HR: 1340h
AN: GP23A-0168    [Abstracts]
TI: Paleomagnetic and rock magnetic investigation of the high magnetic remanence in fault pseudotachylites
AU: * Geissman, J W
EM: jgeiss@unm.edu
AF: University of New Mexico, Department of Earth and Planetary Sciences, Albuquerque, NM 87131 United States
AU: Zechmeister, M
EM: zechmeim@siu.edu
AF: Southern Illinois University, Department of Geology, Carbondale, Ill 62901 United States
AU: Ferre, E C
EM: eferre@geo.siu.edu
AF: Southern Illinois University, Department of Geology, Carbondale, Ill 62901 United States
AU: MathanaSekaran, N
EM: navs_19@yahoo.com
AF: Southern Illinois University, Department of Geology, Carbondale, Ill 62901 United States
AB: Frictional heating during coseismic deformation may melt fault rocks and form pseudotachylite (PSDT) vein networks if slip is important. Limited previous work on PSDT suggests that their remanence properties are similar to those of lightning struck rocks, with anomalously high magnetization, implying that large electric pulses were involved in magnetization acquisition. We are testing the hypothesis that remanence anomalies in PSDT are typical and that coseismic electric currents are responsible for these anomalies. We have sampled young PSDT and immediately adjacent host rocks from three seismically active fault zones (Eastern Peninsular Ranges [SR, Santa Rosa area, 62-56 Ma tonalite host rock], California, Uchinoura shear zone [UC, 14 Ma granodiorite host rock], Kyushu, Japan, and Val Gilba [VG, 37 Ma 3 Gpa gneiss host rock], Dora Maira, Western Alps). All materials collected are oriented; specimen preparation involves the making slabs, cut perpendicular to PSDT vein networks, which are then cut into oriented cubes (about 1 cc). Notably, magnetic properties of PSDT differ considerably from host rock. For SR samples, typical NRM intensities for PSDT range from 2.0 to 7.0 A/m, with the NRM of single component character (median destructive fields are typically about 40 mT and 80 percent of laboratory unblocking temperature spectra between 500 and 580C). NRM intensities of host rock typically decrease away from PSDT and range from 2.0 to 0.01 A/m, host rock adjacent to veins yields magnetizations similar to those in the veins. The characteristics of UC samples are considerably different from those of the SR locality; PSDT has NRM intensities of about 0.08 A/m with host rock of similar NRM intensities. Directions of magnetizations isolated in PSDT and adjacent host rock are similar. Modified Lowrie-Fuller tests suggest that PSDT at both of these localities contains abundant fine, single-domain magnetite particles. VG PSDT have relatively low NRM intensities (2.0 to 7.0 mA/m) and a more complicated demagnetization response. Host rock gneiss is comparable in intensity, regardless of distance from PSDT veins and has no coherent magnetization, even adjacent to veins. A tentative explanation of the VG results involves PSDT formation at considerably deeper levels (about 10km) than those from the other localities. Ultimately, our results will be compared with those from artificially generated PSDT to obtain a better understanding of coseismic electric phenomena.
DE: 1519 Magnetic mineralogy and petrology
DE: 1525 Paleomagnetism applied to tectonics (regional, global)
DE: 1527 Paleomagnetism applied to geologic processes
DE: 1540 Rock and mineral magnetism
SC: Geomagnetism and Paleomagnetism [GP]
MN: 2004 AGU Fall Meeting