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.