ANOMALOUSLY SHALLOW PALEOMAGNETIC INCLINATIONS AND THE QUESTION OF THE AGE OF THE CANARIAN ARCHIPELAGO

Extensive geological, geophysical and geochronological data available from the Canary Islands establish conclusively that formation of the o ldest subaerial volcanic structure of the islands began during the Mio cene. A mid-Cretaceous age for these volcanic formations has been post ulated in previous works on the basis of palaeomagnetic determinations . The results obtained in the present palaeomagnetic study of Lanzarot e include analysis of the record of variations of magnetic inclination in boreholes that penetrate the whole of the oldest volcanic series. They show that the excessive age previously assigned to these formatio ns is due to the utilization of volcanic units with abnormally low (<1 5 degrees) magnetic inclinations (LGIs). In Lanzarote, lavas exhibitin g LGIs appear interbedded in a volcanic series that, overall, shows a typical Miocene inclination (similar to 45 degrees). The units stratig raphically beneath and above the LGI horizons give, in fact, direction s consistent with the Middle-Upper Miocene field direction (D = 359 de grees, I = 45 degrees, with k = 29, a(95) = 6.7 degrees and a palaeopo le of 87 degrees N, 178 degrees E), thereby confirming the Miocene age of the oldest subaerial volcanics of Lanzarote. Short excursions of t he geomagnetic field seem likely to be the explanation for these LGIs, since other factors such as tectonic tilting, post-eruptive modificat ion of the primary remanence or errors in sample orientation can be di sregarded. The detection in the Canary Archipelago of volcanic units w ith abnormally low magnetic inclinations seems to be related to the re latively continuous record of the geomagnetic field in rapidly growing volcanic edifices, as seems to be indicated by the presence of a few (usually one or two) short polarity events in volcanic suites of sever al hundred metres thickness.