Cosmogenic 3He exposure dating of the Quaternary lavas at Fogo, Cape Verdes: Dating flank collapse and magmatic reorganisation

Construction and destruction of young ocean island volcanoes is often episodic and governed by periods of intense volcanism and flank collapse. Volcano flank collapses can potentially trigger large tsunamis, which pose significant natural hazard to coastal communities when reaching the main land. However factors leading up to flank instability followed by collapse are not well understood. Both endogenetic (increased volcanism, reorganisation of magmatic system, shift in stress direction, etc) as well as exogenetic (sea level rise, climate change, etc) processes have been proposed (Keating and McGuire, 2000). The lack of quantitative age constraints on the duration and timing of volcanic processes leading to flank instability hamper interpretation to the underlying causes. Here we present cosmogenic 3He exposure age results from a study to date Quaternary lavas of Fogo, Cape Verdes. The growth of Fogo occurred in two main stages, separated by a large flank collapse removing the summit and eastern flank of the pre-collapse Monte Amarelo volcano. The collapse produced a 9 km wide escarpment with a headwall cliff that is still 1 km high despite the infilling with over 1.5 km of post-collapse lava flows from the Cha das Caldeiras volcano. Dyke swarms in the Monte Amarelo volcano indicate that the collapse was preceded by a structural reconfiguration of an initially radial dyke swarm to a single, dominantly north-south swarm across the summit region. This coincided with an eastward shift in magmatic activity. The recent history of the Cha das Caldeiras volcano has seen similar structural reconfiguration of dyke swarms and eastward shift of volcanism resulting in the extinction of post-collapse volcanic rift zones in the west of the island. Dating these events is vital for understanding the timing of (future) volcano flank collapses of Fogo. Lava flows erupted immediately prior to, and after the collapse bracket its age and exposure ages from two pre- collapse ankaramite flows yield 24 and 110 ka, while 8 post-collapse flows yield 55 - 48 ka (2 flows), 25 - 22 ka (2 flows) to 14 - 8 ka (4 flows). We infer two possible scenarios; (i) the collapse occurred between 22 and 24 ka, (ii) the young exposure age of the pre-collapse flow is an artefact of ash cover/erosion and collapse occurred between 55 and 110 ka. This has profound implications for the interpretation of future flank collapses on Fogo. The post-collapse flows are among the youngest flows on the western side and the early Holocene ages imply that the magmatic reconfiguration and possible onset of renewed instability of the eastern flank has been developing for several thousand years. If the collapse occurred between 22 and 24 ka, flank instability may be in its early stages. If the collapse occurred between 55 and 110 ka, the duration of the pre-Monte Amarelo collapse reorganization may be comparable to the duration of the present one, with the prospect of a near-future collapse likely.