HR: 1340h
AN: GP13A-0034    [Abstracts]
TI: Paleomagnetic constraints of seismic thermal signatures and coseismic geo-current in the Nojima fault gouge, Japan
AU: * Kitagawa, T
EM: pantani_55@hotmail.com
AF: Department of Geo-Environmental Sciences, Tohoku University, 6-3, Aoba, Aramaki-aza, Aoba-ku, Sendai, 980-8578 Japan
AU: Nakamura, N
EM: n-naka@mail.tains.tohoku.ac.jp
AF: Department of Geo-Environmental Sciences, Tohoku University, 6-3, Aoba, Aramaki-aza, Aoba-ku, Sendai, 980-8578 Japan
AB: The Nojima fault gouge is an alternating layer of fine-grained gouge (gray, cohesive) and pseudotachylyte (black, brittle), derived from the adjacent low-magnetic granite. Each layer is thinner than a few millimeters, and corresponds to one seismic slip event. Previous X-ray diffraction study showed that the gray-color gouge includes siderite and lepidocrocite, while the black-color pseudotachylyte includes magnetite. The bulk sample showed a strong intensity of natural remanent magnetization (NRM), being 400 times more intense than the granite. Two hypotheses to describe this higher remanence have been presented as 1) the thermal decomposition of mafic minerals to magnetite by friction melting of fine-grained gouge, and 2) the thermal dehydration of lepidocrocite to ferromagnetic maghemite through flash heating of the gouge. Both hypotheses indirectly estimated the temperature rise of up to at least 700°C, resulting that the pseudotachylyte should acquire a thermal remanence during seismic slip. Although thermal remanence records a thermal history at the time of flash heating during seismic slip, the bulk sample analysis averages the thermal history information of each seismic slip event. Nevertheless, there has been no paleomagnetic study of the alternating gouge by 'layer-by-layer'. Here, we report a direct estimation for the temperature rise of pseudotachylyte by the layer-by-layer analysis through step-wise thermal demagnetization by SQUID magnetometer and scanning electron microscopy (SEM). The thermal demagnetization was performed for small chips of pseudotachylyte in total 64 measurements, and 42 subsamples show relatively strong intensity and stable demagnetization patterns. Moreover, the orientations of high coercivity component are randomly oriented even in a centimeter apart along the same layer, being no hemispherical bias. SEM observation reveals that a remanence carrier is submicron-sized inclusions of iron, which is identified as magnetite through powder X-ray diffraction analysis. Interestingly, 16 subsamples (38%) in the stable samples unblocked the NRM in 450-540°C, lower than the Curie temperature of magnetite (580°C). This result shows direct evidence that the interior of the pseudotachylyte has not been heated to more than these temperatures. Moreover, our flash heating test of fine-grained gouge confirms that magnetite was newly crystallized by subsequent decomposition of previously precipitated siderite by heating up to 400-500°C. Possible heat source for the temperature rise is not only frictional heat due to the fault slip, but also Joule heating due to coseismic electric current associated with the earthquake lightning (EQL). Our paleomagnetic study therefore reveals that some of the pseudotachylyte in the Nojima fault gouge has not been heated up to more than 540°C and has been magnetized in a randomly directed external field produced by an inhomogeneous coseismic electric current during seismic slip.
UR: http://www.dges.tohoku.ac.jp/igps/nakamura_e.html
DE: 1500 GEOMAGNETISM AND PALEOMAGNETISM
DE: 1527 Paleomagnetism applied to geologic processes
DE: 3324 Lightning
DE: 8163 Rheology and friction of fault zones (8034)
SC: Geomagnetism and Paleomagnetism [GP]
MN: Fall Meeting 2005