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Nature of a low-velocity zone atop the transition zone in northwestern Canada Schaeffer, Andrew John


Seismic studies over the past decade have identified a S-wave low-velocity zone (LVZ) above the transition zone at various locations around the globe. This layer is hypothesized to be a lens of dense, fluid-rich silicate melt ponding atop the 410 km discontinuity, beneath the silicate melt-density crossover predicted to exist within the upper mantle. We have assembled a P- and S-receiver function (PRF and SRF, respectively) dataset from the CNSN Yellowknife Array (YKA), the CANOE array, and the POLARIS-Slave array, to quantify the physical properties and geographical extent of the layer in Northwestern Canada. In order to compute the Poisson's ratio, an important discriminant of possible composition and/or fluid content, we generated a suite of 1-D velocity models based on IASP91, but with varying thicknesses and velocity ratios for a hypothetical layer above the 410 km discontinuity. From these models we computed moveout curves for the range of slowness represented in the YKA data. A grid search was performed over the model space of interval thickness and Poisson's ratio to obtain an estimate of the model that best accounts for the data. In addition, we performed a linearized inversion of transmission coefficient amplitudes to estimate the shear velocity contrast at the bounding interfaces of the LVZ. Results indicate a LVZ of thickness 36 km with a shear velocity contrast of -7.8%, and Poisson's ratio of 0.42. In combination, these two results require an associated increase in compressional velocity into the LVZ. The Poisson's ratio lies well above the IASP91 average of 0.29-0.3 for this depth range and favours the presence of high melt or fluid fractions. Geographic profiles of PRFs and SRFs 1-D migrated to depth from CANOE and POLARIS-Slave arrays reveal 410 km and 660 km discontinuities at nominal depths with little variation in transition zone thickness. PRF results from the Slave craton indicate a potential LVZ beneath many stations at an average nominal depth of 340 km, highlighted by events from the northwest. The CANOE array SRF profile images an emergent LVZ beginning at 280 km depth dipping eastwards to 310 km approaching YKA.

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