GEOPS, Univ. Paris-Sud, CNRS, Université Paris-Saclay, 91405, Orsay, France.
Universidade de Lisboa, Lisboa, Portugal.
Sci Rep. 2018 Sep 17;8(1):13898. doi: 10.1038/s41598-018-32253-0.
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.
小型板块内火山岛（高出海底总高度<2500 米）被认为是重力稳定的。地形、地层、结构和新的 K/Ar 数据表明，弗洛雷斯小岛（亚速尔群岛）强烈不对称，由嵌套的火山序列组成。在西北海岸线，约 120 万年前的熔岩流与较年轻的火山单元（约 70 万年前）侧向接触，反映出存在陡峭的侧向不连续面。根据熔岩流的一般倾角、年龄和接触的弧形几何形状，我们推断出一次重大的滑坡事件，导致较老火山的西侧崩落。在岛内更深处，岛屿山顶处 E 向倾斜的熔岩流约有 130 万年，这表明存在另一个滑坡构造，使前火山体的整个西侧移位。可用的近海数据显示，弗洛雷斯以西有一个大型的丘陵状地貌，这里被解释为大量的碎屑崩落堆积物。与亚速尔群岛的东部和中部岛屿不同，弗洛雷斯位于一个相对稳定的构造环境中。因此，我们提出，小型火山岛可能会因重力不稳定而经历多次大规模物质损失事件，这些事件的触发机制并非构造地震，因此是一个潜在的危险源，需要进行风险评估。
