Geofluids and Mountain Building: Integrated Isotopic Studies of Deformed, Clay-Rich Rocks

Abstract

Hypotheses on the origins and pathways of geofluids in Earth’s crust are numerous and conflicting, ranging from the release of volatiles from magmatic and metamorphic bodies to km-scale infiltration of surface waters. In contractional orogens, the release of metamorphic fluid from the orogenic core would be expected to cause fluids to travel towards the low-temperature and low-pressure foreland region. Alternatively, the high elevation of the orogenic core could allow surface-sourced meteoric fluids to infiltrate the crust and gravitationally travel toward the foreland. These two primary fluid sources have unique stable isotopic signatures, which they impart to minerals that precipitate as a result of their passing. By investigating the isotopic signatures preserved in secondary clay minerals, this dissertation examines the origin of ancient fluids in four fold-and-thrust belts. Both hydrogen and oxygen isotopes are used to examine the provenance of deformational fluids, while 40Ar/39Ar dating is used to constrain the timing of fluid passage and mineralization. These integrated stable and radiogenic isotopic techniques explore the characteristics of several mountain chains from different locations in North and South American and different periods of geologic time from the Paleozoic to the Cenozoic. Stable hydrogen isotopes in fault gouge of the Argentine Precordillera show that fault rock fluids preserve signals of the aridifying Miocene climate in Central South America. Comparing the hydrogen isotopic signature in deformational clays in the Sevier fold-thrust belt of the North American Cordillera to the isotopic proxies of ancient surface fluids shows that clays track the isotopic composition of low-elevation surface fluids in the late Mesozoic. Combining oxygen and hydrogen isotope analyses in the Canadian Cordillera of Alberta on previously dated clay gouge shows variable mixing of metamorphic fluids and high-latitude meteoric fluids. Lastly, paired stable and radiogenic isotope studies and dating of clays from the Late Paleozoic Central Appalachian orogen reveal similar mixing of metamorphic and isotopically buffered meteoric fluids. Appendix A represents the early development and application of paired H-isotopic analysis and Ar-dating, showing that ancient meteoric fluids dominate Basin and Range crust during relatively young tectonic extension. Appendix B describes the Ar-dating correction procedure that corrects a subset of erroneous sample size fractions in Chapters III and V. Appendix C describes a preliminary comparison with Southern Appalachian fault gouge clay isotopic composition, revealing both the dominance of metamorphic fluid signatures in some areas and the mixing of meteoric and metamorphic fluids in others. Lastly, Appendix D explores a novel application of mercury isotopes to deformation fluid studies in samples San Andreas Fault cores, indicating that mercury isotopes are not a unique tracer of fluid sources in these samples, but they may be used to constrain the fluid sources and fractionation processes that occur in crustal rock reservoirs. This set of studies shows that ancient, meteoric fluids play an important role in upper crustal deformation. The mixing of metamorphic and meteoric fluids implies an open fluid system and a balancing of driving forces, gravitational and buoyancy forces, which together promote foreland-directed fluid flow in compressional tectonic belts.