A low‐velocity zone atop the transition zone in northwestern Canada

Abstract

Seismic studies over the past decade have identified an 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, hydrous, 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 data set to quantify the physical properties and geographical extent of the layer in northwestern Canada. Geographic profiles formed from 1‐D migration of receiver functions computed for the Canadian Northwest Experiment (CANOE) and Portable Observatories for Lithospheric Analysis and Research Investigating Seismicity (POLARIS) Slave arrays reveal an LVZ beneath many stations at a nominal depth of ∼340 km. To constrain layer thickness and Poisson's ratio, we performed a grid search over a suite of 1‐D velocity profiles to model the relative delay times of direct conversions and reverberations from the top of the LVZ and 410 km discontinuity, as recorded at the Yellowknife array. In addition, we performed linearized inversion of transmission coefficient amplitudes to estimate S velocity contrasts at the bounding interfaces. The LVZ is characterized by a thickness of ∼36 km with an S velocity contrast of −7.8% and Poisson's ratio of 0.42. Taken at face value, the two latter results require an increase in P velocity into the LVZ. The Poisson's ratio lies well above the IASP91 average of ∼0.29–0.3 for this depth range and favors the presence of high melt or fluid fractions.