Project

extensional detachments; orogen; fluids; fluid flow; isotopes; Extension; Detachment; Cordillera

Lead
Lay summary
The project explores deep crustal hydrothermal systems, formed during the Cenozoic in the North American Cordillera and exhumed by tectonics and erosion. A combined field, laboratory, and modeling study of these regions helps understand how fluid circulation influences the thermal budget and deformation of the crust. In addition, retrieving the composition of water that may have originated at the Earth surface gives information on the topographic evolution of the Cordillera. The interaction between fluids and minerals and the relevance to deformation processes still remain unclear and are the focus of this research. Six different sections of detachments from British Columbia to Nevada were sampled. The exposed transition between detachment and overlying crust offer insights into the plumbing system (faults, veins, cataclasites). Hydrogen isotope analyses of micas of these detachments indicate that muscovite grains grew in the presence of surface-derived water. Quartz-muscovite oxygen isotope thermometry indicates deformation-recrystallization at about 350°C. In some parts of the sections though, isotopic disequilibrium may be related to channelized fluid flow, with preferential flow in regions of dynamic recrystallization. The latter is indicated by changes in chemical composition of micas and EBSD analyses that give constant pole figures suggestive of non-coaxial, simple shear deformation. High angle cataclasite zones, brittle faults, and veins, potentially channelizing fluid flow, compared to quartzite mylonite and schist layers from detachments have not been affected by late stage semi-brittle to brittle deformation. Distributions of fluid inclusions within microstructures and analyses thereof indicate that aqueous fluids were trapped in planes crosscutting quartz grain boundaries at temperatures below 300°C. Aqueous-carbonic fluids occur in grain boundaries of dynamically recrystallized quartz at trapping conditions above 300°C. Throughout the section, hydrogen isotope compositions of fluid inclusions in quartzite mylonite and schists are different from those of high angle cataclasites and brittle faults. As only fluids from the latter are similar in value to fluids in equilibrium with muscovite, two separate fluid-flow events are indicated for the different zones. The isotopic compositions of muscovite in the detachment are compatible with an early pervasive influx of meteoric water within the quartzites. During exhumation, the fluids are, however, buffered by the schists. The differentiated meteoric fluids are considered to flow upwards within convection cells induced by an elevated geothermal gradient and deformation-enhanced permeability.

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Last update: 21.02.2013

Active participation

Number	Title	Start	Funding scheme

The proposed project aims at understanding the circulation of surface fluids in the context of extensional detachments, particularly during orogenic collapse. The main hypothesis is that fluids convect from the surface to the detachment along faults and fractures in the brittle crust that serve as zones of recharge and discharge. This buoyancy-driven fluid flow is controlled by a high heat flow at the base of the system, beneath the detachment, where heat is advected by crustal thinning and magma intrusions. This hydrothermal convective flow is focused in the detachment for the duration of activity of the detachment and at high temperature, resulting in fluid-rock interaction. As such, detachments may be privileged locations for isotopic exchange. Indeed, preliminary studies of the North American cordilleran core complexes have shown that meteoric fluids permeate detachment zones, as recorded by the hydrogen isotope composition of hydrous phases such as muscovite and biotite. These minerals can record the original isotopic composition of the fluids and, therefore, provide information on surface paleo-environment and in particular paleo-elevation. One important goal of the project is to understand the dynamic link that may exist between this fluid flow and the thermomechanical behavior of detachments.We are studying this problem by using a combined field, analytical, and numerical modeling approach. One Ph.D. student and one postdoc at UNIL (University of Lausanne) are currently involved in a field-based project on several detachment zones in the Cordilleran core complexes from British Columbia to Montana. These studies include mapping, structural, and microstructural charactarization, and analyses of fluid inclusions as well as their structural, geochemical, and stable isotope composition, all performed at UNIL and the University Henri Poincaré in Nancy. The PhD student and postdoc are part of a larger team that includes the UNIL project director (Vennemann) and collaborators in the USA (Teyssier and Saar, U. Minnesota), France (Jessell, Toulouse), Germany (Mulch, Hannover), and Australia (Rey, Sydney). Regarding the modeling component of the project, the PhD student is spending part of his academic year 2008-09 at the University of Minnesota where he is conducting numerical modeling of crustal scale fluid flow under the guidance of Saar (fluid dynamicist) and Teyssier. This modeling will focus on the role that fluid flow plays in the thermal exchange between deep and hot orogenic crust and the Earth’s surface. Results from the models will serve as a guide for thermochronological strategies in detachment systems and will likely shed light on the metamorphic evolution of footwall regions of detachments. The postdoc at UNIL will focus on fluid inclusion studies in order to trace the history of fluids preserved in footwall to hanging wall sections of detachments. Scientific impact: The project will provide a set of numerical tools to study the relationship between fluid flow and deformation on the crustal scale to the grain scale. Models will be developed using the Ellipsis numerical code, coupled to Lattice-Boltzman models of fluid dynamics, to simulate crustal deformation and fluid flow. Microdynamical models will also be developed using ELLE in collaboration with Mark Jessell (U. Toulouse) and Martin Saar (U Minnesota). These models will be made available to the geoscience community. The project will also produce original geologic work on classic detachments and will demonstrate the importance of integrating structural and geochemical studies to address a complex geologic system, in this case the coupling between fluid flow and deformation.Broader impact: This project will also enhance our understanding of hydrothermal systems and the conditions that lead to mineral and ore deposits. Long-term fluid flow in the crust (104-105 yrs) will be modeled and tested with isotopic data, and will therefore serve as a guide for hydrologic studies aimed at, for example, investigating the storage of high-level radioactive waste. In addition, the study of this type of hydrothermal system will have implications for the possible exploitation of geothermal energy away from magmatic/volcanic centers.