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

合作研究：胡安德富卡海岭及其侧翼的地震结构和洋壳演化

• 批准号: 0648923
• 负责人: Juan Pablo Canales
• 金额: $ 17.65万
• 依托单位: Woods Hole Oceanographic Institution
• 依托单位国家: 美国
• 项目类别: Continuing Grant
• 财政年份: 2007
• 资助国家: 美国
• 起止时间: 2007-04-01 至 2010-03-31
• 项目状态: 已结题
• 来源: https://www.nsf.gov/awardsearch/showAward?AWD_ID=0648923&HistoricalAwards=false
• 关键词: Collaborative; Research; Seismic; Structure; Evolution

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.

地球海洋地壳内部的热液循环是影响固体地球与海洋之间化学和物理交换的一个基本过程。海水通过裂缝深入海洋地壳几公里，当它遇到岩浆库和热岩石时被加热，与周围的岩石发生反应，然后带着矿物质上升到海底，这些矿物质被排入海洋，并被形成深海生态系统的庞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影响。在此框架下，本项目将研究洋中脊形成过程中海洋地壳热液蚀变的物理影响、深度范围和水平空间尺度，以及随后的演化和成熟过程。特别是，我们想了解(1)洋中脊的高温热液对流与下伏构造（岩浆储层位置、地壳孔隙度和渗透率等）之间的关系是什么？(2)随着地壳的成熟，蚀变渗透的深度有多大？(3)地形对低温热液流动的控制是否与地壳结构的精细尺度变化有关？(4)热液冷却与观测到的洋中脊附近海洋地壳结构变化之间的联系是什么？热液蚀变对地震波通过地壳传播的速度（地震速度）有显著影响。因此，详细测量地壳上部~1-2公里的地震速度可以给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，该数据是美国国家科学基金会资助的一个项目的一部分，该项目沿着俄勒冈州、华盛顿州和不列颠哥伦比亚省东北太平洋的胡安德富卡洋中脊和构造板块收集的。“尤因”号科考船产生的地震波被安装在该船拖拽的6公里长的拖缆上的传感器记录下来。通过测量地震波从震源到接收器穿过地壳的时间，以及它们沿其路径的振幅变化，我们将能够使用称为传播时间和波形地震层析成像的最先进的计算技术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海洋地壳的密度结构，并结合地震观测，研究热液冷却对地壳厚度表观变化的影响。了解世界海洋下发生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。迄今为止，我们对海洋岩石圈内热液循环的了解大多来自过去几十年对胡安·德·富卡海脊和构造板块的研究。该研究将通过量化热液蚀变对岩石圈地震结构的影响和规模，并将其与地形和沉积史等地质环境变量联系起来，填补这一空白。这个项目的目标与海洋科学的几个大型倡议和正在进行的项目的科学目标直接相关，并将受益。我们对热液喷口下结构特征的研究与奋进号国际空间站的NSF RIDGE-2000项目的目标直接相关。CanNeptune和海洋观测倡议的区域电缆观测站组成部分将在Juan de Fuca板块上铺设光纤电缆，以促进对脊轴和侧翼过程的长期监测。该项目的轴向结构层析研究将为这些监测研究提供适当尺度的物理背景的新限制。我们的山脊侧翼工作将有利于ORION设想的板块尺度实验，以及正在进行的ODP-IODP研究的水文地质目标。该项目将成为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断层成像技术的培训。这将有助于培养具有这些方法专业知识的劳动力，使美国学术研究和工业界都受益。