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首页> 外文期刊>Journal of Volcanology and Geothermal Research >Gravitational slope-deformation of a resurgent caldera: New insights from the mechanical behaviour of Mt. Nuovo tuffs (Ischia Island, Italy)
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Gravitational slope-deformation of a resurgent caldera: New insights from the mechanical behaviour of Mt. Nuovo tuffs (Ischia Island, Italy)

机译：重生火山口的引力坡度变形：山的机械行为的新见解。 Nuovo凝灰岩（意大利伊斯基亚岛）

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Ischia Island (Italy) is an impressive example of the rare phenomenon of caldera resurgence. The emplacement and replenishment of magmas at shallow depth resulted in a vertical uplift of about 900 m, concentrated in the western portion of Mt. Epomeo (789 m a.s.l.). As a consequence of this uplift, the island has experienced several slope instabilities at different scales since the Holocene, from shallow mass movements to large rock and debris avalanches. These mass wasting events, which mobilised large volumes of greenish alkali-trachytic tuff (the Mt. Epomeo Green Tuff, MEGT), were strictly related to volcano-tectonic activity and the interaction between the volcanic slopes and the hydrothermal system beneath the island. Deep-Seated Gravitational Slope Deformation (DSGSD) at Mt. Nuovo, located adjacent to densely populated coastal villages, is an ongoing process that covers an area of 1.6 km(2). The Mt. Nuovo DSGSD involves a rock mass volume of 190 Mm(3) and is accommodated by a main shear zone and a series of sub-vertical fault zones associated with high-angle joint sets. To improve our understanding of this gravity-induced process, we performed a physical (porosity and permeability) and mechanical (uniaxial and triaxial deformation experiments) characterisation of two ignimbrite deposits - both from the MEGT - that form a significant component of the NW sector of Mt. Epomeo. The main conclusions drawn from our experiments are twofold. First, the presence of water dramatically reduces the strength of the tuffs, suggesting that the movement of fluids within the hydrothermal system could greatly impact slope stability. Second, the transition from brittle (dilatant) to ductile (compactant) behaviour in the tuffs of the MEGT occurs at a very low effective pressure, analogous to a depth of a couple of hundred metres, and that this transition is likely moved closer to the surface in the presence of water. We hypothesise that compactant (porosity decreasing) behaviour at the base of the layer could therefore facilitate slope instability. Although our results show that transient exposure to 300 degrees C does not influence the short-term strength of the tuff, we speculate that the high in situ temperature could increase the efficiency of brittle and compactant creep and therefore increase the rate of slope deformation. Taken together, our experimental data highlight a potentially important role for the hydro thermal system (that reaches a minimum depth of similar to 1 km) in dictating the DSGSD at Mt. Nuovo. An understanding of this deformation process is not only important for the proximal coastal villages, at risk of engulfment by a large debris avalanche, but also for the towns and cities along the coast of the Gulf of Naples that are at risk to a secondary consequence of such an avalanche - a tsunami wave. (C) 2017 Elsevier B.V. All rights reserved.
机译：伊斯基亚岛（意大利）是破火山口复活的罕见现象的一个令人印象深刻的例子。浅层岩浆的进入和补给导致大约900 m的垂直隆起，集中在山的西部。埃波美欧（A.s.l. 789 m。）由于这种隆升，自全新世以来，该岛经历了不同规模的几次边坡失稳，从浅质量运动到大型岩石和碎屑雪崩。这些大规模浪费事件动员了大量绿色的碱性-特拉弗特质凝灰岩（Met。Epomeo Green Tuff，MEGT），与火山构造活动以及火山坡与岛下热液系统之间的相互作用密切相关。山峰的深部重力坡变形（DSGSD）。 Nuovo位于人口稠密的沿海村庄附近，是一个持续进行的过程，占地面积1.6 km（2）。山。 Nuovo DSGSD的岩体体积为190 Mm（3），由一个主剪切带和与高角度节理组关联的一系列次垂直断层带容纳。为了增进我们对这种重力诱发过程的理解，我们对两个火成岩沉积物（均来自MEGT）进行了物理（孔隙度和渗透率）和机械（单轴和三轴变形实验）表征，形成了西北西北地区的重要组成部分。公吨。 Epomeo。从我们的实验中得出的主要结论是双重的。首先，水的存在极大地降低了凝灰岩的强度，这表明流体在热液系统中的运动会极大地影响边坡的稳定性。其次，在MEGT凝灰岩中，从脆性（膨胀）到延性（致密）的转变发生在非常低​​的有效压力下，类似于几百米的深度，而且这种转变很可能移近了水存在下的表面。我们假设在该层底部的压实（孔隙度降低）行为可能会促进边坡的不稳定性。尽管我们的结果表明，短暂暴露于300摄氏度不会影响凝灰岩的短期强度，但我们推测，较高的原位温度可以提高脆性和压实蠕变的效率，从而增加边坡变形率。综上所述，我们的实验数据突显了水热系统（达到最小深度约1 km）对指示山峰DSGSD的潜在重要作用。 Nuovo。对这种变形过程的理解不仅对于近端沿海村庄，被大碎片雪崩吞没的风险非常重要，而且对于那不勒斯湾沿岸的城镇和城市也具有重要的次级后果。这样的雪崩-海啸浪潮。 （C）2017 Elsevier B.V.保留所有权利。

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