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首页> 外文期刊>Journal of Volcanology and Geothermal Research >Elliptical calderas in active tectonic settings: an experimental approach
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Elliptical calderas in active tectonic settings: an experimental approach

机译：活动构造环境中的椭圆火山口：一种实验方法

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Caldera volcanoes form due to collapse of a magma chamber roof into the underlying magma chamber. Many field, theoretical and experimental studies have postulated that calderas are delimited by reverse ring faults and are surrounded by peripheral concentric normal faults. In the simplest theoretical scenario, circular magma chambers produce circular calderas. Many calderas, however, are elliptical in shape, particularly those in extensional and compressive tectonic settings. Several factors may explain elliptical calderas. The first is the presence of an elliptical magma chamber, established by, for instance, preferential intrusion along pre-existing basement structures or differential spalling of the magma chamber walls. The second is the overlap (nesting) of several discrete calderas to form a single, larger elliptical structure. The third is asymmetric subsidence. The fourth is variable pre-collapse topography. A fifth possible factor is distortion of the caldera faults by the regional stress field during caldera formation. A sixth factor is the post-collapse distortion of the caldera structure due to continued regional deformation. To better understand relationships between caldera surface expression, reservoir geometry and regional tectonic stresses, we conducted scaled analogue experiments. These experiments examined the impact of regional stress and associated structures on calderas formed during evacuation of reservoirs (circular rubber balloons) of known dimensions and depths. The results show that, in principle, calderas produced in compression/extension experiments are elongated parallel to the direction of minimum horizontal compressive stress, despite the chamber beneath being circular in plan view. As a consequence, model ring fault orientation varied from steeply dipping where striking perpendicular to the minimum horizontal regional compressive stress, to shallower dips where striking parallel to the minimum horizontal regional compressive stress. This leads us to suggest that the influence of a regional stress field on caldera fault orientation during and/or after caldera formation may be significant in the development of elliptical calderas. In addition, such variation of caldera ring fault dip from steep to relatively shallow could influence location and behaviour of ring fissure eruptions.
机译：破火山口火山的形成是由于岩浆室屋顶坍塌到下面的岩浆室中。许多领域，理论和实验研究都假定破火山口由反向环断层划定，并被外围同心正断层包围。在最简单的理论情况下，圆形岩浆室会产生圆形破火山口。但是，许多火山口的形状都是椭圆形的，特别是在伸展和压缩构造环境下的椭圆形。几个因素可以解释椭圆形破火山口。首先是椭圆形岩浆室的存在，例如，通过沿既有的基底结构的优先侵入或岩浆室壁的不同剥落来建立。第二个是几个离散火山口的重叠（嵌套），以形成单个较大的椭圆形结构。第三是不对称沉降。第四个是可变的塌陷前地形。第五个可能的因素是在破火山口形成过程中区域应力场引起的破火山口断层变形。第六个因素是由于持续的区域变形导致的火山口结构坍塌后变形。为了更好地了解火山口表面表达，储层几何形状和区域构造应力之间的关系，我们进行了大规模的模拟实验。这些实验检查了区域应力和相关结构对已知尺寸和深度的储层（圆形橡胶气球）撤离时形成的破火山口的影响。结果显示，原则上，尽管在平面图中下方的腔室为圆形，但在压缩/延伸实验中产生的火山口在平行于最小水平压缩应力的方向上是细长的。结果，模型环断层的方向从垂直于最小水平区域压应力的陡峭倾角变化到平行于最小水平区域压应力的浅倾角变化。这使我们建议，在椭圆形火山口的形成过程中，局部应力场对火山口断层定向的影响在火山口形成期间和/或之后可能是重要的。另外，破火山口环断层倾角从陡变到相对浅的这种变化可能影响环裂缝爆发的位置和行为。

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