EVOLUTION OF THE EASTERN VOLCANIC RIDGE OF THE CANARY-ISLANDS BASED ON NEW K-AR DATA

The results of 64 new K-Ar age determinations, together with 32 previously published ages, show that after a period of erosion of the basal complex, Miocene volcanic activity started around 20 Ma in Fuerteventura and 15 Ma in Lanzarote, forming a tabular succession of basaltic lavas and pyroclastics with a few salic dykes and plugs. This series includes five separate volcanic edifices, each one with its own eruptive history. In Fuerteventura, several Miocene eruptive cycles have been identified: in the central edifice one around 20-17 Ma, followed by two others centred around 15 and 13 Ma; in the southern edifice the maximum of activity took place around 16-14 Ma, whereas in the northern one the main activity occurred between 14 and 12 Ma. In Lanzarote a first cycle of activity took place in the southern edifice between 15.5 and 14.1 Ma, followed by another between 13.6 and 12.3 Ma. In the northern edifice three pulses occurred: 10.2-8.3, 6.6-5.3 and 3.9-3.8 Ma. An important temporal gap, greater in Fuerteventura than in Lanzarote, separates Series I from the Plio-Quaternary Series II, III and IV, formed by multi-vent basaltic emissions. In Fuerteventura the following eruptive cycles have been identified: 5, 2.9-2.4, 1.8-1.7, 0.8-0.4 and < 0.1 Ma. In Lanzarote, the activity was fairly continuous from 2.7 Ma to historic times, with a maximum in the Lower Pleistocene. Eruptive rates in the Series I edifices were on the average 0.1-0.01 km3/ka, comparable but slightly smaller than in similar edifices in Tenerife and La Gomera, but much lower than in Gran Canaria. Average post-Miocene eruptive rates were about 0.013-0.027 km3/ka in Lanzarote and 0.003-0.007 km3/ka in Fuerteventura. All these volcanic edifices show a similar general sequence (fissural eruptions, erosion, multi-vent volcanism), repeated at different periods in different parts of the eastern islands of the Canaries. The model of growth of the Series I edifices is comparable to those in Tenerife and La Gomera: long periods of activity, sometimes greater than 6 m.y., with pulses separated by gaps. However, salic and intermediate differentiates, frequent in Tenerife and La Gomera, are very scarce in these islands. The Fuerteventura-Lanzarote ridge shows a decrease in volcanic activity with time, and also a certain SSW-NNE polarity in the temporal development of volcanism.