Internal deformation of the Gorda plate and its tectonic significance within the Cascadia subduction zone

摘要

In this review paper, we compile observations that have been made over decades of research on the Cascadia convergent boundary and re-examine the deformation using structural kinematic models, thereby providing new insights. Deformation in the subducting Juan de Fuca slab and the overriding North American plate is primarily accommodated by fracturing. We show that the along-strike deformation patterns are a function of fracture density and activity, which influences coupling with the overriding North American plate and the underlying mantle wedge. We analyze the deformation within a fracture kinematics framework and show that the orientations of the fractures and motion history follow expected empirical fracture mechanics models and are used to provide a more detailed kinematic history of the Cascadia convergent boundary. We focus on the Gorda plate of the Juan de Fuca subducting slab and show that it is intensely fractured with curved magnetic anomalies such that the brittle deformation is collectively expressed macroscopically as a deforming continuum. The Gorda plate has changed shape via NS shortening and clockwise rotation without losing cohesion at the plate-scale, which suggests that the fractures form an interlocking network to accommodate ductile deformation via cataclastic flow. In the forearc, the fracture and velocity patterns suggest that deformation is accommodated by a continental-scale right-lateral shear zone, trending 320/140 and is assisted via crustal-scale cataclastic flow. We suggest that ETS activity is strongly correlated to deformation in the over-riding plate. The Gorda plate is also used to better understand larger-scale plate dynamics. We demonstrate that the Gorda plate acts as a ductile deformation zone to assist global plate mobility. Additionally, details of the magnetic lineation patterns in the Gorda plate indicate that slab pull overtakes plate rotation post-subduction. This suggests that plate behavior is strongly influenced by its internal deformation and coupling with the mantle post-subduction.