Geologic observations of linked pseudotachylyte-filled seismogenic faults and related mylonite-ultramylonite plastic shear zones exposed in the Sawatch Range, Colorado, to investigate the mechanics of seismic faulting at mid-crustal hypocentral depths. Detailed field and laboratory observations of deformation structures at all scales focus on three related aspects of faulting at depth: (1) rupture directivity and processes related to barriers and asperities in en echelon fault arrays and fanning splays; (2) mechanisms of fault initiation, linkage, and stress/strain transfer between individual faults within the system; and (3) coupling and partitioning between frictional faulting (pseudotachylyte system) and coeval plastic creep (mylonite-ultramylonite shear system). The project combines detailed field studies with laboratory analysis and imaging (optical microscopy as well as scanning electron microscope, electron microprobe, and electron backscatter diffraction methods) to develop a model of fault/shear zone kinematics and dynamics and to elucidate the linkage between microscopic deformation mechanisms and seismic processes. Project results will provide empirical geologic constraints for models of mid-crustal earthquake processes and plastic instabilities and shed light on how transient elastic strain in the upper crust may be coupled to slow plastic creep and flow in the lower crust.
This project will investigate earthquake processes by exploiting two large and well-mapped pseudotachylyte fault systems. Pseudotachylytes are produced by flash melting along faults during seismic slip and can record a detailed record of seismic events. More than a billion years of erosion and uplift in the Sawatch Range of Colorado has exposed fault rocks from the middle crust where the largest earthquakes typically originate. Thus, structures produced by deep seismic processes that are far beyond the range of direct observation or scientific drilling can be observed. The project will contribute empirical observations to help improve models of how faults rupture to produce earthquakes. The geologic results will complement geophysical observations of seismic waves, geodynamic computer models of earthquake processes, and direct observations of faults at shallower depths to give a more complete picture of earthquake mechanics. Training of the next generation of earth scientists will be promoted by building a connection between the undergraduate field geology program at Concord University and the research-intensive graduate program at Montana State University. The educational impact of the project will be enhanced through close cooperation with the McNair scholars program at Concord to promote science careers for first generation college students and under-represented groups.
