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

Abstract

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 lm in thickness). Authigenic chlorite provides essential information about the origin of fluids and their temperature. d18 O and dD 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 largescale 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 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.

Microtextural investigation (SEM and TEM study) of phyllosilicates in a major thrust fault zone (Monte Perdido, southern Pyrenees): impact on fault reactivation

“…This author suggests that fabric formation has an important role in clay mineral formation in this context. The clay mineralogical assemblage and the nature of chlorite and illite polytypes described here are consistent with the P-T conditions established by Lacroix et al (2011Lacroix et al ( , 2012 who suggest a temperatures of about 200 and 240°C and a maximum depth of 6 km during Monte Perdido thrust fault emplacement et reactivation. The TEM observations of the fault core samples display the preferential orientation of phyllosilicate.…”

“…6.1 Origin and behaviour of phyllosilicates during deformation Lacroix et al (2012) demonstrate that the clay fraction of the studied host sediment is dominated by illite in limestones (Alveolina limestone from the hanging wall), whereas the pelites and sandstones (Hecho Group turbidites from the footwall) contain chlorite and illite in similar proportions. They also found detrital chlorite-illite aggregates of various sizes in pelites, sometimes accumulated along pressure-solution surfaces as an ''insoluble residue''.…”

“…1c, 2) related to the Monte Perdido thrust was studied and sampled (see also Lacroix et al 2011Lacroix et al , 2012. The hanging wall comprises the Alveolina limestone of the Monte Perdido thrust unit and the footwall comprises Hecho Group turbidites (alternating sandstone and pelite layers) of the Gavarnie thrust unit.…”

“…The latter structures are related to the late exhumation of the massif. Lacroix et al (2011Lacroix et al ( , 2012 demonstrated that the first stage of deformation, related to the Monte Perdido thrust unit emplacement, occurred at a temperature around 210°C and a burial depth of about 6 km (estimated from calcite to quartz d…”

“…The second deformation episode corresponds to a fault reactivation associated to the development of the chlorite shear veins in highly foliated clay-rich core zone. Coupling compositional chlorite geothermometry with fluid inclusion microthermometry, Lacroix et al (2012) estimated a temperature of 240°C and a burial depth of about 7 km for this second stage of deformation. The O-isotope fractionation between cogenetic chlorite and quartz gives a similar range of temperatures (234 to 265°C).…”