Historical Volcanism and the State of Stress in the East African Rift System
Historical Volcanism and the State of Stress in the East African Rift System
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DOI：
10.3389/feart.2016.00086
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发表时间：
2016-09
影响因子：
2.9
通讯作者：
G. Wadge;J. Biggs;R. Lloyd;J. Kendall
中科院分区：
文献类型：
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作者：
G. Wadge;J. Biggs;R. Lloyd;J. Kendall
东非裂谷系（EARS）的地壳伸展，作为一种构造理想，应该包括一个应力场，其中最小水平应力的方向垂直于裂谷。在这样的环境中，火山应该会产生与裂谷平行的堤坝和裂缝。那么，EARS的火山与此有多密切呢？我们通过研究历史上（自1800年以来）爆发的21座火山来回答这个问题，并发现其中7座符合（近似）几何理想。在其他14个火山的喷发裂缝/岩墙和/或主机裂谷段的轴的方向是倾斜的理想值。为了解释这些火山的爆发，我们调用本地（非板块构造）的应力场变化所造成的：地壳的不均匀性和各向异性（占主导地位的NW结构在原生中生代基底），转移带构造在偏移裂谷段的两端，重力负荷的火山建筑物（通常有1-2公里的救济）和中央水库的岩浆压力。我们发现，更倾斜的火山往往有大的大厦，大喷发量和进化和混合的岩浆能够爆炸行为。9个火山有不同的椭圆度的破火山口，其中6个是大的，主要是横跨裂谷（如Kone）和3个较小的，平行于裂谷延长，并包含活跃的熔岩湖（如Erta Ale），这表明不同的形成机制和应力场。Nyamuragira是EARS火山中唯一一个有足够充分记录的火山爆发来推断其长期动态行为的火山。火山7公里范围内的喷发持续时间相对较短（<100天），但具有更远裂缝的喷发往往具有更大的连续性和更长的持续时间，表明远离火山的应力场正在变化。有重大变化，长期岩浆挤出率在1977年（也许在2002年），由于主要沿裂谷岩墙事件，有效地改变了Nyamuragira应力场和侵入/喷发挤出比。
Crustal extension at the East African Rift System (EARS) should, as a tectonic ideal, involve a stress field in which the direction of minimum horizontal stress is perpendicular to the rift. A volcano in such a setting should produce dykes and fissures parallel to the rift. How closely do the volcanoes of the EARS follow this? We answer this question by studying the 21 volcanoes that have erupted historically (since about 1800) and find that 7 match the (approximate) geometrical ideal. At the other 14 volcanoes the orientation of the eruptive fissures/dykes and/or the axes of the host rift segments are oblique to the ideal values. To explain the eruptions at these volcanoes we invoke local (non-plate tectonic) variations of the stress field caused by: crustal heterogeneities and anisotropies (dominated by NW structures in the Protoerozoic basement), transfer zone tectonics at the ends of offset rift segments, gravitational loading by the volcanic edifice (typically those with 1-2 km relief) and magmatic pressure in central reservoirs. We find that the more oblique volcanoes tend to have large edifices, large eruptive volumes and evolved and mixed magmas capable of explosive behaviour. Nine of the volcanoes have calderas of varying ellipticity, 6 of which are large, reservoir-collapse types mainly elongated across rift (e.g. Kone) and 3 are smaller, elongated parallel to the rift and contain active lava lakes (e.g. Erta Ale), suggesting different mechanisms of formation and stress fields. Nyamuragira is the only EARS volcano with enough sufficiently well-documented eruptions to infer its long-term dynamic behaviour. Eruptions within 7 km of the volcano are of relatively short duration (<100 days), but eruptions with more distal fissures tend to have greater obliquity and longer durations, indicating a changing stress field away from the volcano. There were major changes in long-term magma extrusion rates in 1977 (and perhaps in 2002) due to major along-rift dyking events that effectively changed the Nyamuragira stress field and the intrusion/extrusion ratios of eruptions.