" Palaeomagnetism of ca. 0-5 Ma lavas from Western Iceland: New results from Borgarfjörður" by Maxwell Brown, Institue for Rock Magnetism, University of Minnesota.

Talk presented at the virtual 2021 MagIC Workshop (https://earthref.org/events/MAGIC/2021/) - Rock and Paleomagnetism through Time and Space. Jan 19th-21st, 2021.

Abstract:
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 2.5-4.5 Ma from the Lundarreykjardalur valley in the Borgarfjörður region of western Iceland. In total over 2000 specimens from over 300 lavas spanning 16 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 seven 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 GPTS spanning approximately 4.3 to 2.5 Ma. Ar/Ar ages confirm that the composite lava section records the Gauss-Matuyama and Gilbert-Gauss reversal boundaries, as well as field behaviour during the Cochiti (GPTS age: 4.187-4.300 Ma) and Kaena subchrons (GPTS age: 3.023-3.116 Ma).

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. We compare our results from other studies in Iceland obtained from the recently compiled ICEPMAG database. Our initial analysis suggests the field recorded in Iceland over the past 5 Ma 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.