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

Abstract

Thrusting fault zone in foreland basins are characterized by highly foliated zones generally enriched in phyllosilicates which can play a major role on the mechanical behaviour of the fault. In this context, investigations of synkinematic clay minerals permit to determine the origin of the fluid from which they precipitated as well as the mechanisms of deformation. Our study is focused on clay mineral assemblages (illite and chlorite) in a major thrust fault located in the Monte Perdido massif (southern Pyrenees), a shallow thrust that affects upper cretaceouspaleocene platform carbonates and lower Eocene marls and turbidites. It implied 3 km of displacement of the Monte Perdido thrust unit with respect to the underlying Gavarnie unit. In this area the cleavage development by pressuresolution is linked to the Monte Perdido and Gavarnie thrust activity. The core zone of the fault, about 6 m thick, consists of an interval of intensely deformed clay-bearing rocks bounded by major shear surfaces. The deformed sediment is markedly darker than the protolith. Calcitequartz shear veins along the shear planes are abundant. Detailed SEM and TEM observations of highly deformed fault zone samples indicate that clay mineral enrichment in the core zone of the fault is not only related to passive increase by pressure-solution mechanism but that dissolution-recrystallization of phyllosilicates occurs during deformation. A mineral segregation is observed in the highly deformed zone. Newly formed 2M 1 muscovite is present along the cleavage whereas IIb chlorite crystals fill S V 2 shear veins suggesting syntectonic growth of phyllosilicates in the presence of fluids in low-grade metamorphic conditions. These mineralogical reactions act as weakening processes and would favour Monte Perdido fault creeping.

Microstructural evolution of an incipient fault zone in Opalinus Clay: Insights from an optical and electron microscopic study of ion-beam polished samples from the Main Fault in the Mt-Terri Underground Research Laboratory

“…11E suggest that smectite-illite transformation could have occurred but this hypothesis should be further tested by analysis of the minerals in the micro shear zone and in the protolith (cf. Buatier et al, 2012b;Sasseville et al, 2012).…”

Quantification of mass transfers and mineralogical transformations in a thrust fault (Monte Perdido thrust unit, southern Pyrenees, Spain)

“…6E). Recrystallisation of clay minerals in the Monte Perdido thrust fault zone was observed by Buatier et al (2012) on the Torla outcrop (Fig. 2).…”

“…SEM images of the Millaris fault zone show also chlorite veins and well developed cleavage planes, but the SeC structures and the mineral segregation (calcite veins but also newly-formed chlorite and illite) are less developed than in the Torla fault zone. Therefore, the microstructures of the Monte Perdido (Torla) fault zone suggest a stronger accumulation of deformation than in the Millaris fault zone Buatier et al, 2012).…”

“…The pressure solution deformation mechanism might cause concentration of insoluble clay minerals and mechanical weakening within the fault zone (Lacroix et al, 2013). In the same fault zones, Buatier et al (2012) demonstrated that clay minerals did not only act as passive insoluble mineralogical phases but that phyllosilicates dissolution and precipitation occurred during deformation in the most deformed sediments.…”

“…Previous studies (e.g. Buatier et al, 2012;Lacroix et al, 2012Lacroix et al, , 2013 were realised on three selected outcrops, two outcrops corresponding to the Monte Perdido sole thrust with footwall and hanging wall of different lithologies (the Torla thrust) and the third one is a detachment fault inside the thrust unit (the Millaris fault). The latter fault was chosen for the present study because the similar marl lithology of the footwall and hanging wall is compatible with the assumption that the initial states of the sediments on both sides of the fault zone before deformation were identical.…”