11:00 AM - 11:15 AM
[SEM13-07] Preliminary paleodirection results during the high reversal rate period from lava sequences in eastern Iceland
The polarity of the Earth’s magnetic field has been frequently reversing between normal polarity and reversed polarity at various time intervals. The history of these reversals has been known back to 160 million years ago (e.g. Ogg, 2020), with the highest reversal frequency during the period from 12 to 13 million years ago. Paleomagnetic direction variations including this period have been reported from lava sequences in Iceland, for example, the Mjoifjördur area in eastern Iceland (Kristjansson et al., 1995), where lava with intermediate polarity is relatively common. In this study, total of 66 lava samples were systematically collected from two adjacent sections (lava sequences) of the EP and EO in this area to investigate paleomagnetic direction variations during this period.
Paleomagnetic direction was estimated based on stepwise AF demagnetization for each lava, with additional stepwise thermal demagnetization for some lavas. The results of the demagnetization data were analyzed using Zijderveld diagrams. Then the mean direction of each lava was calculated using Fisher statistics, and also the location of the virtual geomagnetic pole (VGP) was calculated. As a result, from bottom to top, we recognized a polarity change of R-N-R-N in the section EP and that of R-N-R in the section EO (though some missing outcrops in the section EO). The two sections can be correlated by the key bed, Gerpir Lava, and the overall polarity change is interpreted as R-N-R-N from bottom to top as the composite section EP+EO.
According to the geological map published by the Icelandic Institute of Natural History, the R polarity at the bottom of the composite section EP+EO corresponds to Chron C5AAr. Therefore, the polarity change in the composite section is interpreted to correspond to Chron C5AAr-C5Aan-C5Ar.3r-C5Ar.2n. Considering the number and thickness of the lava, it is likely to record paleomagnetic direction variations for the duration of about 500 kyr at 13 Ma. With the exception of approximately 100 kyr during the Chron C5Ar.3r, VGP latitudes tend to fluctuate around and within ±45°. This tendency might be an important feature of the earth's magnetic field during periods of high frequency of reversals.
Paleomagnetic direction was estimated based on stepwise AF demagnetization for each lava, with additional stepwise thermal demagnetization for some lavas. The results of the demagnetization data were analyzed using Zijderveld diagrams. Then the mean direction of each lava was calculated using Fisher statistics, and also the location of the virtual geomagnetic pole (VGP) was calculated. As a result, from bottom to top, we recognized a polarity change of R-N-R-N in the section EP and that of R-N-R in the section EO (though some missing outcrops in the section EO). The two sections can be correlated by the key bed, Gerpir Lava, and the overall polarity change is interpreted as R-N-R-N from bottom to top as the composite section EP+EO.
According to the geological map published by the Icelandic Institute of Natural History, the R polarity at the bottom of the composite section EP+EO corresponds to Chron C5AAr. Therefore, the polarity change in the composite section is interpreted to correspond to Chron C5AAr-C5Aan-C5Ar.3r-C5Ar.2n. Considering the number and thickness of the lava, it is likely to record paleomagnetic direction variations for the duration of about 500 kyr at 13 Ma. With the exception of approximately 100 kyr during the Chron C5Ar.3r, VGP latitudes tend to fluctuate around and within ±45°. This tendency might be an important feature of the earth's magnetic field during periods of high frequency of reversals.