Fault Related Alteration of Gouge in Sandstones
Texas A&M University, Department of Geology & Geophysics, Center for Tectonophysics, College Station, TX USA
I seek to establish a relationship between alteration of fault gouge and seismic displacement of the host fault. The fault sample, a portion of the San Andreas Fault Observatory at Depth (SAFOD) Phase One core, is arkosic sandstone and siltstone separated by several centimeters of fine-grain gouge material. A left-lateral subsidiary fault of the San Andreas juxtaposes the two rock types in this sample. I propose that thermochemical and mechanical alterations of fault gouge record the micro-mechanisms of frictional slip, strength characteristics, and slip-rate history (seismic versus aseismic) of the fault. Characterization of localized chemical changes, microstructural evidence of dissolution/precipitation, mineral alterations, and maturation of organic particles will be characterized and quantified. Mechanical and fluid flow properties of the fault will be inferred from these analyses. Previous work in fault scaling relationships and results of data analysis will constrain the magnitude of frictional heating and the coefficient of sliding friction. Rotational vitrinite reflectance measurements of dispersed particles within the gouge will be analyzed to determine mean reflectance (%Ro mean) and orientation of max/min reflectance. Mean reflectance will be used to infer maximum temperature. Orientation of %Ro will constrain stress anisotropy during maturation and differentiate diagenetic and seismogenic maturation. XRF and electron microscopy will be used to characterize the mineralogy of gouge and protoliths, and to compare structural fabrics to experimentally and naturally deformed gouge samples. Preliminary X-ray fluorescence (XRF) data show distinct chemical boundaries and evidence of neoformed minerals corresponding to optically observed structural domains within the shear zone.
AAPG Search and Discovery Article #90183©2013 AAPG Foundation 2013 Grants-in-Aid Projects