FIELD AND PETROGRAPHIC OBSERVATIONS OF FLUID INCLUSION DISTRIBUTION AND RELATED STRUCTURES WITHIN THE NORTHERN SNAKE RANGE, NV DETACHMENT SYSTEM AT HAMPTON CREEK

Fluid circulation is an essential process associated with the evolution of metamorphic core complexes. Brittle normal faults and fractures provide pathways for surface fluids to reach the footwall of detachments, where fluids likely play a major role in the thermomechanics of extensional systems. In order to study fluid rock interaction, samples within a 150 m section of quartzite mylonite exposed in the footwall of the northern Snake Range detachment were analyzed using three orthogonal thin sections. Within the upper 15 m of the footwall, fractures, veins, cataclasites, fluid inclusion trails (FITs) and fluid inclusion (FI) clusters are abundant, but are rarely seen 20-30 m below the detachment fault. FITs are associated with fractures, quartz cataclasites and veins of epidote and quartz. FITs share orientations with these structures, and are at high angles to the mylonite foliation and lineation, or are sub-parallel to the lineation. Where fractures end, FITs commonly continue or locally fan upward or downward. FITs are composed of multiple 1-10 µm size FIs that are organized into bands 5-20 µm wide, and 200-400 µm long, locally longer (> 1 cm). FITs are observed in sets spaced 5-10 µm apart, but are locally closer (< 5 µm) or farther (> 100 µm) apart. Sections oriented parallel to lineation and perpendicular to foliation display quartz grains with c-axes nearly perpendicular to the thin section. In these crystals, 1-10 µm FIs cluster roughly the area of elongate crystal (250-2000 µm²). The other two thin section orientations contain quartz crystals with flash figures. Locally, these crystals contain en echelon FITs that are 50-100 µm long and 10-20 µm wide. The geometric relationships of these FI structures indicate that fluids may have been trapped along basal planes of quartz. Lastly, 1-3 µm FIs are abundantly present along grain boundaries and sub-grain boundaries of quartz crystals throughout the sampled section. These observations suggest a dynamic interaction of brittle and crystal plastic deformation mechanisms within the upper 15-20 m of the footwall, and also support the distribution of fluids via brittle-ductile cycling in these systems.