HR: 0800h
AN: S41B-1000    [Abstracts]
TI: Frictional behavior of natural clay-rich fault gouges from ODP Leg 190, Nankai Trough, offshore Japan
AU: McKiernan, A
EM: amckiern@geosc.psu.edu
AF: Pennsylvania State University, 522 Deike Building, University Park, PA 16802 United States
AU: Lockner, D
EM: dlockner@usgs.gov
AF: US Geological Survey, 345 Middlefield Road MS 977, Menlo Park, CA 94035 United States
AU: * Saffer, D M
EM: dsaffer@geosc.psu.edu
AF: Pennsylvania State University, 522 Deike Building, University Park, PA 16802 United States
AB: The decollement at subduction interfaces localizes within sediments carried by the subducting plate, and therefore understanding the frictional behavior of these sediments provides insight into the initiation of the decollement and the nature of slip along it. Variations in frictional behavior due to the conversion of smectite to illite may affect the strength of subduction interfaces and the updip limit of the seismogenic zone. Recent work has shown that illite exhibits velocity strengthening behavior, and this study expands on previous experimental conditions by testing saturated, intact, natural samples, which are sheared parallel to the incipient plate-boundary fault. Here we present results from a mineralogically-varied suite of intact samples from Ocean Drilling Program (ODP) Leg 190, site 1174, offshore Japan. These samples range in bulk illite abundance from 21-35 wt. % and in total clay content from 45-67 wt. %. Thus our experiments provide a natural test of mineralogically-controlled frictional variability, and act as a pilot study into the role of depositional fabrics in shearing. Samples were prepared as ``wafers,'' where ODP cores were cut parallel to the depositional surface to produce thin, intact, rectangular layers, which were then sheared between Berea Sandstone and Westerly Granite driving blocks. Prepared layers of intact gouge were saturated at a controlled fluid pressure of 1 MPa, and sheared in a triaxial apparatus at normal stresses of 20, 50, 120, and 150 MPa. Initial sample thickness varied from ~1-7 mm, with maximum shear strains of ~1.5-10. Corrections were made primarily for displacement-based area change (contact area is decreased by ~40% at a maximum displacement of 10 mm) and jacket strength (linear increase in measured shear stress with displacement). Preliminary results show 1) values of μ (~0.25-0.4) similar to that previously reported for clay mixtures; 2) velocity strengthening behavior (a-b > 0) in all cases, consistent with the stably-sliding behavior of similar clay mixtures noted by other researchers under a variety of conditions; 3) no clear, systematic variations in strength with either total clay fraction or illite content; and 4) no obvious effect of depositional fabric on frictional behavior.
DE: 3036 Ocean drilling
DE: 3060 Subduction zone processes (1031, 3613, 8170, 8413)
DE: 3613 Subduction zone processes (1031, 3060, 8170, 8413)
DE: 5199 General or miscellaneous
DE: 8034 Rheology and friction of fault zones (8163)
SC: Seismology [S]
MN: Fall Meeting 2005