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

慢速扩张的洋壳中地壳增生的时空尺度（亚特兰蒂斯地块、大西洋中脊 - IODP 站点 U1309）

基本信息

批准号：
NE/E003079/1
负责人：
Antony Morris
金额：
$ 22.16万
依托单位：
Plymouth University
依托单位国家：
英国
项目类别：
Research Grant
财政年份：
2007
资助国家：
英国
起止时间：
2007 至无数据
项目状态：
已结题

来源：
https://gtr.ukri.org/projects?ref=NE%2FE003079%2F1
关键词：
Spatial temporal scales crustal accretion

项目摘要

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.

通过洋中脊海底扩张形成海洋岩石圈是地球上运行的基本地质过程之一。最重要但最棘手的问题之一是了解山脊下方的岩浆库如何形成下地壳。来自下地壳的辉长岩通常是难以接近的，但通过缓慢扩张的山脊段末端附近的地壳大规模断层（在所谓的“海洋核心复合体”的发展过程中），科学的海洋钻探可以到达。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1.4公里长的下地壳岩石垂直剖面，显示出岩石类型在数米至数十米范围内的快速变化。对这些岩石自然磁化强度的初步船上分析表明，该部分记录了在地球磁场不同极性（即不同时间）获得的复杂磁化强度序列。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，在该模型中，下地壳是由岩浆在一段漫长的时间内以次水平岩层（或岩台）的形式连续侵入而形成的，跨越了几次极性反转。这种“板状基台”机制之前已根据蛇绿岩（即造山作用期间放置在大陆上的古代洋壳碎片）的现场证据提出，但从未在真正的大洋中脊环境中得到明确测试。我们建议通过对亚特兰蒂斯地块的样本进行详细的古地磁分析来进行这样的测试，以便精确地表征具有不同磁极性的核心区间之间的过渡性质，并确定由多个标记的区间中磁叠印的起源。 - 分量磁化强度。这些分析将使我们能够明确识别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，并将为地壳构造的时间和亚特兰蒂斯地块的热状态提供宝贵的约束。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）从相同岩心样本中获得的矿物化学数据的支持。此外，我们还将把岩心中可见的物理结构与钻孔壁定向地球物理图像中观察到的物理结构进行匹配。这将使我们能够将从原始方位角不受约束的核心获得的古地磁数据恢复到真实的地理参考框架，从而使用完整的磁化矢量（方位角方向和倾角）来确定构造绕水平轴和垂直轴旋转的程度为地块的结构发展做出了贡献。因此，该项目提供了一个独特的机会，可以同时建立下地壳在缓慢扩张轴上的吸积模式（从而测试洋中脊岩浆房功能的相互冲突模型）并测试当前下地壳折返模型。海洋核心复合体中的地壳。