Shear Velocity Structure of the Mantle Beneath the North American Plate

The long-period tangential components of seismograms recorded by the WWSS network over the last 20 years are used to constrain the shear velocity variations in the mantle beneath the North American plate. The first two chapters derive vertical shear velocity profiles beneath three tectonic provinces: the East Pacific Rise, the Canadian shield and the old northwest Atlantic ocean. The SH motion seismograms were interpreted as multi-bounce shear bodywaves and use was made of Cagniard-de Hoop and WKBJ synthetic seismogram techniques to model the data with the assumption that the structure within each province was laterally homogeneous. The three models were constructed such that the S-waves and up to 4 minutes of later arrivals in the data were fit in terms of travel-time, phase and amplitude. The most useful arrival in the study was the SS phase. The three regions studied were found to have very similar structure below 400 km depth. Anomalously large increases in velocity of about 6.5% from 375 to 425 km depth and 11% from 625 to 700 km depth were found beneath each tectonic area with no indication of lateral variations in the structures. These high gradient zones were modeled as discontinuities of 4.5% at 405 km depth and 7.5% at 660 km depth. Other common features of the models are a high gradient from 425 to 625 km depth and an anomalously high gradient beneath the discontinuity at 660 km depth to about 750 km depth. Above 400 km depth, large lateral differences in structure exist beneath the three provinces. Velocities beneath the shield were found to be about 10% higher at 100 km depth, 5% higher at 200 km depth and 3% higher at 300 km depth, relative to the Rise model. The old Atlantic has high velocities to about 100 km depth, but below 150 km the structure is similar to that beneath the East Pacific Rise. The shield model has a very low gradient and high velocity in the upper 175 km. The velocity decreases by about 3% at 200 km depth with a low gradient below to 400 km depth. Beneath the Atlantic and East Pacific, very low velocities occur near 200 km depth. The Rise model has the lowest velocities (4.3 km/sec) near 100 km depth. Both models have very high gradients (about .035%/km) from 175 to 375 km suggesting a partial melt or dislocation relaxation explanation for the low velocities near 200 km depth. Chapter 3 is an attempt to develop a fully three-dimensional model of mantle shear velocity beneath the entire North American plate to 1700 km depth. S and SS arrival time anomalies were measured relative to a simple starting model. The raypaths of the different measured phases were found using the experience gained in Chapters 1 and 2. The travel-time anomalies were then used as the input for a tomographic back projection inversion for velocity fluctuations within blocks on the order of 500 by 500 by 100 kms. A high velocity root to 400 km depth is found directly beneath almost all of the shield and platform of North America. Eastern North America was found to have similar structure to the old Atlantic with high velocities to 140 km depth and average to low velocities to 400 km depth. The Basin and Range, Carribean, Mid-Atlantic Ridge and the East Pacific have the lowest velocities in the upper 400 km. From the southwest Carribean to the northern United States, a major high velocity feature is observed to exist from 700 to 1700 km depth with about a 1% increase in velocity over an 800 km width in an east-west direction. The structure has a slab-like shape and dips to the east. It is hypothesized that the anomalous high velocities are due to remnants of the subducted Farallon plate going back to 100 Ma.

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