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Hydrothermal systems, thermal boundary layers and detachment faults in slow-spread ocean crust

缓慢扩张洋壳中的热液系统、热边界层和滑脱断层

基本信息

批准号：
NE/I015035/1
负责人：
Andrew McCaig
金额：
$ 53.15万
依托单位：
University of Leeds
依托单位国家：
英国
项目类别：
Research Grant
财政年份：
2011
资助国家：
英国
起止时间：
2011 至无数据
项目状态：
已结题

来源：
https://gtr.ukri.org/projects?ref=NE%2FI015035%2F1
关键词：
Hydrothermal systems thermal boundary layers

项目摘要

We have known for the last 50 years that Europe and America have been moving apart at about 2cm/yr by processes of seafloor spreading that generate new oceanic crust at the submarine mid-Atlantic Ridge. This is one of the fundamental processes of Plate Tectonics, and has shaped the planet that we live on. Yet because we cannot use standard remote sensing techniques using electromagnetic radiation to study the seafloor, in many ways we know more about the surface of Mars than we do about the floor of the Atlantic! Over the last 12 years improved sonar surveys of the mid Atlantic Ridge have revealed a new mode of seafloor spreading where a significant part of the plate divergence is taken up by slip on long-lived, convex upward detachment faults, rather than mainly by magmatic intrusion. Up to half of the Atlantic seafloor may have formed in this way. These detachment faults are associated with large hydrothermal systems producing black smokers venting 400 C fluids on the seafloor. On fast (10-15 cm/yr) spreading ridges such as the East Pacific Rise, black smoker systems are small, short-lived, and located in zones of active volcanism, and are supplied with heat by shallow (1-2 km) magma chambers that are there more or less all the time. These systems have been modelled extensively, and a key element is the existence of a thin conductive boundary layer between molten magma and the hydrothermal fluid. On the mid-Atlantic ridge, black smoker systems are more widely spaced, larger, and longer lived, and often are located a few km away from the zone of active volcanism. These systems may in some cases be controlled by fluid flow up detachment faults, with heat supplied by episodic magma chambers as deep as 7km below seafloor, and much less numerical modelling work has been done on them. We have identified a fossil thermal boundary layer in a detachment fault sampled by drilling. In this proposal we plan to investigate this boundary layer more thoroughly, as well as the complex interrelationships between faulting, magmatism and hydrothermal circulation at slow spreading ridges. We will address this problem by building thermal and hydrothermal numerical models to predict both the asymmetric thermal structure produced by detachment faulting and the hydrothermal circulation patterns associated with permeable fault zones and localised magmatism. The hydrothermal models have to be very sophisticated because of the complicated properties of water, which changes density and viscosity very rapidly in the temperature range of black smoker systems. Hence we will work with experienced modellers in Paris to achieve our aims. We will test these models using data on cooling rates of rocks from IODP core in the footwall of an exposed detachment fault in the Atlantic - these cooling rates are calculated by comparing the compositions of natural minerals with experimental data on diffusion rates of trace elements. The aim of our models is not to replicate nature precisely (there are too many unknowns to do that) - but to test the range of parameter values that generate acceptable results. For example, the model must generate vents with the temperature measured on the seafloor and the heat output estimated from geochemical data - what are the minimum values of fault zone thickness and permeability that allow this to happen? These values can then be compared with physical models of permeability based on fracture densities and seismicity distributions. Because it is hard to observe subsurface geology or fluid flow directly, modelling is often the only way of determining whether hypotheses are realistic. At the end of this project we will have a better understanding of one of the most important but least accessible parts of the Earth System - the formation of new lithosphere at ridge crests, and the complex interactions between the ocean and the crust that occur as a result of this process.

在过去的50年里，我们已经知道，欧洲和美洲一直在以每年约2厘米的速度移动，这是由于海底扩张过程在大西洋中脊产生了新的洋壳。这是板块构造学的基本过程之一，也塑造了我们所居住的星球。然而，由于我们无法使用标准的电磁辐射遥感技术来研究海底，在许多方面，我们对火星表面的了解比对大西洋海底的了解还要多！在过去的12年中，改进的大西洋中脊声纳调查揭示了一种新的海底扩张模式，其中很大一部分板块扩张是由长期存在的向上凸起的剥离断层上的滑动所引起的，而不是主要由岩浆侵入所引起的。多达一半的大西洋海底可能是这样形成的。这些剥离断层与大型热液系统有关，这些热液系统产生黑烟囱，在海底排放400 C的流体。在快速（10-15厘米/年）扩张的海脊上，如东太平洋隆起，黑烟系统很小，寿命很短，位于活火山活动区，并由浅（1-2公里）的岩浆房提供热量，这些岩浆房或多或少一直存在。这些系统已经被广泛地模拟，一个关键因素是熔融岩浆和热液流体之间存在薄的导电边界层。在大西洋中脊，黑烟系统分布更广，更大，寿命更长，通常位于离活火山活动区几公里的地方。在某些情况下，这些系统可能受到沿拆离断层向上流动的流体的控制，热由海底以下7公里深处的幕式岩浆房提供，对它们进行的数值模拟工作要少得多。我们在钻探取样的滑脱断层中发现了化石热边界层。在本提案中，我们计划更彻底地调查这一边界层，以及在缓慢扩张的海脊中断层、岩浆活动和热液循环之间复杂的相互关系。我们将通过建立热和热液数值模型来预测由拆离断层产生的不对称热结构和与渗透性断层带和局部岩浆活动相关的热液循环模式来解决这个问题。由于水的性质复杂，在黑烟系统的温度范围内，水的密度和粘度变化非常迅速，因此热液模型必须非常复杂。因此，我们将与巴黎经验丰富的建模师合作，以实现我们的目标。我们将测试这些模型使用的数据从IODP核心的岩石冷却速率在下盘暴露的剥离故障在大西洋-这些冷却速率计算比较天然矿物的成分与实验数据的微量元素的扩散速率。我们的模型的目的不是精确地复制自然（有太多的未知数），而是测试产生可接受结果的参数值范围。例如，模型必须利用海底测量的温度和根据地球化学数据估计的热输出来生成喷口-允许这种情况发生的断层带厚度和渗透率的最小值是多少？然后，这些值可以与基于裂缝密度和地震活动性分布的渗透率物理模型进行比较。由于很难直接观察地下地质或流体流动，建模往往是确定假设是否现实的唯一方法。在这个项目的最后，我们将有一个更好的了解地球系统中最重要的，但最难访问的部分之一-新的岩石圈在脊峰的形成，以及海洋和地壳之间的复杂的相互作用，发生作为这一过程的结果。

项目成果

期刊论文数量（9）

专著数量（0）

科研奖励数量（0）

会议论文数量（0）

专利数量（0）

Magmatism, serpentinization and life: Insights through drilling the Atlantis Massif (IODP Expedition 357)

DOI：
10.1016/j.lithos.2018.09.012
发表时间：
2018-12
期刊：
Lithos
影响因子：
3.5
作者：
G. Früh-Green;B. Orcutt;S. Rouméjon;M. Lilley;Y. Morono;C. Cotterill;S. Green;J. Escartín;
通讯作者：
G. Früh-Green;B. Orcutt;S. Rouméjon;M. Lilley;Y. Morono;C. Cotterill;S. Green;J. Escartín;

Crustal Permeability

地壳渗透率