OCGC Seminar - What happens to the Juan de Fuca plate boundary beneath Northern Cascadia? Insight into the metamorphism of the oceanic crust


Dr. Nicola Piana Agostinetti 
Dublin Institute for Advanced Studies in Ireland and
Istituto Nazionale di Geofisica e Vulcanologia in Italy  

Thursday, November 17th 
1:30 a.m.  

123 Marion Hall 
140 Louis Pasteur
University of Ottawa



In subduction zones, the plate boundary is the locus where the largest earthquakes occur worldwide. The exact knowledge of the geometry of the plate boundary and the physical properties of the materials brought together in contact are fundamental to understand the earthquake nucleation process and to better assess the seismic hazard in densely populated areas. Due to the metamorphism of the subducted materials, and the release of fluids from the subducted plate (e.g. breakout of the serpentinized oceanic upper mantle), the seismic properties of the materials distributed along the plate boundary change with depth (i.e. at different age of subduction). Thus, the clear recognition of the plate boundary at depth using indirect method (e.g. seismic tomography) might be a very complex task.

In Northern Cascadia, the position of the plate boundary between the North America (NAM) plate and the Juan de Fuca (JdF) plate is still debated. While along the coast, plate boundary models almost agree, differences in the estimated depth of such interface arise in-land between different models, exactly where large earthquakes enucleate.

In this study, we investigate the seismic properties of the subducted JdF crust as it plunges in the upper mantle beneath Northern Cascadia. Harmonic decomposition of a huge Receiver function data-set is used to image both isotropic and anisotropic structures along a trench-normal profile, to better constrain the metamorphism of the crustal materials during the subduction process. The analysis of the seismic anisotropy of the materials is fundamental to better recognize the different components of the subducted plate, where large changes in the seismic velocity of the materials are expected.

Our results confirm the 2-layer structure of the JdF crust: basalts over gabbros, and allow us to depict their metamorphism as they plunge into the upper mantle. In the western part of our profile an East-dipping low S-velocity layer is interpreted as fluid-filled basalts, in a region where the uppermost interface of the JdF is sealed. In such region, the JdF upper mantle is strongly anisotropic and the occurrence of intermediate-depth events suggests its progressive de-serpentinization. Further East, fluids coming from the JdF upper mantle trigger the metamorphism of the JdF crust. In the same region, the sealed boundary is finally cracked and fluids hydrate the lower-crust of the NAM plate. Finally, in the eastern portion of our profile, our results clearly depict the final eclogization of the JdF crust and the fluid migration in the NAM upper mantle.