A Low Velocity Zone Atop the Transition Zone in Northwestern Canada

A Low Velocity Zone Atop the Transition Zone in Northwestern Canada, Figure 1 Figure 1:
Radial PRFs for the Northwest corridor (274 - 313 ) YKA dataset. Top panel is windowed for direct conversions converting between the transition zone and the surface, whereas bottom panel is windowed for back-scattered reverberations. The relevant features are indicated in 1D mantle reectivity pro les on the right hand side. The phases utilized include Pds and Ppds.
<p>
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, hydrous silicate melt ponding atop the 410 km discontinuity, beneath the silicate melt-density crossover theorized to exist within the upper mantle [Bercovici and Karato, 2003]. We have assembled a P- and S-receiver function (PRF and SRF, respectively) dataset from the CNSN Yellowknife Array (YKA), the CANOE array (obtained from the IRIS DMC), and the POLARIS-Slave array, in order to quantify the physical properties and geographical extent of the layer in Northwestern Canada.
</p><p>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. Through utilization of differential times of forward- and back-scattered arrivals from the LVZ bounding interfaces, the Poisson’s ratio and thickness is isolated indepedently from the overlying column of material [Audet et al, 2009]. From these models, we computed moveout curves over the range of slowness for the main scattering modes (Pds and Ppds) observed 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 moveouts represented in 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 Poisson’s ratio of 0.42, and shear velocity contrasts of minus and plus 7.5% into and out of the LVZ, respectively. Bootstrap resampling error estimates for thickness and Poisson’s ratio are ±3km and ±0.011. In combination, our results require an increase in compressional velocity associated with the shear velocity drop 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.
</p><p>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.
</p><p>Reference
</p><p>Audet, P., Bostock, M., Christensen, N. I., and Peacock, S. M. (2009). Seismic evidence for over-pressured subducted oceanic crust and mega- thrust fault sealing. Nature, 457, 76-78.
</p><p>Bercovici, D. and Karato, S. (2003). Whole-mantle convection and the transition-zone water filter. Nature, 425(6953), 39-44.</p>

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