A PALEOMAGNETIC AND PALEOINTENSITY STUDY ON A RAPIDLY EMITTED UPPER MIOCENE LAVA FLOW SEQUENCE FROM CAPE VERDE

We report results obtained in a paleomagnetic and paleointensity study carried out on an upper Miocene lava flow sequence in São Vicente island (Cape Verde) consisting of eleven nephelinitic flows. We have also performed a K-Ar age determination on the lowermost flow of the sequence, which yielded an age of 6.07 ± 0.16 (2σ) Ma. Paleomagnetic analysis showed that all flows record a normal polarity magnetisation, a result which allowed to narrow the age of the sequence to an interval between 6.03 and 6.23 Ma.

Reliable paleomagnetic results could be obtained in all eleven flows. If the mean paleomagnetic direction of the sequence is compared with the expected one, which was obtained from the African Apparent Polar Wander Path, it can be observed that the paleodeclination of the sequence is westwardly deviated by 26.5° ± 2.9°, but inclinations approximately agree. A possible explanation of these observations could be a vertical-axis counterclockwise rotation, although the effect of non-averaged secular variation cannot be discarded. Analysis of paleosecular variation shows virtually no scatter of virtual geomagnetic poles around their mean value. Comparison of the scatter of paleomagnetic directions in the present study with the scatter observed in directions obtained from geomagnetic field model SHA.DIF.14k suggests that the sequence was emplaced in only a few centuries.

Paleointensity determinations were carried out with two different methods: Thellier-Coe paleointensity experiments on 66 specimens yielded 22 successful determinations and multispecimen paleointensity experiments on 11 samples yielded 8 successful determinations. A mean paleointensity F = (18.5 ± 1.9) µT was obtained from the joint analysis of the results from both methods. This result lies clearly below the present-day value F_P = 33.1 mT.