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内容記述 |
Magnetic anomaly data provide valuable information on size and morphology of buried impact craters. An understanding of how stress waves change mag-netic properties of rock is critical for correct interpretation of magnetic data. Although previous studies demonstrated effects of strong shock on magnetic properties of rocks beneath impact craters, effects of relatively weak shock for those of rocks in crater wall have been poorly studied. In this context, we estigated shock effects on magnetic properties through studies of experimentally impacted basaltic andesite, and basalt from natural impact crater (Lonar crater). <br /> An initial peak pressure of 5 GPa was generated in a block of basaltic andesite containing Ti-rich titanomagnetite with the impact of a cylindrical projectile. Effects of decaying stress waves on magnetic properties were subsequently quantified. Natural remanent magnetization (NRM) was partially but significantly demagnetized at peak pressures higher than 1 GPa. Highcoercivity part of NRM, even higher than 80 mT, was partially demagnetized. At higher pressure (3-5 GPa), low-field magnetic susceptibility was significantly reduced and coercivity was increased, probably due to increased internal stress. Different patterns of change in AMS were observed at different distance from the impacted surface. In high-pressure range (＞3 GPa), the anisotropy degree was increased, the minimum susceptibility was oriented toward the shock direction, and the average susceptibility was decreased. This feature is consistent with the result of a previous shock experiment. The initial orientations of AMS were however significantly changed at around 0.4-3 GPa; The maximum susceptibility was induced parallel to the shock direction, and superposed on the initial AMS. This kind of changes in the AMS parameters has been never reported. <br /> Basalt samples were collected from flows in the crater wall, ejecta clasts, and flow outside the rim of Lonar crater. Irreversible thermomagnetic curves and the maximum Curie temperature of 500-560±C indicated presence of Ti-poor titanomagnetite and its oxidized phase as the main magnetic minerals in Lonar basalts. The result of AMS measurement of both inside and outside samples showed relatively weak anisotropy degree (P＜1.03), which is similar to that of basalt from outside the crater rim. The samples from the crater wall showed predominantly oblate shape of AMS ellipsoid, with tightly clustered vertical distribution of the minimum principal axes. Substantial, but not strict, parallelism between the maximum principal axes and the radial direction from the crater center was observed only for the samples from the lower part of the crater wall. This fact and the result of the shock experiment indicate that radially expanding stress waves reoriented the initial AMS. Stepwise thermal demagnetization of NRM and anhysteretic remanent magnetization (ARM) demonstrated that the main NRM carriers of Lonar basalts are titanomagnetite and titanomaghemite. The primary NRM component was demagnetized above 200±C, indicating that the post-impact temperature did not exceed the value. The site-mean and overall-mean directions were deter-mined for both the high and low coercivity components isolated by stepwise alternating field demagnetization. The intensities of the primary NRM component were generally decteased toward the lower altitude, and positively correlated with the intensity of ARM after AFD with a peak field of 15 mT. This fact indicates that the NRM intensity is not simply a function of distance from the crater center as previously argued. Meanwhile, the site-mean directions of the low-coercivity NRM component were more tightly clustered before tilt correction, and the in-situ overall mean direction was not distinguishable with the present field direction. The ejecta samples also showed two NRM components, whose remagnetization circles intersected at an orientation close to the present earth field. These results suggest that the dominant fraction of the low-coercivity NRM component for the basalt flows in the crater wall and ejecta clasts were not shock remanent magnetization as previously suggested, but were viscous remanent magnetization acquired since the formation of the crater. |
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