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

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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.