Crustal anisotropy beneath Hudson Bay from ambient noise tomography: Evidence for post‐orogenic lower‐crustal flow?

Abstract

The crust underlying Hudson Bay, Canada records a long and complex tectonic history. In this study, we investigate this region using tomographic inversion based on continuous ambient noise recordings from 37 broadband seismograph stations that encircle Hudson Bay. The ambient noise data were processed to obtain group‐velocity dispersion measurements from 10–35 s period, which were inverted using an algorithm that incorporates the effects of anisotropy. This work is among the first in which ambient noise data have been used to investigate azimuthal anisotropy. The inversion method uses smoothing and damping to regularize the solution; due to the significantly increased number of model parameters relative to the isotropic case, we performed a careful analysis for parameter selection to determine whether “leakage” occurs between isotropic and anisotropic model parameters. We observe a robust pattern of anisotropic fast directions in the mid‐crust that are consistent with large‐scale tectonic trends based on magnetic‐anomaly patterns. In particular, a distinctive double‐indentor shape for the Superior craton is clearly expressed in both data sets. This pattern breaks down deeper in the crust, suggesting that some degree of lithospheric decoupling in the lower crust, such as channel flow, occurred during orogenesis. Given regional evidence for vertically coherent deformation in the crust and underlying mantle, we interpret this pattern in the lower crust as a tectonic overprint that post‐dates the main phase of Trans‐Hudson deformation. At most levels in the crust, we observe a profound change in direction of anisotropic fast direction across an inferred suture beneath Hudson Bay.