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Crustal structure from an ocean bottom seismometer survey of the Nootka fault zone Au, C.Y. Daniel


The Nootka fault zone is the boundary between the small Explorer and Juan de Fuca plates which lie off western Canada between the America and Pacific plates. To investigate the crustal structure in the region, three refraction lines were shot with explosives and a large airgun into three ocean bottom seismometers (OBS's) each equipped with three-component geophone assemblies. The P and S wave profile data are analysed primarily by synthetic seismogram modelling using the WKBJ algorithm. The interpretation gives relatively consistent results for the upper crust. Below the 1 km thick sediment layer, the P wave velocity ranges from 3-7 to 4.7 km/s and increases with depth at a moderate velocity gradient of 0.5 km/s/km to a depth of 1.9 km. A zone of high velocity gradient marks the transition from layer 2A to 2B, below which the velocity again increases moderately with depth to a range of 6.0 to 6.4 km/s at the base of layer 2B. The S wave velocity also increases with depth in the upper crust but no detail is available for the v[sub s] structure in the shallow region of layer 2A. Within layer 2B, the P and S wave velocity values give a Poisson's ratio with an unusually low value of 0.24. This may be related to the presence of quartz-rich trondhjemites. Both v[sub p] and v[sub s] are relatively uniform at the top of layer 3A. Good agreement is found between the seismic velocities of layer 3A and the seismic velocities of rock samples from the corresponding depths of ophiolite complexes measured in the laboatory, resulting in consistent values for Poisson's ratio. Sub-bottom crustal thickness varies from 6.4 to 11.2 km among the various profiles. Some aspects of this variation can be explained by consideration of a recent tectonic model for the development of the fault zone. This requires, within the past one million years, variation in the process of crustal formation at the ridge, crustal 'maturing', or both. The abnormally thick crust may result in part from the complex interaction of the Juan de Fuca and Explorer plates with the larger and older America and Pacific plates. Apparent velocities of compressional waves in the upper mantle, calculated for areally distributed ray paths, show significant anisotropy. A maximum velocity of 8.3 km/s is found in the inferred direction of plate motion and a minimum velocity of 7.5 km/s is found parallel to the spreading ridge. This type of velocity variation can be approximated by a mixture of 42% transversely isotropic olivine and an isotropic material with a constant velocity of 7.0 km/s. Velocity of shear waves in the upper mantle, varying from 4.5 to 4.6 km/s, is isotropic within the resolution of the interpretation. This causes a prominent anisotropy in the values of Poisson's ratio. The variation in velocity with depth in the crust, values of Poisson's ratio, the P wave velocity anisotropy, and the approximately isotropic S wave velocity are consistent with laboratory measurements on rock samples from ophiolite complexes.

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