HR: 1340h
AN: T23A-0552    [Abstracts]
TI: Occurrence of pseudotachylytes from cataclastic-plastic transition and near-surface crustal levels
AU: * Takagi, H
EM: hideo@waseda.jp
AF: Department of Earth Sciences Waseda University, 1-6-1 Nishiwaseda, Shinjuku, Tokyo, 169-8050 Japan
AB: This paper introduces natural examples of pseudotachylytes formed from different crustal levels from the cataclastic-plastic transition (10-15km) to near-surface (-1km). Pseudotachylytes from the cataclastic-plastic transition regime have been investigated from two shear zones in Japan: the Hatagawa Fault Zone in NE Japan and the Asuke Shear Zone in SW Japan. All studied pseudotachylytes have the following characteristics. 1. Each pseudotachylyte-bearing shear zone is relatively narrow, and a close occurrence of pseudotachylyte, cataclasite and mylonite (especially ultramylonite) is commonly observed. 2. Mylonitized pseudotachylytes or mylonitized cataclasites occur as well as fractured mylonites. 3. Polyphase generation of pseudotachylytes (pseudotachylyte vein transecting previous pseudotachylyte or co-occurrence of non-deformed and deformed pseudo-tachylyte) is observed. The Asuke Shear Zone is a sinistral-extensional shear zone dominantly composed of small-scale cataclasite zones often associated with pseudotachylyte and ultramylonite. One of the major factors that determines whether pseudotachylyte is associated or not is the attitude of fault surface: faults parallel to the P shear sense tend to be associated with pseudotachylyte when compared to faults parallel to the R1 shear sense. This implies that transpressional shear zones, where normal stress is relatively high, has greater frictional melting compared to transtensional shear zones. On the other hand, pseudotachylytes from Langtang, Nepal occur in a near-surface (about 700 m depth) regime and are considered to have been generated during large-scale landsliding. The evidence for landslides is not only topographic features but also kinematic indicators in foliated fault breccia overlying pseudotachylytes showing normal fault sense, and the pseudotachylyte itself contains many large bubbles and is fairly glassy. Even the normal stress must have been quite low there, but frictional melting during the landslide was possible due to the high velocity of sliding. Recent Fission-Track ages of zircon in pseudotachylyte from the Asuke Shear Zone (c. 53 Ma: Murakami et al., 2003) and from the Langtang (c. 50 ka: Takagi unpublished data) are successful examples of the dating of frictional melting.
DE: 8110 Continental tectonics--general (0905)
DE: 8159 Rheology--crust and lithosphere
SC: Tectonophysics [T]
MN: 2004 AGU Fall Meeting