Interaction of Tectonics and Magmatism in the Askja spreading segment of Iceland

关于裂谷作用和熔岩在裂谷扩张中心注入地壳的问题，存在着一个长期存在的“鸡和蛋”的争论：熔岩的注入是裂谷作用的驱动力，还是反之亦然？另一个问题是，熔体是否从深地幔流入在中心点扩展的裂谷段的上覆地壳，然后在地壳内沿着该段流动，或者熔体是否在沿整个段沿着的多个点以近乎垂直的馈线进入地壳，在地表以下几公里处的高级岩浆库最终喷发之前，在地壳的不同深度形成积水。70%的地壳是在海洋扩张段产生的：在冰岛，底层地幔柱的存在将扩张段推到海平面以上，使我们能够比在海洋下面更好地使用地震仪来测量它们。冰岛北方80公里长的Askja扩散段是一个非凡的自然实验室：它是一个非常安静的地震环境，因为没有植被，也没有人类活动产生的噪音（只有在夏天才能进入，然后只有四轮驱动车辆）。我们经常记录到非常小的0.2级局部事件。自从我们在2006年夏季开始在Askja火山周围季节性地部署地震仪（地震探测器）以来，Askja部分的活动异常增加。到目前为止，我们已经记录了超过10，000次地震，包括在Upptyppingar附近15-25公里深处的熔体注入事件，几个月后，在3-7公里深处的上覆脆性地壳中发生了数百次构造地震。我们将这些地震模拟为由深处堤坝中的熔体注入引发上覆地壳的构造破坏引起的。侵入和随后的脆性破坏的时间尺度为18-24个月，因此需要长期监测以捕获整个周期。到目前为止，我们只捕获了周期的一部分。在我们确定下地壳地震的基础上，我们认为是由Askja裂谷段中部的熔体运动引起的，我们建议沿整个Askja段沿着部署一个由15个地震仪组成的阵列，从2010年9月起记录3年的地震活动。除此之外，还将另外配备10个地震仪，这些地震仪将沿着这一段每年移动一次，以提供极为密集的采样覆盖面，我们将绘制沿着Askja段地震活动的空间和时间发展图，并将调查熔融运动引起的下地壳地震与上覆脆性地壳破裂引起的下地壳地震之间的关系。如果我们幸运的话，我们将捕捉到包括火山喷发在内的整个周期，尽管由于扩张段中80%的火成地壳由侵入岩组成，这些侵入岩在到达地表之前就在深处冻结，但我们更有可能记录到地震活动。由岩脉或岩床入侵引起。人们对北方冰岛裂谷的地壳结构知之甚少，特别是对下地壳。我们将使用约束良好的地震作为震源，绘制沿着整个Askja段的地壳结构和各向异性图。这不仅可以描绘出远离火山中心的沿着地壳结构的变化，而且通过地震，我们还可以利用剪切波丰富的地震来限制下地壳中熔融物的存在和分布，这是可能的，因为剪切波对熔融岩石的存在非常敏感，因为它们不能穿过液体。2011年夏天，我们将安装更多的临时地震仪，以增加其密度，然后向阵列发射大型爆炸源，以提供常规的广角可控震源剖面，以校准和补充地震层析成像。

There is a long-standing 'chicken and egg' debate over rifting and the injection of molten rock into the crust at rifted spreading centres: does the molten rock injection drive rifting or vice-versa? A further question is whether melt flows from the deep mantle where it is generated into the overlying crust of a segment of the rift that is spreading at a central point, and then flows within the crust along the segment, or whether it moves into the crust in sub-vertical feeders at multiple points along the entire segment, ponding at different depths in the crust prior to eventual eruption from a high-level magma chamber a few kilometres below the surface. Seventy percent of the earth's crust is generated at oceanic spreading segments: in Iceland the presence of an underlying mantle plume pushes the spreading segments above sea level, allowing us better access to instrument them with seismometers than below the oceans. The 80-km long Askja spreading segment in northern Iceland is a remarkable natural laboratory: it is an extremely quiet environment seismically because there is no vegetation, and no noise created by human activity (it is only accessible in the summer, and then only by 4-wheel drive vehicles). We routinely record down to extremely small local magnitude 0.2 events. There has been an extraordinary increase of activity in the Askja segment since we started deploying seismometers (earthquake detectors) in it seasonally around Askja volcano in the summer months of 2006. So far we have recorded over 10,000 earthquakes, including episodes of melt injection at 15-25 km depth near Upptyppingar, followed some months later by hundreds of tectonic earthquakes in the overlying brittle crust at 3-7 km depth. We model these earthquakes as caused by melt injection in a dyke at depth triggering tectonic failure in the overlying crust. The time-scale of the intrusions and succeeding brittle failure is 18-24 months, so it requires long-term monitoring to capture an entire cycle. So far we have only captured portions of the cycle. Building on our identification of lower-crustal earthquakes thought to be caused by melt moving in the central part of the Askja rift segment, we propose to deploy an array of 15 seismometers along the entire Askja segment to record seismicity for 3 years from September 2010. This will be supplemented by a further 10 seismometers which will be moved once per year along the segment to provide extremely densely sampled coverage overall.We will map the development in space and time of earthquake seismicity along the Askja segment and will investigate the relationship between lower crustal earthquakes attributed to melt movement and those caused by failure in the overlying brittle crust. If we are fortunate we shall capture an entire cycle including a volcanic eruption, although since 80% of the igneous crust in the spreading segment consists of intrusive rocks that freeze at depth before they reach the surface, it is more likely that we shall record seismicity resulting from dyke or sill intrusion. The crustal structure of the northern Icelandic rifts is only poorly known, especially in the lower crust. We shall use well constrained earthquakes as seismic sources to map the crustal structure and anisotropy along the entire Askja segment. This will give not only a picture of the variation in crustal structure along the segment away from its central volcano, but by using earthquakes, which are rich in shear waves, we shall be able to constrain the presence and distribution of melt in the lower crust.This is possible because shear waves are very sensitive to the presence of molten rock, because they do not travel through liquids. In summer 2011 we shall install additional temporary seismometers to increase their density, and then fire large explosive sources into the array to provide a conventional wide angle controlled source profile to calibrate and to supplement the earthquake tomography.