The Mantle Seismic Structure below Canada and Alaska Constrained by a
New Absolute P-wavespeed Tomographic Model
Abstract
Mapping absolute P-wavespeeds in the Canadian and Alaskan mantle will
further our understanding of its present-day state and evolution.
S-wavespeeds are relatively well constrained, especially across Canada,
but are primarily sensitive to temperature while complimentary
P-wavespeed constraints provide better sensitivity to compositional
variations. One technical issue concerns the difficulties in extracting
absolute arrival-time measurements from often-noisy data recorded by
temporary seismograph networks. Such processing is required to ensure
that regional Canadian datasets are compatible with supplementary
continental and global datasets provided by global pick databases. To
address this, we utilize the Absolute Arrival-time Recovery Method
(Boyce et al., 2017). We extract over 180,000 new absolute arrival-time
residuals from seismograph stations across Canada and Alaska that
include both land and ocean bottom seismometers. We combine these data
with the latest USArray P-wave arrival-time data from the contiguous US
and Alaska. Using an adaptively parameterised least-squares tomographic
inversion we develop a new absolute P-wavespeed model, with focus on
Canada and Alaska (CAP21). Initial results suggest fast wavespeeds
characterise the upper mantle beneath eastern and northern Canada. A
sharp transition between the slow wavespeeds below the North American
Cordillera and the fast wavespeeds of the stable continental interior
appears to follow the Cordilleran Deformation Front (CDF) in southwest
Canada. Slow wavespeeds below the Mackenzie Mountains may extend further
inland of the CDF in northwest Canada. In Alaska, CAP21 illuminates both
lithospheric structure and the along strike morphology of the subducting
slab. The newly compiled data may also improve resolution of subducted
slab remnants in the mid-mantle below the North American continent,
crucial to help constrain the formation of the Alaskan peninsular at
≥50Ma.