PROPAGATOR WAKES OF THE JUAN DE FUCA PLATE - ARE THEY WEAKER OR STRONGER THAN THE SURROUNDING OCEANIC CRUST?

The Cascadia Subduction Zone is one of the most tectonically complex regions of the world. The oceanic microplates of this region have propagator wakes (sometimes also referred to in the literature as pseudofaults) — the zones of distorted oceanic crust formed as a result of the propagation of spreading ridge segments that are mapped from apparent disturbances in the magnetic anomalies. As most of the propagator wakes are being subducted, the associated variations in the strength of the oceanic slab should impact the entire subduction process. We assessed the density distribution across two propagator wakes of the Juan de Fuca plate from a joint analysis of previously published gravity and seismic data. Previous 2D gravity modeling of Marjanović et al. (2011) suggested that these features represent zones of denser than surrounding oceanic crust interpreted as evidence of iron enrichment through subcrustal magma lenses.

We performed 2.75D gravity modeling along the same seismic profiles and arrived at an opposite conclusion. Our modeling suggests that the propagator wakes correspond to zones of lower density than the surrounding oceanic crust. This disagreement relates to the difference in the modeling approaches. Our 2.75D modeling allowed us to incorporate gravity signals from nearby seamounts - both exposed (bathymetric) and buried ones imaged by the vintage seismic data. The seamounts were not directly crossed by the modeled lines but were in the vicinity, so their up to 5 mGal gravity signal should be properly accounted for. In addition, a general increase in density of the oceanic crust away from the spreading center must be included in the model to explain observed gravity data.

Our results suggest that the densities within the propagator wakes are up to 0.05 g/cc lower than the ones of the surrounding oceanic crust which we interpret as evidence of crustal weakness within the propagator wakes related to faulting during the formation of those features. This conclusion is consistent with variations in crustal thickness within propagator wakes documented by other researchers. We postulate these weaker crustal zones facilitate the formation of seamounts that cluster near the propagator wakes. However, the spatial and temporal distribution of seamounts with respect to propagator wakes require further investigations.