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

我们知道，在过去的50年里，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部海脊产生了新的洋壳。这是板块构造的基本过程之一，塑造了我们赖以生存的星球。然而，由于我们不能使用使用电磁辐射的标准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底部的了解更多！在过去的12年里，对大西洋中脊的改进声纳测量揭示了一种新的海底扩张模式，其中很大一部分板块散布是通过长期凸起的向上拆离断层上的滑动来吸收的，而不是主要通过岩浆侵入。多达一半的大西洋海底可能是以这种方式形成的。这些滑脱断层与大型热液系统有关，这些热液系统产生的黑烟在海底喷发400摄氏度的液体。在东太平洋隆起等快速(10-15厘米/年)扩张的山脊上，黑色烟雾系统很小，寿命很短，位于活火山活动区，由或多或少一直存在的浅(1-2公里)岩浆室提供热量。这些系统已经被广泛地模拟，一个关键因素是在熔融岩浆和热液流体之间存在一个薄的导电边界层。在大西洋中部海脊，黑烟系统分布更广、规模更大、寿命更长，通常位于距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滑脱断层向上流动的流体的控制，幕式岩浆室提供的热量最深可达海底7公里，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。在这项提议中，我们计划更深入地研究这一边界层，以及断裂作用、岩浆作用和缓慢扩张山脊上的热液循环之间的复杂相互关系。我们将通过建立热和热液数值模型来解决这个问题，以预测由滑脱断裂作用产生的不对称热结构，以及与可渗透断裂带和局部岩浆活动相关的热液循环模式。由于水的复杂性质，在黑烟系统的温度范围内，水的密度和粘度变化非常迅速，因此水热模型必须非常复杂。因此，我们将与巴黎经验丰富的模特师合作，以实现我们的目标。我们将使用大西洋一个暴露的滑脱断层下盘IODP岩心的岩石冷却速度数据来测试这些模型-这些冷却速度是通过比较天然矿物的组成与微量元素扩散速度的实验数据来计算的。我们的模型的目的不是精确地复制自然(有太多的未知因素无法做到这一点)，而是测试产生可接受结果的参数值范围。例如，模型必须用在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裂带厚度和渗透率的最小值是多少？然后，可以将这些值与基于裂缝密度和地震活动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。在这个项目结束时，我们将更好地了解地球系统中最重要但最难接近的部分之一--在山脊形成新的岩石圈，以及这一过程导致的海洋和地壳之间复杂的相互作用。

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.