RUI: Integrated Strain and Fluid Composition, Temperature and Pressure Histories of Orogens

Colgate University, Hamilton NY

Investigators

Abstract

The investigators present a plan of research aimed at determining the strain, fluid composition, fluid temperature and fluid pressure history of orogenic belts. The presence of fluids and fluid pressure are believed to exert strong controls on a variety of tectonic phenomena, including earthquake cyclicity, the geometry of accretionary prisms and both brittle and ductile rock deformation. Fluid migration is also a significant factor in the transfer of heat and matter in the lithosphere and is one of the more significant factors in the formation of mineral deposits. However, determining the nature and conditions of fluids at depth in actively deforming regions remains highly difficult. Even when drilling does yield information about fluid composition, temperature and pressure we are given only a brief snapshot rather than an indication of possible fluid variations over geologic time. Some information has been gleaned from the study of fluid inclusions and stable isotopes in veins, but the timing and duration of vein formation in a structural sequence is obscure in most cases. The PI's propose that analysis of fluid inclusions and stable isotopes in fibrous strain fringes around rigid objects is the best way to get a nearly complete record of fluid conditions over the course of orogeny. They have conducted an incremental strain, stable isotope and fluid inclusion study of some unusually large strain fringes in the Taconic slate belt of Vermont and New York, with interesting results. The data indicate a very strong correlation between the directions of principal elongation and fluid pressure and temperature. Their methods rely, however, on being able to serially section the strain fringes and measure O isotopes in coexisting quartz and calcite using laser fluorination of quartz and carbonate dissolution. This precludes using most strain fringes, because they are either too small or contain other minerals, such as chlorite or white mica. Stable isotope analyses of such features become quite difficult because pure mineral separates in small volume must be produced. The PI's propose using the ion microprobe to overcome these drawbacks. Doing so will allow unprecedented spatial resolution in large strain fringes and will allow the use of smaller strain fringes as well. They will also be able to use strain fringes with more mineralogical diversity, opening up new avenues for research. They propose performing a test of this method by conducting both silicate laser fluorination and carbonate dissolution stable isotope analyses along with ion microprobe stable isotope analyses on identical samples of unusually large strain fringes from the Taconics. Following this, they will expand their geographic coverage in the Taconics to test the hypothesis that strain and fluid conditions are inter-related and controlled by motion on the Bird Mountain thrust fault. They will also analyze samples from well studied sites in the Pyrenees and Helvetic Alps, with both very simple and complex strain histories. The PI's believe that this work will demonstrate the utility of this approach, opening new avenues to other workers. They also plan to include undergraduates in the research and contend that this will provide a stimulating and significant research experience for three students each summer. Each year one student will work on each aspect of the research; incremental strain, fluid inclusion and stable isotope analysis, working in close collaboration with one of the PI's. Broader impacts of the proposed work include expanding the utility of the ion microprobe, taking full advantage of NSF-purchased stable isotope and SEM instrumentation at Colgate, exposure of undergraduates to research, inclusion of faculty from an undergraduate institution in research and expansion of techniques available for the study of orogenic fluids.