40Ar/39Ar stratigraphy of pyroclastic units from the Cañadas Volcanic Edifice (Tenerife, Canary Islands) and their bearing on the structural evolution

Many felsic pyroclastic units of various types are exposed in different sectors of Tenerife. New 40Ar/39Ar determinations allow them to be placed more precisely in the general volcano-stratigraphic succession. According to geographic distribution, stratigraphic position and isotopic ages, four main pyroclastic phases may be identified. The first, San Juan de la Rambla phase (2.1 Ma), is only known in the north of Tenerife in the Tigaiga massif. The second, Adeje phase (1.8–1.5 Ma), is most completely developed in the southwest of the island, but occasionally occurs in the other sectors. The third, Las Américas phase (1 Ma), is only presently known in the southern region. The fourth, Bandas del Sur phase (0.7–0.15 Ma), is essentially exposed in the southeast sector. The results of this work emphasise the complexity of the pre-1-Ma eruptive history of Tenerife and underline the fact that explosive volcanic activity has taken place for at least the last 2 Ma. Vertical collapse structures have developed as a result of pyroclastic flow activity and these may be as old as 1.6–1.8 Ma, therefore much older than generally considered. The precise location of calderas is difficult to ascertain as a result of the repeated lateral flank collapse during the construction of the Cañadas volcano

Evolution of the Cañadas edifice and its implications for the origin of the Cañadas Caldera (Tenerife, Canary Islands)

The volcano-stratigraphic and geochronologic data presented in this work show that the Tenerife central zone has been occupied during the last 3 Ma by shield or central composite volcanoes which reached more than 3000 m in height. The last volcanic system, the presently active Teide-Pico Viejo Complex began to form approximately 150 ka ago. The first Cañadas Edifice CE. volcanic activity took place between about 3.5 Ma and 2.7 Ma. The CE-I is formed mainly by basalts, trachybasalts and trachytes. The remains of this phase outcrop in the Cañadas Wall CW. sectors of La Angostura 3.5–3.0 Ma and 3.0–2.7 Ma., Boca de Tauce 3.0 Ma., and in the bottom of some external radial ravines 3.5 Ma.. The position of its main emission center was located in the central part of the CC. The volcano could have reached 3000 m in height. This edifice underwent a partial destruction by failure and flank collapse, forming debris-avalanches during the 2.6–2.3 Ma period. The debris-avalanche deposits can be seen in the most distal zones in the N flank of the CE-I Tigaiga Breccia.. A new volcanic phase, whose deposits overlie the remains of CE-I and the former debris-avalanche deposits, constituted a new volcanic edifice, the CE-II. The dyke directions analysis and the morphological reconstruction suggest that the CE-II center was situated somewhat westward of the CE-I, reaching some 3200 m in height. The CE-II formations are well exposed on the CW, especially at the El Cedro 2.3–2.00 Ma. sector. They are also frequent in the S flank of the edifice 2.25–1.89 Ma. in Tejina 2.5–1.87 Ma. as well as in the Tigaiga massif to the N 2.23 Ma.. During the last periods of activity of CE-II, important explosive eruptions took place forming ignimbrites, pyroclastic flows, and fall deposits of trachytic composition. Their ages vary between 1.5 and 1.6 Ma Adeje ignimbrites, to the W.. In the CW, the Upper Ucanca phonolitic Unit 1.4 Ma. could be the last main episode of the CE-II. Afterwards, the Can˜adas III phase began. It is well represented in the CW sectors of Tigaiga 1.1 Ma–0.27 Ma., Las Pilas 1.03 Ma–0.78 Ma., Diego Hernández 0.54 Ma–0.17 Ma. and Guajara 1.1 Ma–0.7 Ma.. The materials of this edifice are also found in the SE flank. These materials are trachybasaltic lava-flows and abundant phonolitic lava and pyroclastic flows 0.6 Ma–0.5 Ma. associated with abundant plinian falls. The CE-III was essentially built between 0.9 and 0.2 Ma, a period when the volcanic activity was also intense in the ‘Dorsal Edifice’ situated in the easterly wing of Tenerife. The so called ‘valleys’ of La Orotava and Gu¨imar, transversals to the ridge axis, also formed during this period. In the central part of Tenerife, the CE-III completed its evolution with an explosive deposit resting on the top of the CE, for which ages from 0.173 to 0.13 Ma have been obtained. The CC age must be younger due to the fact that the present caldera scarp cuts these deposits. On the controversial origin of the CC central vertical collapse vs. repeated flank failure and lateral collapse of mature volcanic edifices., the data discussed in this paper favor the second hypothesis. Clearly several debris-avalanche type events exist in the history of the volcano but most of the deposits are now under the sea. The caldera wall should represent the proximal scarps of the large slides whose intermediate scarps are covered by the more recent Teide-Pico Viejo volcanoes