The Mendocino Fracture Zone: A natural laboratory to study aging of the lithosphere and asthenosphere

门多西诺断裂带：研究岩石圈和软流圈老化的天然实验室

• 批准号: 1736590
• 负责人: Steven Constable
• 金额: $ 29.28万
• 依托单位: University of California-San Diego Scripps Inst of Oceanography
• 依托单位国家: 美国
• 项目类别: Continuing Grant
• 财政年份: 2017
• 资助国家: 美国
• 起止时间: 2017-09-01 至 2020-08-31
• 项目状态: 已结题
• 来源: https://www.nsf.gov/awardsearch/showAward?AWD_ID=1736590&HistoricalAwards=false
• 关键词: Mendocino; Fracture; Zone; natural; laboratory

The Mendocino Fracture Zone (FZ) off the west coast of North America is a fault system with a large difference in seafloor age (about 27 My) from north to south across the FZ. Because seafloor cools as it ages, there is an associated temperature difference (300 degrees at 60 km depth) that is well understood from cooling models. It is known from laboratory studies of mantle rocks that electrical conductivity and seismic velocity both depend on temperature, but there are uncertainties associated with these laboratory predictions because it is difficult to predict, and then replicate, conditions in Earth's mantle beneath the crust. One significant uncertainty is the role of water, which decreases the strength of rocks and increases electrical conduction. By measuring electrical conductivity and seismic velocity across the Mendocino FZ, this project will effectively calibrate the combined effect of temperature and age, while keeping other factors, such as mantle chemistry, largely constant. Since seismic and electrical methods are widely used to study the geology of Earth's interior, these results will be broadly useful to the Earth science community. This project will also provide important constraints on the nature of the lithosphere-asthenosphere boundary (LAB) at the base of the oceanic tectonic plate. While the oceanic ridges, ocean trenches, and transform faults that form the lateral boundaries of the tectonic plates are well studied, the largest plate boundary of all, the LAB, is poorly understood. This project aims to assess the extent of partial melting as a function of temperature difference, and also determine any depth dependence of the LAB with age. The ship time for this project is provided by the University of California ship funds program, and through this project will provide seagoing experience for 16 graduate students. The data will be used as the basis for the PhD thesis of a female graduate student.The project plans to deploy 21 ocean-bottom electromagnetic (OBEM) recorders fitted with seismic sensors (hydrophones and differential pressure gauges) during a transit of the R.V. Roger Revelle from Newport, Oregon, to San Diego, California. The instruments will occupy a 200 km long north-south line and will be left to record for several months before being recovered on another cruise out of San Diego. Magnetotelluric response functions will be estimated from the time series data, and inverted for electrical conductivity across the FZ. Earthquake arrival times will be picked from the seismic recordings to estimate travel time differences north and south of the Mendocino FZ. Acoustic chirp data will be used to correct for the effect of variations in sediment thickness on arrival times. The lithosphere and asthenosphere across the fracture zone were presumably formed from compositionally similar mantle upwelling, so any differences in the conductivity and seismic velocity across the fracture zone can be attributed to differences in temperature and partial melt. Sediment thickness and underway data collected across the Mendocino and Pioneer FZs will be used to refine models of flexure due to differential thermal subsidence and estimate the strength of the FZ by comparing the data with models of partially locked faults.

北美西海岸附近的Mendocino骨折区（FZ）是一个断层系统，海底年龄差异很大（大约27 my），从北部到南面。由于海底随着年龄的增长而冷却，因此从冷却模型中可以很好地理解了相关的温度差（60 km深度）。从对地幔岩石的实验室研究中可以知道，电导率和地震速度都取决于温度，但是与这些实验室预测相关的不确定性是因为难以预测地球下地面的地下室中的条件，然后复制。一个重要的不确定性是水的作用，它降低了岩石的强度并增加了电传导。通过测量整个Mendocino FZ的电导率和地震速度，该项目将有效校准温度和年龄的综合效果，同时保持其他因素，例如地幔化学，在很大程度上恒定。由于地震和电气方法被广泛用于研究地球内部的地质，因此这些结果将对地球科学界广泛有用。该项目还将提供有关海洋构造板底部岩石圈 - 心理边界（LAB）性质的重要限制。虽然对构成构造板的侧向边界的海洋山脊，海沟和变形断层进行了充分的研究，但最大的板块边界是实验室的，但对实验室的了解很少。该项目旨在评估部分熔化作为温度差的函数的程度，并确定实验室随着年龄的深度依赖性。该项目的船舶时间由加利福尼亚大学的船舶基金计划提供，通过该项目将为16名研究生提供海上体验。该数据将用作女性研究生的博士学位论文的基础。该项目计划在R.V的运输过程中部署21个海洋底电磁（OBEM）记录器，该记录器配备了地震传感器（氢气和差分测量测量值）。罗杰·雷维尔（Roger Revelle）从俄勒冈州纽波特（Newport）到加利福尼亚州的圣地亚哥。这些乐器将占据200公里长的南北线，并将留下几个月的记录，然后在另一辆从圣地亚哥的巡游中回收。磁铁响应函数将从时间序列数据中估算，并在整个FZ上倒置电导率。地震到达时间将从地震记录中选出，以估计门多西诺FZ以北和南部的旅行时间差异。声学chirp数据将用于纠正沉积物厚度对到达时间的变化的影响。跨断裂带的岩石圈和小圈是由构图相似的地幔上升流形成的，因此整个断裂区的电导率和地震速度的任何差异都可以归因于温度和部分熔体的差异。由于差异热沉降，将使用跨门多西诺和先驱FZ收集的沉积物厚度和正在进行的数据，并通过将数据与部分锁定故障的模型进行比较，并估算FZ的强度。