HR: 08:15h
AN: S31B-02    [Abstracts]
TI: Effects of Frictional Melting on Seismic Slip in a Subduction Zone: Inference from Pseudotachylytes in the Shimanto Accretionary Complex, Southwest Japan
AU: * Ujiie, K
EM: ujiiek@jamstec.go.jp
AF: Institute for Research on Earth Evolution, JAMSTEC, 3173-25 Showa-machi, Kanazawa-ku, Yokohama, 236-0001 Japan
AU: Yamaguchi, H
EM: harukay@jamstec.go.jp
AF: Institute for Research on Earth Evolution, JAMSTEC, 2-15 Natsushima-cho, Yokosuka, 237-0061 Japan
AU: Sakaguchi, A
EM: arito@jamstec.go.jp
AF: Institute for Research on Earth Evolution, JAMSTEC, 3173-25 Showa-machi, Kanazawa-ku, Yokohama, 236-0001 Japan
AB: Recently, pseudotachylytes (i.e. fault-generated friction melt) have been discovered in exposed ancient accretionary complexes. They are key for understanding the dynamics of earthquake faulting in subduction zones. However, the descriptions of pseudotachylytes in accretionary complexes are severely limited, and it is uncertain how the frictional melting of subducted material affects earthquake faulting. In this study, we analyzed the pseudotachylytes in the Shimanto accretionary complex in southwest Japan. The pseudotachylyte-bearing fault zones represent the ~1-5m thick cataclastic thrust zones originating from melanges, which developed at the top of the melanges during subduction or underplating in seismogenic depths (3.2-6 km). Pseudotachylytes occur in narrow dark veins less than a few millimeters thick that are sharply bounded by foliated cataclasites, demonstrating that the coseismic slip in the subduction zone is concentrated into a narrow zone. The pseudotachylyte matrix has the optical character of glass, and the embayed grains, vesicles, and cracks surrounding the grains are visible in a homogeneous, glassy matrix under a backscattered electron image. The pseudotachylyte displays a fragment-laden, glass-supported texture resulting from rapid cooling of the frictional melt, which is consistent with the very short cooling time (less than 10 seconds) of the pseudotachylyte layer calculated using thermal modeling. The rapid cooling of the pseudotachylyte layer is due to its narrow thickness, resulting in the fast healing of the coseismic slip zone by the solidified melt layer. The compositions of the pseudotachylytes and characteristics of the unmelted grains suggest that frictional melting occurred in an illite-rich slip zone with a minimum melting temperature of 1100 degrees C, which is ~850-920 degrees C greater than the maximum temperatures recorded in the host rocks. The melting of the illite-rich slip zone is likely to form a hydrous melt layer, possibly leading to a higher H2O content in these pseudotachylytes as compared with the pseudotachylytes of continental plutonic or metamorphic rocks in other geological settings. The viscosities of the pseudotachylytes were calculated from the pseudotachylyte matrix composition as well as the volume fraction and aspect ratio of the unmelted grains. The viscosities at 1100 degrees C were of the order of 10 to 10² Pa s, and the corresponding shear resistance along a 1-mm-thick pseudotachylyte layer at a slip rate of 1 m/s was 0.1-0.3 MPa. The formation of a pseudotachylyte layer in an illite-rich slip zone is suggested to induce the dynamic weakness of the fault, acceleration of the seismic slip, and propagation of the instability, which together could affect the earthquake magnitude in a subduction zone. The frictional melting of the illite-rich slip zone may be applicable to subduction thrusts and faults in other accretionary complexes where illite is the dominant material.
DE: 7209 Earthquake dynamics (1242)
DE: 7240 Subduction zones (1207, 1219, 1240)
DE: 8004 Dynamics and mechanics of faulting (8118)
DE: 8030 Microstructures
DE: 8034 Rheology and friction of fault zones (8163)
SC: Seismology [S]
MN: Fall Meeting 2005