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

北美西海岸外的门多西诺断裂带（FZ）是一个横跨FZ南北的海底年龄差异较大（约27 My）的断裂系统。由于海底随着年龄的增长而冷却，因此存在相关的温差（60公里深处300度），从冷却模型中可以很好地理解。从地幔岩石的实验室研究中得知，电导率和地震速度都取决于温度，但这些实验室预测存在不确定性，因为很难预测并复制地壳下地幔的条件。一个重要的不确定性是水的作用，它降低了岩石的强度，增加了导电性。通过测量门多西诺自由区的电导率和地震速度，该项目将有效地校准温度和年龄的综合影响，同时保持地幔化学等其他因素基本不变。由于地震和电学方法被广泛用于研究地球内部的地质，这些结果将对地球科学界广泛有用。该项目还将对大洋构造板块底部岩石圈-软流圈边界的性质提供重要的限制。虽然形成构造板块横向边界的洋脊、海沟和转换断层得到了很好的研究，但对最大的板块边界LAB却知之甚少。该项目旨在评估部分熔化的程度作为温差的函数，并确定LAB随年龄的任何深度依赖性。该项目的船舶时间由加州大学船舶基金项目提供，并通过该项目将为16名研究生提供海上航行经验。该项目计划在罗杰·雷维尔号从俄勒冈州的纽波特到加州的圣地亚哥的航行期间，部署21个装有地震传感器（水听器和压差计）的海底电磁记录器。这些仪器将占据一条200公里长的南北线，并将在圣地亚哥的另一次巡航中恢复之前记录几个月。大地电磁响应函数将根据时间序列数据进行估计，并反演整个FZ的电导率。将从地震记录中挑选地震到达时间，以估计门多西诺自由区北部和南部的旅行时差。声波啁啾数据将用于校正沉积厚度变化对到达时间的影响。断裂带两侧的岩石圈和软流圈可能是由成分相似的地幔上涌形成的，因此断裂带两侧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融的差异。沉积物厚度和在门多西诺和先锋自由区收集的正在进行的数据将用于改进由于差异热沉降引起的挠曲模型，并通过将数据与部分锁定断层模型进行比较来估计自由区的强度。

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.