14:00 〜 14:15
[SEM13-02] アイスランドのボルガルフィヨルズル地域に分布する約300-400万年前の溶岩群の古地磁気 － 古地磁気層序と古地磁気方位永年変化
Iceland is a unique location for palaeomagnetism. Glacially and fluvially eroded valleys expose sequences of hundreds of easily accessible lavas that dip gently towards rift zones: ideal candidates for reconstructing past variations in Earth’s magnetic field. In this study we present new palaeomagnetic directions from lavas spanning approximately 3.1-4.3 Ma from the Lundarreykjardalur valley in the Borgarfjörður region of western Iceland. In total ~1600 specimens from ~250 lavas spanning 14 sections along an 18 km transect of the Lundarháls ridge have been measured for palaeodirectional analysis. This provides one of the most detailed records of field changes recorded by lavas for this time.
This study is the culmination of two phases of field work and laboratory measurements. The first phase began in 1994 with the sampling and analysis of nine sections comprising the older half of the magnetostratigraphy. The second phase began in 2016, with an additional five sections expanding the magnetostratigraphy to younger ages. Through a combination of pilot K-Ar ages carried out in the 1990s and new 40Ar/39Ar ages we assigned a series of reversed and normal polarity lavas to chrons within the geomagnetic polarity time scale (GPTS) spanning approximately 4.3 to 3.1 Ma. 40Ar/39Ar ages confirm that the composite lava section records the reversal boundaries of C3n.1r/Cochiti (GPTS age, 4.300 Ma), Cochiti/C2Ar (4.187 Ma), C2Ar/C2An.3n (Gilbert/Gauss, 3.596 Ma) and C2An.2n/Kaena (3.116 Ma), but misses the Mammoth subchron.
Detailed variations in Earth's magnetic field can relate to changes in the geodynamo. We look at the distribution of virtual geomagnetic poles (VGPs), as well as their dispersion. Our analysis suggests the field recorded in the Lundarháls lavas is consistent with the geocentric axial dipole assumption, but that VGP dispersion may differ between chrons. This has implications for our understanding of the long-term variability of the field.
This study is the culmination of two phases of field work and laboratory measurements. The first phase began in 1994 with the sampling and analysis of nine sections comprising the older half of the magnetostratigraphy. The second phase began in 2016, with an additional five sections expanding the magnetostratigraphy to younger ages. Through a combination of pilot K-Ar ages carried out in the 1990s and new 40Ar/39Ar ages we assigned a series of reversed and normal polarity lavas to chrons within the geomagnetic polarity time scale (GPTS) spanning approximately 4.3 to 3.1 Ma. 40Ar/39Ar ages confirm that the composite lava section records the reversal boundaries of C3n.1r/Cochiti (GPTS age, 4.300 Ma), Cochiti/C2Ar (4.187 Ma), C2Ar/C2An.3n (Gilbert/Gauss, 3.596 Ma) and C2An.2n/Kaena (3.116 Ma), but misses the Mammoth subchron.
Detailed variations in Earth's magnetic field can relate to changes in the geodynamo. We look at the distribution of virtual geomagnetic poles (VGPs), as well as their dispersion. Our analysis suggests the field recorded in the Lundarháls lavas is consistent with the geocentric axial dipole assumption, but that VGP dispersion may differ between chrons. This has implications for our understanding of the long-term variability of the field.