This project is a combined field-based and numerical modeling study of the circulation of surface fluids in zones of orogenic collapse. The study focuses on extensional detachment zones that define metamorphic core complexes and are located between the brittle upper crust and the hot and commonly partially molten lower crust. Surface fluids penetrate the crust down to the detachment zones and likely play a major role in the heat transfer and deformation localization that characterize detachments. Several detachment sections along the North American Cordillera, from British Columbia to Arizona, are sampled and analyzed for microstructures, oxygen isotopic ratios, and thermochronology. These results are used as input to develop and test permeability models of the upper crust and detachment zones. Models address fluid flow from the crustal scale, where fluid circulation is a heat-transfer agent, to the grain scale where fluids interact with ductile deformation processes. This study evaluates the degree of fluid-rock interaction in detachment zones and the thermal and mechanical role of fluids in orogenic collapse. In addition, knowing the isotopic fluid composition of surface fluids promotes an evaluation of the topographic and environmental changes that took place during the collapse of the North American Cordillera in Cenozoic time.
Who would think that rain water can descend 10 miles into the Earth?s crust? There is now ample evidence this is the case in regions that meet two conditions: (1) the crust is being pulled apart by the forces of tectonics, resulting in open cracks and fractures that canalize fluids; and (2) the region has a high geothermal gradient because it is heated from below by the rapid upward transfer of hot rocks and magma. These conditions promote the convective circulation of surface waters that remove heat from the crust as a radiator coolant fluid cools a car?s engine. Convective water flow is active today in regions such as the Great Basin in the southwestern United States where recharge of cool fluids and discharge of hot fluids (hot springs, mineralized areas) define hydrothermal systems. This project explores the deepest regions of fossil hydrothermal systems that formed during Cenozoic time (last 50 million years) in the North American Cordillera and that have been brought to the surface by tectonics and erosion. A combined field and modeling study of these regions helps understand how fluid circulations influence the thermal budget and the deformation of the crust. In addition, retrieving the composition of water that originated at the Earth surface at different times during the Cenozoic gives information on the topographic evolution of the Cordillera that evolved from a majestic Andean-like plateau to a dissected mountain range.
This project investigated the circulation of fluids, such as rain water and groundwater, that in some circumstances infiltrate the Earth and circulate to great depth. Minerals like mica that grew at depth and at high temperature within extremely deformed rocks during stretching of the crust contain small amounts of hydrogen atoms. Isotopic analysis fingerprints this hydrogen to have come from the surface of the Earth, which means that water must convect within the upper layer of Earth from the surface to the zone of intense deformation that separates the brittle upper crust from the hot and ductile lower crust. This water convects and therefore influences the thermal and mechanical state of the crust. In an attempt to better understand this fluid circulation, this project was designed with three main components: field, analytical/numerical, and experimental. The field component of the project has demonstrated that, from British Columbia to Arizona, all zones of high deformation that bound metamorphic core complexes (ductile rocks that were exhumed during stretching of crust) display evidence of circulation of surface fluids and intense geochemical/isotopic exchange between fluid and rock. In some cases, when the temperature of isotopic exchange is well known, the composition of the surface water can be retrieved; because the isotopic composition of surface water scales with latitude and altitude, results can be used as a proxy to determine the paleoelevation of the catchment areas where surface fluids originated. Numerical modeling has demonstrated that fluids can circulate in the crust provided there are zones of high porosity and permeability, like faults, that create privileged plumbing systems for fluid flow. Numerical models have also demonstrated the thermal consequence of fluid circulation, since cool fluids tend to refrigerate rocks at depth, and fluids that have been heated can transfer this energy to the shallow crust. Finally, the experimental part of this project has uncovered a new process that may explain how ductile rocks, that are thought to be tight (low permeability) can transfer significant amounts of fluids. Olivine aggregates were deformed in the lab at high temperature and to high strain, and in the presence of excess water that generated fluid inclusions and fluid-filled pores; the deformed aggregate produced the stretching, recrystallization, and crystallographic preferred orientation that we commonly see in highly deformed rocks in nature. During deformation, pores and inclusions line up in planar bands and likely communicate, creating a deformation-induced permeability that allows fluid flow while to rock is deforming by ductile processes (dislocation creep). These bands are in effect a form of high-porosity pathways that coexist with high-temperature deformation processes. These experiments offer a new hypothesis to explain fluid flow without having to resort to fracture instabilities. The combination of field, analytical, numerical, and experimental approaches in this project has necessitated team work; in the end, in addition to PIs and colleagues, a core of junior scientists consisting of one postdoc and three PhD students benefited form the synergies that emerged as the project was being developed. In addition to the training and educational benefits offered by this project, broader impacts for society include a better understanding of the water cycle, one of the primary resources on Earth. Results from this project may help with applications in geothermal energy, understanding of hydrothermal systems, and the transfer of metals from depth to the near-surface through fluid flow.
