45444-B8
Dynamic Development and Reactivation of a Newly Discovered Frictional Plastic Deformation System
Understanding the origin and early geologic evolution of faults is important to petroleum exploration and reservoir development because they serve as zones of weakness in the crust that are preferentially reactivated and rejuvenated through time. This project focuses on the development of coeval pseudotachylyte and mylonite in the newly discovered Grizzly Creek shear zone exposed in Glenwood Canyon, Colorado. Pseudotachylyte is generated by frictional melting during seismic slip at very high strain rates (~10-2 s-1) and is a unique fossil record of individual earthquakes. In contrast, mylonite forms at strain rates at least nine orders of magnitude lower (~10-11 s-1) compatible with slow aseismic creep. The field site is one of only a dozen in the world that exhibit coeval and mutually overprinting pseudotachylyte and mylonite, and is an extraordinary field laboratory that allows us to glimpse into the middle part of the crust and examine the dynamics of fault development through time. Field research completed in 2007 successfully outlined the structural framework of the shear zone as a template for process-oriented field and laboratory studies in 2008 and 2009. During the 8/31/07-8/31/08 reporting period, research focused on geochemical characterization of the rare fault rock assemblage outlined in 2007; continued field work scheduled for summer 2008 was postponed due to injury. Fault rocks and their undeformed host rocks were characterized by X-ray fluorescence (XRF), and LA-ICPMS U-Pb and 207Pb/206Pb ages were determined in order to provide constraints on the timing of shear-zone development. Geochemical work was completed at three external laboratories by collaborators. Preliminary results indicate that the shear zone formed in a closed environment after 1741 ± 10 Ma, the age of the youngest host rock. Current results were presented at a professional field trip led by the PI for the 2007 Geological Society of America annual meeting in Denver. The trip was attended by 20 academic scientists and graduate students from the United States, Canada, and Australia. For the reporting period, one undergraduate researcher was involved with the project. His results were presented in April 2008 at our campus undergraduate research day. We intend to present his ongoing research next March at an undergraduate research symposium at the state capitol, and in May at a regional scientific conference. Three to four undergraduate students will join the project for field work during summer 2009. Undergraduate involvement in this project has led to acquisition of a new scanning X-ray Analytical microscope (SXRAM) and Raman spectrometer using institutional and EPSCoR grant funds. This is the first analytical laboratory for solid materials on our campus and acquisition of funds and renovation of the laboratory space is directly attributable to PRF support of this research. The instruments will be used extensively in this project, as well as for undergraduate teaching purposes.
