HR: 0830h
AN: T11D-0428    [PDF]
TI: Investigation of Microscale Fault Textures Associated with Aseismic Creep and Coseismic Rupture in Active Fault Zones
AU: * Cashman, S M
EM: smc1@humboldt.edu
AF: Humboldt State University, Department of Geology, 1 Harpst St., Arcata, CA 95521 United States
AU: Baldwin, J N
EM: baldwin@lettis.com
AF: William Lettis and Associates, 1777 Botelho Dr., Suite 262, Walnut Creek, CA 94596 United States
AU: Cashman, K V
EM: cashman@oregon.uoregon.edu
AF: University of Oregon, Department of Geological Sciences, 1272 University of Oregon, Eugene, OR 97403-1272 United States
AB: Micro-scale textures in poorly consolidated sand collected from active fault zones show differences between deformation fabrics produced by aseismic creep and those produced by coseismic rupture. Oriented samples of late Pleistocene to Holocene sand were collected from creeping sections of the Green Valley, Southern Calaveras, and San Andreas Faults, all parts of the San Andreas Fault system in north-central CA. Samples were also collected from two structures thought to record coseismic rupture: the McKinleyville Fault, CA, an active thrust fault in the forearc of the Cascadia Subduction Zone, and the hypothesized New Madrid North fault, MO, a possible active strike-slip fault in the New Madrid Seismic Zone. Samples were cemented with low-viscosity epoxy and examined using image analysis of photomicrographs and SEM images. Fault zone sediment samples from the active faults studied share several textural characteristics at the microscopic scale. Fault zone samples from both coseismic slip and creeping faults have finer grain size and less pore space than sediments outside of ($>$10 cm from) the fault zone, as well as some degree of preferred grain orientation. These textures record a combination of fault zone kinematics (translation, rotation, strain history of sediments) and authigenic processes. Of particular interest to paleoseismologists are textural characteristics that differ between fault zones with histories of coseismic slip and those experiencing fault creep. One such texture in the coseismic slip examples studied is a pronounced anastomosing shear zone structure consisting of relatively undeformed sand lenses surrounded by fine-grained shear zones. Grains within lenses in the McKinleyville fault deformation band appear undamaged, while those in the New Madrid North fault show some grain size reduction relative to sand several meters from the fault zone. Abrupt decrease in grain size at the boundaries of these lenses suggests that grain size reduction accompanied concentration of shear in localized shear zones. This texture may be an indicator of velocity weakening, potentially unstable stick-slip behavior. Another textural characteristic that differs between the samples studied is the orientation of elongate grains relative to the fault. In the McKinleyville and New Madrid North fault samples, preferred grain orientation is oblique to the fault. In contrast, in samples from creeping segments of the Southern Calaveras Fault at Costa Ranch, and San Andreas Fault at Flook Ranch, the highest concentration of elongate grains is oriented parallel to the fault. Factors other than the creep vs. stick-slip history of these faults may influence preferred grain orientation in fault zone sand. Possible factors include differences in sense of motion, mechanics of fracture initiation, interplay between fault strands, etc. We plan to extend our studies to additional sites in order to investigate further the relationships between fault texture and slip history.
DE: 7221 Paleoseismology
DE: 8010 Fractures and faults
DE: 8030 Microstructures
DE: 8107 Continental neotectonics
SC: Tectonophysics [T]
MN: 2003 Fall Meeting