Structural and magmatic evolution of the ocean crust from re-oriented IODP cores

由于海底难以接近，研究海洋地壳形成的动力过程是有问题的。然而，在过去的35年里，大型科学海洋钻探船已经投入了数百万英镑，这些船现在能够钻到海底数百米深的地方，直接对地壳的横截面进行取样。通过在所有主要的海洋和海洋进行钻探，并分析超过300公里的硬岩石和软沉积物，我们已经能够对我们对固体地球循环的理解做出根本性的贡献。然而，由于岩心在钻井过程中会在钻筒内旋转，因此对回收的岩心样品进行任何定向测量的解释都受到严重限制。例1。洋中脊的板块分裂产生了60%的地壳，但目前有两种相互竞争的模型来描述所涉及的大规模断层过程：一种认为位移发生在平坦的低角度断层上；另一种说法是，断层一开始更陡，然后随着时间的推移旋转到更浅的角度。我们可以利用岩石凝固时所记录的磁场方向的倾角随深度的变化来追踪所经历的构造旋转的数量。然而，由于地核在钻筒内自由旋转，我们不知道磁场方向相对于地理北方（称为赤纬）是如何变化的。因此，我们被迫对旋转轴的方向和角度做出假设，并且有无数的解可以解释数据。例2。研究表明，在缓慢扩张的地壳中，岩浆沿着扩张轴从孤立的岩浆房向外横向移动，然后向上移动到海底火山。然而，在快速扩张的脊上，被动研究表明，较小的岩浆透镜沿着脊的长度延伸，很少或根本不需要水平熔融迁移来形成上地壳。当岩浆侵入穿过钻芯时，排列的磁晶体记录下岩浆流动方向。然而，如果我们没有地理坐标系中关于流动方向的信息，我们就不能定量地评价融化路径。作为该方案的一部分，我将开发一种将核心部件恢复到原始方向的技术，并将其计算机化，从而恢复大量丢失的方向信息。该方法使用的数据来自钻井完成后放入空井眼的工具。该工具测量构成井壁的岩石的电阻率变化，并可以挑选出脆性特征，如裂缝、岩浆和热液脉。关键是，这些特征的倾角是在地理参考系中测量的，因为在同一工具串上包括一个记录磁北方向的磁力计。然后，可以将这些井壁图像上看到的特征与岩心上测量到的特征相匹配，并计算重新定位的角度，将岩心带回其原始位置。重新定向过程的自动化将使定向信息的恢复更加有效和稳健。只有有了完全定向的数据，我才能检验假设，并约束描述海洋地壳结构和岩浆演化的模型，就像上面的例子中提出的那样。

Studying the dynamic processes involving the creation of oceanic crust is problematic due to the inaccessibility of the seafloor. However, over the last 35 years, millions of pounds have been invested in large scientific ocean drilling vessels that are now capable of drilling hundreds of meters into the ocean floor to directly sample a cross section of the Earth's crust. By drilling in all the major oceans and seas, and analysing over 300km of hard rock and soft sediment, we have been able to make fundamental contributions to our understanding of the solid Earth cycle. However, the interpretation of any directional measurements made on recovered core samples is severely limited because the core is allowed to rotate inside the drill barrel during drilling. Example 1. Plate divergence at mid-ocean ridges generates 60% of the Earth's crust, but there are currently two competing models describing the large-scale faulting processes involved: one suggests that displacement occurs along flat low angle faults; the other suggests that the faults start steeper and then rotate to shallower angles over time. We can use the variation in the dip of the magnetic field direction (known as inclination) with depth, recorded by the rocks as they solidify, to track the amount of tectonic rotation experienced. However, because the core is free to spin inside the drill barrel, we do not know how the magnetic field direction changes relative to geographic north (known as declination). Therefore, we are forced to make assumptions about the direction and angle of the rotation axis, and there are an infinite number of solutions that can explain the data. Example 2. In slow-spreading crust, studies have shown that magma is transported laterally away from isolated magma chambers along the spreading axis, before moving upward to feed seafloor volcanoes. However, at fast-spreading ridges passive studies suggest that smaller magma lenses extend along the length of the ridge and little or no horizontal melt migration is needed to build the upper crust. Where magmatic intrusions cut through a drill core, the magmatic flow direction is recorded by aligned magnetic crystals. However, if we do not have information about the flow direction in terms of the geographic co-ordinate system, we cannot quantitatively comment on melt pathways. As part of this proposal I will be developing and computerising a technique that restores core pieces to their original orientation, thus recovering a wealth of lost directional information. The method uses data from a tool that is lowered into the empty borehole after the completion of drilling. The tool measures the variation in resistivity of the rocks making up the borehole wall and can pick out brittle features such as fractures, and magmatic and hydrothermal veins. The key is that the dip of these features is measured in the geographical reference frame because included on the same toolstring is a magnetometer that records the direction of magnetic north. It is then possible to match features seen on these borehole wall images to those measured on the core and calculate a re-orientation angle that brings the core back into its original position. Automating the re-orientation process will allow directional information to be recovered much more efficiently and robustly. Only with fully oriented data can I test hypotheses and constrain models describing the structural and magmatic evolution of ocean crust, like those proposed in the examples above.