Multimethod palaeointensity results from a rapidly emitted upper Miocene lava flow sequence in São Vicente (Cape Verde): new data for the African record

Abstract

SUMMARY We present new radiometric ages and full-vector palaeomagnetic data from São Vicente Island, Cape Verde. Samples were obtained from a sequence of 11 flows. A K-Ar age determination was performed on the lowermost flow of the sequence, yielding an upper Miocene age of 6.07 ± 0.16 (2σ) Ma. Considering the normal polarity observed in all flows, the age of the sequence could be narrowed down to an interval between 6.03 and 6.23 Ma. The mean palaeomagnetic direction of the studied sequence was compared with the expected direction, calculated from the African Apparent Polar Wander Path (5 Ma). While inclinations approximately agree, the palaeodeclination appears westwardly deviated by 26.5° ± 2.9°. Moreover, nearly no scatter of virtual geomagnetic poles around their mean value is observed and its comparison with the scatter observed in palaeomagnetic directions obtained from geomagnetic field model SHA.DIF.14k suggests that the sequence was very rapidly emplaced in only a few centuries. Thus, the obtained anomalous palaeomagnetic direction seems to reflect the effect of non-averaged secular variation, although a local vertical-axis counter-clockwise rotation of the sequence, cannot be entirely ruled out. Thellier–Coe palaeointensity experiments on 66 specimens yielded 22 successful determinations and multispecimen (MS) palaeointensity experiments on 11 samples yielded 8 successful determinations. In the upper part of the sequence, Thellier–Coe determinations fail due to the presence of multidomain (MD) grains, but the MS method yields successful results in several of these flows, demonstrating that this method can provide reliable palaeointensity determinations on samples with a higher MD grain fraction. A mean palaeointensity BSV = (18.5 ± 1.9) µT is obtained from the joint analysis of the results from both methods. This value lies clearly below the present-day value BP = 33.1 μT. The virtual axial dipole moment (VADM = 4.3 × 1022 Am2), however, cannot be considered anomalous if compared to available upper Miocene–Pliocene data and represents a new high-reliability contribution to the very limited African palaeointensity database.