Fluid circulation in a ductile shear zone (Tinos, Cyclades, Greece): Mechanisms and tectonic implications
Incursion de fluides dans une zone de cisaillement ductile (Tinos, Cyclades, Grèce) : Mécanismes de circulation et implications tectoniques
Résumé
Fluid flow along mid-crustal shear zones has been extensively described on the structural, petrological and chemical aspects. Yet, quantitative parameters of deep fluid circulation, although of considerable importance for a real understanding of lithosphere shearing, are still poorly constrained at the present. We have combined structural investigation, ion probe ?18O analysis and infrared microspectrometry on metamorphic veins in order to determine the timing of occurrence, temperature, source and mechanisms of fluid flow along an exhumed shear zone (Tinos Island, Cyclades, Greece). Vein evolution at the ductile-brittle transition Three main types of metamorphic veins have been recognised from field observation. Type I veins are synfolial boudins stretched by ductile flow. Type II veins are vertical joints folded by late ductile deformation. Type III are brittle tension gashes. The three vein types represent a continuous set of structures from quasi-plastic to cataclastic deformation regime. The volume concentration of each vein type has been estimated along a transect running toward the shear zone. Type I veins are homogeneously spread out on the whole island, whereas Type II and Type III veins concentration gradually increases toward the detachment, to form a connected network just below the fault surface. This feature suggests that the appearance of Type II veins is coeval with a drastic permeability increase in the footwall, while passing the ductile-to-brittle transition. Thermal regime and fluid source Type I and Type II veins have been collected for ?18O thermometry, along a 200 m-long traverse under the detachment. On the basis of cathodoluminescence imaging, a succession of four generations of quartz and calcite presenting an equilibrium texture can be identified in these veins. Early quartz-calcite pairs 1 and 2, found in Type I veins, are plastically deformed, whereas quartz-calcite 3 and 4 (Type II) show evidences for deformation in the brittle regime. Micro-scale ?18O analyses were carried out on each quartz-calcite pair using a Cameca IMS 1270 ion probe. Quartz-calcite 1 yields constant fractionation values, from which a temperature of 360 °C is calculated. In contrast, fractionation values of quartz-calcite 2 and 3 yield a steep cooling gradient of about 120 and 145 °C, respectively, along a distance of 72 m towards the detachment. Such a strong cooling is best interpreted as advective heat extraction by fluid flow along the detachment, during the ductile to brittle evolution. Knowing the temperatures of quartz-calcite crystallisation, the source of fluids can be inferred by comparing the ?18O values of the four quartz-calcite pairs. H2O in equilibrium with quartz-calcite 1 yields steady ?18O of 15-20 ‰ for the fluid phase. In contrast, fluid isotopic composition in quartz-calcite 2 shifts from 20 to 10 ‰ when approaching the detachment. The ?18OH2O deplete is even more obvious in quartz-calcite 3 and 4, falling from 15 ‰ at 60 m to 0 ‰ at 1m below the detachment. The most likely reservoir for such a drastic 18O deplete is oceanic or meteoric water. We thus interpret structural and isotope data as a result of progressive infiltration of surface-derived fluids into the exhuming shear zone, promoted by permeability rise at the ductile-brittle transition. Seismic pumping Ion probe and fluid inclusion analyses have also been conducted on quartz-calcite strain fringes developing around pyrites in the mylonitic footwall of Tinos shear zone. A micro-scale traverse from the edge to the centre of one of the fringes reveals a periodic ?18O fluctuation between 19 and 21 ‰. This shift is interpreted as episodic influx of externally-derived fluids into the shear zone through transient fracture permeability, interspersed by periods of closed system buffering by the wallrock. Primary fluid inclusions trapped in quartz and calcite show increasing salinities (0 to 4 wt% eq. NaCl) and decreasing densities from the edge to the centre of the fringes. Results are interpreted to reflect pressure variations from lithostatic to hydrostatic values in the time span of fringe growth. Oxygen isotope and fluid inclusion data are in excellent agreement with models of dilatancy pumping in shear zones triggered by earthquake cycles in the overlying seismogenic zone. We propose that seismic pumping was efficient in Tinos detachment beneath the ductile-brittle transition, and developed suction forces sufficient to drive the downward penetration of surface-derived fluids into the ductile crust. Wetting properties of fluids We have also studied the OH structure of metamorphic fluids by high temperature infrared (IR) microspectroscopy on natural fluid inclusions contained in Type I and Type II veins, over the temperature range 25-270 °C. Blueschist-facies veins (Type I) display H2O-NaCl-CaCl2-CO2 inclusions whereas greenschist-facies veins (Type II) contains H2O-NaCl±CO2 inclusions. With heating, peak positions of OH stretching IR absorption bands increase quasi-linearly with slopes of 0.25 and 0.50 for inclusions trapped under blueschist and greenschist conditions, respectively. Extrapolation to ~400 °C yield peak positions of 3475 cm-1 for blueschist inclusions and 3585 cm-1 for greenschist inclusions. Since the smaller wavenumber indicates the shorter hydrogen-bond distance between water molecules, fluids involved in the greenschist event have a “loose” structure compared to blueschist fluids. We suggest that these properties might correspond to a low wetting angle of fluids. This would explain the high mobility of infiltrating fluids predicted by stable isotope and fluid inclusion data. The low porosity level of metamorphic rock precluded the migration of local metamorphic fluids, whereas it formed a connected network for externally derived fluids, allowing their penetration into the system. Model of fluid infiltration By combining our conclusions from structural, oxygen isotope, fluid inclusion and microspectroscopic data, we propose a model of fluid circulation at all the stages of detachment evolution. Over the ductile-brittle transition, flow is triggered by thermal convection along the detachment, promoted by the high permeability level of the fractured footwall. Surface-derived fluids are drawn into the ductile shear zone by seismic pumping. Large-scale diffusion of infiltrated fluids into the quasi-plastic regime is enhanced by their high wetting character compared to local fluids. This model would have the advantage to reconcile geochemical observations and mechanical concepts, by solving the paradox of surface-derived fluid infiltration (at hydrostatic pore pressure level) in a ductile lower crust at lithostatic pore pressure.
Les dômes métamorphiques portent les marques d'un hydrothermalisme intense concomitant de leur exhumation de long de grandes zones de cisaillement crustales. Sur l'exemple du détachement de l'île de Tinos (Cyclades, Grèce), nous proposons une approche qui combine géologie structurale et techniques analytiques de pointe, pour contraindre l'origine et les modalités de circulation des fluides responsables du métasomatisme. Une succession continue de veines métamorphiques, ouvertes durant l'évolution de la déformation depuis le domaine ductile jusqu'au domaine cassant, est identifiée dans le mur du détachement. Chaque stade d'ouverture occasionne la précipitation de quartz et de calcite. Le fractionnement isotopique de l'oxygène des couples quartz-calcite précipités à la transition fragile-ductile, mesuré à la sonde ionique, passe de 2 à 5 ‰ dans une frange de 50 m vers la faille, soit une chute de température de ~150 °C. Le ?18O de l'eau en équilibre avec ces couples passe de 15 à 0 ‰ dans cette même frange. Seules des circulations convectives de fluides surfaciques peuvent expliquer ces variations thermiques et isotopiques. Un profil isotopique réalisé dans l'ombre de pression d'une pyrite montre une chute brutale du ?18O de 3 ‰, entre deux maxima. Les inclusions fluides piégées dans des ombres de pression similaires, synchrones du cycle isotopique, montrent une forte fluctuation des isochores de stabilité. Les données isotopiques et microthermométriques, qui suggèrent une arrivée cyclique de fluides superficiels conjointe à des variations de la pression des fluides, concordent point à point avec les modèles de pompage sismique en domaine ductile initiés par la zone sismogénique sus-jacente. Enfin, une comparaison des spectres infrarouge d'inclusions fluides piégées en faciès schiste vert et schiste bleu permet d'évaluer les propriétés mouillantes des fluides métamorphiques. A 400 °C, le pic d'absorption des liaisons OH des fluides piégés dans les inclusions schiste bleu est situé à 3475 cm-1, tandis que celui des fluides schiste vert est à 3585 cm-1. Plus fort est le nombre d'onde, plus l'agencement des molécules d'eau est désordonné, occasionnant des tensions d'interface fluide-grain faibles. Les fluides du faciès schiste vert ont donc une capacité à connecter la faible porosité des roches métamorphiques très supérieure à celle des fluides schiste bleu, et peuvent infiltrer le mur du détachement en cours d'exhumation, auparavant en système clos. Ces interprétations permettent de proposer un modèle global de circulation à tous les étages d'un détachement, commandé –dans l'ordre de profondeur croissante- par la convection thermique, les cycles sismiques et la migration diffuse. Ce modèle synthétique résout le paradoxe qui consiste à introduire des fluides surfaciques -à pression hydrostatique- dans une croûte inférieure ductile à pression de pores lithostatique.