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[1] The Northern Snake Range (Nevada) represents a spectacular example of a metamorphic core complex and exposes a complete section from the mylonitic footwall into the hanging wall of a fossil detachment system. Paired geochronological and stable isotopic data of mylonitic quartzite within the detachment footwall reveal that ductile deformation and infiltration of meteoric fluids occurred between 27 and 23 Ma. 40 Ar/ 39 Ar ages display complex recrystallization-cooling relationships but decrease systematically from 26.9 ± 0.2 Ma at the top to 21.3 ± 0.2 Ma at the bottom of footwall mylonite. Hydrogen isotope (dD) values in white mica are very low (−150 to −145 ‰) within the top 80-90 m of detachment footwall, in contrast to values obtained from the deeper part of the section where values range from −77 to −64 ‰, suggesting that time-integrated interaction between rock and meteoric fluid was restricted to the uppermost part of the mylonitic footwall. Pervasive mica-water hydrogen isotope exchange is difficult to reconcile with models of 40 Ar loss during mylonitization solely by volume diffusion. Rather, we interpret the 40 Ar/ 39 Ar ages of white mica with low-dD values to date syn-mylonitic hydrogen and argon isotope exchange, and we conclude that the hydrothermal system of the Northern Snake Range was active during late Oligocene (27-23 Ma) and has been exhumed by the combined effects of ductile strain, extensional detachment faulting, and erosion.

In the Limousin area, Variscan leucogranitic plutons are spatially associated with normal faults and major strike-slip shear zones that are a continuation of the South Armorican shear zone. Our study focuses on the large N-S-trending Millevaches granitic massif (Massif Central, France), and intends to highlight, through gravity modelling, structural and anisotropy of magnetic susceptibility (AMS), the massif structure at depth and to discuss the mode of emplacement of granites within a strike-slip tectonic context. The mica subfabric suggests that the magnetic foliations display a general NW-SE sub-horizontal pattern on both sides of the N-S Pradines dextral wrench fault zone that deforms the core of the massif on 5 km width. The magnetic lineation trend exhibits a sigmoïdal pattern, N-S in the Pradines fault zone and NW-SE on both sides of it, which are consistent with a dextral wrench component. The horizontal magnetic foliations and lineations are consistent with the thin granite laccolith model. There is no significant imprint of the extensional Variscan belt collapse on the internal fabric of Millevaches granites than the tectonic dextral transcurrent movement prevailing in this area.

Combined petrofabric, microstructural, stable isotopic, and 40 Ar/ 39 Ar geochronologic data provide a new perspective on the Cenozoic evolution of the northern Snake Range metamorphic core complex in east-central Nevada. This core complex is bounded by the northern Snake Range detachment, interpreted as a rolling-hinge detachment, and by an underlying shear zone that is dominated by muscovite-bearing quartzite mylonite and interlayered micaschist. In addition to petrofabric, microstructural analysis, and 40 Ar/ 39 Ar geochronology, we use hydrogen isotope ratios (δD) in synkinematic white mica to characterize fl uid-rock interaction across the rolling-hinge detachment. Results indicate that the western fl ank of the range preserves mostly Eocene deformation (49-45 Ma), characterized by coaxial quartz fabrics and the dominant presence of metamorphic fl uids, although the imprint of meteoric fl uids increases structurally downward and culminates in a shear zone with a white mica 40 Ar/ 39 Ar plateau age of ca. 27 Ma. In contrast, the eastern fl ank of the range displays pervasive noncoaxial (top-tothe-east) fabrics defi ned by white mica that formed in the presence of meteoric fl uids and yield Oligo cene-Miocene 40 Ar/ 39 Ar ages (27-21 Ma). Evolution of the Oligocene-Miocene rolling-hinge detachment controlled where and when faulting was active or became inactive owing to rotation, and therefore where fl uids were able to circulate from the surface to the brittle-ductile transition. On the western fl ank (rotated detachment), faulting became inactive early, while continued active faulting on the eastern fl ank of the detachment allowed surface fl uids to reach the mylonitic quartzite. The combined effects of synkinematic recrystallization and fl uid inter action reset argon and hydrogen isotope ratios in white mica until the early Miocene (ca. 21 Ma), when the brittle-ductile transition was exhumed beneath the detachment. GEOLOGICAL SETTING AND PREVIOUS WORK The northern Snake Range detachment defi nes the Snake Range metamorphic core complex in east-central Nevada (Fig. 1), which resulted from Oligocene-Miocene extension of the Basin and Range Province (Wernicke, 1981;