A.McCaig - research: Fluid sources and flow patterns in an Alpine Thrust Belt: The Pyrenees

Fluid Sources and Flow Patterns in an Alpine Thrust Belt: the Pyrenees

Our study of syntectonic fluid flow in the Pyrenees has demonstrated the power of integrating a wide range of geochemical techniques in order to constrain different aspects of the fluid flow history. Datasets include Sr and Pb isotope data from fluid inclusions and whole rocks (Banks et al., 1991; McCaig et al., 1995, 2000a; Wayne and McCaig, 1998), fluid inclusion microthermometry (Grant et al., 1990; Henderson & McCaig, 1996); fluid inclusion crush-leach data on halogens and cations (McCaig et al., 2000b); SEM and electron microprobe data on deformed rocks (Knipe and McCaig, 1994); and stable isotope data. This study has demonstrated patterns of crustal fluid behaviour never previously documented:

Brines from a wide range of host rocks and structural settings are linked by high Br/Cl ratios, suggesting a common origin as evaporated seawater in the Upper Triassic. This fluid was stored in Triassic redbeds and fractured basement and acquired a range of cation and isotopic signatures related to host rock type. It appears to have been involved in the formation of Mesozoic Pb-F deposits before being expelled through thrusts and shear zones in the Tertiary Alpine orogeny.
Unidirectional expulsion of high pressure ⁸⁷Sr-enriched brine, derived from Triassic redbeds, through carbonate mylonites beneath the Gavarnie Thrust
Change in the fluid-flow regime to dilatancy pumping during growth of a culmination beneath the thrust. Evidence is a new trend towards ²⁰⁸Pb enrichment seen only in quartz-hosted fluid inclusions (Figure 1, 2), with ²⁰⁸Pb-rich fluid derived in part from Devonian phyllites in the hangingwall of the thrust. Isotopic trends correlate with Ca/Na ratio (Figure 3)
Higher-level mixing of the expelled brine with dilute fluids involved in a gravity-driven flow system in carbonate thrust sheets, with some fluids showing evidence for leaching of evaporite beds in the form of low Br/Cl ratios.
Fluid-flow along thrusts was dominated by transient cyclic fracture events, with healing of fractures and chemical redistribution during ductile deformation.

Earlier work on shear zones in the Aston and Bassiès Massifs showed that surface-derived fluids moved upwards in steep greenschist facies shear zones of Alpine age, and that fluid flow can be clearly related to syndeformational metamorphic reactions (McCaig, 1984; 1986; 1987; 1988; McCaig and Miller, 1986; McCaig et al., 1990; McCaig and Knipe; 1990; Tempest, 1991)

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Fig.1: Detailed cross-section through Pic de Port Vieux (after Banks et al., 1991). 1: Silurian graphitic slate. 2: Cambro-Ordovician schists and gneisses. 3: Triassic redbeds. 4: Cretaceous carbonates. G.T.: Gavarnie Thrust. PPVT: Pic de Port Vieux Thrust. Dots are fluid inclusion sample localities and triangles whole rock sample sites. Other samples were collected outside the immediate area shown.

Fig. 2. Carbonate mylonites (thrust zone Kls) show ⁸⁷Sr enrichment, but not the ²⁰⁸Pb enrichment trend seen in the inclusion fluids (adapted from McCaig et al., 2000a)

Fluid inclusion crush-leach data from Pic de Port Vieux

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Bibliography of Leeds-based publications related to fluid-flow in the Pyrenees

McCaig, A. M. (1984) Fluid-rock interaction in some shear zones from the Pyrenees. Journal of Metamorphic Geology, 2, 129-141.

McCaig, (1986) Thick- and thin-skinned tectonics in the Pyrenees. Tectonophysics 129, 319-342

McCaig, A.M. and Miller, J. (1986). 40Ar-39-Ar age of mylonites along the Merens Fault, central Pyrenees. Tectonophysics, 129, 140-172.

McCaig A. M. (1987) Deformation and fluid-rock interaction in metasomatic dilatant shear bands. Tectonophysics, 135, 121-132.

McCaig, A. M. (1988) Deep fluid circulation in fault zones. Geology, 16, 867-870.

McCaig, A. M. and Knipe, R. J. (1990) Mass-transport mechanisms in deforming rocks: Recognition using microstructural and microchemical criteria. Geology, 18, 824-827.

Grant N. T. (1990) Episodic discrete and distributed deformation: consequences and controls in a thrust culmination from the central Pyrenees. J. Struct. Geol. 12, 835-850.

Grant N. T. (1992) Post-emplacement extension within a thrust sheet from the central Pyrenees. J. Geol. Soc. Lond. 149, 775-792.

McCaig, A. M., Wickham, S. M. and Taylor H. P. Jr. (1990) Deep fluid circulation in alpine shear zones, Pyrenees, France: field and oxygen isotope studies. Contributions to Mineralogy and Petrology, 106, 41-60.

Tempest, S. A., (1991) Fluid-rock interaction in ductile shear zones, Central-eastern Pyrenees. PhD dissertation, Univ. Leeds, 202pp.

Knipe, R.J & McCaig, A.M., (1994) Microstructural and microchemical consequences of fluid flow in deforming rocks. Geol. Soc. Spec Pap. 78, 99-111

McCaig, A. M., Wayne, D. M., Marshall, J. D., Banks, D. A. & Henderson, I, (1995). Isotopic and fluid inclusion studies of fluid movement along the Gavarnie Thrust, central Pyrenees: Reaction fronts in carbonate mylonites. American Journal of Science, 295, 309-343.

Henderson, I. H. C., (1994) Fluid pressure variations in contrasting types of faulting: a fluid inclusion and cathodoluminescence study. PhD dissertation, Leeds Univ. 210pp.

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

McCaig, A. M. (1997) The Geochemistry of volatile fluid flow in shear zones In: Holness, M. (ed.) Deformation enhanced melt segregation and metamorphic fluid transport. Chapman and Hall, 227-260.

Wayne, D. M. & McCaig A. M., 1998. Dating fluid flow in shear zones: Rb-Sr and U-Pb studies of syntectonic veins in the Néouvielle Massif, Pyrenees. Geol Soc Spec Publ. 144, 129-135

McCaig, A. M. & Kirby, J. G., 1998. Fluid flow during folding and thrusting in carbonates: 2-D patterns of Sr and O isotope alteration. Proceedings of the 9^th international symposium on water-rock interaction - WRI-9, 785-788 (Balkema)

McCaig, A.M., 1998. Fluid flow mechanisms in mylonites: Evidence from compositional zoning patterns. In: Snoke, A. W., Tullis J. A. & Todd, V. R. (Eds) Fault-related rocks: A photographic atlas, 208-209, Princeton University Press

McCaig, A.M., 1998. Fluid flow mechanisms in mylonites: Evidence from microcracks. In: Snoke, A. W., Tullis J. A. & Todd, V. R. (Eds) Fault-related rocks: A photographic atlas. 210-211, Princeton University Press

McCaig, A. M, and Gong, L-Y 1998. Fluid flow in mylonites: Pervasive and channelised flow in carbonates. In: Snoke, A. W., Tullis J. A. & Todd, V. R. (Eds) Fault-related rocks: A photographic atlas, 212-213, Princeton University Press

Kirby, J.G., (1998) Fluid flow and isotopic exchange during deformation in the Pyrenees. PhD dissertation, Univ. Leeds, 289 pp.

McCaig, A. M., 1999. The influence of fluids on fault rock microstructures. J. Conf. Abs. v. 4, p. 726

Wibberley, CAJ & McCaig, AM 2000, Quantifying orthoclase and albite muscovitisation sequences in fault zones Chem Geol. 165, 181-196

McCaig, AM, Tritlla, J. & Banks DA, 2000. Fluid flow patterns during Pyrenean thrusting. J. Geochem Expl. 69-70, 539-544.

McCaig A. M., Wayne, D. M. and Rosenbaum, J. M. (2000a) Fluid expulsion and dilatancy pumping during thrusting in the Pyrenees: Pb and Sr isotope evidence. Geol. Soc. Amer. Bull. 112, 1199-1208.

McCaig A M, Tritlla, J and Banks D A (2000b) Fluid mixing and recycling during Pyrenean thrusting: evidence from fluid inclusion halogen ratios. Geochim. Cosmochim. Acta, 64, 3395-3412

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