The Evolution of the Hydration State of the Juan De Fuca Plate From Ridge to Trench Offshore Washington State

Date: 4/24/2019

Time: 04:30 PM

Room: Puget Sound

Characterizing the evolution, hydration state, and extent of faulting on the Juan de Fuca (JdF) plate is an essential component of understanding the structure and convergent tectonics of Cascadia subduction zone. We present results from a 2D tomography study of the JdF plate from east of the ridge axis to the Cascadia subduction zone offshore Washington. We compare our results with a transect across the southern JdF plate from Axial Volcano to offshore Oregon (Horning et al., 2016), facilitating a regional assessment of the velocity structure in the crust and shallow mantle. Velocity models are compared to predicted velocities for crustal and upper mantle lithologies at temperatures estimated from a plate-cooling model and used to provide constraints on water contents in these layers. Co-location of velocity models with multi-channel seismic reflection images (Han et al., 2016), allows for comparison of the relationship between velocities and crustal structure.

Results show upper crustal velocities consistently less than predicted, indicating a fractured and hydrated upper crust. Lower crust and uppermost mantle velocities are similar to predicted values for all plate ages, suggesting a lack of significant lower crust and upper mantle fracturing or hydration. The Washington Transect shows a modest decrease in velocities approaching the deformation front, where crustal faults are few and confined to the upper crust, indicating little fracturing and hydration due to subduction bend faulting, in contrast to observations on the Oregon Transect. Washington and Oregon Transects show distinct crustal structure and velocity regimes at comparable plate ages: increased crustal velocities at 6-8 Ma, decreased velocities coincident with a zone of rough basement topography at 4-6 Ma, and higher velocities and smooth basement crust at 1-4 Ma. Distribution of crustal types across JdF plate suggests differing potential for hydration within the plate inherited from ridge processes.

Presenting Author: Bridgit Boulahanis

Authors