Nature of a low-velocity zone atop the transition zone in Northwestern Canada

Seismic studies over the past decade have identified a low-velocity layer 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 theorized to exist within the upper mantle. In order to quantify the nature and geographical extent of the layer in Northwestern Canada, we have assembled a P- and S-wave dataset from the Canadian National Seismograph Network Yellowknife Array (YKA), the Canadian Northwest Experiment (CANOE), and the POLARIS Slave array. P- and S- receiver functions (PRFs and SRFs) for all stations were generated via standard procedures utilizing earthquakes at epicentral distances of 30° to 95° for PRFs and 55° to 85° for SRFs. Totals of RFs computed include ~ 2500 (P) and ~ 400 (S) for YKA, ~ 7000 (P) and ~ 800 (S) for POLARIS Slave, and ~ 25000 (P) and ~ 1600 (S) for CANOE. In order to compute 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 from travel-times of five different scattering modes (Ps, Sp, Pps, Pss, & Ppp) for all combinations of source-receiver geometry represented in the data. A grid search was performed over all possible models, where we shifted and stacked autocorrelations to obtain an estimate of the model that most accurately predicts the interval thickness and Poisson's ratio. Results from geographic profiles of PRFs and SRFs and 1-D migrated depth stacks indicate upper mantle and transition zone arrivals with moveout compatible with both direct and reverbatory phases from the layer. We obtain estimates for the interval Poisson's ratio in the range 0.37 to 0.40, which falls well above the 1-D velocity model average of ~ 0.25 for this depth range and favours the presence of high melt or fluid fractions. The nature of these signals, their spatial distribution, and their implications for the nature of the low-velocity zone will be discussed.