TECTONIC EVOLUTION OF THE EMERGENT PART OF A VOLCANIC OCEAN ISLAND - LANZAROTE, CANARY-ISLANDS

A reconstruction of the structural evolution of Lanzarote confirms the existence of two principal stages of construction of the island, sepa rated by a main erosional event. The first (pre-erosional) is the shie ld stage, Miocene-Pliocene in age, which is divided in three stratigra phic groups, where different formations can be distinguished using unc onformities mappable over the whole area of outcrop. In the second (po st-erosional) stage, developed during the Quaternary, nine stratigraph ic levels are tentatively identified and are grouped in two phases of volcanic activity (Shields phase and Pyroclastic cones phase). Data fr om 580 photogeologic lineaments, as well as field data from 204 fault planes (195 with sense of movement), 273 dykes, and 45 volcanic alignm ents, are presented. The tectonic style of the deformations is brittle and polyphasic. Strike-slip faults are dominant, but normal and rever se faults are also present. Both the so-called ''Atlantic'' and ''Afri can'' directions are well developed in trends of strike-slip faults an d volcanic indicators. At least two deformation phases can be identifi ed from the geometrical analysis of faults and from the stress tenser calculated using lineations on fault planes. The reconstructed stress field responsible for the measured deformations changed orientation du ring the evolution of the island of Lanzarote. It is suggested that a permutation between principal stress axes sigma(1) and sigma(3) occurr ed subparallel to the length of the island between the pre-erosional a nd post-erosional stages.