HR: 14:40h
AN: T42D-05    [PDF]
TI: DEFORMATION OF SERPENTINITE AND ITS IMPLICATIONS FOR THE ASEISMICITY AND SEISMIC LOW VELOCITY ANOMALY IN SUBDUCTION ZONES
AU: * Jung, H
EM: hjung@citrus.ucr.edu
AF: Inst. Geophys. & Planet. Phys., University of California, Riverside, CA 92521 United States
AU: Green, H W
EM: hgreen@mail.ucr.edu
AF: Inst. Geophys. & Planet. Phys., University of California, Riverside, CA 92521 United States
AU: Green, H W
EM: hgreen@mail.ucr.edu
AF: Dept. of Earth Sciences, University of California, Riverside, CA 92521 United States
AB: Recent seismic data suggest that a low velocity layer up to $\sim$ 7 km thick exists at the top of the subducting slab in many subduction zones (Abers, 2000). Both P and S-wave velocities are consistently 5-7 % lower than the surroundings. There are also observations of a distinct low seismicity zone in the Japan Trench (Fujie et al., 2002) and even an aseismic zone in central Oregon (Brocher et al., 2003). We have investigated a possible cause of these observations by deforming a natural serpentinite at the pressures of 1.0 - 3.4 GPa and the temperatures of 923 - 1023 K. We conducted triaxial deformation experiments at a constant strain rate using a Griggs-type apparatus. Oxygen fugacity was buffered by Ni/NiO. We find that under conditions where serpentine is dehydrating during deformation, Mode-I cracks are commonly observed in serpentine as well as in relict olivine. A similar phenomenon (Mode-I cracks) may occur at the top of the subducting slab due to the deformation of a hydrated mantle wedge above a dehydrating subducting slab. We propose that such aligned Mode-I cracks, which fill with water as they open, may result in the low seismic velocity in subduction zones (i.e. Alaska, the Aleutians, Kuriles, Honshu, the Marianas, and Nicaragua). We also find that deformation of the serpentinite under a differential stress display faults and localized shear zones, delineated by ultra fine-grained solid reaction products, formed as byproducts of antigorite dehydration. We observe this phenomenon under all conditions tested. A distinct low seismicity zone in the Japan Trench region, aseismicity beneath central Oregon, and other aseismic zones in mantle wedges may be attributable to "superplastic" flow under low stresses within the ultra-fine-grained dehydration products in dehydration-induced fault zones. Abers, G. A. Earth Planet. Sci. Lett., 176, 323-330 (2000). Fujie, G. et al., Geophys. Res. Lett., 29, 7 (2002). Brocher, T. M. et al., Geology, 31, 267-270 (2003).
DE: 3924 High-pressure behavior
DE: 5104 Fracture and flow
DE: 7209 Earthquake dynamics and mechanics
DE: 8123 Dynamics, seismotectonics
SC: Tectonophysics [T]
MN: 2003 Fall Meeting