993) as salts such as sylvite join halite in precipitating. Data symbols as Fig. 2.
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A.M. McCaig et al. / Journal of Geochemical Exploration 69–70 (2000) 539–543
4. Fluid inclusion halogen data Fig. 2 shows Br and Cl data from the three flow regimes above. In the Ne´ouvielle Massif (Pic Long and La Gle`re data), Cl/Br ratios of inclusion fluids are all lower than seawater and plot on or close to the seawater evaporation curve. Such fluids could have been produced by evaporation past the point of halite precipitation—this interpretation is supported by a plot of Cl/Br vs. Na/Br (Fig. 3). The change from a 1:1 slope in the Pic de Port Vieux samples to a steeper slope in the Ne´ouvielle samples is consistent with a change from halite precipitation to halite ⫹ sylvite (or other salts). Concentration of the brines by retrograde hydration may have been partly responsible for the observed salinities, but it is unlikely that high temperature precipitation of halite was involved since the samples plot well below the predicted evolution curve at 300⬚C (Fig. 2). The Pic de Port Vieux samples are less saline and plot below the seawater evaporation curve (Fig. 2). This is consistent with evaporation followed by mixing with dilute fluids such as metamorphic dehydration waters or carbonate-hosted formation waters. At Plan de Larri, fluids appear to have more diverse origins. One sample collected from just below the Gavarnie Thrust has a very low Cl/Br ratio, suggesting it contains a more diluted version of the Pic de Port Vieux fluid. Other samples collected from both the footwall and hangingwall of the thrust have Cl/Br ratios close to seawater suggesting evaporation within the gypsum field or possibly some dissolution of evaporites. A calcite-hosted sample from within the South Pyrenean thrust belt at Mediano has a similar composition. Finally, three samples collected from veins in Triassic redbeds at Gistain show high Cl/Br ratios suggesting halite dissolution, although they remain quite dilute. This locality is along strike from Plan de Larri and upper Triassic evaporites are still present between the Triassic redbeds and the Gavarnie Thrust. At Plan de Larri and Pic de Port Vieux, the redbeds are overlain unconformably by Cretaceous carbonates and we infer that the evaporites were eroded away before the Cretaceous. Also plotted in Fig. 2 are data from the Cierco Pb– Zn deposit hosted by Devonian basement rocks on the southern margin of the Axial Zone south of the Maladeta granodiorite (Johnson et al., 1996). Similar Pb–
Zn deposits occur at Parzan, along normal faults cutting Triassic redbeds in the footwall of the Gavarnie Thrust south of Pic de Port Vieux. These deposits are pre-Cretaceous in age and presumably formed during Mesozoic extension. The similarity of the Cierco fluids to those found in syntectonic veins within the thrust belt is striking, and we infer that essentially the same brines were involved in each case. 5. Discussion and conclusions Fig. 1 shows our fluid circulation model. Brines are inferred to have formed during Upper Triassic evaporite formation, sinking into underlying Triassic redbeds and fractured basement rocks and displacing any older more dilute formation waters. Circulation of these brines was responsible for the formation of faultrelated Pb–Zn deposits during Mesozoic extension. However, the brines were not expelled during this phase but remained in situ before being overpressured and expelled during Tertiary thrusting. At this level fluid flow was largely restricted to faults and shear zones. Mixing occurred at the limit between the overpressured zone and a topographically driven flow system in carbonate thrust sheets, with local dissolution of evaporites. Further south, expulsion of fluid again occurred along thrust faults cutting shale-rich lithologies of the foreland basin. References Banks, D.A., Davies, G.R., Yardley, B.W.D., McCaig, A.M., Grant, N.T., 1991. The chemistry of brines from an Alpine thrust system in the Pyrenees: an application of fluid inclusion analysis to the study of fluid behaviour in orogenesis. Geochim. Cosmochim. Acta 55, 1021–1030. Fontes, J.Ch., Matray, J.M., 1993. Geochemistry and origin of formation brines from the Paris Basin, France: 1. Brines associated with Triassic salts. Chem. Geol. 109, 149–175. Henderson, I.H.C., McCaig, A.M., 1996. Fluid pressure and salinity variations in shear-zone-related veins, central Pyrenees, France: implications for the fault-valve model. Tectonophysics 262, 321–348. Johnson, C.A., Cardellach, E., Tritlla, J., Hanan, B., 1996. Cierco Pb–Zn–Ag vein deposits: isotopic and fluid inclusion evidence for formation during the Mesozoic extension in the Pyrenees of Spain. Econ. Geol. 91, 497–506. Knipe, R.J., McCaig, A.M., 1994. Microstructural and microchemical consequences of fluid flow in deforming rocks. In: Parnell, J.
A.M. McCaig et al. / Journal of Geochemical Exploration 69–70 (2000) 539–543 (Ed.). Geofluids: Origin, Migration and Evolution of Fluids in Sedimentary Basins, Geol. Soc. Lond. Spec. Publ., vol. 78., pp. 99–111. McCaig, A.M., Wickham, S.M., Taylor Jr., H.P., 1990. Deep fluid circulation in alpine shear zones, Pyrenees, France: field and oxygen isotope studies. Contrib. Mineral. Petrol. 106, 41–60. McCaig, A.M., Wayne, D.M., Marshall, J.D., Banks, D.A., Henderson, I.H.C., 1995. Isotopic and fluid inclusion studies of fluid movement along the Gavarnie thrust, central Pyrenees: reaction fronts in carbonate mylonites. Am. J. Sci. 295, 309–343. McCaig, A.M., Wayne, D.M., Rosenbaum, J.M., 2000. Fluid expulsion and dilatancy pumping during thrusting in the Pyrenees: Pb and Sr isotope evidence. Geol. Soc. Am. Bull. (in press). Roure, F., Choukroune, P., Berastagui, X., Munoz, J.A., Villien, A., Matheron, P., Bareyt, M., Se´guret, M., Camara, P., De´ramond, J., 1989. ECORS deep seismic data and balanced cross-sections,
543
geometric constraints to trace the evolution of the Pyrenees. Tectonics 8, 41–50. Rye, D., Bradbury, H.J., 1988. Fluid flow in the crust: an example from a Pyrenean thrust ramp. Am. J. Sci. 288, 197–235. Trave´, A., Labaume, P., Calvet, F., Soler, A., Tritlla, J., Buatier, M., Potdevin, J.-L., Se´guret, M., Raynaud, S., Briqueau, L., 1998. Fluid migration during Eocene thrust emplacement in the south Pyrenean foreland basin (Spain): an integrated structural, mineralogical and geochemical approach. In: Mascle, A., Puigdefabregas, C., Luterbacher, H.P., Fernandez, M. (Eds.). Cenozoic Foreland Basins of Western Europe, Geol. Soc. Lond. Spec. Publ., vol. 134., pp. 163–188. Wayne, D.M., McCaig, A.M., 1998. Dating fluid flow in shear zones: Rb–Sr and U–Pb studies of syntectonic veins in the Ne´ouvielle Massif, Pyrenees. In: Parnell, J. (Ed.). Dating and Duration of Fluid Flow and Fluid-Rock Interaction, Geol. Soc. Lond. Spec. Publ., vol. 144., pp. 129–135.
Fluid flow patterns during Pyrenean thrusting A.M. McCaig a,*, J. Tritlla 1,b, D.A. Banks 2,a b
a School of Earth Sciences, The University of Leeds, Leeds LS2 9JT, UK Instituto de Geologia, Universidad Nacional Autonoma de Mexico, Ciudad Universitaria, 14510 Mexico, DF, Mexico
Abstract Geochemical data for fluid circulation patterns during thrusting in the Pyrenees are reviewed. New halogen data from fluid inclusions suggests that brines responsible for metasomatic alteration in shear zones, and expelled along the Gavarnie Thrust, were of evaporitic origin. These brines mixed with more dilute formation waters in a topographically driven flow system at higher levels. The brines probably formed in the Triassic and were also involved in the formation of Mesozoic Pb–Zn deposits. Dense fluids are hard to completely expel from upper crustal rocks, and recycling of such fluids through several metasomatic events is probably a common process. 䉷 2000 Elsevier Science B.V. All rights reserved. Keywords: halogens; fluid inclusions; thrust; shear zones; brine; Pyrenees
1. Introduction A number of important questions can be posed regarding fluid flow during orogenesis, for example: What are the driving forces for fluid flow in mountain belts, and do different driving forces operate in distinct fluid flow regimes? What are the origins of fluid reservoirs in mountain belts, and do they retain distinctive chemical signatures during orogenic events? What is the permeability structure of mountain belts, and how do lithology and deformation combine to determine fluid flow paths? In this abstract we review almost two decades of work on Alpine fluid flow in various parts of the Pyrenean thrust belt, and present new crush-leach halogen analyses from fluid
* Corresponding author. Fax: ⫹ 113-2335-259. E-mail addresses: [email protected] (A.M. McCaig), [email protected] (J. Tritlla), [email protected] (D.A. Banks). 1 Fax: ⫹ 525-6224-317. 2 Fax: ⫹ 113-2335-259.
inclusions that enable the large-scale patterns of fluid flow to be constrained.
2. The Pyrenees The Pyrenees were formed by limited convergence between Iberia and France during the early Tertiary. The geometry of the chain has been well constrained by seismic reflection profiles (Roure et al., 1989), and consists of a doubly verging thrust belt with the larger movements on southward-verging thrusts. Many of these thrusts appear to be reactivated normal faults formed during Mesozoic crustal thinning. The belt is conventionally divided into three main zones. The north Pyrenean Zone contains northward-verging structures and is bounded to the south by the North Pyrenean Fault, which was the locus of strike–slip movement and metamorphism in the middle Tertiary. The Axial Zone consists of uplifted Palaeozoic metamorphic rocks cut by late Hercynian granodiorites. The South Pyrenean Zone consists of southwardverging thrust sheets of Mesozoic carbonates and
0375-6742/00/$ - see front matter 䉷 2000 Elsevier Science B.V. All rights reserved. PII: S0375-674 2(00)00060-1
540
A.M. McCaig et al. / Journal of Geochemical Exploration 69–70 (2000) 539–543
Fig. 1. Cross-section through the Gavarnie Nappe showing hypothetical geometry above present erosion level and inferred fluid flow patterns. Pic Long and La Gle`re are shear zones cutting the Ne´ouvielle granodiorite. E is approximate present-day peak top erosion level. GT is Gavarnie Thrust; MPT is Mont Perdu Thrust. Stippled areas are Triassic redbeds originally deposited in fault-bounded basins. K is Cretaceous carbonates unconformable on Triassic redbeds and Variscan basement rocks (V). Upper Triassic rocks including evaporites are only present in the footwall of the Gavarnie Thrust, beneath the Cretaceous, at the extreme southern end of the section.
Tertiary foreland basin sediments. This paper concentrates on the Axial Zone and the northern part of the South Pyrenean Zone where Axial Zone basement rocks are thrust over Mesozoic sediments. Fig. 1 shows a schematic cross-section through this part of the Pyrenees showing the locations of the localities described below. 3. Fluid flow regimes in the Pyrenees Three distinct fluid flow regimes can be distinguished related to the Alpine structure of the Pyrenees: 1. In the northern Axial Zone, steep conjugate reverse shear zones cut granodiorites and high grade metamorphic massifs. In the Ne´ouvielle granodiorite (Fig. 1), fluid flow under greenschist facies conditions has been dated as Alpine (Wayne and McCaig, 1998), and resulted in redistribution of Sr isotopes. Fluid inclusions in syntectonic quartz veins contain hypersaline calcic brines (15–35% dissolved salts) with a wide range of homogenisation temperatures (Henderson and McCaig, 1996), interpreted to reflect large pressure fluctuations during formation of the veins. There is little evidence for major fluxes of fluid in the Ne´ouvielle shear zones. In contrast, in similar shear zones cutting the Aston Massif, further east in the Axial Zone, large upward fluxes of fluids with low d 18O
ratios (⬍⫹4‰) were inferred by McCaig et al. (1990) on the basis of metasomatic reactions and oxygen isotope data. 2. On the southern margin of the Axial Zone, the Gavarnie Thrust places Silurian and Devonian phyllites onto Triassic redbeds unconformably overlain by Cretaceous carbonates. At the Pic de Port Vieux (Fig. 1), a small culmination occurs beneath the Gavarnie Thrust with imbrication of Cretaceous, Triassic and basement rocks. Abundant syntectonic quartz veins within this culmination contain hypersaline (16–23% dissolved salts) calcic brines (Banks et al., 1991). A Pb and Sr isotope study of the fluids has revealed systematic variations in isotopic ratios related to position in the culmination. These are interpreted to indicate sucking of fluid into the growing culmination from both hangingwall and footwall lithologies (Banks et al., 1991; McCaig et al., 2000). 3. Southwards from the Pic de Port Vieux, carbonate mylonites beneath the Gavarnie Thrust are systematically enriched in 87Sr compared with undeformed carbonates. McCaig et al. (1995) interpreted this to reflect southward expulsion of brines, stored in Triassic redbeds and underlying fractured basement, along the thrust. The carbonate mylonites do not show the same Pb isotope signature as the fluid inclusions and do not appear to be affected by fluid derived from the hangingwall of
A.M. McCaig et al. / Journal of Geochemical Exploration 69–70 (2000) 539–543
541
Fig. 2. Halogen data from crush-leach fluid inclusion analyses. Seawater evaporation curve is from Fontes and Matray (1993). Cierco data from Johnson et al. (1996).
the thrust. McCaig et al. (2000) suggest that an early smooth slip stage with expulsion of fluid was followed by dilatant growth of the culmination with sucking of fluid into the thrust zone (Fig. 1). Permeability in the mylonite zone was probably dominated by transient fractures, although grainscale mass-transport was important in homogenising isotopic signatures (Knipe and McCaig, 1994). 4. Carbonate-dominated thrust sheets in the South Pyrenean Zone also show evidence for channe-
lised flow along faults and into vein arrays (Rye and Bradbury, 1988), although isotopic evidence can be interpreted either in terms of downward movement of high-level formation waters or upward movement of metamorphic dehydration waters. At higher levels and to the south, upward movement of fluid appears to have been channelised into thrust faults cutting Tertiary marls in the foreland basin (Trave´ et al., 1998), although there is no evidence for generation of fluid outside the sedimentary pile.
Fig. 3. Plot showing change in trend of seawater evaporation curve (Fontes and Matray, 1993) as salts such as sylvite join halite in precipitating. Data symbols as Fig. 2.
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A.M. McCaig et al. / Journal of Geochemical Exploration 69–70 (2000) 539–543
4. Fluid inclusion halogen data Fig. 2 shows Br and Cl data from the three flow regimes above. In the Ne´ouvielle Massif (Pic Long and La Gle`re data), Cl/Br ratios of inclusion fluids are all lower than seawater and plot on or close to the seawater evaporation curve. Such fluids could have been produced by evaporation past the point of halite precipitation—this interpretation is supported by a plot of Cl/Br vs. Na/Br (Fig. 3). The change from a 1:1 slope in the Pic de Port Vieux samples to a steeper slope in the Ne´ouvielle samples is consistent with a change from halite precipitation to halite ⫹ sylvite (or other salts). Concentration of the brines by retrograde hydration may have been partly responsible for the observed salinities, but it is unlikely that high temperature precipitation of halite was involved since the samples plot well below the predicted evolution curve at 300⬚C (Fig. 2). The Pic de Port Vieux samples are less saline and plot below the seawater evaporation curve (Fig. 2). This is consistent with evaporation followed by mixing with dilute fluids such as metamorphic dehydration waters or carbonate-hosted formation waters. At Plan de Larri, fluids appear to have more diverse origins. One sample collected from just below the Gavarnie Thrust has a very low Cl/Br ratio, suggesting it contains a more diluted version of the Pic de Port Vieux fluid. Other samples collected from both the footwall and hangingwall of the thrust have Cl/Br ratios close to seawater suggesting evaporation within the gypsum field or possibly some dissolution of evaporites. A calcite-hosted sample from within the South Pyrenean thrust belt at Mediano has a similar composition. Finally, three samples collected from veins in Triassic redbeds at Gistain show high Cl/Br ratios suggesting halite dissolution, although they remain quite dilute. This locality is along strike from Plan de Larri and upper Triassic evaporites are still present between the Triassic redbeds and the Gavarnie Thrust. At Plan de Larri and Pic de Port Vieux, the redbeds are overlain unconformably by Cretaceous carbonates and we infer that the evaporites were eroded away before the Cretaceous. Also plotted in Fig. 2 are data from the Cierco Pb– Zn deposit hosted by Devonian basement rocks on the southern margin of the Axial Zone south of the Maladeta granodiorite (Johnson et al., 1996). Similar Pb–
Zn deposits occur at Parzan, along normal faults cutting Triassic redbeds in the footwall of the Gavarnie Thrust south of Pic de Port Vieux. These deposits are pre-Cretaceous in age and presumably formed during Mesozoic extension. The similarity of the Cierco fluids to those found in syntectonic veins within the thrust belt is striking, and we infer that essentially the same brines were involved in each case. 5. Discussion and conclusions Fig. 1 shows our fluid circulation model. Brines are inferred to have formed during Upper Triassic evaporite formation, sinking into underlying Triassic redbeds and fractured basement rocks and displacing any older more dilute formation waters. Circulation of these brines was responsible for the formation of faultrelated Pb–Zn deposits during Mesozoic extension. However, the brines were not expelled during this phase but remained in situ before being overpressured and expelled during Tertiary thrusting. At this level fluid flow was largely restricted to faults and shear zones. Mixing occurred at the limit between the overpressured zone and a topographically driven flow system in carbonate thrust sheets, with local dissolution of evaporites. Further south, expulsion of fluid again occurred along thrust faults cutting shale-rich lithologies of the foreland basin. References Banks, D.A., Davies, G.R., Yardley, B.W.D., McCaig, A.M., Grant, N.T., 1991. The chemistry of brines from an Alpine thrust system in the Pyrenees: an application of fluid inclusion analysis to the study of fluid behaviour in orogenesis. Geochim. Cosmochim. Acta 55, 1021–1030. Fontes, J.Ch., Matray, J.M., 1993. Geochemistry and origin of formation brines from the Paris Basin, France: 1. Brines associated with Triassic salts. Chem. Geol. 109, 149–175. Henderson, I.H.C., McCaig, A.M., 1996. Fluid pressure and salinity variations in shear-zone-related veins, central Pyrenees, France: implications for the fault-valve model. Tectonophysics 262, 321–348. Johnson, C.A., Cardellach, E., Tritlla, J., Hanan, B., 1996. Cierco Pb–Zn–Ag vein deposits: isotopic and fluid inclusion evidence for formation during the Mesozoic extension in the Pyrenees of Spain. Econ. Geol. 91, 497–506. Knipe, R.J., McCaig, A.M., 1994. Microstructural and microchemical consequences of fluid flow in deforming rocks. In: Parnell, J.
A.M. McCaig et al. / Journal of Geochemical Exploration 69–70 (2000) 539–543 (Ed.). Geofluids: Origin, Migration and Evolution of Fluids in Sedimentary Basins, Geol. Soc. Lond. Spec. Publ., vol. 78., pp. 99–111. McCaig, A.M., Wickham, S.M., Taylor Jr., H.P., 1990. Deep fluid circulation in alpine shear zones, Pyrenees, France: field and oxygen isotope studies. Contrib. Mineral. Petrol. 106, 41–60. McCaig, A.M., Wayne, D.M., Marshall, J.D., Banks, D.A., Henderson, I.H.C., 1995. Isotopic and fluid inclusion studies of fluid movement along the Gavarnie thrust, central Pyrenees: reaction fronts in carbonate mylonites. Am. J. Sci. 295, 309–343. McCaig, A.M., Wayne, D.M., Rosenbaum, J.M., 2000. Fluid expulsion and dilatancy pumping during thrusting in the Pyrenees: Pb and Sr isotope evidence. Geol. Soc. Am. Bull. (in press). Roure, F., Choukroune, P., Berastagui, X., Munoz, J.A., Villien, A., Matheron, P., Bareyt, M., Se´guret, M., Camara, P., De´ramond, J., 1989. ECORS deep seismic data and balanced cross-sections,
543
geometric constraints to trace the evolution of the Pyrenees. Tectonics 8, 41–50. Rye, D., Bradbury, H.J., 1988. Fluid flow in the crust: an example from a Pyrenean thrust ramp. Am. J. Sci. 288, 197–235. Trave´, A., Labaume, P., Calvet, F., Soler, A., Tritlla, J., Buatier, M., Potdevin, J.-L., Se´guret, M., Raynaud, S., Briqueau, L., 1998. Fluid migration during Eocene thrust emplacement in the south Pyrenean foreland basin (Spain): an integrated structural, mineralogical and geochemical approach. In: Mascle, A., Puigdefabregas, C., Luterbacher, H.P., Fernandez, M. (Eds.). Cenozoic Foreland Basins of Western Europe, Geol. Soc. Lond. Spec. Publ., vol. 134., pp. 163–188. Wayne, D.M., McCaig, A.M., 1998. Dating fluid flow in shear zones: Rb–Sr and U–Pb studies of syntectonic veins in the Ne´ouvielle Massif, Pyrenees. In: Parnell, J. (Ed.). Dating and Duration of Fluid Flow and Fluid-Rock Interaction, Geol. Soc. Lond. Spec. Publ., vol. 144., pp. 129–135.