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）

• 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

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.

海底散布在海山山脊上的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面的岩浆储层如何产生较低的地壳。来自下层外壳的gabbroic岩石通常是无法访问的，但通过在慢速扩散山脊段的末端的大规模断层（在所谓的“海洋核心综合体”的发展过程中）带来了科学的海洋钻孔。这样的地方是亚特兰蒂斯地块，在大西洋中部山脊北30度。这里的钻探最近恢复了下层地壳岩石的垂直截面1.4 km，该部分显示了岩石类型的快速变化，以数十米至数十米的尺寸变化。对这些岩石的自然磁化的初步分析表明，本节记录了在地球磁场的不同极性（即在不同时间）中获得的复杂磁化序列。在海洋钻探史上，此类多组分磁化曾经曾经遇到过一次。数据与一个模型最一致，在该模型中，岩浆连续入侵以泥浆片（或窗台）的形式构建了下层地壳，该模型是在旷日持久的一段时间内，跨越了几种极性逆转。以前已经根据蛇绿岩的现场证据提出了这种“床单”机制（即在造成造成期间，它放在大陆上的古老海洋壳的碎片），但从未在真正的中洋山脊设置中进行过明确测试。我们建议通过对来自亚特兰蒂斯地块的样品进行详细的古磁分析来进行此类测试，以精确地表征具有不同磁性极性的核心间隔之间的过渡性质，并以多组分磁力磁化标记的间隔来建立磁性过电的起源。这些分析将使我们能够明确地确定在不同的地磁极性时期注入的入侵的离散入侵/包装之间的边界，并将在地壳结构的时机和Atlantis massif的热状态下提供宝贵的约束。古磁数据将由约翰（Wyoming大学项目合作伙伴）博士（项目合作伙伴）和矿物化学数据的互补辐射测量来支持。此外，我们将在核心壁的定向地球物理图像中观察到的物理结构与可见的物理结构匹配。这将使我们能够恢复从最初具有方位角不受限制的核心获得的古磁数据，以实现真正的地理参考框架，从而使用完整的磁化载体（Azimuthalworded和Dip）确定围绕水平和垂直轴的构造旋转的程度有助于水平和垂直轴的结构发展。因此，该项目代表了一个独特的机会，可以同时建立在慢速轴线（从而测试中海洋山脊岩浆腔室功能的冲突模型）和测试当前模型以在海洋核心复合物中挖掘下壳的模型。

项目成果

Earth and Life Processes Discovered from Subseafloor Environments - A Decade of Science Achieved by the Integrated Ocean Drilling Program (IODP)

从海底环境中发现的地球和生命过程 - 综合海洋钻探计划 (IODP) 十年的科学成就

• DOI: 10.1016/b978-0-444-62617-2.00017-7
• 发表时间: 2014
• 作者: Ildefonse B