Collaborative Research: Seismic Structure and Evolution of Oceanic Crust along the Juan de Fuca Ridge and its Flanks

地球海洋地壳内的热液循环是影响固体地球和海洋之间化学和物理交换的一个基本过程。海水通过裂缝向下渗透几公里，在遇到岩浆库和热岩时变热，与周围的岩石发生反应，然后上升到海底，携带的矿物质被排入海洋，被形成深海生态系统的巨大生物群落用作营养物质。此外，热液流体与岩石之间的相互作用影响岩石圈的化学和物理性质，因此对深海矿床的形成等过程具有重要意义。在这个总的框架内，这个项目将解决沿大洋中脊形成过程中洋壳热液变化的物理影响、深度范围和水平空间尺度，以及随后的演化和成熟的问题。我们特别想了解(1)大洋中脊高温热液对流与下伏结构(岩浆库位置、地壳孔隙度和渗透率，(2)随着地壳的成熟，蚀变的深度有多深？(3)地壳结构是否存在与地形控制低温热液流动有关的细微尺度变化？(4)水热冷却与大洋中脊附近洋壳结构的观测变化之间有什么联系？热液蚀变对地震波在地壳中传播的速度(地震速度)有重大影响。因此，详细测量地壳上部1-2公里的地震速度可以为我们提供有关上述过程的丰富信息。为了回答上述问题，我们将分析2002年收集的海洋地震反射数据，这是NSF资助的一个项目的一部分，该项目位于东北太平洋、俄勒冈州、华盛顿州和不列颠哥伦比亚省海岸外的胡安德富卡大洋中脊和构造板块沿线。尤文号科考船产生的地震波是由该船拖着的6公里长的拖缆上的传感器记录下来的。通过测量波从震源到接收器穿过地壳的时间，以及它们沿路径的幅度变化，我们将能够使用最先进的计算技术，即旅行时间和波形地震层析成像，构建地壳弹性特性的高分辨率图像。由此产生的地震层析成像图像将被解释为热液对流引起的蚀变。我们还将利用重力测量来推断洋壳的密度结构，并与地震观测一起调查热液冷却对地壳厚度表观变化的影响。要了解发生在世界海洋下面的热液循环和变化的原因和后果，需要多学科的方法，包括生物、地球化学、地质和地球物理研究，以及数值计算机模型和现场和远程观测。到目前为止，我们对大洋岩石圈内热液循环的了解大多来自过去几十年来对胡安德富卡海脊和构造板块的研究。这项研究将通过量化热液改造对岩石圈地震结构的影响和规模来填补这一空白，并将其与地形和沉积历史等地质和环境变量联系起来。该项目的目标与海洋科学中的几个大型倡议和正在进行的计划的科学目标直接相关，并将从中受益。我们对热液喷口下方结构的研究与奋进国际空间站的NSF Ridge-2000计划的目标直接相关。加拿大海王星和海洋观测站倡议的区域有线天文台将铺设一条横跨胡安德富卡板块的光缆，以促进对脊轴和侧翼过程的长期监测。该项目轴向结构的层析成像研究将在适当的尺度上为这些监测研究提供新的物理环境约束。我们的山脊侧翼工作将有助于在猎户座计划下进行的板块尺度实验，以及正在进行的ODPIODP研究的水文地质目标。该项目将是两名研究生博士研究的一部分，他们将接受海洋地震反射和层析成像技术方面的培训。这将有助于发展一支拥有这些方法专业知识的劳动力队伍，使美国学术研究和产业界都受益。

Hydrothermal circulation within the Earth's oceanic crust is a fundamental process that affects the chemical and physical exchange between the solid Earth and the oceans. Sea water penetrates down the ocean crust several kilometers through cracks, becomes heated up as it encounters magma reservoirs and hot rocks, reacts with the surrounding rocks, and rises back to the seafloor carrying minerals that are expelled into the oceans and used as nutrients by the vast biological communities that form deep-sea ecosystems. In addition, the interaction between the hydrothermal fluids and the rocks affects the chemical and physical properties of the lithosphere, therefore having important implications for processes like the formation of deep-sea mineral deposits. Within this general framework, this project will address the issue of what are the physical impact, depth extent, and horizontal spatial scales of hydrothermal alteration in the oceanic crust during its formation along a mid-ocean ridge, and its subsequent evolution and maturation.In particular, we want to understand (1) what is the relationship between high-temperature hydrothermal convection in a mid-ocean ridge and the underlying structure (location of magma reservoirs, porosity and permeability of the crust, etc.)? (2) How deep does the alteration penetrate as the crust matures? (3) Are there fine-scale variations in crustal structure related to topographic controls on low-temperature hydrothermal flow control? (4) What are the links between hydrothermal cooling and the observed variations in the structure of the oceanic crust near a mid-ocean ridge?Hydrothermal alteration has a significant impact on the speed at which seismic waves propagate through the crust (seismic velocity). Thus measuring in detail the seismic velocity of the upper ~1-2 km of the crust can give us a wealth of information about the processes described above. To answer the above-mentioned questions we will analyze marine seismic reflection data collected in 2002 as part of a NSF-funded project along the Juan de Fuca mid-ocean ridge and tectonic plate in the northeast Pacific, off the coast of Oregon, Washington, and British Columbia. Seismic waves generated from the research vessel M. Ewing were recorded by sensors located in a 6-km-long streamer towed by the ship. By measuring the time that waves traveled through the Earth's crust from the sources to the receivers, as well as their amplitude changes along their paths, we will be able to construct high resolution images of the elastic properties of the crust using state-of-the-art computing techniques known as travel-time and waveform seismic tomography. The resulting seismic tomography images will then be interpreted in terms alteration due to hydrothermal convection. We will also use gravity measurements to infer the density structure of the ocean crust, and together with the seismic observations, investigate the impact of hydrothermal cooling on the apparent variability of crustal thickness.Understanding the causes and consequences of hydrothermal circulation and alteration that takes place under the world's oceans requires a multidisciplinary approach, including biological, geochemical, geological, and geophysical studies, as well as numerical computer models and in-situ and remote observations. Much of what we know to date about hydrothermal circulation within oceanic lithosphere comes from studies at the Juan de Fuca ridge and tectonic plate during the past decades. This study will fill a gap by quantifying the impact and scales of hydrothermal alteration on the seismic structure of the lithosphere, and relate it to geological and environmental variables such as topography and sedimentation history.The objectives of this project are directly linked to the science goals of, and will benefit, several large initiatives and ongoing programs within Ocean Science. Our studies to characterize the structure beneath hydrothermal vent sites are directly linked to objectives of the NSF RIDGE-2000 program at the Endeavour ISS. CanNeptune and the Regional Cabled Observatory component of the Ocean Observatory Initiative will place a fiber optic cable spanning the Juan de Fuca plate to facilitate long-term monitoring of ridge axis and flank processes. The tomographic studies of axial structure of this project will provide new constraints on physical context at appropriate scales for these monitoring studies. Our ridge flank work will benefit plate-scale experiments envisioned here under ORION, and hydrogeologic objectives of ongoing ODP-IODP studies. This project will be part of the Ph.D. research of two graduate students, who will be trained in marine seismic reflection and tomography techniques. This will contribute to the development of a workforce with expertise in these methods, benefiting both the US academic research and industry communities.