Structural controls and diagenetic conditions associated with fluid migration on the Moab Fault, UT
Hodson, Keith Robert
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Faults and fault zones can have strong effects on fluid migration within the Earth’s crust, with important implications for resource management and earthquake hazards. The research presented within this dissertation addresses the fluid migration history of the Moab Fault, Utah. Using the stable isotope compositions of fault-hosted carbonate cements in conjunction with field and petrographic observations, new insight is gained into the structural context, thermal and diagenetic conditions, and source fluids associated with cementation and vein formation along the fault zone. In particular, clumped isotope thermometry is used to directly estimate the formation temperature of carbonate minerals. Two generations of cement are identified, featuring distinct isotopic compositions, formation temperatures, and source fluids. The two cements are related to different styles of deformation within the fault zone, which we use to interpret the timing and burial conditions for both cementation and deformation. Spatial distributions of different cements suggest that the distribution of permeability was primarily controlled by fault-segment interaction, producing localized zones of relatively intense deformation and fracturing. Relationships between carbonate and fluid isotopic compositions indicate that cementation and diagenesis occurred under rock-buffered conditions with low fluid-to-rock ratios. Through the combination of clumped isotope thermometry with fluid inclusion microthermometry, a complementary thermometry method, a new approach is developed to aid in interpreting temperature measurements from carbonate veins. Expected relationships between these two methods allow a researcher to identify whether a measured temperature likely reflects primary crystal growth or secondary alteration. Additionally, the combination of these methods allows for the estimation of the fluid pressure associated with crystal growth, which may help identify elevated geothermal gradients or pressure conditions during the seismic cycle. Included with this thesis are two supplementary files with additional data tables for chapters three and four. Supplementary tables for chapter 3 include a list of all replicate clumped isotope analyses, bulk isotope analyses, and recalculated data discussed in the text. Supplementary tables for chapter 4 include a list of all measurements of carbonate standards used in the study, a list of all replicate clumped isotope analyses, and a list of individual fluid inclusion measurements for each sample.