Abstract
We have performed an extended palaeomagnetic study of the Oslo Graben volcanics, compared to the study of half a century ago by van Everdingen, using modern techniques and a four times larger amount of sites, plus additional rock magnetic experiments. We conclude that the average direction (D = 204.0, I = −37.9, k = 46.9, α95 = 2.0) and associated palaeomagnetic pole (λ = 48.3, ϕ = 155.5, K = 52.2, A95 = 1.9) of the Krokskogen and Vestfold volcanics together are statistically identical to those of the earlier study. This gives confidence in the fact that older palaeomagnetic studies can be reliable and robust, even though methods have improved. Our larger number of samples, and better age constraints, enable us to separate the data into two major intervals: the younger, on average, Krokskogen area and the older Vestfold area. The results show firstly that palaeolatitudes are slightly higher than predicted by the latest apparent polar wander path (APWP) for Eurasia by Torsvik et al. These data support an early Permian Pangaea A configuration and do not necessitate a Pangaea B configuration. The larger data set also allows us to assess the distribution of the characteristic remanent magnetization directions of the Oslo Graben in terms of geomagnetic field behaviour, which were acquired during a long period of dominantly single polarity the Permo-Carboniferous Reversed Superchron (PCRS). The distributions show a significantly lower virtual geomagnetic pole (VGP) scatter at the observed (low) latitudes than expected from a compilation from lavas of the last 5 Myr. The data do however show excellent agreement with the scatter observed both during the Cretaceous Normal Superchron and the PCRS. A comparison of the directional distributions in terms of elongation is less discriminating, since the large errors in all cases allow a fit to the predicted elongation/inclination behaviour of the TK03.GAD model.
Highlights
The Earth’s magnetic field is produced by a geodynamo process in the molten outer core of our planet
palaeosecular variation (PSV) analyses are represented as virtual geomagnetic pole (VGP) dispersion (S) of our data and existing data from the Oslo Graben plotted versus their palaeolatitude, while we determine the elongation (E) of the directional distributions for comparison to the TK03.geocentric axial dipole (GAD) field model (Tauxe & Kodama 2009)
The characteristic remanent magnetizations (ChRM) of the lava specimens was determined by applying thermal (TH) demagnetization, conducted with steps of 100 to 300 ◦C or 400 ◦C; for higher temperatures, the intervals were reduced to 50 ◦C and down to 10 ◦C steps, to permit a more detailed study of the NRM demagnetization
Summary
The Earth’s magnetic field is produced by a geodynamo process in the molten outer core of our planet. Longer term variations are preserved in rocks, which can contain reliable records of the direction and intensity of the magnetic field at the time of their formation Palaeomagnetic measurements from these rocks allow us to study the variations of the field over thousands, millions, or even billions of years. Our significantly enlarged data set provides important information for analysing the behaviour of the geomagnetic field and for comparison to the PSV database for lavas of the last 5 Myr and Giant Gaussian Process (GGP; Constable & Parker 1988) model predictions using TK03.GAD (Tauxe & Kent 2004). PSV analyses are represented as VGP dispersion (S) of our data and existing data from the Oslo Graben plotted versus their palaeolatitude, while we determine the elongation (E) of the directional distributions for comparison to the TK03.GAD field model (Tauxe & Kodama 2009). It aims to update and extend older palaeomagnetic studies of the Oslo Graben (van Everdingen 1960; Storetvedt et al 1978)
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