Seismic imaging of Santorini: Subsurface constraints on caldera collapse and present-day magma recharge
Seismic imaging of Santorini: Subsurface constraints on caldera collapse and present-day magma recharge
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DOI：
10.1016/j.epsl.2019.02.033
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发表时间：
2019-05-15
影响因子：
5.3
通讯作者：
Warner, M. R.
中科院分区：
文献类型：
--
作者：
Hooft, E. E. E.;Heath, B. A.;Warner, M. R.
火山破火山口是由于岩浆从下伏的储层中喷出而导致顶板塌陷而形成的地表洼地。对火山口形成的机制进行了讨论，并预测了火山口底演化的差异和岩浆补给的不同风格。在这里，我们使用密集的活源地震层析成像研究来揭示圣托里尼火山口的地下物理特性，以了解火山口的形成。我们在火山口中北部下方3公里处发现了一个类似于3公里宽的圆柱形低速异常，该异常位于2011-2012年暴胀的压力源正上方。我们将该异常解释为低密度体积，由孔隙度超过4%至28%引起，孔隙空间被热海水填充。在3.6 ka晚青铜时代（LBA）喷发的前三个阶段形成的通风口位于靠近图像结构的边缘。较老的火山口与低速异常之间的相关性表明，这一特征可能是长期存在的。我们推断，火山口底部有限区域的崩塌导致了一个高孔隙度、低密度的圆柱形体，其形成可能是沿逆断层的混乱崩塌、大面积沉降和凝灰岩和火成岩的充填、气岩浆压裂或这些过程的组合。第4期喷发口位于地形破火山口边缘，速度结构表明较宽的地形破火山口在较有限的北部破火山口崩塌之后连续下降。这种递进式崩塌序列与沿共轭逆断层和正断层多阶段形成巢状破火山口的模式一致。由地震速度模型推断出的上地壳密度差预测了地下重力载荷的差异，这些差异与2011-2012年高楼膨胀的位置有关。这一结果支持了地下密度异常可能影响当今岩浆补给事件的假设。我们假设过去的崩塌以及由此产生的圣托里尼火山的地形和密度变化影响了岩浆在喷发周期之间的集中，这是一个反馈过程，在其他火山中可能很重要。(C) 2019 Elsevier B.V.版权所有
Volcanic calderas are surface depressions formed by roof collapse following evacuation of magma from an underlying reservoir. The mechanisms of caldera formation are debated and predict differences in the evolution of the caldera floor and distinct styles of magma recharge. Here we use a dense, active source, seismic tomography study to reveal the sub-surface physical properties of the Santorini caldera in order to understand caldera formation. We find a similar to 3-km-wide, cylindrical low-velocity anomaly in the upper 3 km beneath the north-central portion of the caldera, that lies directly above the pressure source of the 2011-2012 inflation. We interpret this anomaly as a low-density volume caused by excess porosities of between 4% and 28%, with pore spaces filled with hot seawater. Vents that were formed during the first three phases of the 3.6 ka Late Bronze Age (LBA) eruption are located close to the edge of the imaged structure. The correlation between older volcanic vents and the low-velocity anomaly suggests that this feature may be long-lived. We infer that collapse of a limited area of the caldera floor resulted in a high-porosity, low-density cylindrical volume, which formed by either chaotic collapse along reverse faults, wholesale subsidence and infilling with tuffs and ignimbrites, phreatomagmatic fracturing, or a combination of these processes. Phase 4 eruptive vents are located along the margins of the topographic caldera and the velocity structure indicates that coherent down-drop of the wider topographic caldera followed the more limited collapse in the northern caldera. This progressive collapse sequence is consistent with models for multi-stage formation of nested calderas along conjugate reverse and normal faults. The upper crustal density differences inferred from the seismic velocity model predict differences in subsurface gravitational loading that correlate with the location of 2011-2012 edifice inflation. This result supports the hypothesis that sub-surface density anomalies may influence present-day magma recharge events. We postulate that past collapses and the resulting topographical and density variations at Santorini influence magma focusing between eruptive cycles, a feedback process that may be important in other volcanoes. (C) 2019 Elsevier B.V. All rights reserved.