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A Low Velocity Zone Atop the Transition Zone in Northwestern
Canada
Andrew J. Schaeffer (The University of British Columbia, now at Dublin Institute for Advanced Studies), Michael G.
Bostock (University of British Columbia)
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.
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.
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.
Reference
Audet, P., Bostock, M., Christensen, N. I., and Peacock, S. M. (2009). Seismic evidence for over-pressured subducted oceanic crust and megathrust
fault sealing. Nature, 457, 76-78.
Bercovici, D. and Karato, S. (2003). Whole-mantle convection and the transition-zone water filter. Nature, 425(6953), 39-44.
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.
Figure 2: P and SRF pro les for the CANOE and POLARIS-Slave array datasets
computing using analogous techniques to that of YKA. Top panels are SRFS and
bottom panels are PRFs for CANOE (left) and POLARIS-Slave (right). The LVZ can
be seen clearly in the CANOE SRFs at approximately 280 km depth beginning at
station A11 and dipping gently eastwards. The Slave PRF image also clearly illustrates
a potential LVZ at 330km depth in the Northern stations, and potentially
dipping southwards from station MLON.
II-202 | 2010 IRIS Core Programs Proposal | Volume II | Upper Mantle Structure and Dynamics