Paleointensity of the Earth's magnetic field in the Jurassic: New results from a Thellier study of the Lesotho Basalt, southern Africa

Abstract

Thellier paleointensity experiments carried out on the Jurassic (180 m.y. old) Karoo basalt, Lesotho, southern Africa, provided paleointensity estimates for 40 samples from 10 distinct lava flows with reversed magnetization. Mean paleointensity was found to be equal to 24.2 ± 11.3 μT (standard deviation), with a minimum of 12 μT and a maximum of 41.3 μT. This corresponds to a mean virtual dipole moment (VDM) of 4.6±2.0×1022 Am2. The between‐flow dispersion is ascribed to paleosecular variation, which thus would approach up to 40% of the mean paleointensity, as is observed from the published paleointensity data set covering last 20 m.y. This result yields further evidence for the period of low geomagnetic field in Jurassic to Early Cretaceous time (Mesozoic Dipole Low). A symmetrical intensity versus time pattern is suggested for this time interval, with a minimum VDM of 3×1022 Am2 in Late Jurassic and substantially higher (∼ 4.5×1022 Am2) VDMs in Early to Middle Jurassic and Early Cretaceous, respectively.