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Plate Torture: The Gorda Deformation Zone

The Gorda Deformation Zone, the southernmost fragment of the larger Juan de Fuca plate system, is a highly deformed tectonic accommodation zone, allowing the surrounding larger plates to act in a more rigid fashion. Numerous interpretations of its deformation are extant, the most recent model is as an asymmetrical flexural-slip buckle with a vertical axis, utilizing reactivation of spreading-ridge fabric normal faults as strike-slip faults. Second-generation faults and reactivated rift-propagators crosscut the inherited structural grain. This model suggests that spreading-rate variations along the Gorda Ridge may be controlled by internal deformation of the plate rather than the reverse, as previously hypothesized. Southern Cascadia is strongly influenced by the subduction of rift propagator wakes (RPW) which are more extensive than previously realized, both in lateral extent and in topographic expression. The sub-surface expression includes not only tall seamounts, but deep holes as deep as 2.0 seconds TWT, or about 1800m. While a typical expression includes vertical features of 300-500m, in some cases as much as 1600m of basement topography is observed. The modern positions of the RPW may be consistent with paleoseismic segmentation proposed using onshore and offshore paleoseismic data. Southern Cascadia upper plate deformation includes a strong transition from outboard mixed-vergence compression, to inboard transpressive deformation. It also includes an anomalous region of listric normal faulting and subsidence which could be related to this transition, or some other origin. Offshore paleoseismic records include beds from full and segmented subduction events, as well as historic smaller earthquakes from the NSAF (1906, 1700) and Petrolia 1992. These observations serve to validate submarine paleoseismology in this difficult region with multiple fault sources, as well as address the possible stress-triggering relationship between Cascadia and the NSAF.

Session: Characteristics, Hazards and Evolution of the Gorda Region of the Cascadia Subduction Zone I

Type: Oral

Room: Grand B

Date: 4/21/2022

Presentation Time: 08:45 AM Pacific

Presenting Author: Chris Goldfinger

Student Presenter: No

Additional Authors