ROQUE NUBLO CALDERA - A NEW STRATOCONE CALDERA IN GRAN-CANARIA, CANARY-ISLANDS

An inferred elliptic structure 5 x 4 km in diameter, has been identified as the caldera that brought the Roque Nublo Pliocene (5-3.5 Ma.) volcanic cycle in Gran Canaria to an end. This buried caldera, whose morphological expression is no longer evident, is now filled with almost 400 m of interbedded lacustrine sediments and nephelinite lava flows, ashes and tuffs. Mapping of the postcaldera sedimentary layers, which comprise debris-flow lobes, turbiditic sandstones and muds, has permitted the paleogeographic reconstruction of a saucerlike volcanic depression. This has been confirmed by a gravimetric survey, whose data clearly define a double gravity minimum in the area of the inferred caldera. Some postcaldera vents are located near the caldera rim, but most of them define a NW-SE line, across the caldera, which apparently marks an important tectonic lineation. The paucity of remains of Roque Nublo breccias in the caldera infilling suggests a powerful explosive terminal phase for the stratocone, after which the center of the residual edifice collapsed to form the caldera. The very early resumption of activity in the caldera is not a definite proof for a resurgent activity in the area. Whereas the Roque Nublo Formation was formed after a quiescent period of almost four million years on Gran Canaria, this event was soon followed by the third and last cycle of volcanic activity on the island.