We present new 40Ar/39Ar ages and palaeomagnetic directions from two re-examined sequences of basaltic lava flows on the island of Santo Antão, Cape Verde: Tarrafal and Agua Nova. We find no evidence for the fully-reversed, Brunhes age excursions reported in Knudsen et al., Earth Planet. Sci. Lett., 206, 199-214, 2003. At Tarrafal, complex geology, including repeated gravitational collapses and/or considerable erosion followed by in-filling of subsequent valleys by younger lavas, coupled with an insufficient number of dated reversely and normally magnetised flows, led earlier researchers to suggest a series of Brunhes age excursions. At Agua Nova, the use of single samples to determine palaeodirections resulted in sampling of anomalous reversed directions, creating apparent excursions. We report new palaeomagnetic directions from Agua Nova, based on multiple measurements per flow determined by both alternating field and thermal demagnetisation. All flows of Brunhes age have virtual geomagnetic pole latitudes > 60°. Our findings highlight the need for precise age constraints and a high sampling density of dated flows, with multiple directional results per flow, when using long lava sequences to reconstruct palaeofield behaviour.
- Ar/Ar dating
- Cape Verde