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

关于裂谷作用和在裂谷扩展中心向地壳注入熔融岩石的问题，长期以来一直存在着“先有鸡还是先有蛋”的争论：是熔融岩石注入推动裂谷作用，还是相反？一个进一步的问题是，熔体是否从产生的深层地幔流到在中心点扩张的裂谷部分的上覆地壳中，然后沿着裂谷部分在地壳内部流动，或者它是否在整个裂谷部分的多个点上以亚垂直馈线进入地壳，在地壳的不同深度淤积，最终从地表以下几公里的高层岩浆库喷发出来。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。冰岛北部80公里长的Askja延伸段是一个非凡的自然实验室：它是一个极其安静的地震环境，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，而且只有4轮驱动的车辆）。我们经常记录到极小的局部0.2级地震。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1万次地震，包括乌普蒂平加尔附近15-25公里深度的熔体注入事件，几个月后，在3-7公里深度的脆性地壳上发生了数百次构造地震。我们将这些地震模拟为由岩脉深处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捉整个周期。到目前为止，我们只捕获了周期的一部分。基于我们对被认为是由Askja裂谷段中部融化运动引起的下地壳地震的识别，我们建议在整个Askja裂谷段部署一个由15个地震仪组成的阵列，从2010年9月开始记录3年的地震活动。除此之外，还将增加10个地震仪，每年沿着这一段移动一次，以提供极其密集的采样覆盖范围。我们将绘制沿Askja段地震活动性的时空发展图，并将研究由熔体运动引起的下地壳地震与由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%的火成岩地壳由侵入岩组成，这些侵入岩在到达地表之前就在深处冻结，因此我们更有可能记录到由岩堤或岩壁侵入引起的地震活动。人们对冰岛北部裂谷的地壳结构知之甚少，尤其是在地壳下部。我们将使用约束良好的地震作为震源，绘制整个Askja段的地壳结构和各向异性图。这不仅可以描绘出远离其中心火山的部分地壳结构的变化，而且通过利用富含剪切波的地震，我们将能够限制下地壳中熔体的存在和分布。这是可能的，因为剪切波对熔岩的存在非常敏感，因为它们不通过液体传播。在2011年夏天，我们将安装额外的临时地震检波器以增加其密度，然后将大型爆炸源发射到阵列中，以提供传统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.