The dyke swarms of the Old Volcanic Edifice of La Gomera (Canary Islands): Implications for the origin and evolution of volcanic rifts in oceanic island volcanoes

Abstract

Rift zones are fundamental features in oceanic island volcanoes, although their origin is still controversial, especially at the Canary Islands where the role of volcanic spreading is under debate. For this reason, we have performed a field study of > 400 dykes at 20 outcrops in the Lower Old Edifice and 2 outcrops of the Upper Old edifice of La Gomera Island, identifying different dyke swarms. We propose the development of an unnoticed E-W volcanic rift (Hermigua Rift) restricted to the Lower Old Edifice East flank, with up to 25% of extension and structural characteristics compatible with the spreading of the edifice over a basal weak decollement. Contemporaneously a dyke swarm of outward-dipping dykes intruded at the W-SW zones, indicating a progressive radial sliding of the edifice. This asymmetric volcano structural pattern suggests that volcano stresses were predominately local in origin, probably related to the buttressing effect of the neighbor contemporaneous Roque del Conde edifice (Tenerife Island). Additionally, we have deduced two different rift zones (NW-SE Enchereda and N-S Erque rifts) during the growth of the Upper Old Edifice, evidencing that La Gomera rifts were dynamic evolving structures. The structure and regional setting of the Upper Old Edifice rifts suggest a change to a predominance of regional stresses. Our results have implications for the understanding of the origin and evolution of rifts at oceanic island volcanoes, since: 1) confirm a principal role for volcanic spreading processes; 2) support that the location of neighbor volcanic loads strongly influences rifts development; and 3) suggests, in some situations, a possible main role for the regional stress field.