Magnetotelluric Imaging of the Cascadia Subduction System
|A map showing MT data coverage in Cascadia. This project collected data along the Cafe line and also reanalysed data along the EMLAB profile. (Wannamaker et al., (2014))|
|Seismic and MT models for the Café Line Data. The top model shows a seismic velocity distribution in the subsurface [McGary et al, submitted]. The bottom model shows the primary MT inversion image for the CAFE line and includes both a thermal profile [Van Keken et al., 2010] and the location of earthquake hypocenters (red circles) [McCrory et al., 2012]. Fluids and Melts are more conductive (orange-red colors in the model). Fluid is released from the subducting slab and enters the overlying mantle wedge at A. Melt is initiated at or very near the interface and is transported upward by buoyancy (A1) and dragged down (A2) by convection in the wedge. The fluid/melt phase rises through the mantle wedge (B) until it reaches the crust, where it is joined by fluids released from shallower reactions (D). The combined fluid/melt continues to rise until reaching a reservoir (C) in the crust. Mount Rainier is shown for both models as a red triangle. The two models share the same scale, with the coastline set at x=0 in the horizontal. (McGary et al., (2014))|
|Similar images from further south in Oregon. Here, the MT model (lower) shows evidence for shallow fluid release (A) but does not image a deeper fluid release and melt generation, suggesting significant variations in patterns of melt generation in different sections of the Cascadia system. (Evans et al., (2013))|
Shane McGary (now at The College of New Jersey) , Phil Wannamaker (University of Utah)
This grant funded collection and analysis of geophysical Magnetotelluric (MT) data across a portion of the Cascadia subduction system, just to the south of Seattle, Washington. The profile crossed the Cascadia volcanic arc just to the north of Mount Rainier. The project formed the core of a PhD dissertation for a student in the WHOI/MIT Joint Program.
Offshore Washington State, the seafloor is in collision with the continent and, as a result, is pushed beneath the continent and down into the mantle. This process of subduction results in the generation of large earthquakes, some of them capable of generating tsunamis, and is also responsible for the volcanism along the Cascadia arc at volcanoes such as Mount St. Helens. The volcanic process begins deep beneath the Earth’s surface when fluids carried down by the seafloor plate are released causing the mantle to melt.
Magnetotellurics (MT) is a geophysical technique used to image deep into the Earth. The method measures naturally occurring electric and magnetic fields at the Earth’s surface, generated by the interaction of the Earth’s magnetic field and the charged particles of the solar wind. These charged particles create electric currents in the ionosphere which, in turn, induce current flow within the Earth. The pattern of current flow beneath Earth’s surface is dependent on the electrical conductivity structure, a physical property sensitive to temperature, composition and particularly the presence of fluids, in essence properties that are important for understanding key fluid release and melting processes beneath the Cascadia system.
MT data were collected in an East-West profile through Washington state, along a line roughly coincident with the previous Café seismic experiment (Figure 1). The PhD student working on the MT data also analysed a subset of the seismic data and used constraints from the seismic image to inform the MT data modeling.
The MT data have provided a cross sectional image of the downgoing slab and overlying mantle (Figure 2) and shows where melt is generated in the mantle and how it travels from its source region to the base of the continental crust. This is one of the first such images to be collected. It contrasts with another image generated during the project through reanalysis of existing data further south in Oregon (Figure 3) which shows much weaker evidence for substantial melt generation at depth. We already know that there are significant differences in earthquake episodicity along the Cascadia system and we are now beginning to link that variability to differences in fluid release from the downgoing slab.