Large-scale mass wasting on small volcanic islands revealed by the study of Flores Island (Azores)

Abstract

Small intra-plate volcanic islands (total height above seafloor <2500 m) have been considered gravitationally stable. Topographic, stratigraphic, structural and new K/Ar data show that the small island of Flores (Azores) is strongly asymmetric and made up of nested volcanic successions. Along the northwestern coastline, ca. 1.2 Ma lava flows are in lateral contact with a younger volcanic unit (ca. 0.7 Ma), reflecting the existence of a steep lateral discontinuity. From the general dip of the lava flows, their age and the arcuate geometry of the contact, we infer a major landslide that removed the western flank of the older volcano. Further inland, E-dipping lava flows at the summit of the island are ca. 1.3 Ma, suggesting another landslide structure that displaced the whole western half of the former volcanic edifice. Available offshore data show a large hummocky field west of Flores, here interpreted as voluminous debris-avalanche deposits. Unlike the eastern and central Azores islands, Flores sits on a relatively stable tectonic setting. Therefore, we propose that small-size volcanic islands can be sufficiently gravitationally unstable to experience recurrent episodes of large-scale mass wasting triggered by mechanisms other than tectonic earthquakes and thus represent an under-evaluated potential source of hazard and, therefore, risk.

The evolution of Santa Maria Island in the context of the Azores Triple Junction

“…In this regard, geochronological data is used to constrain the age of the different volcanic complexes, the age of the inferred large-scale mass wasting (LSMW), and the age of the different fossil deposits. Third, it is a very small island, therefore seemingly not prone to LSMW (Mitchell, 2003;Hildenbrand et al, 2018), which is in apparent contradiction with the LSMW inferred in Santa Maria.…”

“…Despite the small size of the Azorean islands, several LSMW events have been described recently (Hildenbrand et al, 2012a;Marques et al, 2013b;Sibrant et al, 2014Sibrant et al, , 2015aSibrant et al, , b, 2016Costa et al, 2014Costa et al, , 2015Marques et al, 2018;Hildenbrand et al, 2018). Exceptions in the eastern Azores are the islands of Faial (Hildenbrand et al, 2012b;Marques et al, 2014b;Trippanera et al, 2014) and Terceira Quartau et al, 2014), where LSMW has not been described (Mitchell, 2003;Casalbore et al, 2015;Quartau et al, 2012Quartau et al, , 2015.…”

The Complex Vertical Motion of Intraplate Oceanic Islands Assessed in Santiago Island, Cape Verde

“…The samples were dated in Laboratory GEOPS (Univ. Paris-Sud, Orsay, France) with the unspiked K-Ar Cassignol-Gillot technique Gillot & Cornette, 1986), which allows the precise determination of small amounts of radiogenic argon ( 40 Ar*) and is particularly appropriate to date young volcanic rocks, including late quaternary low-K basalts and andesites (e.g., Bablon et al, 2018Bablon et al, , 2019Boulesteix et al, 2012Boulesteix et al, , 2013Costa et al, 2014Costa et al, , 2015Germa et al, 2010Germa et al, , 2011Hildenbrand et al, 2003Hildenbrand et al, , 2004Hildenbrand et al, , 2008Hildenbrand et al, , 2012Hildenbrand et al, , 2014Hildenbrand et al, , 2018Marques et al, 2015Marques et al, , 2018Quidelleur et al, 2008;Ricci et al, 2015Ricci et al, , 2017Ricci et al, , 2018Salvany et al, 2012;Sibrant et al, 2014;Sibrant, Hildenbrand, Marques, Weiss, et al, 2015;Silva et al, 2012Silva et al, , 2018. The technique has even been extended to the last millennium with an uncertainty of only a few centuries in the case of high-K lavas (Quidelleur et al, 2001).…”

Volcanic Record of the Last Geomagnetic Reversal in a Lava Flow Sequence From the Azores

“…We used the decay constants and isotopic ratios of Steiger and Jäger (1977). The uncertainty on each age determination was obtained from the quadratic sum of all three independent sources of uncertainty involved in the calculation: (1) relative uncertainty on K-content determination, (2) relative uncertainty on calibration of the 40 Ar signal, and (3) relative uncertainty on the correction of the atmospheric contamination (full description in Hildenbrand et al, 2018).…”

“…During the last two decades, numerous studies on the Azores archipelago brought new information on the timing of volcanism at both local and regional scales (e.g., Calvert et al, 2006;Costa et al, 2014;Sibrant et al, 2015;Hildenbrand et al, 2018;Marques et al, 2018). Johnson et al (1998) and Sibrant et al (2015) identified lava flow sequences in São Miguel island with ages close to the M-B reversal.…”