Unconsolidated pyroclastic flow deposits of the 1993 eruption of Lascar Volcano, Chile, have, with time, become increasingly dissected by a network of deeply penetrating fractures. The fracture network comprises orthogonal sets of decimeter-wide linear voids that form a pseudo-polygonal grid visible on the deposit surface. In this work, we combine shallow surface geophysical imaging tools with remote sensing observations and direct field measurements of the deposit to investigate these fractures and their underlying causal mechanisms. Based on ground penetrating radar images, the fractures are observed to have propagated to depths of up to 10 m. In addition, orbiting radar interferometry shows that deposit subsidence of up to 1 cm/year−1 occurred between 1993 and 1996 with continued subsidence occurring at a slower rate thereafter. In situ measurements show that 1 m below the surface, the 1993 deposits remain 5°C to 15°C hotter, 18 years after emplacement, than adjacent deposits. Based on the observed subsidence as well as estimated cooling rates, the fractures are inferred to be the combined result of deaeration, thermal contraction, and sedimentary compaction in the months to years following deposition. Significant environmental factors, including regional earthquakes in 1995 and 2007, accelerated settling at punctuated moments in time. The spatially variable fracture pattern relates to surface slope and lithofacies variations as well as substrate lithology. Similar fractures have been reported in other ignimbrites but are generally exposed only in cross section and are often attributed to formation by external forces. Here we suggest that such interpretations should be invoked with caution, and deformation including post-emplacement subsidence and fracturing of loosely packed ash-rich deposits in the months to years post-emplacement is a process inherent in the settling of pyroclastic material.

中文翻译：

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浮石流沉积物的沉积后压裂和沉降：智利拉斯卡火山

随着时间的推移，智利拉斯卡火山 1993 年喷发的松散火山碎屑流沉积物越来越多地被深度穿透的裂缝网络解剖。裂缝网络包括正交组的分米宽线性空隙，形成在沉积表面上可见的伪多边形网格。在这项工作中，我们将浅表地球物理成像工具与遥感观测和矿床的直接现场测量相结合，以研究这些裂缝及其潜在的因果机制。根据探地雷达图像，观察到裂缝已传播至 10 m 的深度。此外，轨道雷达干涉测量显示，1993 年至 1996 年间发生了高达 1 cm/year−1 的沉积物沉降，此后以较慢的速度继续沉降。现场测量表明，在地表以下 1 m 处，1993 年的矿床在就位 18 年后仍比相邻矿床高 5°C 至 15°C。根据观测到的沉降以及估计的冷却速率，推断这些裂缝是沉积后数月至数年内脱气、热收缩和沉积压实的综合结果。重要的环境因素，包括 1995 年和 2007 年的区域地震，在不规则的时刻加速了沉降。空间可变的裂缝模式与地表坡度和岩相变化以及基底岩性有关。类似的裂缝在其他熔结块中也有报道，但通常仅在横截面中暴露，并且通常归因于外力形成。