Palaeomagnetism of flood basalts in the Pilbara Craton, Western Australia: Late Archaean continental drift and the oldest known reversal of the geomagnetic field

Abstract

[1] A late Archaean (circa 2775-2715 Ma) succession of terrestrial continental flood basalts, mafic tuffs, felsic volcanic rocks, and clastic sedimentary rocks in the Nullagine Synclinorium (and Meentheena Centrocline) of the East Pilbara Basin, Western Australia, has been sampled for a palaeomagnetic study. Over 500 oriented, mostly basalt, drill cores were collected from the supracrustal succession and associated dykes. Thermal and alternating field demagnetization revealed two distinct components. Positive fold, conglomerate, and reversal tests confirm that the primary natural remanent magnetization (NRM) is still preserved. The secondary component is interpreted as the record of remagnetization during a major thermal event, possibly in the Early Proterozoic. Analysis of the primary NRM directions results in a magnetostratigraphy and an apparent polar wander path (APWP) for the 60 Myr interval covered by the sampled succession. Assuming a geocentric axial dipole during this time interval, the APWP shows that the Pilbara Craton was drifting during the late Archaean and that drift rates probably varied significantly. In particular, a mean 27.2°shift in palaeolatitude is recorded across an unconformity that represents a relatively short time period and that marks a significant change in basalt geochemistry. This study suggests that continents moved horizontally during the late Archaean and that the rates of movement were significantly faster than in the Phanerozoic. In addition, a reversed polarity interval, with a positive reversal test, is recorded. We argue that it documents the oldest known geomagnetic reversals of the geomagnetic field.

The intensity of the geomagnetic field in the late-Archaean: new measurements and an analysis of the updated IAGA palaeointensity database

“…A regional correlation is currently in progress (Blake, in prep. ) and recent palaeomagnetic work (Strik et al, 2003;Strik, 2004) seems to confirm the stratigraphic framework. The Nullagine and Mount Jope Supersequences have recently been dated with high precision (Fig.…”

Secular variation of the Late Archean–Early Proterozoic geodynamo

“…Squares (M) show estimates derived from the Matachewan dikes (this work). Circles show estimates based on paleomagnetic data from the Karelia (K) [Mertanen et al, 1999] and Pilbara (P) [Strik et al, 2003] cratons. Confidence intervals shown for M, K and P are 1s and were calculated using a N-1 jackknife [Efron, 1982].…”

“…[17] To exclude a component of dispersion potentially associated with motion of the Pilbara craton, we use only the data of Strik et al [2003] corresponding to a relative pole standstill (representing the Nullagine and lower Mount Jope supersequences, Packages 1 to 7). These data yield s = (8.9 ± 1.3)°for 23 paleomagnetic directions.…”

The Gediz River fluvial archive: A benchmark for Quaternary research in Western Anatolia

“…Right column shows decay curves, which for site 9 (Figure d) shows typical fast decay in the early demagnetization steps, in line with lighting induced remanence (Strik et al, 2003). D=declination, I=inclination, ΔD=95% confidence in…”

“…Only in the site KU 9 it was impossible to resolve the original direction: extremely high initial intensities of 4 A/m, together with rapid decrease of the magnetic intensity in the early demagnetization steps (compare Figure A2d with h; see also Strik et al (2003)) are indicative of a strong lightning-induced Page 58 of 61 overprint. The steep inclination and strong deviation of the declination therefore cannot be interpreted in terms of a conclusive original polarity.…”