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Crustal structure of the northern Juan de Fuca plate McClymont, Alastair F.

Abstract

Anomalous crustal structure of the northern Juan de Fuca plate is revealed from wide-angle seismic and gravity modelling. A 2-D velocity model is produced for refraction line II of the 1980 Vancouver Island Seismic Project (VISP80). The refraction data were recorded on three ocean bottom seismometers (OBSs) deployed at the ends and middle of a 110 km line oriented parallel to the deformation front of the Cascadia subduction zone and immediately south of the Nootka fault zone. The velocity model is constructed via ray tracing and conforms to travel time observations of direct, converted and reflected phases. Synthetic seismograms are produced from calculated first-arrival phases for comparison with the recorded seismogram sections. Travel time picks for turning ray phases with shot offsets of > 60 km indicate significantly different lower crustal and upper mantle velocity structure toward each end of the profile. The variation in travel times is modelled by a pronounced increase in igneous crustal thickness of the Juan de Fuca plate, from ~ 7 km at the southern part of the model to ~ 10 km at the northern part (a thickness increase of 43%). A complementary 2-D gravity model using the geometry of the velocity model and velocitydensity relationships characteristic of oceanic crust is produced. The densities required to match the gravity field indicate the presence of serpentinized peridotites rather than excess gabbroic crust. Anomalous travel time delays and unusual reflection characteristics observed from proximal seismic refraction and reflection experiments suggest a broader zone of serpentinized peridotites that coincides with the trace of a pseudofault. This correlation implies that the inferred partial serpentinization of the upper mantle may be a consequence of slow-spreading at the tip of a propagating rift.

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