FAULTS AND ILLITE: SOURCES, PATHWAYS AND TIMING OF GEOFLUIDS (Invited Presentation)

Fluids in the upper crust affect strength properties, rock composition and mineralization, but the sources, timing and pathways of geofluids are still not well constrained. Stable isotopic studies of clay-rich rocks, particularly newly formed illitic clays, shed new light on the role of geofluids during faulting and folding. Hydrogen isotopic (δD) measurements of illite in fault gouge and folded clay-rich sediments from various fault systems, including thrusts in the Argentine Precordillera, normal faults in the US Basin-and-Range and transforms in N Turkey and New Zealand give highly negative values that reflect meteoric, surface-sourced fluid involvement. Timing of neomineralized clays using encapsulated Ar dating of grain size fractions is key to the interpretation of these fluid source data, showing that ancient meteoric fluids infiltrated several km down into the upper crust (e.g., in the Basin and Range), while elsewhere (e.g., the Alpine Fault of New Zealand), mineralization is young and shallow, overprinting the fault system. We discuss these illite isotopic approaches, critically examine their assumptions, limitations and any misconceptions, and present key outcomes from new and recent studies from deformation zones in a range of structural settings. While technically challenging, the results of integrated radiogenic and stable isotope geochemistry of illitic rocks offer robust information on the role and history of geofluids in deformed shallow crustal rocks.