Spatial and temporal scales of crustal accretion in slow-spreading rate oceanic crust (Atlantis Massif, Mid Atlantic Ridge - IODP Site U1309)

洋中脊海底扩张形成海洋岩石圈是地球上的基本地质过程之一。最重要但也是最棘手的问题之一是了解洋脊下的岩浆库如何生成下地壳。来自下地壳的辉长岩通常是无法接近的，但由于缓慢扩张的脊段末端附近的地壳大规模断层（在所谓的“海洋核杂岩”的发展过程中），科学海洋钻探已经进入了范围。其中一个地点位于大西洋中脊以北30度的亚特兰蒂斯地块。最近在这里钻探发现了一个1.4公里长的下地壳岩石垂直剖面，显示岩石类型在几米到几十米的范围内迅速变化。船上对这些岩石自然磁化的初步分析表明，这一部分记录了在地球磁场的不同极性（即不同时间）获得的复杂磁化序列。这种多分量磁化在海洋钻探历史上只遇到过一次。这些数据与一个模型最为一致，在这个模型中，下地壳是由岩浆在跨越几次极性反转的漫长时间内以近水平片（或岩床）的形式连续侵入而形成的。这种“席状岩床”机制以前曾根据蛇绿岩（即在造山作用期间侵位到大陆上的古洋壳碎片）的实地证据提出，但从未在真正的洋中脊环境中得到明确的检验。我们建议进行这样的测试，进行详细的古地磁分析样品从亚特兰蒂斯地块，以精确地解释不同的磁极性的核心间隔之间的过渡的性质，并建立起源的磁性叠印在间隔标记的多分量磁化。这些分析将使我们能够明确地确定在不同地磁极性时期注入的离散侵入体/侵入体包之间的边界，并将对亚特兰蒂斯地块的地壳构造和热状态的时间提供宝贵的约束。古地磁数据将得到John博士（项目合作伙伴，怀俄明州大学）的补充放射性测年和Meurer博士（项目合作伙伴，休斯顿大学）从同一岩心样品中获得的矿物化学数据的支持。此外，我们还将岩心中可见的物理结构与井壁定向地球物理图像中观察到的物理结构进行匹配。这将使我们能够恢复从原来的方位角不受约束的核心获得的古地磁数据，以一个真正的地理参考框架，从而使用完整的磁化矢量（方位角方向和倾角），以确定在何种程度上围绕水平和垂直轴的构造旋转，有助于构造发展的古地磁场。因此，该项目提供了一个独特的机会，可以同时确定下地壳在缓慢扩张轴上的增生模式（从而测试洋中脊岩浆房运作的相互冲突的模式），并测试目前大洋核心复合体下地壳折返的模式。

Generation of ocean lithosphere by seafloor spreading at mid-ocean ridges is one of the fundamental geological processes operating on Earth. One of the most important yet most intractable problems is to understand how the magma reservoir beneath ridges generates the lower crust. Gabbroic rocks from the lower crust are normally inaccessible, but are brought within reach of scientific ocean drilling by large-scale faulting of the crust near the ends of slow spreading ridge segments (during the development of so called 'oceanic core complexes'). One such locality is at the Atlantis Massif, at 30 degrees north on the Mid-Atlantic Ridge. Drilling here has recently recovered a 1.4 km vertical section of lower crustal rocks that shows rapid changes in rock type on a scale of metres to tens of metres. Preliminary shipboard analyses of the natural magnetizations of these rocks shows that this section records a complex sequence of magnetizations acquired in different polarities of the Earth's magnetic field (i.e. at different times). Such multi-component magnetizations have only once been previously encountered in the history of ocean drilling. The data are most consistent with a model in which the lower crust was built by successive intrusions of magma in the form of sub-horizontal sheets (or sills) over a protracted period of time spanning several polarity reversals. This 'sheeted sill' mechanism has been previously proposed on the basis of field evidence from ophiolites (i.e. fragments of ancient oceanic crust emplaced onto the continents during orogenesis), but has never been definitively tested in a true mid-ocean ridge setting. We propose to carry out such a test by performing detailed palaeomagnetic analyses of samples from the Atlantis Massif, in order to precisely characterise the nature of the transitions between core intervals with different magnetic polarities and to establish the origin of magnetic overprinting in intervals marked by multi-component magnetizations. These analyses will allow us to unequivocally identify the boundaries between discrete intrusions/packages of intrusions injected during different geomagnetic polarity periods, and will provide invaluable constraints on the timing of crustal construction and the thermal state of the Atlantis Massif. The palaeomagnetic data will be supported by complementary radiometric dating by Dr. John (Project Partner, University of Wyoming) and mineral chemistry data obtained from the same core samples by Dr. Meurer (Project Partner, University of Houston). In addition, we will match physical structures visible in the cores to those observed in oriented geophysical imagery of the borehole wall. This will allow us to restore the palaeomagnetic data obtained from the originally azimuthally unconstrained cores to a true geographic reference framework and hence to use the complete magnetization vector (azimuthal direction and dip) to determine the extent to which tectonic rotations about both horizontal and vertical axes have contributed to the structural development of the massif. The project therefore represents a unique opportunity to simultaneously establish the mode of accretion of the lower crust at a slow-spreading axis (thereby testing conflicting models for the functioning of mid-ocean ridge magma chambers) and test current models for the exhumation of the lower crust in oceanic core complexes.