DISTRIBUTION AND MICROTHERMOMETRY OF FLUID INCLUSIONS 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 structures provide pathways for surface fluids to reach the footwall of detachments, where fluids likely play a major role in the thermomechanics of extensional systems. To best interpret the interaction between fluids and rock deformation, samples were collected from a 150 m exposed section of quartzite mylonite in the footwall of the Northern Snake Range detachment and were analyzed using three orthogonal thin sections. This well studied system was principally exhumed by Oligo-Miocene SE-directed extension.

Within the footwall, two sets of fractures and veins are observed at a high angle to the mylonite foliation. NE-SW striking fractures are more prominent than SE-NW striking fractures. Thin sections contain veins, microfractures, fluid inclusion planes (FIPs), and fluid inclusion (FI) clusters that are associated with these fracture sets. FIPs are composed of multiple 1-10 µm diameter FIs that form aligned bands 5-20 µm wide and 200-400 µm long. Spacing of FIPs ranges 5-10 µm, but FIPs are locally closer (< 5 µm) or farther (> 100 µm) apart. At the end of microfractures and single FIPs, FIPs bifurcate upward or downward forming horsetail structures. Unique FIPs are perpendicular to c-axes of quartz crystals (lying in the basal slip plane), and locally form en echelon arrays.

Three types of FIs have been identified and are related to these structures: a proportionate mixture of CO2 and H2O (type I), CO2-rich (type II), and H2O-rich (type III) inclusions. Type I inclusions are mostly associated with SE-NW striking fractures (local occurrences in NE-SW fractures), whereas type II and III FIs are only correlated to NE-SW striking fractures. Isochores for these FIs suggest conditions where CO2 and H2O was miscible above 1 kbar and between 270 and 345 degrees C.

It is interpreted that this detachment system experienced an earlier NE-SW extension followed by a later and a more dominant SE-NW extension. Principal SE-directed exhumation of the detachment system is interpreted to have generated a high geothermal gradient (~70 C/km), and at P-T conditions where CO2-H2O was immiscible.