High remanence intensity of breccia samples: Taiwan Chelungpu-fault Drilling Project (TCDP)
Abstract
A negative pulse-like geomagnetic fluctuation had been detected for more than a month prior to the Taiwan Chi-Chi earthquake (M7.6) in 1999 (Yen et al., 2004). Freund (2003) interpreted from his laboratory experiments that a nearly 106 ampere electric current along the fault zones caused the geomagnetic fluctuations. If such electric currents discharged, we can hypothesize that the fault zone rocks might have recorded the strong current-induced magnetic field. Here, we present paleomagnetic evidence for the hypothesis from thousand-meter deep drilled fault breccia samples of Taiwan Chelungpu-fault Drilling Project (TCDP). Three major fault zones at 1136m, 1194m and 1243m in depth showed anomalously strong natural remanent magnetizations (NRM) which were 10~100 times larger than those of host rocks, and the ratio (REM) of NRM over saturation isothermal remanences of breccia samples shows a one-order-of-magnitude higher value of 0.12 than that of normal volcanic lavas in thermal origin. To evaluate if these samples acquired the thermal remanent magnetization (TRM) accompanying the fault motions, we carried out simple heating experiments of host rock samples to acquire a TRM. In the result, the REM ratio of a sample heated to 600°C and cooled in 100µT (twice as strong as Earth’s field) is 0.03, suggesting that 0.12 cannot be explained by TRM in Earth’s field. Additionally, the bedding-corrected paleomagnetic directions for these samples showed a much shallower or deeper paleomagnetic inclination data, which are no correspondence to the geomagnetic inclination at Taiwan. These suggest the presence of strong ambient fields, if the magnetic carrier is magnetite. We are carrying out X-ray diffraction experiments (heavy liquid differentiation) to make solid identifications of magnetic minerals about the other samples now.
- Publication:
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AGU Fall Meeting Abstracts
- Pub Date:
- December 2009
- Bibcode:
- 2009AGUFMGP41B0822T
- Keywords:
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- 1540 GEOMAGNETISM AND PALEOMAGNETISM / Rock and mineral magnetism