Parsing the structurally-controlled fluid migration history of the Moab Fault, UT with carbonate clumped isotope thermometry
Keith Hodson, University of Washington
Juliet Crider, University of Washington
Katharine Huntington, University of Washington
Brittle deformation and faulting alter host rock permeability, but feedbacks associated with fluid migration and diagenesis produce temporal and spatial variability. The complexity of these interacting systems makes it difficult to reconstruct fluid migration histories, especially in cases of varying styles of structural deformation. Fault-hosted cements are a physical record of fault-controlled fluid flow, and their chemistry reflects source fluid composition and mineralization conditions. For carbonate cements and veins, stable isotopes of carbon and oxygen provide valuable insight into mineralization temperatures and fluid sources, but cannot give a unique solution unless one of these components is known. This study employs carbonate clumped isotope thermometry to independently constrain mineralization temperatures of carbonate cements. Our results highlight connections between structural deformation and fluid migration at scales from microstructures to kilometer-long fault segments, and reveal the lasting influence of the earliest structures on fault zone permeability.
We apply clumped isotopes to a well-characterized natural laboratory: the Moab Fault, Utah. At Courthouse Junction, a beautifully exposed and intensively studied fault-segment intersection, fault-hosted cementation is closely associated with a well understood sequence of structural deformation, including two styles of cataclastic deformation bands and mode I fractures. Prior work by others identified cements derived from multiple fluid sources with a range of precipitation temperatures, but did not clearly separate the influences of source and temperature or connect periods of cementation to different styles of structural deformation. Our new textural observations and clumped isotope analyses identify three episodes of cementation at this outcrop, featuring both marine and meteoric fluid sources. Cool marine fluids are the source for an early period of cementation, closely associated with deformation bands and fractures along their cores. Second, warm meteoric fluids record subsurface fluid migration following the formation of joints associated with the main period of fault slip, with a range of temperatures indicating continued permeability during exhumation. Late stage cements appear to mark a return to cool cementation conditions.
Cement properties from different segments of the Moab Fault and around segment intersections reveal the fault-scale fluid migration history during the growth and interaction of independent segments. Using the distributions of discrete cementation episodes, we gain insight into the important controls on fault zone permeability, such as growth of the damage zone and focused deformation around segment intersections and relay zones. These findings provide a new perspective on fault-fluid interaction, and demonstrate the power of clumped isotopes for problems in structural diagenesis.