HR: 10:20h
AN: T41E-01 INVITED     [PDF]
TI: Friction Mechanics at the Updip Limit of Seismogenic Faulting Along Subduction Megathrusts.
AU: * Marone, C
EM: cjm38@psu.edu
AF: Dept. of Geosciences, Penn. State Univ. 536 Deike Bld., University Park, PA 16803 United States
AU: Saffer, D M
EM: dsaffer@uwyo.edu
AF: Dept. of Geology and Geophysics, Univ. of Wyoming, Laramie, WY 82071 United States
AB: The mechanical behavior of plate boundary fault zones can be divided into three main zones: a deep aseismic zone, the seismogenic zone, and an updip aseismic zone. Identifying and understanding the stability transitions from seismic to aseismic faulting are key goals in understanding subduction zone megathrusts. We focus on the mechanics and frictional properties of the upper stability transition from stable to unstable faulting. Two hypotheses for the updip limit of subduction seismicity have been proposed. The clay mineral hypothesis posits that a thermally- driven transition from dominantly smectite to dominantly illite clay produces a transition from aseismic to seismic behavior. The consolidation/lithification hypothesis posits that the stability transition is the result of a change from distributed granular shear, in which aseismic behavior is related to grain crushing, consolidation, and strain- rate dependent dilatancy, to localized shear within highly consolidated material, for which unstable friction behavior results from properties of adhesive contact junctions. We summarize laboratory friction data and constitutive laws in the context of requirements for unstable faulting. We report on laboratory experiments designed to investigate the frictional behavior of smectite-illite clays and clay-quartz mixtures, with emphasis on processes that control frictional stability. Double-direct shear friction experiments were carried out on powders (2-500 5 $\mu$m) at normal stresses from 5-150 MPa, sliding velocities from 0.1-200 $\mu$m/s, and shear strains up to 20 at room temperature. We find that the coefficient of friction ($\mu$) is 0.42-0.68 for illite shale, consistent with previous work. Over the full range of conditions studied, illite shale exhibits only velocity-strengthening behavior, opposite to the widely expected, potentially unstable velocity- weakening behavior assumed in the clay mineral hypothesis. Smectite sheared under identical conditions exhibits low friction ($\mu$ = 0.15-0.32) and a transition from velocity weakening at low normal stress to velocity strengthening at higher normal stress ($>$35 MPa). Our data suggest that the transformation of smectite to illite results in an increase in friction, but do not support the hypothesis that the smectite-illite transition is responsible for the seismic-aseismic transition in subduction zones. We show that mixtures of smectite and quartz undergo a transition from adhesive frictional behavior, in which contact junctions exhibit time-dependent behavior and friction exhibits rate and state properties, to viscous behavior in which shear strength is purely rate dependent. We suggest that processes, such as cementation, consolidation, and slip localization, play an important role in determining the updip limit of the seismogenic zone in subduction zones, and that these processes, in addition to clay mineralogy, should be the focus of future investigations.
DE: 3902 Creep and deformation
DE: 5104 Fracture and flow
DE: 7209 Earthquake dynamics and mechanics
DE: 8123 Dynamics, seismotectonics
SC: Tectonophysics [T]
MN: 2003 Fall Meeting