Construction and destruction of a volcanic island developed inside an oceanic rift: Graciosa Island, Terceira Rift, Azores
Résumé
There is a great lack of knowledge regarding the evolution of islands inside active oceanic rifts, in particular the meaning of the different evolutionary steps. Therefore, we conducted an investigation in Graciosa Island, which lies at the northwestern end of the Terceira Rift in the Azores Triple Junction, with the objective of constraining the evolution of the island in terms of volcanic growth and mass wasting, in particular the meaning and age of the destruction events. From digital elevation model (DEM) analysis, stratigraphic and tectonic observations, K/Ar dating on key samples, and available bathymetry and gravity data, we propose that Graciosa comprises five main volcanic complexes separated by major unconformities related to large scale mass wasting: (1) The older volcanic edifice (Serra das Fontes Complex) grew until ca. 700 ka, and was affected by a major flank collapse towards the southwest, which removed the whole SW flank, the summit and a part of the NE flank. (2) The Baía do Filipe Complex developed between at least 472 ka and 433 ka in two different ways: in the SW (presently offshore) as a main volcano, and in the NE unconformably over the sub-aerial remnants of the Serra das Fontes Complex, as secondary volcanic edifices. (3) The Baía do Filipe Complex was affected by a major flank collapse towards the SW, again removing most of the edifice. (4) The remnants of the Baía do Filipe Complex were covered in unconformity by the Serra Dormida Complex between ca. 330 and 300 ka, which in turn was unconformably covered by the younger Basaltic Cover Complex between ca. 300 ka and 214 ka. These two units were affected by a third major sector collapse that removed the whole western flank, the summit and part of the eastern flank of the Serra Dormida and Basaltic Cover complexes. (5) Despite the relatively young age of Graciosa, the collapse scars are not well preserved, and not active anymore. (6) A central-type volcano has been growing since at least 60 ka at the southeastern end of the island, inside the scar left by a fourth sector collapse towards the SE, which affected most previous complexes. Contemporaneously, parasitic strombolian cones formed all over the island. Despite the location of Graciosa inside the active Terceira Rift, the new data indicate that the evolution of the island has been driven by a competition between volcano growth and repeated destruction by catastrophic sector collapses, rather than by slow incremental faulting associated with the tectonics of the rift.